IBC Foundation Walls
IBC Foundation Walls
(OP)
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????
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????






RE: IBC Foundation Walls
RE: IBC Foundation Walls
DaveAtkins
RE: IBC Foundation Walls
Mike McCann
MMC Engineering
RE: IBC Foundation Walls
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!!!
RE: IBC Foundation Walls
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.
RE: IBC Foundation Walls
RE: IBC Foundation Walls
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 :)
RE: IBC Foundation Walls