Drift Question

[Lion06]

I came across something in AISC that I wanted to get some clarification on. It says that drift shall be limited to ensure that one building doesn't whack into another (that's me paraphrasing, in case you couldn't tell) at strength level loading. Is that to that if you have a building with an expansion joint that the joint needs to accomodate frist with a 1.6 factor on the 50-year wind (since that is strength level loading)? I know that we don't typically do that.

We typically provide an expansion joint to accomodate drift from a 50 year wind (with no 1.6 factor), but we are currently doing a project where the expansion joint under those conditions was too big and we're using 10 year wind to size the expansion joint.

I know I've seen in IBC that the drift needs to accomodate full seismic drift (including Cd), but I've never seen anything in the IBC for wind.

 

[msquared48]

Screw the 1.6 factor for wind. That just confuses the issue here. The bottom line is that you do not want the buildings to hammer each other with either wind or seismic.

I feel that seismic deflection is far less predictable than a steady wind pressure due to the amplification factor Ft, and the possibility of 2nd and third order deflection patterns. Hence their inclusion of the 1.6 factor for seismic, and not for wind.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[miecz]

I would use the wind factor. That said, I would try to use ASCE's method 2, which assigns a windward pressure to your windward building, and a leeward pressure to your leeward building. As I read 6.1.3, the expansion joint needs to accommodate algebraic sum of the effects of the two pressures, in this case the difference in deflection between the two buildings.

 

[BAretired]

miecz,

That could be a very wide expansion joint. Why not provide enough room for expansion, then link the two buildings together with shock absorbers between? I've never done it, but it seems like a possibility.

BA

 

[Lion06]

miecz-

Wouldn't that be the algabraic difference? We would assume they drift in the same direction at the same time, right?

I did consider windward on one only and leeward only on the other.

 

[slickdeals]

We would assume they drift in the same direction at the same time, right?

This is not always true. For a wind blowing parallel to the expansion joint, the buildings will each experience a side wall pressure that will pull the buildings apart from one another.

However, the wind pressures associated with this might be less than the head-on wind, but nevertheless needs to be checked.

 

[miecz]

BARetired-

I haven't sized a joint by this method, as my recent work has been in bridges. I'm simply giving my read of the code. It seems odd that the code would call for this calculation to be done at strength level loads, but that's what it says.

StructuralEIT-

Yes, for wind perpendicular to the joint, the joint would see the difference in the calculated deflections. Sorry if I insulted you by implying that you missed that.

Slickdeals-

As the wind parallel to the joint tends to pull the buildings apart, it wouldn't cause them to "whack into each other", but as you say, would figure into the design of the joint.