Chapter 22 — Temporary Structures: Formwork, Shoring, Scaffolding, and Dewatering

The Hook: 7:20 on a Tuesday

Week 34. North elevation, level three.

Ashcroft Masonry has been laying the CMU — concrete masonry unit, what everybody on site calls block — backup wall behind the precast on the north face for nine days. Their frame scaffold runs three levels up the elevation, erected by Ashcroft's own crew, on mudsills, tied to the structure, planked out, guardrails on. It was a good scaffold. I want you to hold onto that. It was a good scaffold, put up correctly by people who knew what they were doing.

Monday night, two electricians from Halcyon Electric came in on approved after-hours work to pull conduit for the exterior lighting at the level-three soffit. We had signed the after-hours request ourselves. They needed to get a fish tape past a horizontal brace, so they lifted one scaffold plank about eighteen inches out of its hooks, set it back down roughly where it had been, and went home at 11:40 at night.

They did not re-secure it. They did not tell anybody. It did not occur to either of them that they had done anything at all.

7:20 a.m., Tuesday. Emiliano Serrano — Milo, everybody calls him Milo, a mason tender, four years in the trade, two kids — steps off the ladder onto the level-three platform carrying a bucket. His right foot lands on the plank. The plank slides in its hooks. His leg drops, he goes down onto the platform, and both forearms catch the top rail on the way.

The top rail holds. It is supposed to hold — a guardrail system is required to withstand a substantial load applied in any outward or downward direction, and this one did exactly what it was designed to do. Milo gets up. He is not hurt. He is not even scraped. He stands there holding the top rail with both hands for about fifteen seconds, and then he says a word I am not going to print, and then he calls his foreman.

7:34 a.m. Bea Salgado, Kestrel's corporate safety director and a former ironworker, is on the elevation. By 7:41 she has stopped all work on the north face. Not the mason crew — the elevation. Nobody goes up until we know what happened.

Here is what I want you to notice about that morning. Nobody was hurt. There is no recordable injury, no lost time, no insurance claim, no phone call to a family. On the daily report it is four lines. On the monthly safety metrics it is a near-miss, one of eleven that month. If Kestrel were the kind of company that measured safety by its incident rate, this event would have cost us nothing and taught us nothing, because on paper nothing happened.

The investigation ran three days and found three failures.

One. The scaffold inspection tag at the base was dated Sunday. It was Tuesday. A competent person had not inspected that scaffold before that shift, which is precisely what a competent person is required to do.

Two. A trade that did not erect the scaffold modified it and did not report it. Halcyon's electricians altered a structure that belonged, in every practical sense, to Ashcroft Masonry — and nobody re-inspected it before it was reoccupied.

Three. Ashcroft was eleven working days behind on the north elevation. They were behind because we had accelerated the steel after the twenty-three-day fabrication slip, and the resequenced enclosure pushed their start, and nobody at Kestrel had ever said out loud, or in writing, that Ashcroft was now expected to make it up. Nobody had to say it. Everybody knew.

Findings one and two are engineering and procedure. You fix them with a tag, a rule, and a checklist, and you can fix them by Friday.

Finding three is management. It is the one that took four days to get into the report, because it required somebody at Kestrel — me, actually — to write down that a decision made in a trailer in week 31, about money and about days, put a man on an unsecured plank in week 34.

That is this chapter. Roughly two-thirds of it is engineering: what temporary structures have to carry, how they are designed, how they fail, and what the arithmetic is. The last third is management, because a perfectly engineered temporary structure that gets modified by a stranger at eleven at night is not a structure at all. It is a trap with a certificate.

🏃 Fast Track: If you have run concrete work, skim §22.1 and go straight to §22.2.3 (the form cycle and the cycle-time arithmetic), §22.2.6 (reshoring — the section most experienced people get wrong), and §22.9 (the management system and the register). The 📋 Try it cycle drill in §22.2.3 is worth twenty minutes at any experience level.

🔬 Deep Dive: Concrete and steel as systems — what you are forming and erecting — are Chapter 8. Scaffold access as part of the enclosure sequence is Chapter 9. The safety-system argument this chapter only opens is finished in Chapter 24; competent-person duties and inspection checklists are collected in Appendix F. The arithmetic conventions are Appendix A.

What this chapter gives you. You will be able to explain what formwork actually resists and why placement rate is the variable that blows forms; compare form systems and pick one for a given structure; build and price a form cycle, and show the schedule and dollar consequence of a two-day difference; run the arithmetic on buying versus renting a form set; state the strength criterion for stripping and name who has authority to release it; explain reshoring and backshoring correctly, including the multi-floor load path; select excavation support and dewatering methods; write a scaffold inspection procedure and a modification-control rule; and build a temporary-structures register that names, for every temporary structure on your job, who designed it, who inspects it, and who is allowed to change it.

22.1 Four Things That Make Temporary Structures Different

A temporary structure is an engineered structure that happens to be demolished at the end. That sentence sounds like a definition. It is actually an argument, and most of the failures in this chapter happen because somebody did not believe it.

Formwork, shoring, reshoring, scaffolding, excavation support, erection bracing, and dewatering systems carry real loads, are designed by real engineers, are governed by real standards, and are inspected. What makes them different from the permanent building is not that they are less serious. It is these four things.

1. They are designed for a short service life and one load case. A permanent floor is designed for fifty years of an occupancy load it will probably never see. A deck form is designed for eleven days and exactly one event: the weight of wet concrete, the crew, the buggies, and the wind on a Tuesday morning in October. There is no reserve built up from decades of conservative code evolution, no redundancy from partitions and finishes that stiffen the structure, and no second load path. The margin is what the designer put in, and nothing more.

2. They are assembled by people who did not design them. The formwork engineer's drawing says the shores are at 4 feet 0 inches on center with a stated base condition. The carpenter setting them at 6:40 in the morning in the rain has a drawing, a crew, and a schedule, and the drawing is the only one of the three that knows why 4 feet matters. This is not a criticism of carpenters. It is a description of the gap that management exists to close.

3. They are modified in the field, constantly. Nobody cuts a hole in a permanent column to run a pipe. People cut, move, lift, shim, and borrow from temporary structures every day, because temporary structures are in the way of the work and everybody knows they are temporary. The word "temporary" does enormous damage here. It reads as provisional, and people treat provisional things casually. Milo's plank was moved by two competent electricians who would never in a hundred years have removed a permanent guardrail.

4. Their failure is sudden, not progressive. A permanent structure warns you. It deflects, it cracks, a door starts to stick, and somebody calls an engineer. A form blows in under two seconds. A shore stack goes without a sound. A trench wall gives no notice at all — the first indication is that the wall is now on top of a person. There is no window in which to notice and react, which means the only place you can manage the risk is before.

Put those together and you get the management consequence, which is the whole chapter in one line:

💡 Aha moment. A temporary structure needs the same four things a permanent one needs — a design, a drawing, an inspection regime, and a named person responsible for it — and it needs them more urgently, because it has less margin, more handling, and no warning. The only thing you get to skip is the fifty-year durability.

There is a reason to take this seriously beyond the moral one. Industry and regulator reviews of construction failures have found for decades that a disproportionate share of catastrophic structural collapses during construction — the multiple-fatality events, the ones that make national news — originate in temporary works rather than in the permanent structure: formwork and shoring systems, excavation support, and structures that were not yet laterally braced. The permanent building, designed by a licensed engineer, reviewed by a building official, and inspected, very rarely falls down while you are building it. The thing holding it up while it cannot hold itself up is where the danger lives. And that thing is almost always somebody's means and methods, which means it is almost always yours.

⚖️ What the contract says. In nearly every standard form of construction contract, temporary structures fall under the contractor's means, methods, techniques, sequences, and procedures, and the contractor is solely responsible for them. Read that clause carefully, because it has two edges. The first edge is obvious: it is your risk. The second edge is the one people miss — it is also your authority. The architect cannot tell you how to shore your slab, and if the architect tries, you have a problem to raise immediately and in writing, because a directive that touches means and methods can shift responsibility in ways nobody intended. Where the specification does prescribe something — a specific shoring layout, a required sequence, a stated dewatering method — that is a design decision sitting inside your means-and-methods territory, and you should price it, question it, and get the responsibility boundary written down before you build it.

🔄 Check your understanding. A superintendent says: "It's temporary, it only has to stand for two weeks." Name the two ways that sentence is dangerous.

22.2 Formwork: The Structure That Builds the Structure

On a cast-in-place concrete building, formwork is typically 40 to 60 percent of the in-place cost of the concrete — considerably more than the concrete itself, and usually more than the reinforcing steel. It also sets the schedule, as you saw in Chapter 8. If you manage one temporary structure well in your career, make it this one, because it is the only one that shows up on both the cost report and the critical path.

