Deflection Limits for Mechanical Equipment Framing

[StrucDesignPE]

Designing the support framing for several roof top cooling towers. The structure will be steel and about three feet off the roof top. The dimensions are preliminary but as of right now are 14 ft x 23 ft. Unit operating weight is approximately 34.5 kips. In the equipment cut sheet, no deflection limits are given for the unit's framing and I was wondering what others use in such cases? L/360? L/600?

Thoughts are appreciated.

 

[BAretired]

L/360, but notify the mechanical engineer of the expected deflection.

BA

 

[dhengr]

StrucDesignEIT:
The mech. unit is on its own frame and is usually not particularly sensitive to its exact elevation, +/- ½". But, someone has to determine what the hard piping and other connections will tolerate in the way of vertical and lateral movement. And, this relative movement has to be designed for.

 

[JStephen]

I've seen some large (and unrelated) equipment where a deflection limit was specified, and what they were really getting at was making stiffness adequate to avoid vibration problems, rather than elevation/straightness issues.

 

[KootK]

Last spring an owner called me out to look at a mechanical support beam that was failing. It turned out that it was a very long beam and the visible deflection of about 2" was well within normal L/ what have you. It also vibrated perceptibly but that caused no problems with regard to unit function. In future, I shall be enforcing a 1/2" absolute deflection limit whenever possible. Having me spend an afternoon out there with the operations manager surely cost as much as a meatier beam would have.

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.

 

[StrucDesignPE]

Thanks for your responses. I have seen where the mech. unit cut sheets specify a deflection limit, and with mech. platform structures like this, I always apply a factor for vibration such as unit weight x 1.2 (based on IBC). I think what I was worried about (having never designed a platform this large), is that the deflection would be acceptable by all standards (L/360) but as in KootK's scenario, actual deflection would be enough to cause concern to the owner. I like the idea of keeping deflection to +/-1/2" which for my case is around L/530.

 

[BUGGAR]

I designed structures for processing equipment connected with heavy pipe and we held deflection to l/360 with a max of 3/8". Spans were short enough to accommodate this.