Weight used to calc.Seismic Base Shear

[lutein]

Hi
I have a (1)-story building where the brick veneer is bearing on strip footing. When calculating the total seismic base shear, should I be considering the brick veneer weight?

ASCE 7-02 Section 9.5.3 specifies W = effective seismic weight of the structure, including the total dead load and other loads, such as storage, partition, permanent equipment, and snow loads.

I have been including brick dead weight in my design. But, for 1 story building, the lateral system is significant due to seismic load from brick. therefore, I was wondering if I could reduce the shear by excluding the brick weight, because brick veneer should already be cracked and fall down during seismic event due to excessive deflection. In this case, the building won't be seeing the brick dead weight.

I was wondering what other structural engineers have been doing in this situation.

Thank you for your input in advance.

 

[JedClampett]

I wouldn't count on your brick failing to keep the main lateral force resisting system from being overloaded. If it's a permanent load, you need to account for it in your seismic loads.
And why is your brick veneer failing, anyway? I would make sure it's adequately tied to the frame to ride out the seismic event. Those bricks falling down will kill people.

 

[msquared48]

Your exterior wall system should be designed to provide enough stiffness to the brick veneer that is will not fail in deflection.

And as Jed said - you'd better include it in yourseismic calcs - but, remember that only 1/2 of the wall height from the foundation to the roof diaphragm needs to be included in the seismic calcs. The rest will go directly to the foundation, and not thru the roof diaphragm and shear wall system..

Mike McCann
McCann Engineering

 

[lutein]

thanks for your input. Agree that brick should be included in base shear calc.

Just could not get past my mind that BIA specifies an allowable deflection of L/600, but the allowable seismic drift per ASCE or IBC is 0.02hx or L/50 for SUG 1 building. And the rational of seismic resisting system is provide ductile structure for energy absorbance, which will produce large deflection. I have been wondering how would brick veneer behave under such deflection.

 

[hokie66]

It will likely fail, i.e. crack, but the key is to prevent its dislodgement from the supporting frame.

 

[haynewp]

The L/600 is a serviceability limit under wind; and I expect some brick cracking under a design seismic event. However, you don't want brick panels dislodging and falling off under an earthquake as stated above.

 

[Atomic25]

I think an important point is the seismic code does not aim to create a building that is usable after a design event. It makes a SURVIVABLE building. Therefore, the brick can crack as much as it wants as long as it doesn't rain down on fleeing inhabitants.

 

[hoopvols]

Regarding the L/600...check out "Deflection Limits for Wood Studs Backing Brick Veneer" by Harold Sprague in the Fall 2007 edition of Wood Design Focus.