Wind and Seismic Drift
Wind and Seismic Drift
(OP)
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?
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?






RE: Wind and Seismic Drift
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.
RE: Wind and Seismic Drift
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.
RE: Wind and Seismic Drift
RE: Wind and Seismic Drift
RE: Wind and Seismic Drift
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.
RE: Wind and Seismic Drift
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.
RE: Wind and Seismic Drift
Nigel
RE: Wind and Seismic Drift