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Wind deflections at an expansion joint

Wind deflections at an expansion joint

Wind deflections at an expansion joint

(OP)
Another recent post on building expansion joints got me to thinking about this question - wondering what you all typically do or what you all think is code-correct:

One building with an expansion joint down the middle.  The building might be different construction on one side relative to the other.  For seismic deflections I have always calculated the amplified seismic deflections, added them up and used that value to determine the necessary joint width.  In other words, in a seismic event the two building parts might begin oscillating in opposing directions - thus the joint stand-off is the sum of the seismic deflections.

With wind, for most buildings, I'm not sure I can see the buildings oscillating in opposite ways like in a seismic event.  And I don't recall ever seeing anything like that requirement in the code relative to wind.

So for wind, do we assume that the required wind-based joint width might be the difference between the windward and leeward deflections (assuming no seismic)?

Or do we add the windward and leeward deflections assuming oscillation in the building?

RE: Wind deflections at an expansion joint

JAE...the only time I would be concerned about oscillation for the wind condition you describe is if the two building sections have different flexibility (or stiffness...depending on how you want to look at it).  The gust recovery for one might be significantly different than the other, thus for sequential gusts, there could be enough oscillation to be an issue.

Normally, we are only concerned about oscillation of pieces and parts, not the whole building.

Ron

RE: Wind deflections at an expansion joint

(OP)
<side note off-topic message to Ron>  I obviously didn't make it down to your neighborhood for the tournament this year - just too much going on here to do the trip.  I keep talking to my dad about it and ...well...maybe next year.

 

RE: Wind deflections at an expansion joint

JAE...anytime, my friend. The weather is great!  Beer is cold.  Just let me know.
Ron

RE: Wind deflections at an expansion joint

What have you done for either seismic or wind when you are building adjacent to another existing building on a property line?  When the adjacent building is right on the line, so they have not given you any room for their deflection, and you will not know what defelction it will experience anyhow?

RE: Wind deflections at an expansion joint

Just a question JAE, I do not design buildings in seismic regions so what would the typical width of a seismic joint be for the condition you described?

RE: Wind deflections at an expansion joint

(OP)
asixth - what I mentioned above - we would add the two amplified seismic deflections together to get the joint width - which assumes that the two sides of the buildings are swaying in opposite ways.

I guess its the idea that seismic effects tend to be multi-directional where wind is directional.

 

RE: Wind deflections at an expansion joint

Per code the seismic joint should be based on square root of some of the squares of the two building deflections.  This was in IBC 2003, was removed from IBC 2006 because it was supposed to be in ASCE 7-05 but accidentally got missed, and is now in ASCE 7-10 (Section 12.12.3 Eqn. 12.12-2).  

 

RE: Wind deflections at an expansion joint

The SRSS methond is back in the 2009 IBC as well.  Using this reduces the required size of the joint.

RE: Wind deflections at an expansion joint

(OP)
Thanks, WillisV - appreciate the info.

RE: Wind deflections at an expansion joint

JAE:  

Equal stiffness then maybe, but unless the buildings are identical, and built at the same time, I would say no and be conservative here.  

Even with identical stiffnesses, with the windward and leeward forces being different, and timed differently due to the length of the buildings, you can get different responses, especially due to vortex shedding at the trailing edges.  

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

RE: Wind deflections at an expansion joint

2009 IBC also lists two exceptions.

1.    Smaller Separations or property line setbacks shall be permitted when justified by rational analysis.
2.    Buildings and structures assigned to Seismic Design Category A, B, and C


I assume that the second was added so that large separations joints we not required in areas not prone to seismic activity.
 

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