Story Drift Due to Wind

[joemarch]

I am in the process of evaluating a warehouse building that has been framed with multi beam and column lines. The beams and columns were designed as moment frames. The exterior building has been framed with light gauge metal studs (w/slip connections)and gypsum sheathing. Attached to the metal studs is a brick veneer.

I've gone around and around about wind drift limitations. I know the IBC gives limitations to seismic story drift but doesn't address wind. I feel the seismic drift limitations are pretty liberal for a building that has a gypsum interior finish with an exterior brick finish.

What are your thoughts?

Thanks much!

 

[dbuzz]

For a portal frame building with external masonry walls, I would limit the wind drift to the lesser of:
- (eave height)/250; and
- (bay spacing)/100.

 

[UcfSE]

I limit wind story drift to L/350 to L/400 for wind loads with a 10-year return period. I found that in an AISC Design Guide, I don't remember which. The ASCE 7 has a table with which you can convert 50-year return period wind speed to 10-year return period. Notice that the table provides coefficients for wind speed, not wind pressures. In that guide you may find more information based on different cladding types.

 

[bjb]

Check out AISC Design Guide 3 (second Edition). If you are an AISC member you can download it for free. If you have some partitions in there (say in office areas?) then the Design Guide recommends H/500 in a 10 year wind. ASCE 7-02 says that a 10 year wind pressure is 70% of a 50 year wind, and the design guide uses 75% of a 50 year wind pressure to get to a 10 year wind pressure. The guide also says that for the drift perpendicular to unreinforced masonry walls the drift should be limited to that which results in a 1/16" crack at the wall base. I highly recommend getting a copy of this Design Guide.

 

[joemarch]

Thanks for the help. The iformation was greatly appreciated.

 

[UcfSE]

I looked through the DG 3 and I must respectfully disagree with what is said about ASCE 7-02 giving 10-year wind pressure as about 70% of the 50-year return pressure. The table I mentioned above is Table C6-3 on page 308. The table gives coefficients to be applied to wind speed, not pressure. For example, for 110 mph wind area (50-year) the 10-year return speed is 0.74*110=81.4 mph. Now when you use this speed in the equation for qz and qh notice that V is squared V^2. Everything else, I, enclosure, exposure, etc. will be the same. q (10-year) = (0.74*110)^2 for V ==> 0.55*q(50 year) (0.74^2 is about 0.55). The coefficient gets squared when you calculate wind pressure.

I also see where the authors of DG 3 recommend 75% for 10-year wind. Is my math above incorrect or is there just conflict/disagreement in general about how to obtain 10-year winds from 50-year winds? Maybe I'm just missing something somewhere but I can't find it.

 

[bjb]

As regards to ASCE wind pressure, I was just quoting what it said in the Design Guide. In eq. CB-3 of ASCE7-98, they have a factor of .7 on the wind load, maybe that's what they're talking about. I agree with your math, and probably the approach in your post is the most theoretically accurate, but it is conservative to use a factor of .7 or .75 multiplied by the 50 year wind pressure.

I don't have a copy of ASCE 7-02. In the commentary to appendix B, do they have an equation where they factor the wind load by 0.7? In ASCE 7-98, equation CB-3 reads D+0.5L+0.7W. When I check deflections with wind I don't factor the Live load by 0.5, I just multiply my 50 year wind load by 0.7 because it's quick and conservative.

 

[vmirat]

The "Structural Engineering Handbook" by Gaylord and Gaylord has a very good table of suggested deflection indices. It's actually a reprint of the Weiskopf and Pickworth Deflection Index Guide. It is broken out by commercial and residential and then by type of construction. Values vary from 0.0015 to 0.0030. They do make the statement that, "The stiffening effects of floors and interior and exterior walls are highly indeterminate and are neglected in deflection calculations." Since this is mostly a servicability issue, they also state that, "A single-occupancy corporate or prestige building, however, can and must afford the luxury of added stiffness to minimize unnerving movement, unsightly plaster cracks, and eerie noises." The bottom line is that it is based on engineering judgement.