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Re: Modeling Diaphragm Stiffness of Unreinforced Masonry

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In a message dated 4/26/99 2:25:18 PM EST, polhemus(--nospam--at)insync.net writes:

<< That's great, but I'm here.
 
 BTW, no seismic problems in Houston, Texas. The lateral loading is from
 wind.
  >>

You might still want to get the information anyway. 

 In short, what is often done in seismic areas is to perform an in-plane 
shear test (flatjack test) to determine the strength of the existing brick 
infill wall.  An FEM model is done for several different individual bays of 
the buildings (assuming they have different dimension, etc).  The FEM has the 
single bay of  brick infill wall with the boundaries being the concrete 
columns and beams using actual dimensions and properties of the building 
(f'c, fy, etc).  From the FEM model when applying a lateral force you 
determine when and where failure occurs in the wall based upon what you 
decide is an acceptable strain level for each of the materials (concrete, 
masonry, steel in concrete).  This is where you need to carefully look at the 
computer output to decide what has failed first, and is this important, 
relative to the overall stability of the frame (example, one brick crushes at 
the upper corner of the brick infill wall, so you continue until a certain 
percentage of the bricks have failed or strain failure in the concrete). 

Based upon the effective stiffness/displacement of the FEM model, you create 
an x brace in your computer model with an equivalent stiffness to the FEM 
model.  The sizing of the brace elements is an iterative solution, since you 
are checking the displacements of the building, demand in your brace elements 
and existing framing members so they are not overstressed.

Engineering judgement is required to determine how many different types of 
"x" brace bays you need to have in your model, since each would require their 
own FEM model.   As you can imagine, this gets to be a lot of work, for what 
is most likely a very small design fee.  You may want to just knock out the 
existing URM and put in new shear walls, or somethin similar.

Hope this helps, I am sure there are others on the list that could  explain 
this in greater detail.

Michael Cochran