Allowable or Acceptable Beam Lateral Displacement
Allowable or Acceptable Beam Lateral Displacement
(OP)
Is there a source for allowable lateral displacement for a steel i-beam after placement? I have several W12 x 16 beams that are not straight. I am not yet sure why they are not straight but am tasked with evaluating an acceptable tolerance. They are about 37 feet long and about 5 feet apart. They vary from straight within mill tolerance to a several inches of lateral bow at midspan. None are bowed beyond the elastic limit of the flange (about 13 inches). They are in an industrial setting, support steel roof plate over an unoccupied space with no architectural or aesthetic concerns. On both ends, their bottom flanges are welded to steel angle supports. I've looked through a few steel construction manuals but they seem to be concerned with architectural issues, affecting subsequent alignments and load paths. This all steel structure is very simple with no other connections, walls, etc. I'm only concerned with their value as rafters and their ability to resist environmental loads applied to roof plate that is laid on top of them in contact with their top flanges. They were installed around 1980, have performed without failure to date but in documenting this structure we found that a good number of rafters are not straight, some visibly with by eye.
Essentially my question is: at what lateral midspan deflection do simply supported rafters have reduced carrying capacity?
Thanks
Essentially my question is: at what lateral midspan deflection do simply supported rafters have reduced carrying capacity?
Thanks






RE: Allowable or Acceptable Beam Lateral Displacement
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: Allowable or Acceptable Beam Lateral Displacement
The 3/16" thick deck plate is simply laid on the beams, no connection other than friction. I'm considering using twice the mill tolerance for sweep as a starting point, that would be about 2". This deflection stresses the flange to about 15% of yield. The roof plates are by no means flat, they warp from welding and just because, as you walk on them they oil can a bit.
RE: Allowable or Acceptable Beam Lateral Displacement
RE: Allowable or Acceptable Beam Lateral Displacement
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: Allowable or Acceptable Beam Lateral Displacement
RE: Allowable or Acceptable Beam Lateral Displacement
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: Allowable or Acceptable Beam Lateral Displacement
RE: Allowable or Acceptable Beam Lateral Displacement
RE: Allowable or Acceptable Beam Lateral Displacement
If you're feeling motivated, you could look at the weak axis curvature and see if the bend more closely resembles deflection from a point load or from a distributed load, but that might be going too far.
Then check for combined strong & weak axis bending based on your real vertical load and "imaginary" lateral load.
RE: Allowable or Acceptable Beam Lateral Displacement
RE: Allowable or Acceptable Beam Lateral Displacement
1) it may be the cause of the displacement.
2) it may be they be the the most likely disastrous outcome of the displacement.
Truly, if LTB can be shown to be fine, I doubt that any other effects will matter enough to justify scrapping ainy of the beams.
Can you supply a sketch of the framing, strap bracing, and cross bracing?
Do all of the beams sweep one way or present a pattern?
What is the nature of the live loading?
What kind of temperature swings are typical in this environment?
Are the beams oriented vertically to begin with?
What is the detail at the ends of the beams that provides rotational restraint there?
Is the steel plate really a bunch of smaller steel plates side by side? How are they fastened to their neighbors and to the walls or framing lines that stabilize the diaphragm?
Or are you only interested in specs for sweep tolerance at this time?
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: Allowable or Acceptable Beam Lateral Displacement
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.