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Re: masonry block properties

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Rick,

With regards to your first question...I am not completely sure...anymore.
The seminar where I learned this tidbit was quite a while ago and since I
am getting senile already, my memory is fuzzy.  I believe that it may be
due to the fact that the block dimensions are not constant over the full
height of the block.  The webs and face shells are slightly thicker at the
top than they are at the bottom if I remember correctly.  This is a
product of the forms...so that you can "strip" the forms from the blocks.
I believe the block dimensions typically specified are either the maximum
or average dimensions, while the values for full bed bearing would be
based upon the minimum.

Note: It turns out I remember correctly...amazin', eh?  I dugout my notes
from the seminar and here is what it says (FYI ACI/TMS seminar on the 1995
MSJC):

"The Code (Section 5.13.1) specifies that for stress computations,
stresses are to be computed using section properties based upon minimum
net cross-sectional area of the member.  For most applications this will
be at teh bed joint and the section properties will depend on the mortar
placement, that is, whether the units are laid in face shell or full
bedding.

For stiffness and radius of gyration calculations, however, the Code
(Sections 5.13.2 and 5.13.3) indicates that the average uncracked net
cross-sectional properties of the member should be used.  These properties
are based on the average section properties of the units, irrespective of
the mortar placement on the bed joints.  Since the unit properties are not
constant over the depth of the unit, it is difficult to computer the
average unit section properties and this information is usually best
obtained from the unit supplier or manufacturer."

This gave me the brillant idea to actually look in the Code.  (Like, duh,
why would I not think of something logical like that, eh?)  The above
section are now WAY out of date.  The appropriate code sections in teh
1999 Code are 1.9.1 thru 1.9.3.

This also answers your second question as to the "superficial" reason as
to why.  The background reason for the Code provision is that the h/r is
not really a calculation of the stresses at a "weakest" point, but rather
kind of an average for the full height of the wall that is then used in
the axial load "failure" mechanism.  A block wall (or column) will "fail"
under axial loads much like a steel column does...by buckling.  The h/r
(similar to kl/r or gamma in steel design) value is used to determine if
the wall will "fail" in the elastic buckling (Euler's...longer, slender)
mode or an inelastic buckling (short, stocky) mode.  Thus, we use
something that is more reflective of the overall "average" of the wall
rather than the "weakest" link.

HTH,

Scott
Ypsilanti, MI


On Wed, 30 Jul 2003, Rick Burch wrote:

> Scott,
> Two questions please. For context, I am interested in reinforced masonry
> only.
>
> 1. Do you know why the values you have are different for properties
> based on full mortar bedding and properties based on block dimensions?
>
> 2. Why would the h/r value for determining allowable axial stresses
> always be based on the r value based on block dimensions? I've always
> thought the r had to be calculated based on the I and A accounting for
> actual grouted cells and mortar bedding conditions.
>
> Thanks,
> Rick Burch
> Columbia, SC
>
>
>
>
> Scott Maxwell wrote:
>
>
> >
> > 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
>
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