ACI 347 — the American Concrete Institute's guide to formwork for concrete — is the governing document in most of the United States, and your specification will reference it. It covers design loads, materials, construction and use, shoring and reshoring, and removal criteria. Get a current copy. Do not work from a formwork number you memorized in school, and do not work from an old company standard, because the guidance and the concrete chemistry have both moved.

22.2.1 What Formwork Actually Resists

Formwork carries four categories of load, and only one of them is obvious.

The one that blows forms is lateral concrete pressure, and the reason is that most people's intuition about it is wrong.

Fresh concrete, while it is still fluid, behaves like a liquid — a heavy one. Concrete weighs roughly 150 pounds per cubic foot (pcf). So a column of fluid concrete produces a pressure at its base of about 150 pounds per square foot (psf) for every foot of fluid depth. That is not a code provision. That is hydrostatics.

Now the part that decides everything. Concrete does not stay fluid. It stiffens, and once it stiffens it begins carrying part of its own weight through internal friction and arching instead of pushing on the form. So the real design pressure depends on how much of the pour is still fluid when the top of the pour arrives — which is entirely a function of how fast you place it and how fast the mix sets.

The variables, in the order they will bite you:

• Rate of placement. Place a 12-foot wall in four lifts over three hours and the bottom is stiffening while you fill the top; the design pressure can be well under full liquid head. Place the same wall in twenty-five minutes because the pump is on site and the crew wants to go home, and the whole 12 feet is fluid at once: 1,800 psf, potentially against a form designed for half of that.
• Temperature. Cold concrete sets slowly. At 40°F a mix stays fluid far longer than the same mix at 80°F, so cold weather increases the design pressure at the same placement rate. Winter is when forms blow.
• Admixtures. Retarders, high-range water reducers, and hydration stabilizers all extend the fluid period. Nobody tells the formwork designer when the mix gets adjusted for a hot day.
• Self-consolidating concrete (SCC). SCC flows, and it stays flowing. Treat SCC as full liquid head unless your formwork engineer and the concrete supplier have specifically agreed otherwise in writing. This is one of the most common causes of blown forms on modern jobs, because the crew sees a mix that places beautifully and has no idea what it is doing to the ties.
• Consolidation method. Internal vibration re-liquefies concrete that had begun to stiffen. Over-vibrating, or revibrating deep into a previous lift, can drive the pressure back toward full liquid head at that depth.
• Cement type, slump, and mix constituents. All of them affect setting time, and therefore fluid depth.

The published relationships in ACI 347 give a design pressure as a function of placement rate, temperature, and mix characteristics, with full liquid head as the absolute ceiling and reductions permitted only within stated limits. Use the current edition, and use it through your formwork engineer, not from memory.

🔍 Why this works. Here is the mechanism, because knowing it will keep you out of trouble even after you have forgotten every number above. Fresh concrete transitions from a suspension to a solid. While it is a suspension, the aggregate is effectively floating and the whole mass transmits pressure in all directions like a fluid. As hydration begins and particles interlock, the mass develops shear strength — it starts carrying some of its own weight by arching between the form faces, exactly like grain in a silo. The design question is therefore never "how tall is the wall." It is "how deep is the part that is still a liquid when you reach the top." That depth is set by two clocks running against each other: how fast you fill, and how fast it stiffens. Anything that speeds up the first clock or slows down the second raises the pressure. A pump truck speeds up the first. A cold morning and a retarder both slow down the second. Put all three on the same pour and you have quietly doubled a load on a structure that was designed once, in an office, weeks ago.

💰 Money check: the pour that got ahead of the form. A 10-foot-high, 60-foot-long foundation wall, formed with a gang system designed for a 3-foot-per-hour placement rate. The crew has a pump, the truck rotation is running early, and they place the wall in 40 minutes — a rate of about 15 feet per hour. The form does not fail spectacularly. It deflects, ties elongate, and one panel joint opens about an inch and a half over a 12-foot length before anybody shuts the pump down.

Nobody was hurt, which is luck rather than management — a fully fluid 10 feet is 1,500 psf against a form designed for a fraction of it, and forms that let go at that pressure throw hardware. The prevention cost nothing at all: a placement rate written on the pour card, and a foreman with authority to slow the pump down. The most expensive thing on a construction site is a crew going faster than the plan and being praised for it.

⚠️ Safety alert — the placement is the dangerous hour. Nobody should be inside, under, or directly alongside a form during placement except the people whose work requires it. Set a watch: one competent person whose only assignment during the pour is to watch the forms, the ties, the shores, and the bracing for movement, with authority to stop the placement instantly and without asking anyone. Not the foreman running the pour — a separate set of eyes with nothing else to do. Formwork almost always talks before it fails: a tie creaking, a wale bowing, a shore going out of plumb, water and paste bleeding at a joint that was tight ten minutes ago. Somebody has to be looking, and that somebody must not also be trying to keep the trucks moving. OSHA's construction requirements for concrete and masonry work, including formwork, shoring, and reshoring, sit in 29 CFR 1926 Subpart Q; the governing design requirement is that formwork be designed, fabricated, erected, supported, braced, and maintained so that it will support all vertical and lateral loads that may reasonably be anticipated.

22.2.2 The Form Systems, and Where Each One Fits

Three management observations the table cannot show you.

First, the labor column is the money column. A flying form costs several times what job-built plywood costs per square foot to buy or rent, and on most repetitive structures it is dramatically cheaper anyway, because forming labor dominates. When somebody proposes a "cheaper" form system, ask what it does to man-hours per square foot of contact area — not what it does to the rental invoice.

Second, crane dependency is a schedule risk you are creating. Gang, flying, and jump forms all trade carpenter hours for crane hours. That is usually a good trade, right up until the crane is the constraint — at which point you have converted a labor problem into a critical-path problem. Before you select a flown system, count the picks per cycle and put them on the crane schedule from Chapter 21. I have watched a flying form system get chosen for speed on a job with one tower crane and a heavy façade package, where the two competed for the same hook and both lost.

Third, stay-in-place metal deck is formwork, and people forget it. Composite floor deck on a steel building is a temporary structure for about four hours and a permanent one forever after. During those four hours it has an unshored span limit, a construction-load limit, and a required attachment pattern, all of which live on the deck manufacturer's tables and the erection drawings. Concrete placed too deep on an unshored deck — because the deck deflected, so the crew added concrete to bring the surface up to elevation, which made it deflect further — is a genuine and recurring construction-load problem with a name: ponding. It is why the placement sequence and the depth checks matter, and it is worth a specific conversation before your first deck pour.

🔄 Check your understanding. Your estimator has priced job-built plywood forms for a twelve-story cast-in-place residential tower because the material cost per square foot is the lowest of any system on the market. Explain, in two sentences, why that is probably the most expensive decision on the estimate — and name the number you would ask him to produce before the bid goes out.

22.2.3 The Form Cycle — Where the Schedule Actually Lives

Chapter 8 introduced the form cycle and told you it sets the schedule of a concrete frame. Now we manage it.

A cycle is the repeating sequence of operations that produces one floor. It repeats identically, floor after floor, which is what makes it both a beautiful production system and a brutal amplifier of mistakes: whatever you do wrong, you do it once per floor, and the frame has as many floors as it has.

   THE FORM CYCLE — ONE FLOOR, WITH THE MANAGEMENT CONSTRAINT ON EACH STEP

   1 FORM      →  2 REINFORCE  →  3 EMBED &    →  4 PLACE      →  5 CURE
     Set shores    Place rebar,     SLEEVE          Concrete,       Protect,
     deck, edge    curb bars,       MEP sleeves,    finish,         monitor
     forms,        dowels           inserts,        make            temp,
     bulkheads                      embeds,         cylinders       break
                                    hangers                         cylinders
   ─────────────────────────────────────────────────────────────────────────
   CONSTRAINT:   CONSTRAINT:     CONSTRAINT:     CONSTRAINT:    CONSTRAINT:
   carpenter     rebar delivery  MEP coordina-   crane/pump     concrete
   crew size,    + inspection    tion + the      hours, truck   strength,
   crane picks,  hold point      inspection      supply,        temperature,
   form sets     (Ch 23)         hold point      finishers      cylinder lab
   ─────────────────────────────────────────────────────────────────────────
                          ↓                             ↓
   8 MOVE      ←  7 RESHORE    ←  6 STRIP
     Fly/hoist     Replace          Remove forms
     the set to    shores per       only on a
     the next      the engineer's   written strength
     floor         scheme           release
   ─────────────────────────────────────────────────────────────────────────
   CONSTRAINT:   CONSTRAINT:     CONSTRAINT:
   crane hours   reshore         WHO SIGNS?  ← the single most
                 inventory,      abused step in the whole cycle
                 shore levels
   ─────────────────────────────────────────────────────────────────────────

              REPEAT × (number of elevated floors)

Two things about that diagram deserve your attention.

