Chapter 22 — Key Takeaways

The one sentence: A temporary structure is an engineered structure that happens to be demolished at the end. It needs a design, a drawing, an inspection regime, and a named person — the same as a permanent one, and more urgently, because it has less margin, more handling, and no warning before it fails.

Key Takeaways

• Four properties make temporary structures different: they are designed for a short service life and one load case; they are assembled by people who did not design them; they are modified in the field constantly; and their failure is sudden rather than progressive. Every one of those pushes toward more management, not less.

• A disproportionate share of catastrophic construction collapses originate in temporary works, not in the permanent structure — formwork and shoring, excavation support, and structures not yet laterally braced. The building rarely falls down while you build it. The thing holding it up does.

• Formwork is 40 to 60 percent of the in-place cost of concrete and it sets the schedule of a concrete frame. It is the only temporary structure that appears on both the cost report and the critical path.

• Placement rate, not wall height, decides lateral form pressure. Full liquid head is roughly 150 psf per foot of fluid concrete — a 12-foot fully fluid pour is 1,800 psf. Cold weather, retarders, and self-consolidating concrete all extend the fluid period and raise the pressure at the same placement rate. Treat SCC as full liquid head unless your engineer says otherwise in writing.

• Frame duration = elevated floors × cycle time. Two days of cycle on a six-floor building is 12 work days, about 17 calendar days, $87,550 at $5,150/CD. On a twenty-two-floor tower the same two days is a month.

• Cycles slip at hold points, not at production steps. Waiting on a cylinder break, an inspection, or an MEP sleeve approval is what stretches a cycle. Adding carpenters almost never fixes it.

• The stripping criterion is a specified strength, verified by field-cured cylinders, released in writing by one named person. "It looked hard" measures the top quarter-inch. Lab cylinders qualify the mix; field cylinders qualify the member.

• Reshoring lets the slab deflect under its own weight; backshoring never does. Either way the construction load path runs through several young floors as a coupled system, which is why a local overload becomes a multi-floor collapse and why the scheme is an engineered design, not a field decision.

• Excavation cave-in is one of OSHA's Focus Four. A cubic yard of soil weighs on the order of 3,000 pounds and gives no warning. Protective system at 5 feet, PE design at 20 feet, egress in trenches 4 feet or deeper within 25 feet of travel, spoil set back from the edge, and daily competent-person inspection before each shift and after every rain.

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

• Dewatering is operated, not built. It runs 24/7, and its failure modes are a stopped pump at 2 a.m., a discharge permit nobody read, and settlement at the neighbor's building. Redundancy is cheap; the failure is not.

• The marginal daily cost of a time-related system is not its average daily cost. A delay does not re-buy the mobilization. Answer "what does a week cost?" from time-related costs only.

• A structure is most vulnerable while it is being built. Temporary bracing for steel, precast, and especially ungrouted masonry walls is a load-bearing element with a written release requirement, exactly like a shore.

• Financial risk transfers. Physical risk stays where the person is standing. You can allocate the cost of a scaffold by contract; you cannot allocate away the obligation to keep people from falling off it.

Action Items — What to Do on Your Job This Week

1. Build the temporary-structures register. Every temporary structure gets a row: designer, design document, erector, competent person and backup, inspection frequency, removal authority. Then read the competent-person column aloud in the coordination meeting. Any cell you cannot fill from memory is a gap.

2. Verify every competent-person designation has a name, training records, a backup, and actual stop-work authority. A competent person who cannot stop work is not one.

3. Walk the scaffolds and read the tags. Date, shift, printed name, signature, load class. A tag with a name but no date is decoration. Any scaffold without a current tag gets barricaded at the access point today.

4. Add one question to every morning huddle: "Did anybody touch this scaffold since yesterday?" Eight seconds. It converts users into witnesses.

5. Write or verify your shore-release form and name the single signer. Floor, bay, cylinder ID, criterion, break result, date, signature. Confirm the cylinders being broken for that decision are field-cured.

6. Check your testing agreement for weekend and holiday breaks. If it does not include them and you are running a fast cycle, amend it. A few hundred dollars a visit buys back days worth thousands.

7. Confirm the dewatering system has a standby generator with automatic transfer, spare pumps, and an alarm that calls a phone. Then test the call-out list by actually calling it.

8. Add the modification-control rule to your subcontract template and your orientation, and revise the after-hours work request form to require what structures will be accessed or touched plus a next-morning pre-use inspection.

9. Look at your permit matrix for the dewatering discharge permit. If it is not there, find out today what governs it in your jurisdiction and what the lead time is.

10. If anything on the job has been accelerated, write the hazard statement — which trades' sequence changed, where stacking will occur, which crews are expected to recover time, and what you funded to offset it.

Common Mistakes and the Fix

Decision Framework

The six questions, asked once per temporary structure, at the start of the job

   1. WHO DESIGNED IT?
        └─ "It's standard" / "The foreman's built a hundred" ──▶ STOP.
           That is a form of "nobody." Get a design.

   2. WHOSE SCOPE IS IT IN?
        └─ Check the subcontract. Formwork = concrete. Excavation support =
           earthwork or specialty. SCAFFOLD and DEWATERING are the two that
           fall in the gap. Resolve them at buyout, not in month six.

   3. WHAT SUBMITTAL, AND HOW LONG IS THE REVIEW?
        └─ Delegated design = PE seal + EOR review = WEEKS.
           It gates work. Back-schedule it onto the submittal log.

   4. WHO INSPECTS IT, AND HOW OFTEN?
        └─ Scaffold: before each shift.     Excavation: daily + after rain.
           Formwork: before each placement. Dewatering: continuous + alarms.

   5. WHO IS THE COMPETENT PERSON — BY NAME, WITH A BACKUP?
        └─ No name ──▶ no competent person, just a box on a form.
           No stop-work authority ──▶ same answer.

   6. WHO MAY CHANGE IT, AND WHAT HAPPENS WHEN SOMEBODY DOES?
        └─ Modification control. Request → competent person performs →
           re-inspect → re-tag → reoccupy. Unauthorized change = stop work.

The release test — apply to any removal decision

Before anything comes out — a shore, a reshore, a brace, an excavation support member, or a dewatering pump — answer all four:

1. What is the written criterion? (A strength, a completed permanent element, a structural weight-versus-uplift calculation.)
2. What is the measurement? (A field-cured break, an engineer's field verification, a survey.)
3. Who signs? (One named person, with authority, whose signature goes in the project record.)
4. What is above it, on it, or behind it right now? (Stacked material on a slab you are about to release; a wall behind an excavation support member; a structure that floats when the pumping stops.)

If you cannot answer all four, the answer to "can we pull it?" is no — and the correct next move is to fix the missing answer, not to argue about the schedule.

The acceleration rule

Any decision to accelerate, resequence, or compress work is accompanied by a written statement of the hazard it creates and the controls added because of it — issued before the acceleration is directed. Name the trades whose sequence changes, the areas where trades will be stacked, the crews expected to recover time, and what you funded to offset it.

If your incident reports never contain a finding that implicates a decision made in the trailer, your investigations are not finished. They are polite.