Engineering Structural Steel to Accommodate future rooftop loads
May 07, 2021
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| STATEMENT OF PROBLEM In design-build, the rooftop HVAC units and equipment are typically not final-sized at the time the structural steel framing is completed. Typically the engineer up-sizes the joists or trusses at the unit locations but sometimes they do not increase the size sufficiently to accommodate the fluctuation in the unit size or weight. This missed coordination can lead to having to add steel reinforcing after the fact. Sometimes the unit or equipment weight doubles between a 60% level design and an IFC level design. COURSE OF ACTION It is a good practice to discuss this matter with the structural engineer and the mechanical engineer before the steel is released to determine the risk of units being upsized and the safety factor that the engineer is including to mitigate the risk. If there is still much unknown about the units the structural capacity should be increased 1.5-2x the assumed load to account for unexpected fluctuations. LESSON LEARNED Factoring a heavier steel design when units are not finalized is a small upfront price to pay in coordination, to avoid having to go back and add steel reinforcing when the units turn out to be much heavier than anticipated, or larger than anticipated. |
