Platform Supporting Mechanical Equipment
Platform Supporting Mechanical Equipment
(OP)
I have steel girders, and joists spanning between girders. I am designing a structural platform to support a RTU, the platform will span from girder to girder above the roof.
The span of the platform beams at the two sides of the RTU is approximately 49ft (girder to girder distance); there are two secondary beams to pick up the other two edges of the RTU. There are no horizontal bracing or grating. The RTU is at mid-span.
The RTU weighs approximately 9 kips. I have taken into consideration wind acting on the RTU and the beams, and seismic load. I am taking the full length of the platform (49ft) as the unbraced length.
I have sized the platform beams to be a W14x74. The weak-axis deflection of this beam to wind load only is 1.5” (this is after multiplying the 2” deflection with a serviceability important factor for wind load of 0.75, as permitted in the Canadian code). The deflection comes to be L/400.
Even though I meet the very reasonable deflection requirement of L/400, a deflection of 1.5” due to wind load seems high. This also brings up another question of vibrations. Would vibrations be a concern and how would I go about checking it? (The RTU is a makeup air unit). Any other design considerations?
The span of the platform beams at the two sides of the RTU is approximately 49ft (girder to girder distance); there are two secondary beams to pick up the other two edges of the RTU. There are no horizontal bracing or grating. The RTU is at mid-span.
The RTU weighs approximately 9 kips. I have taken into consideration wind acting on the RTU and the beams, and seismic load. I am taking the full length of the platform (49ft) as the unbraced length.
I have sized the platform beams to be a W14x74. The weak-axis deflection of this beam to wind load only is 1.5” (this is after multiplying the 2” deflection with a serviceability important factor for wind load of 0.75, as permitted in the Canadian code). The deflection comes to be L/400.
Even though I meet the very reasonable deflection requirement of L/400, a deflection of 1.5” due to wind load seems high. This also brings up another question of vibrations. Would vibrations be a concern and how would I go about checking it? (The RTU is a makeup air unit). Any other design considerations?






RE: Platform Supporting Mechanical Equipment
RE: Platform Supporting Mechanical Equipment
RE: Platform Supporting Mechanical Equipment
RE: Platform Supporting Mechanical Equipment
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RE: Platform Supporting Mechanical Equipment
If that resonant frequency is close to any of the motor/fan/compressor rpm the entire platform may have significant vibration even if the motors etc are well balanced. If there are no occupied spaces nearby or attached it //may// not be a big deal.
Mason Industries used to have a copy of a presentation made to ASHRAE back in the 70s about the difficulties isolating HVAC equipment on upper stories but I could not locate it today. Basically very soft isolators are required because the floor and equipment are a 2 mass system.
RE: Platform Supporting Mechanical Equipment
Anytime your span to depth exceeds about 20, you need to start worrying about deflection. When it gets above 30, you're almost always going to have a problem.
His suggestion of a W18 x 76 will cost almost exactly the same. Steel is a commodity, priced by the lb.
RE: Platform Supporting Mechanical Equipment
I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
RE: Platform Supporting Mechanical Equipment
Not many isolator catalogs make a big deal about upper story installations.
Some, especially rubber pad types, kind of mention the weight of the equipment and stop there.
Dan T