Wind and Seismic Drift

[haynewp]

Can anyone explain why, in the IBC for example, there is not a table for allowable wind drift as there is for seismic?
There is a table showing allowable member deflection under serviceability, but it doesn't address drift specifically. I usually use this table anyway for wind drift (maybe more stringent depending on the cladding) but when I compare it with the seismic drift table, it is much more conservative in general.
For example using the wall deflection of L/240 for an allowable wind drift, the limit of L/240 is about 0.4% of wall height. Where a common seismic drift limit is 0.02(h) or 2% times the story height.
I know an earthquake is considered a catastrophic event and allowed greater deflection as long as the building didn't fail, but it would seem that for hurricane type winds, there would also be a separate wind drift table?

 

[JAE]

Building codes, in general over the years, have been somewhat silent in terms of lateral drift of buildings. There was a nice article in the AISC Structural Journal some time ago with a nice table of acceptable drifts based on the frame type and the cladding.

Generally you find that the lateral drift ranges between L/300 and L/600.

Why don't codes more aggressively specify drift? Not sure other than there is such a wide range of acceptable values and it is more subjective.

 

[haynewp]

I think I have seen an AISC design guide with values like that.
From what I know so far,

For example I have 2 similar buildings, one located in 120mph wind with no seismic and the other across the country in seismic zone 4 with 70 mph wind.
The one in the 120mph wind area I must keep say around L/300 for wind drift, where the one in the high seismic zone, I must keep L/300 for the 70 mph wind and L/50 (0.02h) for the seismic drift. It just seems a little strange to me.
Maybe by thinking in terms of serviceability it makes more sense, a building will be hit with wind all the time, maybe not 120mph but say for instance 50mph in that area. The more stringent drift of L/300 at 120mph might be in the L/1700 range when the building is hit with 50 mph wind and acceptable to occupants.

 

[steve111]

not sure i have your question right, is this a P-delta query, but there are definetly lateral deflection limits for wind both for the overall building l/400, and storey drift limits of l/500 and additional requirements with regards to notational loads to consider erection tolerances and out of plumbness. but still exterior and interior finishes may govern, thinking of glazing and masonry.

 

[haynewp]

Where in the IBC did you find l/400 and l/500? Maybe I overlooked it.

 

[ishvaaag]

The requirements of 2.5 per thousandth or L/400 under service level wind is a common traditional limit to drift targeted to reasonable serviceability of common buildings.

Estructuras de Acero
Oscar de Buen
Limusa

a book full of US references gives repeated account of this in its examples.

This would amount to limit drift L/250 at factored loads level if the safety factor was uniform and equal to 1.6.

The limit to drift set for seismic events -under the earthquake hypothesis- is set more to prevent excessive damage to the building and hence people under the event. I would say that the main intent here is to fix in some way the behaviour of the building, or specifically, its structure, when subject to the seismic event as derived from the code.

The good part is that by forcing the building to be quite stiff, structural and nonstructural damage is likely to stay small, and also keep small any effects of accounted or unaccounted structural pounding. The bad part for SOME structures is that they are directed to be stiff when they might benefit from flexibility.

But in general, one may say that drift from wind control derives mainly from outright serviceability (it notwithstanding having life safety virtues), whereas seismic drift control derives mainly from the urgence of the prevention of the destructive effects of the maximum expected earthquake event.

 

[JAE]

The only drift limits in the IBC I'm aware of is for seismic (although I'm sure if there are other limits our great group here at Eng-Tips will find it!).

The seismic drift is usually much higher (higher = more drift) due to the fact that you are limiting drift for a condition where the building will entered a post-elastic deflection - although your calculated deflection is based on elastic properties, the code magnifies it to account for the type of lateral system you have.

 

[NigelB]

The National Building Code in Canada has a limit on wind drift. The basic limit is H/500, which can be relaxed to H/400 if finishes are detailed to accommodate the increased drift. This may have changed in the 13 years since I practiced there - maybe one of our Canadian friends would chime in, eh?
Nigel

 

[j19]

AISC published a booklet titled "Serviceability Design Consdierations for Low-Rise Buildings". It is #3 in the Steel Design Guide Series. It provides suggested drift limitations for various types of wall cladding.