The cycle is a loop of hold points, not a list of tasks. Steps 2, 3, and 6 each contain a gate that somebody outside your crew controls — a rebar inspection, an MEP sign-off, a cylinder break. Every gate is a place where a two-hour delay becomes a one-day delay, because you cannot place concrete at 3 p.m. and finish it in the dark with a crew that started at six.

Step 6 is where the money and the danger meet. Stripping early gains you cycle time. Stripping early is also the single most common cause of formwork collapse. We will come back to it in §22.2.5, and you should notice now that the schedule pressure and the safety risk push in exactly opposite directions at the same moment on the same step. That is theme 4 in a diagram.

The cycle-time arithmetic

The relationship is simple, which is what makes it powerful:

Frame duration (work days) = number of elevated floors × cycle time (work days per floor)

Everything else in this section is a consequence of that one line. A one-day change in the cycle changes the frame by one day per floor. On a six-floor structure that is six work days. On a twenty-two-floor structure it is twenty-two — about a month of calendar time from a single day.

📋 Try it: two cycles, one building, and the price of two days

Kestrel is pricing a cast-in-place structure: six elevated floors at 16,500 SF each (99,000 SF of elevated deck). You own one full-floor set of deck forms and shoring. The estimating team has given you two candidate cycle times, and Jamal Foster, the self-perform concrete superintendent, will run whichever you buy.

Compute four things. (1) The frame duration under each cycle, in work days and calendar days. (2) The general-conditions difference. (3) Whether the second form set is justified. (4) Then the part that matters more than the arithmetic: name at least five things that change when you compress a cycle from 7 days to 5. Give yourself twenty minutes before opening the answer.

🧩 Productive struggle. Before you read the next paragraph: your 5-day cycle has been running at 6.5 days for three floors. The carpenter foreman says he needs two more carpenters. Take three minutes and write down the four questions you would ask before approving them.

Here are mine. (1) Which step of the cycle is actually long? Get the eight steps timed for the last three floors. If forming is on plan and stripping is running two days late waiting on cylinder breaks, two more carpenters buy you nothing at all — you have a laboratory problem, not a labor problem. (2) Is the crew waiting or working? A crew that is idle 90 minutes a day for material has a logistics constraint, and adding people multiplies the idleness (Chapter 18). (3) What does the added crew do to the crane? More carpenters mean more picks; if the hook is already the constraint you have made it worse. (4) What is the marginal productivity? The eleventh and twelfth carpenter on a congested deck do not produce what the first ten did. If the added crew moves the cycle by half a day, price the half day at $5,150 and compare it with two carpenters for the rest of the frame.

Nine times out of ten, a cycle that has slipped has slipped at a hold point, and hold points are not fixed by adding labor. They are fixed by scheduling the inspector, pre-ordering the mix, or getting the MEP sleeve layout approved a floor ahead. That is the difference between managing a cycle and staffing one.

22.2.4 Reuse Economics: Buy or Rent

Formwork is one of the few things on a job where the same physical object gets used ten, fifty, or two hundred times, which means the interesting number is never the purchase price. It is the cost per use per square foot of contact area (SFCA).

Ownership cost per use = (purchase price − residual value + maintenance) ÷ (contact area × number of uses)

Work it with real numbers. You need 6,000 SFCA of gang wall forms for a job that will use them 8 times, over a forming period of 4.5 months.

Buying wins by $8,050 on this job — about 11 percent. And now the four things the table does not say, every one of which has reversed this decision on a real job:

1. Residual value is a forecast, not a fact. It assumes a next job with compatible geometry inside the useful life of the panels. If Kestrel's backlog has no concrete frames for eighteen months, the panels sit in the yard depreciating and taking up space, and the honest residual is lower.
2. Purchase consumes working capital. $132,000 of cash leaves in month one and comes back over years. On a company with $6.8M of cash and a $15M line, that is a real decision, and it belongs to the CFO, not the project manager. Cash is not profit — that argument is Chapter 34.
3. Rental clocks run on calendar time, not on use. This is the one that gets people. A rented form set sitting idle through a three-week weather delay bills every day of it. On a job with schedule risk, rental converts a schedule problem directly into a cost problem — which is theme 2 in its purest form. Always ask what the rental costs during the delay you are already worried about.
4. Rented forms come back to somebody who inspects them. Damage billing on returned formwork is a real and frequently disputed cost. Photograph the set on arrival, and again on return.

💡 Aha moment. Every own-versus-rent question in construction — forms, cranes, hoists, trench boxes, pumps — is really the same question with different units: how many uses will I actually get, and what is this worth to me on the day I no longer need it? You saw it for equipment in Chapter 21. Formwork just runs the arithmetic per square foot instead of per hour.

22.2.5 Stripping and Early Loading: "It Looked Hard" Is Not a Criterion

There are two different removal events, and confusing them is dangerous.

Removing side forms — the vertical faces of a wall, a column, or a beam — releases nothing structural. The member is holding itself up already. The criterion is whether the concrete has enough strength to resist damage from the stripping operation itself: no spalling, no edge damage, no surface tearing. This threshold is comparatively low, and on many jobs it is a matter of hours.

Removing shores — anything that is holding up an elevated slab or beam — is a structural event. The moment the last shore under a bay is loosened, that slab carries its own weight, plus everything stacked on it, plus whatever is coming down through the shores from the floor above. The criterion is specified strength, stated in the contract documents, and it is not negotiable in the field.

Your specification will state the criterion in one of two forms: a required percentage of the specified 28-day compressive strength (f'c), commonly in the range of 70 to 75 percent for removing shores under an elevated slab, or a required strength in pounds per square inch that the structural engineer has calculated for the actual construction loading. Read yours. Use yours. Do not use the number from your last job, because the criterion is a function of that slab's design, span, and construction loads.

Now the part that matters operationally, and that I see done wrong constantly:

You must break the right cylinders. A concrete test cylinder cured under controlled laboratory conditions tells you about the mix. It says the concrete you bought is capable of reaching the strength you specified. It tells you almost nothing about the strength of a slab that spent four nights at 38°F under a tarp. To make a stripping decision, you need field-cured cylinders — cylinders cured on the deck, next to the member, under the same temperature and moisture history — or an accepted maturity method correlating temperature history to strength. The lab cylinder is for acceptance. The field cylinder is for stripping. Two different questions, two different sets of cylinders, and if your testing agreement only bought you the first kind, you have a problem before the first cold snap.

Somebody's name goes on the release. Here is Kestrel's rule, and I recommend it without reservation:

No shore or reshore is removed anywhere on this project until the concrete superintendent has a written strength result in hand that meets the specified criterion, and signs a shore-release form naming the floor, the bay, the cylinder identification, the break result, the criterion, and the date. The form goes in the project record. Nobody else may release shores. Not the foreman. Not the project manager. Not me.

That rule exists because of a conversation I do not enjoy remembering. A carpenter foreman on a job years ago needed panels on Friday afternoon for Monday's pour and told me the deck "was hard, you could park a truck on it." He was not lying and he was not reckless; he was doing what people do, which is substituting a reasonable-sounding observation for a measurement. Concrete gains most of its strength in the first week and a surface can feel entirely solid at 40 percent of f'c. Hardness under a boot heel is a measurement of the top quarter-inch. The question is what the member can carry in flexure and shear across a 28-foot span with a load coming down through the shores from a fresh pour above.

⚠️ Safety alert — stripping and the load you cannot see. Before any shore comes out, ask what is on top of the slab you are about to release. A slab at 70 percent of design strength may be perfectly adequate for its own weight and completely inadequate under 40 stacked bundles of rebar, a pallet of block, and a power buggy. Construction loads on a young slab are routinely heavier per square foot than the occupancy load the slab was designed for. Two rules: material stacking locations on a fresh deck are an engineered decision, not a convenience decision — get them on a drawing, over columns and beam lines, not at midspan — and nothing gets stacked on a floor whose shores are about to be pulled. Walk it before you release it.

🔄 Check your understanding. The lab calls at 2 p.m. Friday: the 7-day cylinder for the level-4 deck broke at 3,180 psi. The specified 28-day strength is 4,500 psi, and the specification requires 75 percent of f'c before shore removal. The carpenter foreman wants to strip level 4 before the weekend. What do you say, and what is the second question you ask?

22.2.6 Reshoring and Backshoring — The Section Most People Get Wrong

This is the most widely misunderstood topic in this chapter, and it is a genuine killer. Read it twice.

