By: Joseph E. Caza III, PE, Partner
Highland Falls – Fort Montgomery (HFFM) High School is replacing its rooftop HVAC units (RTUs) as part of Phase 3 renovations.
The new, heavier equipment is hung over the gymnasium’s “high roof,” where 36 in‑deep plate girders—originally specified as ASTM A36—span between primary frames. A first‑round structural analysis (based on the as‑bid RTUs) showed the girders were acceptable, but a post‑bid
equipment change increased gravity loads beyond the assumed design basis. Pre‑test calculations therefore indicated the need to weld steel continuous steel bottom plates to every affected girder that is a time‑intensive, highly disruptive, and expensive field operation. Before issuing reinforcement details, the design
team investigated whether the actual steel strength of this 1960‑era building might be higher than the nominal 36 ksi specified on the legacy drawings.
Engineering Strategy: Material
Verification
|
Step |
Action |
Rationale |
|
1 |
Select coupon locations at low stress locations |
Negligible effect on capacity; worst‑case |
|
2 |
Cut two 6″ x 12″ coupons (¾‑in |
Produces standard dog‑bone blanks without |
|
3 |
Ship samples to accredited lab |
Independent testing ensures defensible data |
|
4 |
Test per ASTM E8‑24 & ASTM A370‑24 |
Complies with industry standard tension |
Laboratory Results
|
Property |
Coupon #1 |
Coupon #2 |
ASTM A36 Requirement |
|
Yield Strength Fy (ksi) |
44.2 |
43 |
>= 36 |
|
Ultimate Strength Fu (ksi) |
63.1 |
59.7 |
58–80 |
|
Elongation (%) |
34.8 |
30.4 |
>= 23 |
|
Reduction of Area (%) |
63.1 |
59.7 |
— |
Key takeaway: Both coupons comfortably exceeded the legacy 36 ksi specification, with an average Fy ≈ 43.6 ksi (∼ 21 % higher). Ductility indicators (elongation, reduction of area) were excellent, confirming the material is not embrittled despite its age.
Re‑Analysis of Girder Capacity
Using AISC 360‑22, Ch. F, the design team reran the load‑rating model with:
- Dead + snow + HVAC loads reflecting the heavier RTUs and curb dead load increase
- Revised Fy = 43 ksi (conservative rounding of the coupon average)
Critical limit states were considered and we found the existing steel girders had adequate capacity to support the revised heavier units.
Project Impacts
|
Aspect |
Benefit |
|
Cost |
Avoids steel material and labor costs |
|
Schedule |
Removes critical‑path activity, keeps summer |
|
Safety & Disruption |
No hot‑work over gym |
|
Sustainability |
Saves CO₂ |
Lessons for Forensics Engineers:
- Don’t default to original drawings. Material specs in mid‑20th‑century buildings often understate strength; coupon testing can unlock existing capacity.
- Coupon extraction is precise yet economical. Two strategically cut samples yielded data that swung a five-figure reinforcement decision.
- Integrate field testing early. Align testing with destructive demo activities (e.g., roof opening for curbs) to minimize patching costs.
- Communicate results clearly. A concise technical report helped the architect, owner, and contractor grasp the structural implications quickly.
Conclusion
Through a targeted materials‑verification program, the structural engineering team transformed a mandatory reinforcement into an unnecessary scope item. The coupons proved that the gym’s girders, benefiting from a field verified yield strength 20 % above their design value, can safely carry the upgraded rooftop units. This case exemplifies how forensic testing, combined with modern analytical tools, can conserve resources and protect project budgets.
References
- Highland Falls – Fort Montgomery Existing High Roof Girder Strength Testing Results, Elwyn & Palmer Consulting Engineers, 3 April 2025.
- Report of Tests, Ref. S‑140023, ATLAS Evaluation & Inspection Services, 31 March 2025.
- AISC 360‑22, Specification for Structural Steel Buildings.
- ASTM E8‑24 / ASTM A370‑24, Standard Test Methods for Tension Testing of Metallic Materials / Mechanical Testing of Steel Products.