AISC ASD L3.1 beam depth limit
AISC ASD L3.1 beam depth limit
(OP)
I would appreciate a little help on the interpretation of L3.1.1 (ASD Green book p.5-181). In simple terms for floors it states that the depth of fully stressed beams should be a minimum of L.Fy/800 and if the actual depth is less than this the "unit stress in bending should be decreased in the same ratio as the depth is decreased from that recommended above".
No problems with a short span beam: if (say) Fb is 24, the formula gives 15" and your section is 10" then you must not stress it more than 24 x 10/15 =16ksi.
If though the beam is unrestrained with a longer span the various formulae already reduce Fb to take account of this. So it would appear to be double counting to get a stress of (say) 12ksi and then have to reduce this by a further 10/15. Logically it would seem to me that in my hypothetical example if you get an Fb of 18 you have to chop it down to 16, but not if Fb is already 15ksi.
Crawley & Dillon example 1 p.117 has a 30ft beam with intermediate restraints at 10ft so Lb (10ft) is more than Lc and less than Lu so Fb is 0.6Fy and it is this figure they reduce (by 14/16.2). If the restraints were every 6ft then Fb would be 0.66Fy and this figure would then be being reduced. To me it seems more logical to limit the stress to the lesser of 0.66Fy(d1/d2) or (in this case) 0.6Fy, not 0.6Fy(d1/d2)
Comments gratefully received
TonyB UK
No problems with a short span beam: if (say) Fb is 24, the formula gives 15" and your section is 10" then you must not stress it more than 24 x 10/15 =16ksi.
If though the beam is unrestrained with a longer span the various formulae already reduce Fb to take account of this. So it would appear to be double counting to get a stress of (say) 12ksi and then have to reduce this by a further 10/15. Logically it would seem to me that in my hypothetical example if you get an Fb of 18 you have to chop it down to 16, but not if Fb is already 15ksi.
Crawley & Dillon example 1 p.117 has a 30ft beam with intermediate restraints at 10ft so Lb (10ft) is more than Lc and less than Lu so Fb is 0.6Fy and it is this figure they reduce (by 14/16.2). If the restraints were every 6ft then Fb would be 0.66Fy and this figure would then be being reduced. To me it seems more logical to limit the stress to the lesser of 0.66Fy(d1/d2) or (in this case) 0.6Fy, not 0.6Fy(d1/d2)
Comments gratefully received
TonyB UK






RE: AISC ASD L3.1 beam depth limit
RE: AISC ASD L3.1 beam depth limit
Rarely do I find that the allowable bending moment is the same as the actual bending moment. If it is not, then the beam is not fully stressed.
RE: AISC ASD L3.1 beam depth limit
RE: AISC ASD L3.1 beam depth limit
Then....if you go beyond their suggested Fy/800 limit for depth, they are suggesting that, in terms of SERVICABILITY, you will have deflection issues (not strength issues). So they suggest further limiting your fb to a lower limit of Fb (true depth/recommended depth).
They are dealing with two different things here so its not double counting at all.
RE: AISC ASD L3.1 beam depth limit
I think the idea is to keep you from getting too "springy" a beam.
RE: AISC ASD L3.1 beam depth limit