Start with the problem. On a fast cycle, you place floor 4 roughly a week after floor 3 and roughly two weeks after floor 2. When you place floor 4, its wet weight plus the forms plus the crew plus the buggies has to go somewhere. It goes down through the shores, into floor 3 — which is seven days old, has not reached design strength, and was never designed to carry another entire floor by itself.

So you leave shores in below floor 3 as well, and the load distributes into floor 2. And maybe floor 1. The construction load path runs through multiple floors of a partially built structure, and it must be analyzed as a system. That is the whole idea, and it is the idea people skip.

Now the two terms, which are not interchangeable:

Reshoring. The original shores are removed, the slab is allowed to deflect under its own weight, and then shores are placed back and snugged — tight enough to bear, not tight enough to lift the slab. After reshoring, the slab is permanently carrying its own dead load; the reshores carry only the additional construction loads that arrive later.

Backshoring. The original shores are removed and replaced in a sequence that never lets the slab deflect — a small area at a time, so the slab is never asked to carry even its own weight alone. Backshores are typically installed before the slab has reached the strength required for reshoring.

        RESHORING                          BACKSHORING
   ┌───────────────────┐            ┌───────────────────┐
   │  Fresh floor N    │            │  Fresh floor N    │
   ├──┬─────────────┬──┤            ├──┬─────────────┬──┤
   │  ║   shores    ║  │            │  ║   shores    ║  │
   ├──┴─────────────┴──┤            ├──┴─────────────┴──┤
   │  Floor N−1        │            │  Floor N−1        │
   ├──┬─────────────┬──┤            ├──┬─────────────┬──┤
   │  ║  RESHORES   ║  │            │  ║ BACKSHORES  ║  │
   │  ║  installed  ║  │            │  ║ installed a │  │
   │  ║  AFTER the  ║  │            │  ║ small area  │  │
   │  ║  slab has   ║  │            │  ║ at a time,  │  │
   │  ║  deflected  ║  │            │  ║ slab NEVER  │  │
   │  ║  on its own ║  │            │  ║ deflects on │  │
   │  ║             ║  │            │  ║ its own     │  │
   ├──┴─────────────┴──┤            ├──┴─────────────┴──┤
   │  Floor N−2        │            │  Floor N−2        │
   └───────────────────┘            └───────────────────┘

   Slab N−1 carries its OWN weight;   Slab N−1 never carries its own
   reshores carry only ADDED loads.   weight alone; backshores share it.

Five rules that follow from the mechanism, all of which show up in real failures:

1. The reshoring scheme is an engineered design. How many levels, where the shores land, what they are made of, and how they are loaded is a calculation by a qualified engineer — usually a specialty formwork engineer working for the concrete contractor — accounting for the actual construction loads, the actual concrete ages, and the actual sequence. ACI 347 addresses shoring and reshoring; the design is not a rule of thumb and it is not a field decision.
2. Shores in a stack must line up vertically. A reshore that lands at midspan of the floor below, instead of over a column or beam line, has just applied a concentrated construction load exactly where the young slab is weakest. Alignment through the stack is a checklist item on every floor.
3. Reshores are snugged, not jacked. Over-tightening a reshore lifts the slab above and loads the slab below in a direction and magnitude nobody designed. There is a right amount of tightening, and the crew needs to be told what it is.
4. The removal sequence is part of the design. How and in what order shores come out changes how the load redistributes. Pulling all of one level at once, or working the wrong direction across a bay, can transfer more load to a young floor than the scheme assumed.
5. The bottom of the stack matters. Shores bearing on a slab on grade, on backfill, or on a basement slab that was not designed for concentrated construction loads is a real failure mode. Ground bearing pressure was Chapter 21's problem with a crane outrigger under it; it is the same physics here with a shore under it, and the same question: what is the load, over what area, on what material?

🔍 Why this works — and why the failures are catastrophic rather than local. In a completed building, floors are independent: each one carries its own loads to the columns, and a problem on floor 4 stays on floor 4. In a shored structure under construction, floors are coupled. The shore stack ties three or four young slabs into one load-sharing system, and the sharing is by relative stiffness — the stiffer, older floor takes more. Now consider what happens if one floor in that stack is overloaded and fails. Its load does not disappear; it is delivered instantly and dynamically to the floor below, which is younger and weaker and was already carrying its share. That floor fails, delivering two floors' worth of load to the next one down. This is the mechanism behind the multi-floor progressive collapses that have historically killed the most people in cast-in-place construction, and it is why the reshoring scheme gets an engineer and a drawing rather than a conversation. Coupling is what turns a local overload into a building.

22.2.7 How Formwork Fails — The Recurring List

Investigations of formwork failures return the same causes with depressing regularity. Every one of these is a management failure at least as much as an engineering one.

Look at the middle column and notice what almost all of it has in common: it describes somebody doing a sensible-sounding thing quickly. Nobody in that column is careless. They are all solving a real problem in front of them, using a structure they did not design and cannot see the calculations for. That is why the controls in the right-hand column are procedural rather than motivational. You do not fix this by telling people to be careful. You fix it by making the structure impossible to change without a conversation.

22.3 Shoring and Excavation Support: Holding Back the Ground

Soil is a structural material with one inconvenient property: it has almost no tensile strength, so a vertical face of it is held up entirely by friction and cohesion, both of which change with moisture, vibration, time, and whether somebody parked an excavator next to it. Chapter 8 introduced the systems. Here is what a manager needs to decide and control.

Three decisions belong to you as the manager, and only one of them is technical.

Which system, and who designs it. Excavation support is normally a delegated design: a specialty subcontractor engineers it, a professional engineer licensed in the jurisdiction stamps it, and it becomes a submittal with a review cycle and therefore a lead time. Put it on the procurement log in preconstruction (Chapter 17), because the hole cannot open until the support design is approved, and nothing else on the job can start until the hole is open.

Where the tiebacks go, and whose dirt they are in. A tieback that extends 40 feet behind your wall is usually 40 feet under somebody else's property. That requires an easement, negotiated in advance, often for money, and it takes weeks. Contractors discover this constraint late with remarkable consistency.

What the support does to the neighbors. Every excavation support system moves a little. Movement behind the wall shows up as settlement on the adjacent property. That means monitoring points, survey readings on a schedule, agreed thresholds for action, and — before you break ground — the pre-construction condition survey from Chapter 17. Skip the survey and every crack the neighbor's attorney can find becomes yours, whether you caused it or not.

⚠️ Safety alert — excavation and trenching. Cave-in is one of OSHA's Focus Four construction hazards, and it is the one with the worst survival rate. A cubic yard of soil weighs on the order of 3,000 pounds — roughly a small car — and it arrives without warning. Under the U.S. construction standard for excavations (29 CFR 1926 Subpart P), the requirements that a manager must know cold include:

• A protective system is required in excavations 5 feet deep or greater, unless the excavation is entirely in stable rock. Below 5 feet a competent person still evaluates whether protection is needed.
• Excavations 20 feet deep or greater require a protective system designed by a registered professional engineer.
• A competent person must inspect the excavation, the adjacent areas, and the protective systems daily before each shift, as needed throughout the shift, and after every rainstorm or other hazard-increasing event — with authority to remove workers immediately.
• Soil classification governs the allowable slope, and the classification is made by that competent person using visual and manual tests. It is a judgment about this soil, today, in this weather — not a number copied from the geotechnical report.
• Spoil piles, equipment, and material are set back from the edge — at least 2 feet, and often much more, because the setback that matters is the one that keeps the surcharge load off the failure wedge.
• Means of egress — a ladder, stairway, or ramp — in trenches 4 feet deep or greater, within 25 feet of lateral travel of every worker.
• Water, atmosphere, and adjacent structures are all separate hazards with their own requirements: no work in accumulated water without controls, atmospheric testing where a hazardous atmosphere could exist, and support or underpinning for any structure whose stability the excavation could affect.

Requirements outside the United States differ in their numbers and their structure, so check your jurisdiction. But no jurisdiction lets you send someone into an unprotected 6-foot trench, and no schedule pressure on earth is worth what happens if you do. The trench that kills somebody is almost always the one where somebody was in a hurry and it was "only going to take twenty minutes."

🏗️ From the field. I once watched a crew of three drop into an unshielded five-and-a-half-foot trench to make one coupling. The box was thirty feet away on the ground. Their foreman's reasoning was not "safety doesn't matter" — it was "moving the box takes fifteen minutes and this takes five." He was correct about the arithmetic and catastrophically wrong about the question. What made it fixable was not a lecture; it was changing the sequence so the box moved with the crew as a matter of course, which cost about twenty minutes a day and removed the decision entirely. The best safety controls are the ones that stop being decisions.

