Chapter 22 — Exercises

Work these with a calculator, a blank sheet, and the chapter closed. Selected answers are in Appendix J; several calculation items carry their numeric answers here in a <details> block so you can check your arithmetic without checking your reasoning.

Difficulty legend: ⭐ basic · ⭐⭐ applied · ⭐⭐⭐ judgment and integration · ⭐⭐⭐⭐ research and extension

Part A — Conceptual Understanding ⭐

A1. In your own words, state the four properties that make a temporary structure different from a permanent one. Then name the single management consequence that follows from all four together.

A2. A wall form is being filled. Explain, in plain language and without a formula, why how fast the concrete goes in changes how hard it pushes on the form. Then name three things other than placement rate that change the same answer.

A3. Distinguish reshoring from backshoring. Which one allows the slab to deflect under its own weight, and what does that mean for what the shores below are carrying?

A4. What is the difference between a lab-cured and a field-cured concrete test cylinder, and which one governs a decision to remove shores? Explain why.

A5. Define competent person as the construction safety standards use the term. Identify the half of the definition that employers most often fail to provide, and say what happens on a job when they fail it.

A6. A trench shield (trench box) and hydraulic shoring both protect workers in a trench. Explain the mechanical difference between what the two systems actually do to the soil.

A7. Name the five common groundwater-control methods described in §22.6.1 and give a one-line statement of the soil condition or drawdown requirement each one suits.

A8. Why is a freshly laid, ungrouted masonry wall one of the most dangerous temporary conditions on a job site? Name the two controls that address it.

A9. Explain the difference between the average daily cost of a dewatering system and its marginal daily cost, and say which one you use to answer "what does a one-week delay cost?"

A10. Composite metal deck on a steel building is both formwork and permanent structure. For roughly how long is it a temporary structure, and what three limits govern it during that period?

A11. State the eight steps of the form cycle in order, and identify the three steps that contain a hold point controlled by somebody outside your own crew.

A12. A specification requires "75 percent of f'c before removal of shores." Explain what each of the three parts of that phrase means — the percentage, the f'c, and the word "shores" as distinct from "forms" — and state who in your organization is allowed to decide the criterion has been met.

A13. Why is the word "temporary" itself described in this chapter as doing damage? Give the mechanism, not a slogan.

Part B — Applied Analysis ⭐⭐

B1. Your specification requires 75 percent of a 5,000 psi design strength before shore removal. The lab reports a 7-day break of 3,640 psi from a lab-cured cylinder. The carpenter foreman wants to strip. Write the three sentences you say to him, in order, and then write the two things you do next.

B2. A subcontractor's proposed schedule shows a 4-day floor cycle on a 14-story cast-in-place tower. Your estimate assumed 6 days. List the six things you would want to see evidence of before you accept the 4-day cycle in the baseline schedule, and say what each one would cost if it turns out to be missing.

B3. On the Northgate job, the curtain-wall subcontractor asks to use Ashcroft Masonry's scaffold on the north elevation for two weeks of sealant work after the masonry is complete. Neither subcontract addresses it. Describe what you do — the contractual steps, the safety steps, and the commercial terms you would set — and say which of the three you settle first.

B4. An excavation is 22 feet deep in a mixed soil, in an urban location with a two-story masonry building 14 feet from the property line. State (a) what the depth alone requires of the protective system, (b) three specific things you need from the neighbor before you start, and (c) the monitoring regime you would put in place, including what triggers action.

B5. Your dewatering contractor proposes open sump pumping for a 16-foot-deep excavation in a fine sand with the water table 6 feet below grade. Give three technical reasons to be skeptical and one condition under which sump pumping would be the right answer anyway.

B6. Read this sentence from a subcontract scope: "Mason shall provide all scaffolding required for its work and for the use of others." Name five ambiguities in that sentence, and rewrite it so that none of them survive.

B7. A superintendent tells you a flying-form system will save 1.5 days per floor on a 9-story building. Before you approve it, name the four constraints you would test the claim against, and identify which single constraint most often reverses this decision in practice.

B8. Your project has an approved after-hours work request process. Rewrite the form's required fields so that the Milo Serrano event from the chapter's hook could not repeat. List the fields, and for each one say what it prevents.

B9. A renovation project requires removing a 22-foot section of an interior bearing wall on the second floor of a 1961 building to create a new opening. The as-built drawings show the wall as non-bearing. The demolition subcontractor's foreman says he has "done a hundred of these." Write the sequence of steps you require before anything is removed, in order, with a hold point marked at each place work stops until somebody signs. Then say what you do if exploratory demolition reveals a condition the engineer's drawing did not anticipate.

