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Re: CMU Wall question

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Tom,

First, a 20 psf wind load for components and cladding seems a little
light, depending on the trib area of the wall.  It seems in line with a
main force resisting system wind load.  While it certainly may be right
on, I am used to seeing at least about 25 psf for components & cladding
wind loads even for "normal" wind areas.  I did do a quick calc and it
does seem that your apparent trib area (24 ft by 19 ft) is large enough
that the 20 psf might be right on.  But I thought it still wise to offer
some caution.

Regarding the 50 psi allowable stress...as I understand what you are
doing, then the height of the wall has nothing to do with your problem
(other than attracting more overall wind).  You seem to be designing hte
wall to span horizontally.  If so, then changing the height of the wall
will not necessarily make it work.  In such as case, the height of the
wall has no effect on the "applied" stress other than if reduced then your
wind load will increase as the trib area will decrease.  In otherwords,
changing your wall height from 24 ft to 20 feet will not change the
calculation of hte moment since your "moment length" will still be the
horizontal lenght of 19 feet.

Regardless, I should point out that whether you pick the wall to span
horizontally or vertically while mean absolutely nothing to the wall
itself in most cases.  The wall will not "know" that it is supposed to
span horizontally and will in fact "find" the weakest failure mechanism,
which in this case seems to be if it spans vertically.

When spanning vertically, the moment would be about 1440 lb-ft (20 psf and
assuming simple span wl^2/8 with l=24).  This would result in an applied
stress of about 108 psi which compares with the allowable of 25 psi.  This
calculation does not account for the reduction in applied tensile stress
due to vertical gravity loads (self-weight and bearing load if applicable)
nor does it account for the permitted 1/3 allowable increase, which while
debatable is permitted under ACI 530 and other codes.

The end result is that even if you could get the wall to "work" under
horizontal spanning conditions, it will not "work" under vertical spanning
conditions unless you change those conditions (i.e. cut the vertical span
some how).  The end result is that an unreinforced 12" hollow wall does
not appear to work given the conditions that you provided.  The maximum
height that a 12" hollow wall can span with a 20 psf load is about 12 t0
13 feet (per a handy-dandy table in the NCMA publication titled
"Architectural & Engineering Concrete Masonry Details for Building
Construction"...a little out dated since it appears to be using an
allowable stress of 23 psi instead of 25 psi for the vertical spanning
case).

Having said all that, you could look at whether or not the wall will
actually span in two ways.  The same set of handy-dandy tables that I
mentioned has one for a wall with pin supports on four sides, thus for a
two-way span condition.  According to that table, then the wall could be
about 22 feet high and 19 feet wide with an applied wind load of 25 psf
(with type M or S mortar).  Thus, it would appear that your case of 24
feet high and 19 feet wide with 20 psf wind would work for a two-way span
condition assuming that you do have support on all four sides.

HTH,

Scott
Ypsilanti, MI


On Tue, 29 Oct 2002, Tom Bodkin wrote:

> How high can I go with a 12" unreinforced cmu wall before I should be
> concerned about it's integrity?
>
> I am designing a municiple structure in which the facility ( a small
> wastewater treatment plant accessory building with a portion sitting on an
> reinforced concrete tank, and the other walls extending to grade) has three
> walls that need to be 24 feet high above grade but thaey are only 19 feet
> wide which suggests that horizontal bending is more critical than vertical
> bending.
>
> I calculated the moment to be 902.5 ft-lbs (19' span and 20psf wind,
> wl^2/8), and the section modulus of the cmu is listed to be 160 in^3.  It
> looks like the allowable flexural tension is 50 psi (ACI 530) and my bending
> stress is (902.5/160)= 67.7psi which exceeds the 50psi allowed.
>
> Is this reasonable? It doesn't seem like a 24' high cmu wall spanning 19'
> should be too much of a concern.
>
> Thanks in advance for your response.
>
> Tom Bodkin, E.I.T.
>
>


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