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Re: Equivalent concrete thickness for masonry

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Anantha,
 
Tell me more about what you are doing.
 
ETABS does not have a masonry module or perform masonry design.  You may need to analyze the existing, but to what extent?  I assume all you are trying to do is use Etabs to perform a lateral distribution.
 
What is the existing building, are there different wall thicknesses?  Are there different materials, or frames and walls combined?
 
You have to forgive me on the equivalent thickness, percent solid; I did not read that into the question.  Out in CA masonry is typically solid grouted, and partial grout would get the contractors looking at you funny.  But whether it matters or not depends on what you are working with.
 
If the walls are all the same (say 8 inch masonry of what ever grouting or thickness) then you can model the structure as concrete, all you are going to care about is the force distribution based on relative stiffness, and as long as your elements are all similarly the same material, the relative rigidity will be based on geometry and will not change.  Any further analysis will need to be done independent of the computer program.
 
If there is a combination of different materials, then the relative rigidities become dependent on more than just geometry, and the analysis is much more complex.  If you simply create really thin walls to represent the less stiff elements, you need to be careful not to introduce buckling modes or excessive loss of stiffness due to P-Delta effects under gravity load.  You may need to introduce boundary elements to maintain proper shear behavior under load, and then account for the boundary elements in the stiffness model.  Holding the 8 inch thickness and using the stiffness modifiers in the program element definition is one option, modifying EI is another.  With the stiffness modifiers you can limit the in-plane stiffness but increase the out-of plane stiffness to minimize any undesirable secondary effects.  Either way the entire modeling process becomes much more complex and you must be very careful to control the validity of the results.
 
 
----- Original Message -----
Sent: Monday, April 10, 2006 9:50 AM
Subject: Re: Equivalent concrete thickness for masonry

Paul,

I am analyzing an existing building per the new South Florida Building code. It is part of a renovation and we are required per code to analyze the existing.
 
In reference to your comment on issues with thin walls, would calculating the effective thickness of masonry ( % solid * actual thickness) and using the Em, f'm for masonry be more effective than using a ~ 2-3" thick concrete wall?
 
Also, are you suggesting that I use the 8" thickness and then control the stiffness based on EI?
 
Thanks
Anantha

Paul Feather <pfeather(--nospam--at)SE-Solutions.net> wrote:
Anantha,
 
The best you are going to get using Etabs is the lateral distribution based on a rigid diaphragm model, you will not be able to "design" the walls in Etabs, or run a code check verification.
 
Typically it does not matter whether you use an equivalent concrete to masonry thickness; what matters is that the relative stiffness of the walls as modeled is true to life.  If all the walls are 8 inch masonry, and you model 8 inch concrete, the distribution will be the same because the relative stiffnesses are the same.
 
If you have different thicknesses of walls, or masonry in some lines and concrete in others, than you can adjust the wall thickness to "equivalent" thicknesses by using the EI ratio.  E, G, and t are the only real variables in the analysis, the rest is geometry.  G is typically included as a proportion of E, so varying the thickness based on E is appropriate for the model.
 
As an example, for an 8 inch masonry wall (F'm 1500 psi), the equivalent thickness for comparison to an 8 inch concrete wall ( F'c 3000 psi) is 2.88 inches.
 
The next problem is modeling these types of wall thicknesses.  You need to be very clear in understanding the model and boundary parameters to ensure proper results.  Using such thin walls could introduce secondary effects in the model that are not intended and skew the results (garbage in, garbage out).  This is why it is better to model non-mixed systems without equivalent thickness.  Depending on building complexity, a hand analysis is not difficult, or a spreadsheet analysis, and you can have confidence in the results.
 
 
----- Original Message -----
Sent: Monday, April 10, 2006 6:31 AM
Subject: Equivalent concrete thickness for masonry

Gentlemen,
 
I am analyzing an existing building that had reinforced masonry walls for lateral force resistance and I was trying to compute an equivalent concrete thickness for the same in order to model in ETABS.
 
Are there any standard formulae that can be used to arrive at the eq. thickness or is it based on a trial and error basis calculation of the lateral stiffness?
 
Thanks
Anantha


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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134



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ANANTHA NARAYAN, E.I.
Structural Engineer
Bliss and Nyitray Inc.
Miami, FL - 33134


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