Chapter 22 — Quiz

C. Formwork commonly runs 40 to 60 percent of the in-place cost of concrete — more than the concrete itself and usually more than the reinforcing. This is why form-system selection and cycle time drive both the cost report and the schedule on a concrete frame, and why "cheaper forms" that increase carpenter hours are usually more expensive.

B. Wall height sets the ceiling (full liquid head at roughly 150 psf per foot of fluid depth), but the actual design pressure depends on how much of the pour is still fluid when you reach the top — which is governed by placement rate, modified by temperature, admixtures, mix, and consolidation. Two crews placing the same wall at different rates impose very different loads on the same form.

C. 12 ft × 150 pcf = 1,800 psf. This is straightforward hydrostatics, not a code provision, and it is the number your form must be able to resist if the whole depth is fluid at once.

B. A lab-cured cylinder tells you about the mix under controlled conditions and is the basis for acceptance. A field-cured cylinder — or an accepted maturity method — tells you about this member's actual temperature and moisture history, which is the question a stripping decision asks. Using a lab cylinder to make a field decision overstates the strength of a slab that spent four cold nights under a tarp.

B. Reshores go in after the slab has deflected under its own weight, snugged rather than jacked. The slab therefore carries its own dead load permanently, and the reshores carry only the additional construction loads that arrive later. Option A describes backshoring, where the shores are replaced in a sequence that never lets the slab deflect on its own.

C. Five feet or greater, unless the excavation is entirely in stable rock — and a competent person still evaluates whether protection is needed below that depth. Separately, excavations 20 feet or greater require a protective system designed by a registered professional engineer, and means of egress are required in trenches 4 feet deep or greater within 25 feet of lateral travel.

C. Beyond a height-to-base ratio of four to one, guys, ties, or braces are required, installed on a defined vertical and horizontal pattern per the manufacturer or a qualified person. This is one of the handful of scaffold numbers worth carrying in your head, because it is easy to check by eye and frequently violated on narrow towers.

B. Light duty 25 psf, medium duty 50 psf, heavy duty 75 psf. A masonry scaffold carrying block and mortar is a heavy-duty application, and putting block on a light-duty frame because it happened to be the scaffold already standing is a real and common failure.

B. Eductor systems use high-pressure water to create a vacuum at each well, delivering lower flow at higher head — which is exactly what low-permeability soils require. Sump pumping in fine sand pulls fines and destabilizes the base; a single deep well cannot draw water fast enough from silt; and sheet piling is a cutoff, not a dewatering method (though it may be the right answer for a different reason — protecting the neighbors from drawdown settlement).

B. The time-related cost is ($300,000 − $150,000) ÷ 150 = $1,000 per calendar day, so a week is about $7,000. Using the $2,000/day average would have doubled the answer, because the average includes mobilization and installation that a delay does not re-buy. Getting this distinction wrong in front of an owner damages your credibility on everything else you present.

B. Below the water table, pore water pressure carries part of the load. Draw the water down and the effective stress on the soil skeleton increases, consolidating it — and the settlement zone extends well past your property line. Option D is a real and separate mechanism (excavation removing lateral support), which is why monitoring covers both. Controls: monitoring wells, settlement points with action thresholds, recharge wells, and — before anything else — the pre-construction condition survey.

B. The U.S. steel erection standard requires written notification from the controlling contractor that the concrete in footings, piers, and walls has attained adequate strength before erection begins. In practice that means somebody on your team turns cylinder breaks into a piece of paper. Related requirements in the same standard: a minimum of four anchor rods per column, and no field repair, replacement, or modification of anchor rods without the approval of the structural engineer of record.

False. A competent person must inspect the scaffold and its components for visible defects before each work shift and after any occurrence that could affect structural integrity. Nine days of use by multiple trades is precisely the condition in which planks get moved, ties get removed, and material gets stacked past the load class.

False. A temporary structure carries its full design load every day it stands, and it has less margin than a permanent one — no long-term redundancy from finishes and partitions, no second load path, and no time to notice distress before a sudden failure. Short service life does not buy you margin; it removes the warning.

False. A shield protects workers inside it if the walls collapse; it does not support the soil. Shoring systems support the soil. Both are legitimate protective systems, but confusing them leads people to make wrong decisions about spoil setback, adjacent loads, and what happens outside the box.

False. The engineer of record's review of a delegated design is generally limited to conformance with the design intent of the permanent structure — not verification of the specialty engineer's calculations. Responsibility for the temporary structure remains with the party whose means and methods it is. Read the actual language in your specification and your contract, and note that the rules for delegated design vary by jurisdiction.

False, on two counts. Direct-fired heaters put combustion products — including a great deal of water vapor — into the space, so you are humidifying a building you are trying to dry. They are also a carbon monoxide hazard in an enclosed space. Indirect-fired heaters vent combustion products outside and deliver only clean warm air.

False. Reshores are snugged, not jacked. Over-tightening lifts the slab above and loads the slab below in a direction and magnitude nobody designed for. The correct amount of tightening is part of the engineered scheme, and the crew has to be told what it is.

In a shored structure, floors are coupled rather than independent: a shore stack ties three or four young slabs into one load-sharing system, distributing construction loads according to relative stiffness. 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, weaker, and already carrying its share. That floor fails and delivers two floors' worth of load to the next one down. Coupling is what turns a local overload into a building, which is why the reshoring scheme is an engineered design with a drawing, not a field judgment.

The rule: no temporary structure 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 stops work on that structure until the competent person clears it.

Why training is not a substitute: the failure mode is not carelessness, it is unawareness of category. Halcyon's electricians did not decide to modify a scaffold and do it badly. They moved a board, and they had no idea they had touched a structure. You cannot train people to be careful about a hazard they do not know they created. The rule works because it changes the situation — it makes the structure something you have to ask about — rather than trying to change the person.

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

The common structure: a stated criterion, a measurement, and one named person whose signature is on it. Every one of these is a place where schedule pressure and physical risk push in opposite directions on the same step, which is exactly why the decision is removed from the person feeling the pressure.

The criterion. 75% × 4,500 = 3,375 psi. The break was 3,290 psi. It does not meet the criterion. Nothing is stripped. The number is the number, and the gap being small is irrelevant — a criterion you set aside when it is close is not a criterion.

The proposal. "It'll be stronger by then" is a prediction, not a measurement, and the whole point of the criterion is that the release runs on measurement. Concrete does gain strength over a weekend, which is why the right answer is to measure it Saturday, not to assume it.

The laboratory constraint. This is the actual problem and it is solvable this afternoon. Call the lab and buy a Saturday morning break — typically a few hundred dollars as an additional service. If the concrete has reached 3,375 psi Saturday morning, you strip Saturday and lose nothing.

The schedule consequence if the lab cannot come. You strip Monday morning and lose most of a day on the critical path — roughly $10,650. Say it out loud, in the schedule update, so the number is visible. Do not recover it by shortening a different cure.

What changes: 1. Amend the testing agreement to include weekend and holiday breaks at a stated rate, for the duration of the frame. It costs a few thousand dollars and buys back days at $10,650 each. 2. Look at the mix. If the mix cannot reliably make 3,375 psi at the cycle age and the temperatures you are actually getting, that is a preconstruction decision showing up in the field, and it is fixable now with an early-strength mix for the remaining floors. 3. Put the cylinder break on the schedule as an activity with a duration and a responsible name, so nobody discovers on a Friday afternoon that the lab is closed.

And the underlying point: the foreman is not wrong to want the panels. He is being asked to hit a cycle that was not fully resourced, and the honest response includes fixing the resourcing, not just saying no.