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# Re: Equivalent concrete thickness for masonry

• To: seaint(--nospam--at)seaint.org
• Subject: Re: Equivalent concrete thickness for masonry
• From: "Anantha Narayan C.K." <anant_27(--nospam--at)yahoo.com>
• Date: Mon, 10 Apr 2006 17:50:11 +0100 (BST)

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.

Paul Feather PE, SE
www.SE-Solutions.net
pfeather(--nospam--at)SE-Solutions.net
----- Original Message -----
From: Anantha Narayan C.K.
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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