🔄 Check your understanding. A crew is working in a 7-foot-deep trench in a soil the competent person classified this morning as Type B. It rained hard over lunch. Water is standing in the bottom of the trench, and the spoil pile — which has been growing all week — is now about 18 inches from the edge. Name four things that are wrong and say which one you fix first.

22.4 Scaffolding: The Structure Everybody Borrows

Scaffolding gets less engineering attention than formwork and causes more injuries, for a reason that has nothing to do with engineering: scaffolds are shared. A form is used by the crew that built it, for one pour, and then it is gone. A scaffold stands on an elevation for eight weeks and is used by masons, then by waterproofers, then by sealant applicators, then by the electrician who needs to get to a soffit at eleven at night.

22.4.1 The Systems

Selection criteria, in the order that actually decides it: what is the work, how high, how long will it stand, what does it need to carry, can the ground or the structure take the reactions, how much access does the elevation need at once, and how many trades will end up using it. That last one is a management question disguised as a technical one, and it is the one most often skipped.

22.4.2 Capacity, and the Numbers a Manager Carries

• Load classes. Scaffolds are rated by intended load: light duty (25 psf), medium duty (50 psf), and heavy duty (75 psf). A masonry scaffold carrying block and mortar is a heavy-duty application; putting block on a light-duty frame because it was the scaffold already on site is a real and common failure.
• Safety factor. Under the U.S. scaffold standard (29 CFR 1926 Subpart L), scaffolds and their components must be capable of supporting at least four times the maximum intended load without failure; suspension ropes and their connecting hardware must support at least six times the intended load.
• Design and supervision. Scaffolds are designed by a qualified person and erected, moved, dismantled, or altered only under the supervision and direction of a competent person, by trained and experienced workers.
• Height-to-base ratio. A supported scaffold whose height exceeds four times its minimum base width must be restrained from tipping — by guys, ties, or braces, installed on a defined vertical and horizontal pattern per the manufacturer or a qualified person.
• Foundation. Base plates on mudsills or another adequate firm foundation. Not blocks, not bricks, not a scrap of plywood, not the top of a pile of backfill. The bearing question is the same one you asked about crane outriggers in Chapter 21: what load, over what area, on what material.
• Platforms. Fully planked or decked between the front upright and the guardrail, with the gap between the platform and the wall limited (and larger only where the work genuinely requires it, with fall protection to match). Planks must be scaffold-grade or equivalent, overlap or extend over their supports by the required amount, and be secured against uplift and displacement.
• Fall protection. On supported scaffolds, guardrail systems or personal fall arrest are required for workers more than 10 feet above a lower level; on suspended scaffolds, both a guardrail system and personal fall arrest. Toeboards or equivalent protect the people below from falling objects. Falls are the largest single category of construction fatalities, and scaffolds are one of the two places they happen most.
• Access. A ladder, stair tower, ramp, integral prefabricated access frame, or direct access from another surface. Cross braces are not a ladder. Climbing the braces is one of the most common scaffold violations and one of the easiest to eliminate, because a stair tower is not expensive and everybody prefers it.
• Electrical clearance. Scaffolds near energized power lines have required minimum clearances that depend on voltage. On any elevation with an overhead line, this gets planned before the scaffold is erected, not discovered by a mason with a 10-foot piece of rebar.

22.4.3 The Competent Person and the Tag

A competent person is defined in the OSHA construction standards as someone capable of identifying existing and predictable hazards in the surroundings or working conditions that are hazardous to employees, and who has authorization to take prompt corrective measures to eliminate them. Both halves are required, and the second half is where employers most often fail: designating a competent person who cannot actually stop the work is designating nobody.

For scaffolds, the competent person's core duties are to supervise erection, moving, dismantling, and alteration; to determine whether it is safe to erect or use a scaffold in specific conditions; and to inspect the scaffold and its components for visible defects before each work shift and after any occurrence that could affect structural integrity.

The tag is how that inspection becomes visible to the two hundred people who did not perform it. A tag system is not a regulatory requirement in itself; it is the practice that makes the requirement operable on a job with twenty trades. Kestrel's version:

The tag carries the date, the shift, the printed name and signature of the competent person, the company that erected it, and the load class. A tag with a name but no date, or a date but no name, is decoration.

22.4.4 The Rule That Would Have Prevented the Near-Miss

Here it is, and it is the most important sentence in this chapter:

A scaffold may only be erected, moved, dismantled, or altered under the supervision and direction of a competent person. Any trade that modifies someone else's scaffold has created an unsafe structure — even if it puts everything back.

Halcyon's electricians did not think they had modified a scaffold. They thought they had moved a board. That gap — between what a person believes they did and what they structurally did — is exactly where the near-miss lived, and no amount of telling people to be careful closes it, because you cannot be careful about a hazard you do not know you created.

What closes it is a modification-control procedure that treats the scaffold as somebody's property with a lock on it:

1. Every scaffold on the job has an owner — the company that erected it — and a named competent person, both printed on the tag and both listed in the temporary-structures register (§22.9).
2. No trade touches another trade's scaffold. Period. Not a plank, not a brace, not a tie, not a guardrail section.
3. If a trade needs a modification, they request it from the owner's competent person, who performs it or supervises it and re-tags the scaffold. This takes about ten minutes and is written into the subcontract as an obligation of both parties.
4. Any modification triggers re-inspection and a new tag before reoccupancy, regardless of how minor it appeared.
5. After-hours work on an elevation with scaffolding requires a specific authorization that names what will be touched, and a next-morning inspection before first use. This is the control that would have caught Milo's plank at 6:45 a.m. instead of at 7:20.
6. Removing a guardrail or a plank for access is a planned event with fall protection, a barricade, a sign, and a restoration check by a named person before the area reopens.

⚖️ What the contract says. Whose scope is the scaffold? This question causes more disputes than it has any right to. Three answers exist and you must pick one in the subcontract, not in month six: (a) each trade provides its own scaffolding for its own work; (b) one trade provides scaffolding "for the use of others," with the users named and a rate for the time; or (c) the general contractor provides a common-use scaffold system as a general-conditions item. Option (b) is where the money and the arguments live — "for the use of others" is meaningless unless the subcontract says who the others are, how long it must stand, what load class it must be, who inspects it, and who pays for standby time if the users are late. This is the scope-gap lesson from Chapter 16 in its most physical form: the gap between two subcontracts is a place where a man can fall.

And here is the part that is not negotiable regardless of which option you pick: the competent-person duty and the inspection duty run to whoever's employees are on the platform. You can allocate the cost of a scaffold by contract. You cannot allocate away the obligation to keep people from falling off it, and you cannot buy an indemnity that makes a collapse somebody else's problem in any way that matters. That is the deep callback to Chapter 6, and it is worth saying plainly.

22.4.5 A Real Scaffold Inspection Checklist

Use this before each shift, per scaffold. It takes four minutes on a frame scaffold once you know it.

Foundation and base 1. Base plates and mudsills present, level, on firm ground; no blocks, bricks, or loose fill. 2. No undermining from erosion, excavation, water, or vehicle traffic since the last inspection. 3. Screw jacks within their allowable extension; legs plumb.

Structure 4. All frames and braces installed per the manufacturer or the design; nothing removed or missing. 5. Coupling pins and pin locks engaged at every joint. 6. Ties, guys, and braces present at the required spacing; height-to-base ratio satisfied. 7. No visibly bent, cracked, corroded, or field-modified components; components from a single system, not mixed.

Platform 8. Fully planked or decked; no gaps beyond the allowable. 9. Planks scaffold-grade, sound, not split, cracked, painted over, or damaged. 10. Planks properly overlapping or extended over supports, and secured against displacement. 11. Gap between platform edge and the wall face within limits for the work being performed.

Fall and falling-object protection 12. Guardrails — top rail, midrail, and posts — complete on all open sides and ends at the required heights. 13. Toeboards, screens, or debris nets where anyone works or passes below. 14. Anchorage points for personal fall arrest identified where required, and independent of the scaffold where the standard requires it.

Access and housekeeping 15. Safe access provided: ladder, stair tower, ramp, or integral access. No cross-brace climbing. 16. Platforms clear of accumulated material, ice, snow, mud, mortar droppings, and debris. 17. Material stacked within the load class and distributed, not concentrated on one bay.

Environment and administration 18. Required clearance from energized lines maintained; no new overhead exposure. 19. No weather condition that makes the scaffold unsafe (ice, high wind, storm damage since last use). 20. Tag current: today's date, this shift, printed name and signature of the competent person, load class. 21. No unauthorized modification since the last inspection — ask the crew directly, every morning: "Did anybody touch this scaffold since yesterday?"

