Deflection Limitations Question
Deflection Limitations Question
(OP)
Hey guys,
Why are deflection limitations specified as a fraction of the span length, such as L/360 or L/240? I'm having a hard time picturing this criteria, it apears to be more of a deflection slope or gradient, rather than a maximum deflection limit, such as 1" or so. Can someone show a simple example how the total deflection is related mathmatically to this L/360 or L/240 criteria. Is this fraction the total deflection in the member (Delta) divided by the span length (L)?
Why are deflection limitations specified as a fraction of the span length, such as L/360 or L/240? I'm having a hard time picturing this criteria, it apears to be more of a deflection slope or gradient, rather than a maximum deflection limit, such as 1" or so. Can someone show a simple example how the total deflection is related mathmatically to this L/360 or L/240 criteria. Is this fraction the total deflection in the member (Delta) divided by the span length (L)?






RE: Deflection Limitations Question
That's exactly right. Usually midspan deflection or something close to it.
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: Deflection Limitations Question
RE: Deflection Limitations Question
Most of the time, the design guide's recommendations are just related to span. But, other times (skylights, cladding, partitions, cranes) the recommendations may be a little different.
Also, ASCE-7 has appendix C: Serviceability considerations and the associated commentary. That's another good place to look.
There are other documents on the subject that are more technical. These two are just the ones that I'm most comfortable with.
If you're really interested in the subject, Larry Griffis has a book or paper on the subject of more rational drift restrictions where he defines a new "drift measurement index" and a "drift damage index". The only reason why I mention it is because it derives a new method (for lateral drift, not vertical deflection) for more rationally defining what is acceptable vs unacceptable drift.... Sort of the process you'd have to go through if you wanted to replace the simplistic span deflection ratios with something more rational.
RE: Deflection Limitations Question
So, if you're looking at a joist that spans between two girders. The girds may deflect 0.5" at the joist support points and the joist deflects another 0.125", for a total deflection of 0.625". Normally the deflection ratios for the joist are based on the 0.125" not the 0.625.
RE: Deflection Limitations Question
My understanding of 2012 IBC 1604.3 is that you only look at the maximum deflection in each member, and then compare this calculated deflection to the deflection limits. You would need some 3D modeling of the structure to determine relative deflections. If you are using the relative deflection, than what L do you use (multiple members) for the limitations on deflections?
RE: Deflection Limitations Question
The one, and only way, yes.
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: Deflection Limitations Question
Re member deflection versus total deflection, the individual members deflection needs to be checked over its length and the total system deflection needs to be checked over its "length".
eg
- a flat slab needs to be checked for column line/strip deflection in each direction plus overall panel deflection based on the diagonal span length.
- in a structural steel system, each member needs to be checked individually for its own deflection and then each combination of members needs to be checked basically until you get back to a deflection between columns. Same for concrete transfer beam systems.
RE: Deflection Limitations Question
RE: Deflection Limitations Question
RE: Deflection Limitations Question