Masonry wall design and BOCA table 1812.3.2(1)

[drewtheengineer]

So I'm checking an existing hollow 12" CMU wall...

I was referenced to Table 1812.3.2(1) and I see that an 8' wall with 7' of backfill height with a lateral soil load of 30 psf/ft would require a 12" CMU wall. The wall that I'm checking is actually 8'-5 1/2" and the backfill height is 7'-10". When I went through the calcs for my wall, it's not even close. So I decided to go through a calc for the 8' wall/ 7' backfill listed in the BOCA table. Below is how I calc'd it...

Lateral Soil Load = 30 psf/ft
Lateral pressure at base of stem = (7')(30 psf/ft) = 210 psf
Force on stem = (210 psf)(7')/2 = 735 lb.
Location of load = 1/3(7')=2.3' (from bottom)
Moment(max) = 735 lb(2.3')(5.7')/8' = 1204 ft-lb

So far so good?

Section modulus of a 12" CMU block per foot of wall = 160 in^3
fbt (tension stress due to bending) = 1204 ft-lb (12 in/ft)/160 = 90.3 psi

According to Table 6.3.1.1 of ACI 530, the highest tensile stress for a hollow ungrouted unit is 50 psi < 90.3 psi.

Summary:

I highly doubt that BOCA published incorrect material and that the fault lies with me. Hey, give me a break, I've been designing broadcast towers for the past 4 yrs.

Any takers on this one?

Thanks in advance,
Drew

 

[Mattman]

Does this sound reasonable? I don't know if it is correct, but BOCA's Table 1812.3.2(1) works (at least for your test case; I haven't check further) with these conditions:

You did your analysis based on a simply supported beam (BOCA requires lateral restraint at both ends of the wall). However, BOCA 2111.1.2.1 requires full mortar bedding (face and shell) at the starting course for foundations, so let's look at the bottom restraint as fixed (iffy, but...).

This gives us AISC load Case 14, which results in R1= 423#, and M1 (at R1) = 982 ft-#.

Then take in to consideration the weight of the wall and its role in resisting overturning. 12&quot; CMU weighs 85 psf.
85psf*1'*8'= 680#. 6&quot; moment arm * 680# = 340ft-# moment resisting overturning moment.

982ft-# - 340 ft-# = 642 ft-#.

642*12/160 = 48 psi < 50 psi.

How does that sit with you?

 

[drewtheengineer]

I see where your coming from. I'm not too convinced, however, that the base is going to act as a fixed support. That's something I will have to look into.

I don't have the BOCA code in front of me at the moment, but 50 psi is the highest of those given. For my specific case, I was actually using the 15 psi given (I believe it was using the masonry mortar).

Your explanation was very helpful, thanks.

Drew

 

[Mattman]

BOCA 2104.7 calls out ASTM C270 mortar, not ASTM C91 prepared masonry mortar. Also, BOCA Table 1812.3.2(1) calls out for Type M or S mortar for plain masonry (Note e).

These two requirements together give the 50 psi value in ACI 530 table 6.3.1.1, so at least that part checks.