Item 21 is the one that came out of the near-miss. It costs eight seconds and it is worth every one of them, because it turns the crew from users of a structure into witnesses about it.

🔄 Check your understanding. A drywall foreman moves one rolling scaffold tower six feet across a lobby with a laborer standing on the platform, then sets the brakes and goes back to work. Name three separate problems.

22.5 Shoring Existing Structures and Demolition Work

Renovation work adds a category most new-construction managers never meet: temporary support of a structure that already exists, that is carrying real load right now, and whose actual condition is partly unknown.

The techniques are old and mostly self-descriptive. Needle beams are steel beams threaded through openings in a wall to pick up the load above so the wall below can be removed or underpinned. Temporary columns and posts carry a beam or slab while a permanent element is removed and replaced. Dead shores carry vertical load straight down to a foundation or a spreader; raking shores brace a wall laterally from outside; flying shores brace between two walls across a gap. Underpinning extends an existing foundation deeper in a carefully sequenced series of small pits, so that no more than a limited length of the footing is unsupported at any moment.

Four management points, all of which are about information rather than technique:

You are designing against an unknown structure. The as-built drawings are wrong somewhere. Openings have been cut, beams have been notched for ductwork, a bearing wall was removed in 1978 by somebody who did not tell the city. Budget for exploratory demolition and a structural engineer's site investigation before the shoring design, and put it on the schedule as an activity with a duration, because it is one.

Load paths in old buildings are not obvious. In a building that has been renovated four times, the thing that looks like a partition may be carrying a roof, and the thing that looks like a column may be carrying nothing. Never let a demolition crew decide what is structural. The rule is: anything removed above a certain size is removed against a drawing signed by an engineer, and the drawing shows the temporary support that goes in first.

Selective demolition and shoring interlock in sequence. The order is always: install and load the temporary support, verify it, then remove the permanent element. Reversing those two steps for fifteen minutes "just to see" is how people die. Write the sequence into the demolition plan as numbered steps with hold points, and require the engineer's field verification at the defined points.

Vibration and adjacent-property monitoring apply here too. Demolition, breaking, and driving all shake the neighbors. The condition survey and monitoring regime from Chapter 17 is not optional on renovation work in a built-up area; it is the cheapest insurance on the job.

⚠️ Safety alert — demolition and unplanned collapse. Demolition work has its own OSHA construction subpart with an explicit prerequisite: an engineering survey by a competent person of the condition of the structure and the possibility of unplanned collapse, before demolition starts. Treat that survey as the single most important document in the demolition package, and treat any deviation from the demolition sequence — including "we found something unexpected, so we improvised" — as a stop-work event that goes back to the engineer. The pattern in renovation collapses is remarkably consistent: a crew encounters a condition the plan did not anticipate, makes a reasonable adjustment in the field, and removes a load path nobody had analyzed.

22.6 Dewatering and Groundwater Control

Every other temporary structure in this chapter is something you build. Dewatering is something you operate, continuously, and that difference is the whole management problem. A form either holds or it does not. A dewatering system has to keep working at 2 a.m. on the Sunday of a holiday weekend, and if it stops for eight hours you may lose a week.

22.6.1 The Methods, and When Each Applies

The selection logic, in one line: how permeable is the soil, how much drawdown do you need, and what happens to the neighbors if you lower the water table. That last clause turns dewatering from an earthwork problem into a risk problem, which is why it belongs in a chapter on temporary structures rather than in the earthwork estimate.

22.6.2 The Five Management Problems

1. Discharge permitting and water quality. You are taking groundwater out of the ground and putting it somewhere — a storm sewer, a sanitary sewer, a stream, or a recharge system — and every one of those requires permission. Typical conditions include turbidity or total-suspended-solids limits, flow limits, sampling and reporting, and sometimes contaminant testing if the site history warrants it. Meeting the limits usually means settling tanks, filtration, or a treatment train, all of which cost real money and take real space on the site plan. Permit lead times routinely exceed the notice period you will have, so this belongs on the permit matrix in preconstruction, not on the two-week look-ahead. Requirements vary substantially by jurisdiction and by receiving water; get yours in writing and read the conditions, not just the cover page.

2. Settlement of adjacent structures. Here is the mechanism, because it surprises people. Lowering the water table does not just remove water; it removes buoyancy. Soil below the water table is partly supported by pore water pressure. Draw the water down and the effective stress on that soil increases, which consolidates it, which settles everything sitting on it — and the settlement zone extends well beyond your property line. Buildings settle unevenly, which is what cracks them. Controls: monitoring wells to track the actual drawdown, settlement points on adjacent structures surveyed on a defined frequency, thresholds that trigger action, recharge wells to hold the water table up outside your excavation, and — first, always — the pre-construction condition survey from Chapter 17. Without that survey you cannot prove which crack was there before, and you will pay for all of them.

3. Continuous operation. The system runs 24 hours a day, 7 days a week, from the day it starts until the day the structure is heavy enough to resist uplift and the excavation is backfilled. Not during working hours. Continuously. That means somebody checks it on weekends and holidays, and the checking is a scheduled, named, paid task — not a favor.

4. Power redundancy and alarms. A dewatering system is a pump and a power supply, and both fail. The standard package: a standby generator with an automatic transfer switch, spare pumps on site, high-water alarms with telemetry that calls a human being's phone, and a written call-out procedure with names and numbers that somebody has actually tested. If your alarm sends an email to an address nobody reads on Saturday, you do not have an alarm.

5. Cost per day, and the difference between average and marginal. Dewatering is a time-related cost, and you will be asked what an extra week costs. Getting that answer wrong in front of an owner damages your credibility for the rest of the job.

22.6.3 A Worked Dewatering Estimate

Kestrel's Fair Oaks Water Plant Expansion — a $16.2M job in the backlog — has a 26-foot-deep excavation for a new clearwell and pump station. Groundwater sits about 9 feet below existing grade; the design requires drawdown to 2 feet below subgrade, so 19 feet of drawdown. The soil is a silty sand. The specialty dewatering contractor proposes a two-stage wellpoint system, operating for 4.5 months (135 calendar days).

The average daily cost: $308,795 ÷ 135 CD = $2,287 per calendar day.

The marginal daily cost — the one you need when somebody asks what a delay costs. Separate the fixed items (design, mobilization and demobilization, installation of both stages, header, permit, monitoring wells) from the time-related ones:

💰 Money check. If the owner asks what a one-week delay to the clearwell pour costs in dewatering and you answer "about sixteen thousand" — 7 days at the $2,287 average — you have overstated it by more than double, and the first person who checks your arithmetic will discount everything else you tell them for the rest of the job. The correct answer to "what does an extra week cost" is always built from the time-related costs only, because the fixed costs were already spent. This is the same distinction that runs through extended general conditions in Chapter 29 and change-order pricing in Chapter 31: a delay does not re-buy the mobilization.

22.6.4 The Pump That Stopped

💰 Money check: a dewatering failure at 2 a.m. A wellpoint pump on this system fails at about 2 a.m. on a Saturday. There is no generator alarm and no telemetry — the alarm was a light on the pump. Nobody is on site until Monday at 6:30. Fifty-two hours of no dewatering. The excavation refloods to about 4 feet over subgrade, where reinforcing had already been placed for Monday's mat pour.

What would have prevented it: a standby generator with automatic transfer at $2,400/month and a telemetry alarm that dials a phone at roughly $180/month. Over 4.5 months: $11,610. The ratio is about nine to one, and that is before you count the argument with the owner about who owns the delay.

This is the shape of nearly every temporary-structures decision in this chapter. The control is cheap, boring, and easy to value-engineer out of a bid because it produces nothing you can see. The failure is expensive, sudden, and happens at the worst possible moment, because 2 a.m. on a holiday weekend is exactly when nobody is watching. When somebody proposes deleting redundancy from a temporary system, make them write down what happens if the thing they are deleting was needed. That is the whole discipline: not "is it likely," but "what does it cost when it happens, and who is standing there when it does."

🔄 Check your understanding. Your dewatering system has been running for six weeks. The building official's inspector asks why you are still pumping — the excavation is dry and the mat is poured. Give the two reasons you keep pumping, and name the event that would happen if you stopped.

22.7 Temporary Bracing: A Structure Is Most Vulnerable While It Is Being Built

A finished building resists wind and seismic load through a complete lateral system: diaphragms, braced frames or shear walls, connections, and a load path all the way to the foundation. During construction, that system does not exist yet. It arrives piece by piece, and every day until it is complete the structure is standing up because of temporary bracing that somebody designed, installed, and — this is the part that fails — did not remove too early.

