Chapter 8 — Exercises

Work these with a calculator, a pencil, and the chapter open. The calculation problems use the same conventions the chapter does: label every unit, state whether a volume is bank, loose, or compacted, and say whether a duration is calendar days or work days. Selected answers are in Appendix J; several calculation items carry the numeric answer here in a <details> block so you can check your arithmetic without checking your reasoning.

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

Part A — Conceptual Understanding ⭐

A1. In your own words, what is the difference between the recommendations section of a geotechnical report and the boring logs? Which one did the structural engineer design to, and which one is evidence?

A2. Define bank, loose, and compacted cubic yards. For each, name one construction quantity that is naturally measured in that unit.

A3. A specification calls for structural fill compacted to 98% of modified Proctor at moisture within 2 points of optimum. Explain, in plain language, what the Proctor test established and why moisture content is in the requirement at all.

A4. What is the "eighth-inch rule" for reinforcing bar sizes? What is the diameter of a #6 bar? Where does the rule stop working?

A5. Distinguish a construction joint, a control joint, and a cold joint. Which one is a defect?

A6. Why is the water-cement ratio the strongest single predictor of concrete strength and durability? What is the correct way to make a stiff mix more placeable without hurting it?

A7. Name the four things every structural system in this chapter has, and which the construction manager is actually responsible for managing.

A8. What is a mill rolling slot, and why does missing one by six days cost more than six days?

A9. List the three most important reasons masonry productivity varies so much from job to job.

A10. Mass timber erects faster than almost any other structural system. Name the two problems that speed creates for the construction manager.

Part B — Applied Analysis ⭐⭐

B1. You are the project engineer on a 96,000 SF distribution center. The geotechnical report recommends spread footings at 3,000 psf and a slab on grade over 12 inches of compacted granular base. Boring B-4, near the northwest corner, shows N-values of 5, 6, and 4 at the design footing elevation; the other nine borings show N ≥ 18 at that elevation. Describe exactly what you do in the four weeks before excavation starts. Name the documents you produce.

B2. Your contract incorporates a differing-site-conditions clause with a written-notice requirement. Your excavator hits an abandoned concrete foundation, not shown on any drawing, at 6:00 a.m. on a Friday. Write down, in order, the first five actions you take that day. For each, say why it protects the project.

B3. A subcontractor proposes to change your basement excavation from soldier pile and lagging to sloped and benched sides, saving $310,000. The excavation is 22 feet deep and the property line is 26 feet from the building face. Evaluate the proposal. What questions must be answered before you could accept it, and who has to answer each one?

B4. Your concrete superintendent wants to place a 340 CY elevated deck using the tower crane and a 2-yard bucket instead of renting a pump for $4,800. The crane is otherwise scheduled for curtain-wall panel setting that day. Analyze the trade-off. What is the real cost of the "free" crane?

B5. On a mid-rise cast-in-place project, the formwork subcontractor's schedule shows a 6-day floor cycle. Your CPM schedule shows 5 days. Nobody has noticed the difference. The building has 14 typical floors. Quantify the exposure in work days and calendar days, and describe the meeting you call and who is in it.

B6. A ready-mix driver tells your foreman the load is "too stiff to pump" and asks to add 15 gallons of water. The batch ticket shows 8 gallons of allowable trim water remaining. Walk through the decision. Who decides, what gets written down, and what do you do with the remaining 7 gallons of the driver's request?

B7. The Willow Street Community Center has a CMU first floor, a wood-framed second floor, and a long-span roof over the gymnasium. Rank the three structural packages by weather sensitivity, and explain how that ranking should change the order in which you schedule them across a calendar year in a cold climate.

B8. You are handed a mass timber office building as your first project of that type. List six things you will do in preconstruction that you would not do on a comparable steel-framed building.

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

C1 — Swell and shrinkage. A site has 61,000 BCY of cut and requires 24,000 CCY of compacted structural fill. The material has a swell factor of 22% and a shrinkage factor of 14%.

(a) How many bank cubic yards of cut must be diverted to fill to place 24,000 CCY? (b) How many bank cubic yards are left to export? (c) Convert the export to loose cubic yards. (d) At 14 LCY per truck, how many truckloads?

