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Single Story Deflection/drift

Single Story Deflection/drift

Single Story Deflection/drift

(OP)
I am designing a single story building, 17.5 feet in height. The footprint is 51 ft by 56 ft. It consists of bar joists spanning the 56 foot direction (for a number of reasons), supported on WF beams along the 51 dimension. The 51 ft ends are 2-bay rigid frames with steel tube columns and moment connections to the wide flange beams. The 56 foot edges are also 2-bay rigid frames using 30kcs joists due to the longer spans. I am checking the drift on the frames using ASCE 7, Appendix C, cc1.2 = L/400. I am finding this limits me to about 0.6 inches of deflection. The result is my columns are 10x10x3/8 and 30KCS5 joists (highest series) are required. This all seems very heavy for a small, one story building, with basically about 4 kips wind load at each frame. This seems unusual to me but I have checked it several times. Is the L/400 (0.6 inch) drift limit correct? Seems very tight. Have others run into this issue?

RE: Single Story Deflection/drift

You can reduce the MRI to 50 year for deflection checks and modify the pressure or the allowable drift. Personally, I find it easier to reduce the drift. For brick, I use a deflection of H/282 (modified from H/400)for typical wind forces (for in plane racking). The 400 in H/400 gets modified by multiplying it by 0.84^2 (0.74^2 for higher wind regions). Also, what year ASCE 7 are you using? If its the 2010 version, you can reduce the wind load to a service level.

RE: Single Story Deflection/drift

Yes. As Mike20793 described in a bit more detail than I will, use 0.7 times service wind load for lateral deflection check.

RE: Single Story Deflection/drift

(OP)
Excellent! Thank you guys for the guidance.

RE: Single Story Deflection/drift

FYI Jc67roch, if memory serves, I believe there was some guidance on this issue included in the commentary to one of the appendices of ASCE 7-05 or 02. I am not sure if the info is still included in the current edition of ASCE 7. I don't have my references handy right now or I would look it up and tell you definitively where the information was printed. Maybe someone else knows what I am referring to and can chime in.

RE: Single Story Deflection/drift

Also, control of deflections is one reason why I often have a hard time utilizing HSS columns in moment frames. Sounds like it may work for you in this case. Good luck.

RE: Single Story Deflection/drift

I assume that your moment frames and pinned at the base.

For a wind event, it is common to assume that some of the moment is resisted at the base. During a webinar, it was recommended that the point of zero moment be 1/4 to 1/3 of the column height.

I think that this is related to settlement. The base is pinned under sustained loads, and fixed under temporary loads.

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