Steel erection. The U.S. steel erection standard (29 CFR 1926 Subpart R) codifies several things that are worth knowing as principles, not just as rules. Columns are anchored with a minimum of four anchor rods and are designed to resist a specified minimum eccentric load during erection. The controlling contractor must provide written notification to the steel erector that the concrete in footings, piers, and walls has attained sufficient strength before erection begins — which means somebody on your team has to produce a piece of paper based on cylinder breaks before the first column goes up. Anchor rods must not be repaired, replaced, or field-modified without the approval of the structural engineer of record, which sounds obvious right up until a column will not drop over a bolt at four in the afternoon. And a fully bolted or welded connection is not the same as a stable frame: guys and temporary bracing stay until the permanent lateral system is complete, and the erection sequence and bracing plan are the erector's engineered design.

Precast concrete. Every precast panel and every precast frame member is braced from the moment it is set until the permanent connections and diaphragm are complete. Those braces are engineered — sized for a design wind event, anchored into a slab that must itself have sufficient strength for the brace anchor, at a defined angle and spacing. Two failure patterns recur: brace anchors installed into concrete that had not reached the required strength, and braces removed early because they were in the way of the slab pour or the interior trades. The rule is that brace removal requires a written release from the erection engineer, exactly like a shore release.

Masonry walls. This is the one that kills people, and it deserves its own paragraph. A freshly laid CMU wall has almost no flexural capacity until the grout is placed and cured and the wall is connected to a roof or floor diaphragm. It is a tall, heavy, unreinforced cantilever with a large sail area, and a gust front can take it down in seconds. The recurring fatal pattern in the industry is a tall, ungrouted, unbraced or under-braced wall and a thunderstorm outflow. The controls are well established and genuinely effective:

• A written wall-bracing plan, designed by an engineer, with a stated design wind speed for the bracing.
• A defined restricted zone on both sides of an unbraced or partially cured wall, evacuated when the wind exceeds the stated threshold, with somebody responsible for watching the forecast and calling it.
• A limited-access zone on the unscaffolded side of a masonry wall over 8 feet, per OSHA's masonry requirements — established before construction of the wall begins, running the length of the wall plus a defined distance, entry restricted to the crew constructing it, and maintained until the wall is adequately supported.
• Bracing left in place until the wall is grouted, cured, and permanently supported — released in writing.

The masonry industry publishes a standard practice for bracing masonry walls under construction that gives the engineering basis, the wind criteria, and the evacuation-threshold approach. If you build masonry, get it, and require the bracing plan as a submittal.

📊 Diagram (described): the stability gap. Picture a timeline running left to right along the bottom of a page, from "erection begins" to "permanent lateral system complete." Above it, draw two curves. The first curve, rising slowly from zero, is the permanent structure's own lateral capacity — it stays near zero while columns and beams go up, jumps when the first braced bay or shear wall is completed and connected, and climbs in steps as diaphragms are poured and connections are made, reaching 100 percent at the right-hand end. The second curve, starting high and falling, is the temporary bracing's contribution. Between them, the total must never dip below the demand line — the wind and stability load the structure could actually see on any given day.

The management insight is entirely in the shape. The dangerous moment is not the beginning, when everybody is being careful and the structure is short. It is the middle — the point where the frame looks finished enough that people begin removing braces, but the permanent lateral system has not yet been completed and released. That is when the two curves cross, and it is when somebody decides a brace is in the way of a pour. Every brace removal is a structural act, and it needs the same written release that a shore removal needs. If your job has a written shore-release procedure but no brace-release procedure, you have solved half a problem.

🔄 Check your understanding. The precast erector wants to remove the panel braces on the east elevation because they are in the way of Monday's slab pour, and the panels have been standing for three weeks with all their permanent connections welded. Give your answer and the two documents you need.

22.8 Weather Protection and Temporary Enclosure

The last temporary structure is the one that keeps the weather out of a building that has no walls yet.

Temporary enclosure — reinforced poly on scaffold or wood framing, insulated tarps, temporary doors, hoarding at openings — buys you the ability to do interior work in winter. Temporary heat comes in two flavors, and the difference matters: direct-fired heaters burn fuel and put the combustion products, including moisture and carbon monoxide, directly into the space; indirect-fired heaters burn fuel in a separate chamber and vent the exhaust outside, delivering only clean warm air.

⚠️ Safety alert — temporary heat. Direct-fired heaters in a partially enclosed space are a carbon monoxide hazard, and CO poisoning on construction sites is a recurring cause of multiple-casualty events, particularly in winter, in enclosed stairwells and basements, and any time fuel-burning equipment operates indoors. Three rules: ventilate as the manufacturer requires and do not defeat it with the enclosure you just built; use CO monitoring in enclosed areas with fuel-burning equipment; and never run a gasoline or propane engine — a generator, a compressor, a power trowel, a concrete saw — inside an enclosed space, which is the version of this hazard that kills the most people. Fuel storage, hose condition, and clearance to combustibles round out the list; temporary heat is also a leading cause of construction fires.

There is a quality dimension too. Direct-fired heat adds a great deal of water vapor to the space. Running direct-fired heaters while hanging gypsum board, installing wood flooring, or drying out a slab is actively counterproductive — you are heating and humidifying a building you are trying to dry. That interacts with the moisture and drying requirements in Chapter 9 and with the flooring manufacturer's requirements, which are a warranty condition, not a suggestion.

What it costs. Northgate is dried in on March 28 of Year 2, which means the enclosure work runs through a winter. Twelve weeks of temporary conditioning on the lower floors while the upper elevations are still open:

Now read that bottom number as a schedule statement, because that is what it is. Every week the enclosure slips costs about $11,400 of temporary conditioning that buys no permanent work at all. It produces nothing. It goes in the dumpster in April. And it is on top of the $5,150 a day of extended general conditions, and it does not appear in any of the schedule arguments people usually have, because it is buried in a general-conditions line called "temporary facilities."

This is theme 2 with the numbers filled in: the schedule and the budget are the same conversation. A curtain wall panel that arrives three weeks late does not cost you three weeks. It costs you three weeks of extended general conditions, plus about $34,000 of heating a building with no walls, plus whatever it does to the interior trades stacked behind it.

22.9 The Management System for Temporary Structures

Everything above becomes manageable when you stop treating temporary structures as a collection of unrelated field problems and start treating them as one category with one system. Six questions, asked once per temporary structure, at the start of the job.

1. Who designs it? Usually a specialty engineer engaged by the subcontractor who will build it — the formwork engineer for the concrete contractor, the shoring engineer for the excavation subcontractor, the erection engineer for the steel or precast erector, the dewatering contractor's engineer. Sometimes it is a manufacturer's standard design with published tables, which is legitimate only when the actual conditions fall inside the tables. Sometimes it is your own engineer. What is never acceptable is "nobody" — and "the foreman has built a hundred of these" is a form of "nobody."

2. Whose scope is it in? Check the subcontract (Chapter 16). Formwork is almost always in the concrete scope. Excavation support is almost always in the earthwork or a separate specialty scope. Scaffolding is the ambiguous one, and dewatering is the other ambiguous one — dewatering in particular has a habit of being excluded by the earthwork sub, excluded by the concrete sub, and assumed by you. Scope gaps live between subcontracts, and the gaps in this chapter are the expensive kind.

3. What submittal is required, and how long does it take? Delegated designs are submittals: prepared by the specialty engineer, sealed by a professional engineer licensed in the jurisdiction, reviewed by the engineer of record for conformance with the design intent, and returned. That review cycle is weeks, and the work it gates cannot start without it. Put it on the submittal log with a back-scheduled due date (Chapter 25).

4. Who inspects it, and how often? Name the person and state the frequency. Before each shift for scaffolds. Daily and after every rain event for excavations. Before each placement for formwork. Continuously and at defined intervals for dewatering. Quality holds and inspection regimes generally are Chapter 23; the difference here is that these inspections protect people rather than product, so they do not get skipped when the schedule tightens. They get skipped more when the schedule tightens, which is the whole problem.

5. Who is the competent person, by name? Not "the foreman." A name, with training records behind it, with the authority to stop work, and with a designated backup for the days that person is not on site. A competent-person designation without a named backup fails the first time somebody takes a vacation.

6. Who is allowed to change it — and what happens when somebody does? This is the modification-control rule, and it is the one this chapter's near-miss was about. State it once, in writing, for every temporary structure on the job:

No temporary structure on this project may be altered, moved, cut, loaded beyond its rating, or partially dismantled by anyone other than the erecting contractor's competent person. A trade needing a modification requests it. Every modification triggers re-inspection and re-tagging before reoccupancy. Any modification discovered after the fact is a stop-work event on that structure until the competent person clears it.

The temporary-structures register

One page. Reviewed at the weekly coordination meeting. It changes the conversation from "is the scaffold okay?" to "who is Ashcroft's competent person this week and when was it last tagged?"