C2 — Excavator production. An excavator has a 3.0 CY bucket, a 26-second cycle in this material, a bucket fill factor of 0.80, and works at a 45-minute-hour efficiency.

(a) Compute production in LCY per hour. (b) Convert to BCY per hour using a 22% swell factor. (c) How many 9-hour work days to move 33,093 BCY?

C3 — Fleet balance and cost. Using the excavator from C2 and 14 LCY trucks, the haul is 8 miles each way. Loaded average speed 22 mph, empty 28 mph. Spot time 1.0 minute, dump and maneuver 2.5 minutes.

(a) Compute load time per truck, then total cycle time. (b) How many trucks are needed to keep the excavator working? (c) At $210/hr for the excavator and operator, $102/hr per truck, and $175/hr for support, what is the cost per bank cubic yard, before tipping fees? (d) The trucking broker can only supply 18 trucks. Recompute the duration and the total cost, and state in one sentence what you would do.

C4 — Concrete takeoff with waste. A building has:

• 96 spread footings, average 7'-6" × 7'-6" × 30" deep
• 780 LF of foundation wall, 12" thick × 4'-0" tall
• 28,400 SF of slab on grade at 6 inches

(a) Compute the neat concrete volume for each item, in CY. (b) Apply a 5% waste factor to the footings and walls and 4% to the slab. (c) Convert to whole 10 CY ready-mix loads for each item. (d) At $181/CY delivered, what is the material cost of the package?

C5 — The quarter-inch problem. Your slab on grade is 28,400 SF specified at 6 inches. The stone base is graded to an average of 3/8 inch low.

(a) How many extra cubic yards of concrete does that consume? (b) At $181/CY, what does it cost? (c) Express the overrun as a percentage of the neat quantity, and state the FF/FL consequence you would expect.

C6 — Steel lead-time back-schedule. Steel erection must start on August 4. Working backward with these durations — shipping and sequencing 10 CD, fabrication 45 CD, mill rolling and delivery to the fabricator 35 CD, engineer-of-record review 14 CD, general contractor review and transmittal 5 CD, and detailer preparation 10 CD —

(a) On what date must the detailer begin the anchor-bolt and embed submittal? (b) The fabricator holds a mill rolling slot that closes on a fixed date and the next opening is five weeks later. If the general contractor's review takes 11 days instead of 5 and the fabricator recovers 12 days by compressing fabrication, what is the net slip to erection start, and what is the new start date? (c) State the total dollar exposure at $10,650 per calendar day.

C7 — Form cycle resource check. A cast-in-place frame has 16 typical floors. Each floor requires approximately 1,320 carpenter-hours to strip, fly, set, and detail the formwork. The schedule shows a 5-work-day cycle. Carpenters work 9-hour days.

(a) How many carpenter-hours does a 5-day cycle make available per carpenter? (b) How many carpenters does the 5-day cycle require? (c) The superintendent has 19 carpenters. What cycle duration does that crew actually support? (d) Compute the total frame duration at the planned cycle and at the supportable cycle, in work days and in calendar days at 5 work days per week. At $4,900/CD of extended general conditions, what is the exposure?

C8 — Drilled shafts under a unit-price contract. The plans show 32 drilled shafts, 36 inches in diameter, averaging 58 LF, at a bid unit price of $412.00/LF.

(a) What is the plan quantity and the plan value of the pay item? (b) The rock is deeper than shown and the shafts average 79 LF. What is the measured quantity and what does the owner pay? (c) What percentage variation is that from plan quantity, and what contract question does it raise?

Part D — Judgment and Ethics ⭐⭐⭐

D1. A 28-day cylinder break comes back at 3,180 psi against a specified f′c of 4,000 psi, on a set representing an interior column line already covered by two floors of framing. Your superintendent points out that "the 7-day was fine, the cylinders were probably handled badly, and nobody will ever know." Describe the professional path, step by step. Then describe what happens in year three if you take the other path.

D2. Your earthwork subcontractor's foreman is placing 16-inch lifts where the specification requires 8 inches, and the density tests are passing. He argues that a passing test is a passing test. Is he right? Explain the mechanism, and describe how you would handle the conversation without turning it into a fight.

