Rebar Substitution in a Retaining Wall
A 6.0 m high cantilever retaining wall is under construction. The design calls for 25 mm diameter Grade 420 deformed bars at 300 mm spacing in the vertical stem. The contractor has 25 mm Grade 280 bars on site and proposes using them at 250 mm spacing, claiming that more bars compensate for the lower strength. The site engineer is asked to sign a field change before the next concrete pour at 06:00. The wall retains saturated silty sand, and the design firm is unavailable until the next day. The contractor cites a schedule penalty of $8,000 per day. The field file contains the original design calculations, which show a factored flexural demand of 410 kN·m per meter and a resistance factor of 0.9, but the as-designed nominal capacity is not stated. The site engineer has not performed the original design and is not the engineer of record.
1. Determine whether the proposed bar substitution is mechanically equivalent to the specified reinforcement. Show the calculation and state your conclusion.
Read the worked answer
Compute the area of one 25 mm bar: A_b = π(25²)/4 = 491 mm².
As designed: 491/0.300 = 1,637 mm²/m.
Proposed: 491/0.250 = 1,964 mm²/m.
Flexural tension capacity is proportional to A_s f_y, so compare the product:
As designed: 1,637 × 420 = 687,540 N·mm/mm.
Proposed: 1,964 × 280 = 549,920 N·mm/mm.
Ratio: 549,920 / 687,540 = 0.80.
The proposed arrangement provides only 80% of the specified flexural capacity, a 20% reduction. More bars do not compensate for the lower yield strength. The substitution is not equivalent. Other checks also fail or are unverified: development length of the smaller-strength bar, crack width, shear, and seismic detailing. Because the original nominal capacity is not in the field file, the engineer cannot determine whether the 20% loss still leaves adequate capacity. The demand and resistance factor alone do not establish that. Do not approve the substitution on the basis of bar count or spacing.
2. What should the site engineer do before the 06:00 pour? Address technical review, documentation, communication, and professional ethics.
Read the worked answer
The engineer should not sign the field change. Signing would imply review of a design change that the engineer did not originate and may not be competent to approve.
Immediate actions:
- Issue a nonconformance or hold on the stem pour and notify the contractor in writing.
- Notify the owner and the engineer of record (EOR) immediately, even after hours, because a pour is imminent.
- Require either the specified Grade 420 bars or a written substitution approved by the EOR with supporting calculations.
- If the EOR cannot respond before the pour, delay the pour. A schedule penalty is a commercial risk, not a reason to accept an unverified structural change.
- Document the contractor's request, the quantities, the 20% capacity reduction, and all communications.
Ethically, the engineer's duty is to protect public safety and to approve only work over which the engineer has responsible charge. The engineer should not be pressured by the $8,000/day penalty. If the engineer suspects the contractor is substituting without approval, the engineer should follow the organization's escalation procedure and, if necessary, report the matter to the appropriate authority. The engineer may offer to help obtain a rapid EOR review but must not authorize the pour.
3. Suppose the EOR is reached and states that the as-designed nominal flexural capacity was 1.25 times the factored demand. Is the proposed substitution now acceptable? Explain.
Read the worked answer
Original nominal capacity = 1.25 × 410 = 512.5 kN·m/m.
Proposed nominal capacity = 0.80 × 512.5 = 410 kN·m/m.
After applying the resistance factor: φM_n = 0.9 × 410 = 369 kN·m/m.
The factored demand is 410 kN·m/m, so the proposed section is inadequate: 369 < 410.
Even before applying φ, the proposed nominal capacity exactly equals the factored demand, leaving no margin for material variability, dimensional tolerance, deterioration, or load increases. The original 1.25 ratio was a nominal-to-factored margin, not free capacity to give away. A 20% reduction consumes the entire margin and more. The substitution remains unacceptable. The EOR could consider a redesigned section, but that decision belongs to the EOR, not the site engineer. The site engineer should continue to withhold approval and document the calculation.