IBC Foundation Walls

[JKW05]

2006 IBC 1610.1 specifies foundation walls are to be designed for the design pressures in Table 1610.1 (as a minimum). The exception says active pressures may be used for walls not exceeding 8 feet in height and supporting "flexible" floor systems.

Table 1805.5(1) indicates a 12" hollow block wall is acceptable for 7' wall height with 7' backfill. For simple vertical span with triangular load of 30 psf/ft, I get:

M = .1283*W*l = .1283*(30*7^2/2)*7 = 660 ft.lb at 3 feet from the bottom.

NCMA TEK 14-1B, Table 5 lists S=167.1 in^3 and A = 53.1 in^2 for 12" hollow block with full mortar bedding.

So fb = 660*12/167.1 = 47 psi.

ACI 530-05, Table 2.2.3.2 shows maximum allowable flexural tension = 25 psi for ungrouted hollow units.

If I take the weight of the wall at 54 psf (ASCE 7-05, Table C3-1). The axial compression = (7'-3')*54/53.1 = 4 psi.

So even assuming a "flexible" floor system to allow active pressure and deducting axial compression, the net tension stress still exceeds ACI's allowable:

47 psi - 4 psi = 43 psi > 25 psi.

Anyone know how IBC came up with this to work????

 

[Teguci]

Shouldn't underground CMU walls be grouted solid? This would add a lot of vertical weight to the wall.

 

[DaveAtkins]

Must be empirical. I agree with your calculations.

DaveAtkins

 

[msquared48]

Teguci is correct. CMU walls below the ground level must be solid grouted.

Mike McCann
MMC Engineering

 

[JKW05]

2006 IBC Table 1805.5(1) (Table R404.1(2) in the Residential Code) specifically indicates which conditions must be grouted solid. According to the IBC tables, the condition I have presented does not need to be grouted solid.

Is there some other code provision that I am missing that says below-grade walls must be grouted solid? If so, there are thousands of houses in Pennsylvania that are in violation!!!

 

[Teguci]

Note c under Table 1805.5(1) indicates solid grouted hollow units or solid masonry units.

Although the use of solid...hollow in the same statement seems silly, I think they mean fully grouted for the use of the table.

Upon further review it looks like hollow CMU can be used up to a 7'-0" high backfill below grade. However, I would try to convince the Pennsyltuckians that grout really isn't that expensive and it prevents a lot of other problems below grade.

 

[cap4000]

Perhaps IBC is adding some additional nominal dead load from the wood frame weight above. Just not sure.

 

[JKW05]

I would generally specify grouted walls as a minimum and even add some nominal rebar. Not only does the grout add weight, the section modulus is also significantly increased. But owners and large volume home builders shave every penny they can.

In this particular case, I was called in to evaluate an existing wall. It's a bit embarrassing to have to tell the owner that the tables in the building code don't work.

...Pennsyltucky.....I haven't heard that for a long time

 

[PSlem]

Working from memory as home now, but doesn't IRC say there must be a rebar from the footing every 4' grouted? Probably something similar in IBC.