A template version of this register lives in Appendix D. Fill in your own job's rows in the first month, and then do the thing that makes it work: read the "competent person" column out loud in a coordination meeting. If anybody in the room cannot immediately name the person in a cell, that structure does not have a competent person — it has a box on a form.

🔍 Why this works. The register does one specific thing that no amount of safety training accomplishes: it converts a diffuse responsibility into a named one. Diffuse responsibility is the normal state of a construction site — twenty companies, overlapping work areas, shared structures, and everybody reasonably assuming somebody else has it. Diffuse responsibility is not a moral failing; it is an information failure, and it has a signature. It looks like a scaffold that four trades use and nobody owns. It looks like a dewatering pump that runs beautifully for six weeks and stops on a holiday weekend because the person who checks it did not work holidays and nobody knew that. The register does not make anybody more careful. It makes the gaps visible before they are occupied, which is the only time anything on a construction site is cheap to fix.

Spaced Review

Answer each one out loud before you read the response. Retrieval is what makes this available at 6:40 in the morning with a crew waiting, which is the only time it matters.

1. From Chapter 21 — ground bearing pressure. You spent that chapter working out what a crane's outrigger does to the ground beneath it: load ÷ bearing area = pressure, compared against the allowable bearing pressure of the material underneath, with mats sized to spread the load until the arithmetic works. Where does that exact calculation reappear in this chapter, three separate times?

Recall first. — (a) Under a shore. A shore delivers a concentrated load to whatever it stands on. If that is a slab on grade, was the slab designed for it? If it is backfill, is it compacted, and is it still compacted after Tuesday's rain? (b) Under a scaffold leg. Base plate on mudsill on firm foundation — same three-part question: what load, over what area, on what material. A scaffold leg on a block or a scrap of plywood on soft ground is the same failure as a crane outrigger on unprepared subgrade, just quieter and closer to people. (c) Under a reshore stack. The load from the fresh floor travels all the way down and lands somewhere, and "somewhere" is often a slab or a grade that nobody analyzed. The physics never changes. Only the units and the consequences do.

2. From Chapter 8 — the form cycle sets the schedule. Chapter 8 told you that on a cast-in-place frame the cycle, not the concrete, sets the schedule. State the relationship as a formula, and then state the two things that most commonly make a planned cycle run long.

Recall first. — Frame duration = number of elevated floors × cycle time. A one-day change in cycle time moves the frame by one day per floor, which is why two days of cycle on a six-floor structure was worth $87,550 in the drill above, and why the same two days on a twenty-two-floor tower would be worth roughly a month. The two things that most often stretch a cycle are hold points, not labor — waiting on a cylinder break, an inspection, or an MEP sleeve sign-off — and a stripping criterion the mix cannot meet on schedule, which is a mix-design decision made in preconstruction that shows up as a schedule problem in month eight. Neither one is fixed by adding carpenters, which is why the 🧩 drill in §22.2.3 asked you which step was long before you approved anybody.

3. Deep callback to Chapter 6 — the risk you cannot transfer. Chapter 6 gave you five responses to any risk: avoid, transfer, mitigate, accept, exploit. Row R-15 of the Northgate register — serious injury on the north elevation — was the one row marked differently from every other row on the page. What did it say, and why does this chapter prove it?

Recall first. — R-15 was assigned to Bea Salgado, and its response was mitigate — and only mitigate. The disposition column read: never transferred, never accepted. Every other risk on that register could be moved to somebody by a clause, or funded by contingency and carried on purpose. This one could not, and this chapter is why. You can put scaffolding in the masonry subcontract. You can require the specialty engineer's seal on the shoring design. You can carry insurance, require indemnity, and demand additional-insured status on every certificate in the file. And on the Tuesday morning in week 34 when Milo Serrano's foot came down on that plank, not one of those instruments moved one pound of the load, or was standing on the elevation, or would have been the thing anybody remembered if the top rail had not held. Financial risk transfers. Physical risk stays exactly where the person is standing. That is the sentence to carry out of this chapter, and it is what Chapter 24 is built on.

Project Checkpoint: The Willow Street Temporary-Structures Plan

In Chapter 21 you built the equipment plan for the Willow Street Community Center — own versus rent, lift selection, and an equipment schedule with costs. That plan told you what machinery is on the site. This one tells you what structures are on the site that will not be there at the end, and who is responsible for each of them.

Recall the project: $6,800,000, 24,000 SF, two stories, wood-framed second floor over a structural steel and CMU first floor, 425 calendar days, liquidated damages $1,200 per calendar day, City of Rivermont Parks & Recreation, prevailing wage, a flat 2.1-acre site, and one existing 8-inch water main to relocate. Full package in Appendix K.

Deliverable: a five-part temporary-structures plan. File it in the Project Notebook behind the equipment plan.

Part 1 — Formwork approach and cycle. Willow Street is not a concrete frame, so do not force a tower-building answer onto it. Your concrete work is footings, foundation walls and grade beams, the slab on grade, and any elevated slab at the locker rooms or mechanical platform. For each: name the form system (job-built, handset panel, or gang), state the expected number of reuses, and decide buy or rent using the cost-per-SF-per-use arithmetic from §22.2.4. Then write the placement rate and lift height you will require on the foundation-wall pour card, and say what happens if the pump crew wants to go faster.

Part 2 — Shoring and reshoring, with the strength criterion. Write the criterion for stripping side forms and, separately, for removing shores under any elevated concrete — quoting your specification, not a number from this chapter. State whether the cylinders will be lab-cured or field-cured for each decision, who books the testing laboratory, and — the essential line — who signs the shore-release form and who is not allowed to. Then design your one-page shore-release form: floor, bay, cylinder ID, criterion, break result, date, signature.

Part 3 — Scaffold plan for masonry and enclosure. The CMU first floor and the exterior work both need access. Select a system, state the load class you need for block and mortar, sketch the elevations and the tie pattern, and name who owns the scaffold — one trade, several trades, or you as a general-conditions item. Then write two things: the daily inspection procedure (use the checklist in §22.4.5, cut to what applies) with a named competent person and a named backup, and the modification-control rule in the exact words you would put in the subcontract.

Part 4 — Dewatering assessment for the water-main relocation. The relocation is a trench, not a building excavation, so this is an assessment rather than a full design. Determine the likely groundwater condition and depth of cut; decide whether sump pumping is adequate or whether wellpoints are warranted; identify the discharge point and which permit governs it in your jurisdiction; and price it per day with the fixed costs separated from the time-related costs. Add the trench-protection decision: shield, hydraulic shoring, or sloping — and the reason.

Part 5 — The temporary-structures register. Build the table from §22.9 for Willow Street. Every row names the designer, the design document, the erecting contractor, the competent person and backup, the inspection frequency, and the removal authority. Eight to twelve rows will cover this job. When you are finished, read the competent-person column and see whether you can name every one of them.

Next checkpoint: Chapter 23 turns the inspection habit you just built for temporary structures onto the permanent work — an inspection and test plan, mockups and benchmark installations, hold and witness points, and the nonconformance procedure that decides what happens when something gets built wrong.

Chapter Summary

The one sentence: Temporary structures are engineered structures that happen to be demolished at the end, and they need a design, a drawing, an inspection regime, and a named person — the same as permanent ones, and more urgently, because they have less margin, more handling, and no warning.

The six questions to ask about every temporary structure on your job:

The arithmetic you should be able to reproduce:

The four release decisions that need a signature, not an opinion:

1. Shore release — written strength result meeting the specified criterion, field-cured cylinders, one named signer.
2. Brace release (steel, precast, masonry) — written release from the erection or bracing engineer, per area.
3. Excavation support removal — written, in stages, by the shoring engineer.
4. Dewatering shut-off — the structural engineer's uplift criterion, in writing.

The three mistakes that cost the most, and the fix:

And the finding that mattered. The scaffold near-miss had three causes. Two were procedural and were fixed in a week. The third was a management decision about acceleration that nobody wanted to write down. If your incident reports never contain a finding that implicates a decision made in the trailer, your investigations are not finished — they are polite.

What's Next

You have just spent a chapter building an inspection discipline for structures that get thrown away. Chapter 23 turns that same discipline onto the work that stays: the inspection and test plan, mockups and benchmark installations, hold points and witness points, the nonconformance report, and the punch list. The connective tissue is the cylinder break you used in §22.2.5 — the same test that releases a shore also accepts the concrete, and the same question underlies both: what is your criterion, who measures it, and who signs? After that, Chapter 24 finishes the argument this chapter started at 7:20 on a Tuesday morning, and takes it where it has to go: safety is a property of the production system, not a rulebook.