Allowable deflection of beams 1

[Theozz]

Hi All,

Is there any specific code that stipulates the allowable deflection of beams? I notice that different values are used by different engineers. There should be different requirements for:

variable (live) loads only
variable (live) load + permanent load
vibratory loads
support condition (simply supported at both ends or cantilever)

Your comments will be much appreciated.

Thank you

 

[straub46]

In the US, IBC provides deflection limitations for live and dead + live based off material finishes. It also recognizes deflection limitations for the various specific material codes. The support condition doesn't matter for the deflection limitation, only for the deflection a member sees.

I typically see L/360 for live, and L/240 for D+L on floor beams for instance

Outside the US, I'm sure whatever the local code you're using would have some type of similar limits, but I'm not sure where they are.

Go Bucks!

 

[SlideRuleEra]

For beams supporting masonry, L/600 is a commonly accepted maximum allowable deflection.

http://www.SlideRuleEra.net

 

[Hokie93]

As mentioned, deflection is addressed in Chapter 16 of IBC 2015 (and older editions). You should also check with material-specific standards like ACI 318-14, as member deflection is often given some treatment in standards reference by IBC. Two good general resources, though not "codes", are AISC Design Guide 3 (Serviceability Design Considerations for Steel Buildings, second edition) and ASCE 7-10 Commentary to Appendix C (Serviceability Considerations).

 

[Theozz]

Many thanks Gents,

Is there a specific requirement for allowable deflection of a beam supporting say a CAT engine? This is a rotary (vibratory) equipment and where coupled with a compressor requires limited deflection. This os more of structural + mechanical requirement.

 

[Tmoose]

Is it a constant speed design?
Even if constant rpm, engines have some pretty powerful "forcing functions" other than 1X shaft rotation.
Depending on the design and mounting of the engine/compressor skid the static deflection of the floor system could end up with resonances or the floor being the vibration isolator and shaking a bunch.

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I expect the manufacturer has some very specific information and guidelines on this topic.

 

[MotorCity]

This is a completely different problem than you originally stated. Deflection in beam design implies relatively slow (nearly static) deflection of a beam. A beam subjected to vibrations means that you need to be looking into a dynamic or vibration analysis. That will tell you the vibration frequency, amplitude, etc and compare that to the allowable limits of the equipment. The amplitude of vibration may or may not be identical to the L/360 (or whatever) criteria you were initially considering.

 

[dauwerda]

The maximum allowable deflection/out of alignment for something like that would typically be specified by the manufacturer/packager of the compressor package.

 

[Theozz]

Thanks Gents,

The manufacturer doesn't seem to have any limits whatsoever, which I find stange and thus wanted to see if any of you have experience on that. I didn't want to bother you with the details of the project. But looks like I now have to. An existing compressor (with it own driver engine) on an oil & gas platform is to be coupled with with a new compressor. The consultant carrying out the design proposed cantilevering off the existing skid to support the new compressor and my first instinct is that so long as the two compressors are being coupled through a shaft they could be sensitive to deflection and therefore the new compressor needs to be have its own support to achieve close to zero vertical deflection. I proposal has been rejected and I thought I seek opinion elsewhere to see I could be right.

Dynamic/vibrational analysis has been carried out demonstrating low amplitudes and no potential resonance what soever, but still not convinced that supporting the new unit on a cantilevered skid is a proper idea.

Thanks again for your thoughts on this.

 

[JoshPlumSE]

Deflections are a serviceability issue. So, as long as the deflection doesn't inhibit the service of the structure, then it's okay. Some thoughts on this:
1) Live Load deflection for beams in typical residential and commercial buildings is typically limited to L/360. This is meant to limit the perceived deflection of the occupants and limit miscellaneous drywall or flooring issues (i.e. cracks).
2) Deflections for items that support masonry (as someone else said here) is usually something more like L/600 because masonry cracks at lower deflection levels than drywall. Also, similar deflection limits may apply for elements that support certain types of cladding.
3) Roof Live Load Deflection: I tend to use the same L/360. But, you could argue that it needn't be so strict if the roof deflection isn't going to result in drywall cracking.
4) Balcony Live Load Deflection: When I use a balcony live load of 100 psf, I tend to use L/240 as a live load deflection. The 100psf is very much higher than the balcony is likely to see. Over-designing it for safety / stress makes sense based on previously observed failures. But, seems a bit much for a serviceability issue. In fact, in cases like this where the residents are going really overboard in their loading, you kinda want them to feel / see some deflection to know that this is a problem. At least that's my thought.
5) Equipment deflection limits: This is usually not based on live loads, but on operating loads or such. You get information from the manufacturer about the limits of operation of the equipment and you obey that.

 

[Tmoose]

"The details of the project"

As always, this old man needs pictures and drawings of your arrangement with arrows and explanations to help understand what it is you are REALLY asking.

"Twenty seven eight-by-ten color glossy photographs with circles and arrows and a paragraph on the back of each one explaining what each one was to be used as evidence against us. "


For instance, the "coupling" may be either exTREMEly tolerant of static and dynamic misalignment, or it may NOT.

The existing engine has enough capacity to drive both compressors at the same time? Or the "coupling" also includes features to disengage the new compressor so either compressor may be run but one at a time.

Typical shaft colinearity between drive and driven components is often specified to be within .001" or so.

 

[12345abc6ttyui67]

The last offshore pump / motor coupling I looked at required skid displacements <0.01mm. Of course I laughed at them, but last I heard they were still trying to meet that tolerance.