Deflection Criteria
Deflection Criteria
(OP)
Fellows...I have a longstanding question about designing beams with regard to deflection. While many cases meet code minimum deflection criteria per IBC/CBC Table 1604.3, sometime the total delfection seems to be TOO great. For example, if I have a 6X12 garage header spanning 20', i could meet live load, seismic or wind, and dead + live load criteria, but may have 6/10ths of an inch deflection for just dead load. That deflection I think would be visually unacceptable, for a garage header situation or for a residential floor system over a 20' span.
Could any of you comment and or correct my concern here?
Thanks.
Could any of you comment and or correct my concern here?
Thanks.





RE: Deflection Criteria
Common sense should be always used. I'd agree with you that 0.6 inches may be excessive. Use your own judgement.
RE: Deflection Criteria
Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.
RE: Deflection Criteria
RE: Deflection Criteria
The total load deflection was no good but I don't know your ratio of dead load to live load.
Frequently very long beams with a very small tributary load can bite you with deflection, so I put in arbitrary limits on the span of 6", 8", 10", etc deep beams and headers.
RE: Deflection Criteria
RE: Deflection Criteria
RE: Deflection Criteria
Alec...the numbers I threw out were for talking purposes only....they don't reflect an actual condition I have currently, but thank you for looking into it with detail.
Woodman...thanks for your suggestion. In fact, whenever I'm concerned about deflection, I use GLB with camber of 1.5DL.
If anyone still has more thoughts, please feel free to chime in.
RE: Deflection Criteria
Residential GLB's are bought "off the shelf" with standard camber. I have actually heard one builder complain of too much camber in a floor GLB, so I make sure the GLB isn't overdesigned.
RE: Deflection Criteria
I have definitely considered overdesigning them. I suppose this is a little tricky if you don't know what floor material they will install now or in the future...so we've generally decided to design with wood floors unless we absolutely know something heavier will be used.