B10. Your concrete subcontractor submits a shoring and reshoring scheme showing two levels of reshores. The schedule you gave them assumes a 5-day cycle. Name four questions you would ask about the submittal before you forward it to the engineer of record, and identify which one is most likely to reveal a mismatch between the scheme and the schedule.

Part C — Calculations and Deliverables ⭐⭐–⭐⭐⭐

C1. Lateral pressure and the pump. A 14-foot-high foundation wall is formed with a system designed for a maximum lateral pressure of 900 psf.

(a) At 150 pcf, what depth of fluid concrete produces exactly 900 psf? (b) If the entire 14 feet were fluid at once, what would the pressure at the base be? (c) By what factor would the design pressure be exceeded? (d) Write the one line you would put on the pour card to prevent (b), and name the person on site who has authority to enforce it.

C2. The cycle and the money. A cast-in-place structure has 9 elevated floors at 14,200 SF each. You have two candidate cycles: 6 work days and 8 work days. The work week is 5 days. General conditions are $5,150 per calendar day and the contract carries liquidated damages of $3,800 per calendar day; assume the frame is on the critical path.

(a) Frame duration under each cycle, in work days and calendar days. (b) The calendar-day difference. (c) The total daily exposure and the value of the difference. (d) A second form set and an added reshore level to enable the 6-day cycle costs $204,000 to purchase, with a residual value of $118,000. Is it justified? Show the arithmetic. (e) Name three things that get harder when you move from an 8-day cycle to a 6-day cycle.

C3. Buy or rent. You need 9,400 SFCA of gang wall forms, used 6 times, over a 5-month forming period. Purchase price is $24.50/SF with an expected residual of 50 percent; maintenance while owned is estimated at $7,200. Rental is $2.60/SF/month, freight in and out is $9,100, and expected damage billing on return is $3,400.

(a) Total cost to this job under each option. (b) Cost per square foot per use under each. (c) Which wins, and by how much? (d) Name two circumstances that would reverse your answer.

C4. Dewatering, average versus marginal. A dewatering system costs $412,000 for a planned 180 calendar days of operation. Of that, $196,000 is fixed (design, mobilization, installation, permits, monitoring wells) and the rest is time-related.

(a) The average daily cost. (b) The marginal daily cost. (c) The cost of a 12-day delay in reaching the structural shut-off criterion. (d) The owner's representative asks you to justify (c) in two sentences. Write them.

C5. Build a scaffold inspection procedure. For the masonry scaffold on the Willow Street Community Center, write a one-page daily inspection procedure. It must include: who performs it (role, plus named backup), when, the tag regime and what each tag color means, the checklist reduced to the items that apply to this scaffold, what the inspector does when an item fails, who is notified, and how the record is kept. Then write the two-sentence rule that governs what happens if another trade modifies it.

C6. The temporary-structures register. Build a register for the Willow Street Community Center with at least eight rows. Columns: item, location/extent, designed by, design document, erected or operated by, competent person and backup, inspection frequency, and removal or release authority. Then mark, in a ninth column, which rows are a delegated design requiring a PE seal and what the submittal lead time is for each.

C7. Price the temporary heat. You are enclosing a 24,000 SF two-story building through 9 weeks of winter. Assume 14,000 SF of temporary enclosure at $2.75/SF installed, 4 indirect-fired heaters at $1,050 per month each, fuel and power at $1,850 per week, and attendance at $600 per week.

(a) Total cost for 9 weeks. (b) Cost per week. (c) If the enclosure is completed 3 weeks earlier than planned, what do you save — and what is the one cost that does not go down?

C8. The cost of one stopped pump. Your dewatering system has no standby generator and no telemetry alarm. A pump fails on a Friday evening and is discovered Monday morning. Estimate the exposure using the following: re-dewatering $5,400; undercut and replace 280 CY of softened subgrade at $58/CY; pull, clean, and reset reinforcing with 3 workers × 2 days × 8 hours at a loaded rate of $71/hour; a cancelled placement at $4,900; re-inspection at $1,700; and 5 calendar days of critical-path delay at a combined exposure of $8,400/CD.

(a) The direct cost. (b) The total exposure including delay. (c) The redundancy that would have prevented it costs $2,600/month for a system that runs 5 months. What is the ratio, and what do you tell the owner's representative who proposed deleting it?

Part D — Judgment and Ethics ⭐⭐⭐

D1. Your concrete foreman calls at 4:40 p.m. Friday. The 3-day break came back at 3,340 psi against a criterion of 3,375 psi — short by 35 psi, roughly one percent. He needs the panels Monday morning for a pour that is on the critical path, and stripping takes four hours. He proposes stripping Saturday morning, when the concrete will have had another 14 hours of curing. What do you do, and what is the principle you are protecting? Then answer the harder question: if the answer is no, what does that tell you about how the criterion should have been resourced in the first place?

