Platform Supporting Mechanical Equipment

[GalileoG]

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?

 

[XR250]

Why not add some x-bracing between the girders? Seems like that would allow you to lighten up those beams as well.

 

[GalileoG]

Cross bracing would get in the way of maintenance of the RTU.

 

[SAIL3]

even partial X-bracing would help to reduce this deflection....1.5" over 49ft is not excessive in itself...I would assume the RTU unit has some rigidity by itself and may reduce this local lateral deflection in it's vicinity...I would assume the OP is combining the effects of this weak axis deflection with the strong axis bending of the bms..partial X-bracing would also reduce any concern about vibration....a sketch of the layout would help get more acuurate feedback...

 

[SlideRuleEra]

GalileoG - A W14 spanning 49' does not "sound" right (42:1 span to depth ratio). Don't get me wrong, I'm sure you have verified that the numbers work. You may be able to get a better proportioned beam (of similar weight) to carry the load, but with better deflection characteristics. Looking through the tables, suggest taking a look at a W18x76. Both X-X Axis and Y-Y Axis value of "S" are higher than the W14 (for load carrying capacity). Also, the X-X Axis and Y-Y Axis value for "I" are higher (for deflection limitation). Still not an ideal member (33:1 span to depth ratio), but a step in the right direction for a less "springy" platform.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Tmoose]

As a first cut the static deflection is related to the resonant frequency of the platform with the equipment on it.
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.

 

[JedClampett]

Listen to SlideRuleEra. I get 6.5 inches of vertical deflection without self weight.
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.

 

[KootK]

Is this the document Tmoose? Link

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.

 

[Tmoose]

KootK, yep. That's it. thanks
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