Steel Erection Sequence Versus Concrete Strength
A three-story mixed-use building has a cast-in-place concrete podium with structural steel framing above. The podium slabs are specified at 4,000 psi at 28 days, and the steel erector's plan calls for loading the podium roof with a 90-ton crane and staged steel bundles within 10 days of the last podium pour. The concrete supplier's mix design reaches about 2,800 psi at 7 days. The structural engineer's drawings note 'do not load slabs until design strength is achieved' but give no shoring or loading plan. The steel erection is on the critical path, and a 3-week delay costs roughly $45,000 in general conditions.
1. What is the technical and contractual risk here, and who owns it?
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The technical risk is slab cracking, deflection, or punching shear failure under concentrated crane outrigger loads applied before the concrete reaches design strength. A 90-ton crane can impose outrigger reactions well above the slab's capacity at 2,800 psi, and staged bundles add uniform load the slab was not analyzed for during construction.
The contract risk sits with whoever controls the means and methods. The erector controls crane placement and loading; the general contractor controls site access and sequence; the structural engineer set a performance limit without a loading plan. The note 'do not load until design strength' is a prohibition, not a plan, so the erector cannot simply proceed and claim the engineer approved it.
The GC typically owns coordination and the schedule, so the GC should not allow the crane onto the podium without a written engineered lifting and loading plan signed by a qualified engineer, including outrigger load spread, matting, and possibly shoring from below. That plan is the contractual instrument that allocates the risk.
2. The erector proposes using a smaller crane at lower cost but with longer erection time. How do you decide?
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Compare total cost and risk, not crane rental.
Option A: large crane at day 10. Saves up to 3 weeks of general conditions (about $45,000) but requires an engineered loading plan, likely matting or shoring, and carries a real risk of slab damage whose repair cost could exceed the savings. It also requires the concrete to be stronger sooner, which may mean a mix change and a premium.
Option B: smaller crane, longer erection. Lower point loads, easier to justify on the podium, but adds erection duration and crane time, and may consume the schedule float.
A third option is often best: accelerate the concrete (higher early strength mix, or cure the critical bays first) so the large crane can load safely at day 10 with a proper plan. That buys schedule without gambling on slab capacity.
Decision rule: if the engineered loading plan shows the slab is adequate with matting, Option A plus a concrete acceleration premium is likely cheaper than 3 weeks of general conditions. If the plan requires extensive shoring, compare that cost against the delay. Document the engineer's sign-off either way.