D3. Your project is nine days behind after a rain delay. Your steel erector offers to "get creative" with the decking sequence and run a controlled decking zone larger than the plan allows, promising to make up four days. What is your answer, and what does theme 4 of this book — safety is not a line item — have to do with the schedule conversation you have with your VP the next morning?

D4. The geotechnical report was prepared for the owner, and the contract says the contractor may rely on factual data but not on interpretations. Your estimator wants to carry zero contingency for subsurface conditions because "the owner owns that risk anyway." Evaluate that reasoning. What does the owner actually own, and what do you still own?

D5. You discover that a subcontractor cut two post-tensioning tendons while coring for a floor drain three weeks ago and patched the holes without telling anyone. Nobody was hurt. Describe your obligations, in order, and to whom. Then describe the systemic fix — not the disciplinary one.

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

M1 — With Chapter 7. The structural drawings show a footing schedule with F-12 at 9'-0" square by 30" deep. The specification section for cast-in-place concrete requires 4,500 psi at footings. The geotechnical report recommends 3,500 psf bearing. The architectural foundation plan shows F-12 at 8'-6" square. Apply the order of precedence you learned in Chapter 7, identify which document governs each of the three facts in play, and write the RFI.

M2 — With Chapter 6. Build a five-line risk register for the sitework and foundation package on a project of your choosing. For each risk, give the trigger, the probability band, the dollar impact range, the owner of the risk under your contract, the planned response, and the contingency amount. At least two lines must be subsurface risks and at least one must be a testing or inspection risk.

M3 — With Chapter 4. Take the earthwork scope from C1 and price it three ways: as a lump sum, as a unit-price item, and as cost-plus with a guaranteed maximum. For each, state in two sentences who owns the quantity risk, who owns the productivity risk, and what you would put in the contract to protect yourself.

M4 — Looking forward to Chapter 14. Sketch the activity list and logic for a foundation package: mass excavation, over-excavation contingency, footing excavation, subgrade inspection, form and reinforce footings, rebar inspection, place footings, cure, strip, foundation walls, waterproofing, drainage, backfill and compaction, under-slab utilities, stone base, vapor barrier, slab on grade. Show which activities are finish-to-start, which can overlap with a lag, and where the three inspection hold points sit. You do not need durations yet — the logic is the exercise.

M5 — Looking forward to Chapter 12. Using unit cost = quantity × productivity × rate, build a labor estimate for forming and placing the 96 footings in C4. Assume a nine-person crew, a productivity of 6 footings per work day, and a loaded crew rate of $68.00 per man-hour on a 9-hour day. Compute total man-hours, total labor cost, and cost per footing. Then state the two assumptions in your estimate that are most likely to be wrong.

M6 — Integrative. Take the Northgate steel-delay chain from §8.7 and describe how each of the following would have changed the outcome: (a) a submittal log with back-scheduled required-approval dates; (b) a weekly submittal review meeting with the engineer of record; (c) a fabricator who did not hold a mill slot; (d) an owner who cared less about the October 1, Year 2 interim clinic lease. Which of the four is the cheapest intervention?

Part E — Research and Extension ⭐⭐⭐⭐

E1. Obtain a real geotechnical report — many are published as appendices to public project bid documents on state and municipal procurement sites. Read the boring logs, not the recommendations. Write a two-page memo identifying the three locations on that site where you would expect subsurface surprises, and say what evidence in the logs led you there.

E2. Find the excavation and trenching requirements in OSHA 29 CFR 1926 Subpart P and the steel erection requirements in Subpart R, at the source. For each, write down the competent-person duties, the training requirements, and the specific thresholds that trigger a protective system or a fall-protection requirement. Then compare them to the site-specific safety plan on a job you have access to, and list the gaps.

E3. Pick a jurisdiction and find the code edition it has actually adopted, plus any local amendments. Determine what construction types it permits for mass timber, what height and area limits apply, and what encapsulation or noncombustible protection it requires. Then write the one-page memo you would bring to a preliminary meeting with the building official on a proposed six-story mass timber office building — including the three questions you would ask.