Deflection Criteria

[smokiibear]

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.

 

[JAE]

Codes are MINIMUM standards and are primarily concerned with safety, not beauty.

Common sense should be always used. I'd agree with you that 0.6 inches may be excessive. Use your own judgement.

 

[woodman88]

If you are concerned, than specify a GLB with camber or a steel beam. But understand that a GLB is still wood and creep in wood will always occur over time under any high stress loading.

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.

 

[boo1]

Exterior cladding requirements may govern

 

[AELLC]

I use my own deflection critera of maximum L/280 (total load) for headers and I ran a test example of your design on my spreadsheet.

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.

 

[AELLC]

Even with only 1 foot of trib my spreadsheet arbitrarily limits the span of a 6 x 12 DF-L #1 to 21 feet. In your case, you probably have at least 4 feet of trib so I would definitely say the 6 x 12 is no good.

 

[MiketheEngineer]

Deflection in wood can be a real problem... Remember E might be 1.9 vs 29 for Steel.

 

[smokiibear]

Thanks, everyone. I'm getting the confirmation that I expected...both deflection limits and actual deflection need to be looked at. This is a little bit ambiguous sometimes - but that's where engineering judgement comes in, eh? Not sure exactly what deflection over what distance for what applications will be noticable.

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.

 

[AELLC]

With GLB, you only get the camber you specify when its a major commercial/industrial project.

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.

 

[smokiibear]

In our area, most GLB are special order. I usually spec camber if deflection is over 1/2".

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.