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Re: masonry block properties

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Andrew,

See comments interjected below:



> To prove a point, I have to determine the allowable load for an unreinforced
> 8"x 16" section of block wall. Now I know I have to check everything
> according to ACI 530-99, C-20, Sect. 2.2 Unreinforced Masonry. But, to
> calculate the bending stress in the block, ungrouted with no reinforcement,
> is completely different then standard reinforced masonry design. I am
> assuming I would use the following regular engineering mechanics formula for
> bending stress:
>
> Fb= M/S
>

Yes.  If unreinforced, then the problem is a basic mechanics/stress
calculation.

> S- this is the problem, as it is hollow block. I cannot find any tables for
> this, and I have quite a few resources. Does anyone have this value in a
> table? Would it be improper to just hand calc the Sx for a 8"x16" hollow
> block? Now would a simplifed way of doing this, to be calculate Sx for a
> solid 8x16 block, then subtract the Sx for the cavities? This ignores the
> curved cavity walls, and the thickness of walls is different on the top then
> the bottom, etc.

The value of S will potentially vary depending on the use and the mortar
application.  Many tables that you will find that have section properties
are for use in the h/r calculation for use in the axial stress
determinations (Pe...your question below).  These section properties will
be based off of the (minimum) block dimensions.  If you have the
dimensions of the block to use then you can easily calculate these section
properties.

For stress calculations, you need to use the dimension of "contact" in the
mortar bed, not just the block dimensions.  This will then depend on the
whether you are using either face shell mortar bedding (i.e. mortar at
just the front and back face shells but not on the webs) or full mortar
bedding (both face shells AND the webs get "buttered").  This values from
tables that I have BOTH are different that section properties determined
from just block dimensions.  For an 8" block:

			face shell bedding 	full bedding
A			30.0 in^2		41.5 in^2
I			309 in^4		334 in^4
S			81.0 in^3		87.6 in^3
ek (kern ecc)		2.70 in			2.11 in
r			3.21 in			2.84 in


>
> I do not see why this is wrong, then I calc. Fb as I did above. The same
> goes for Ix calc, which I need for formula (2-15) to calc Pe.

For the h/r and other parts of the axial load/stress determination, you do
use the section properties of the block based upon the block dimensions.
For 8" ungrouted block:

A = 50 in^2
I = 364 in^4
S = 95 in^3

HTH,

Scott
Ypsilanti, MI


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