D2. Your subcontractor's superintendent tells you privately that his crew has been removing one guardrail section each morning to land material and replacing it at the end of the day, and that he has told them to stop. Nothing has happened. He asks you not to write it up because it will hurt his company's rating on your prequalification list. Work through your obligations, in order, and say what you actually do. Then describe the version of a prequalification system that would not create this incentive.

D3. In value engineering, the owner's representative proposes deleting the standby generator and telemetry alarm from the dewatering system, saving about $11,600 on a project with a tight budget. You believe the exposure is roughly nine times that. Write the two-paragraph response you would send. Then answer: if the owner directs the deletion anyway, in writing, what do you do — and what does that decision do to your own risk position?

D4. A specialty engineer's sealed shoring drawing shows a condition your superintendent believes is wrong. The engineer, contacted, says the drawing is correct and declines to change it. Describe your obligations and your options, in order, and identify the line between "raising a legitimate concern" and "substituting your judgment for a licensed engineer's." What would you require in writing before proceeding either way?

D5. Return to the third finding in Case Study 1. Argue the case against including it in the written report — the strongest version of the argument, not a straw man. Then answer it. Your answer must address the discoverability concern directly rather than talking past it.

Part M — Mixed and Interleaved Practice ⭐⭐–⭐⭐⭐

M1. With Chapter 6 — risk allocation. Take three temporary structures on a job you know: a scaffold, an excavation support system, and a dewatering system. For each, complete the five-response analysis from Chapter 6 — avoid, transfer, mitigate, accept, exploit — with a cost for each response. Then identify which component of each risk cannot be transferred by any instrument, and say what you do about that component instead.

M2. With Chapter 14 and Chapter 29 — schedule logic. Build a 10-activity CPM fragment for one floor cycle of a cast-in-place frame, including the cure duration as an activity and the cylinder break as a milestone constraint. Run the forward and backward pass. Then answer: which activity would you attack first to compress the cycle by one day, and what would that cost?

M3. With Chapter 16 — scope sheets. Write the scope-sheet section on temporary structures for a concrete subcontract on a six-story cast-in-place building. It must resolve: formwork design responsibility and PE seal, shoring and reshoring design, the early-strength requirement for the mix, who books and pays for testing including weekend breaks, the shore-release procedure and signer, and what happens when the general contractor's schedule compresses the cycle after award.

M4. With Chapter 21 — bearing and load. A shoring tower under a heavy transfer beam delivers 46,000 lb to a slab on grade through four legs with 8-inch × 8-inch base plates. (a) What is the bearing pressure under one plate, in psi and psf? (b) The slab on grade was designed for a uniform load of 250 psf. What do you do? (c) Size a timber spreader that brings the pressure under the design value, and state what other question you must ask about the slab before you accept your own answer.

M5. With Chapter 23 — inspection and test planning. Build the concrete testing and inspection portion of an inspection and test plan for one floor of a cast-in-place frame. Show, for each test: what is tested, at what frequency, by whom, what the acceptance criterion is, whether it is a hold point or a witness point, and — the item people forget — which decision the result releases. Mark clearly which cylinders are lab-cured and which are field-cured.

M6. With Chapter 18 and Chapter 27 — constraints. Take the eight steps of the form cycle and build a constraint list for each one: what must be true before that step can start, who removes each constraint, and how many days ahead it must be removed. Then identify the two constraints on your list that are most likely to be discovered late, and design the check that would find them early.

Part E — Research and Extension ⭐⭐⭐⭐

E1. Obtain the current edition of ACI 347 (a library, a university, or your company's technical shelf). Read the sections on design loads and on shoring and reshoring. Then write a two-page memo comparing what it actually requires against what you would find on a typical job site, and list the five requirements most likely to be honored in name only. Do not quote the document at length; summarize and cite it properly.

E2. Pick a real jurisdiction. Find (a) what permit governs construction dewatering discharge there, (b) what the discharge limits and monitoring requirements actually are, (c) the application lead time, and (d) whether discharge to sanitary sewer, storm sewer, and surface water are treated differently. Then write the one-page procedure a project engineer in that jurisdiction would follow, starting from "the geotechnical report shows groundwater at 8 feet."

E3. Find your jurisdiction's requirements for delegated design — the rules governing when a specialty engineer's seal is required, what the engineer of record's review of a delegated design does and does not mean, and how responsibility is allocated. Then interview, or find published guidance from, a structural engineer about what they are actually reviewing when a shoring submittal crosses their desk. Write 800 words on the gap between what contractors believe an "approved" stamp means and what it legally means.