masonry - minimum reinf reqd

huangyhg

masonry - minimum reinf req'd?
i am trying to figure out the need/requirement for minimum steel in a masonry wall.  my specific problem is amount of steel needed for an 8" foundation/basement wall.  the wall is 4ft below grade on both sides, ie no unbalanced load.  in some references i see as(min) = 0.002ag with no less than 0.0007ag in either direction.  so, for an 8" wall, i would need 0.002(12")(8") = 0.19in2 / ft of wall, vertically (conservative), which would be #5@16 (0.23in2)
however, why when i check design tables for 8" cantilever retaining walls, noise barriers, etc... i see steel req'd of much less than this minimum.
final state is no unbalanced load, but make sure to consider that they'll backfill one side at a time.
i don't know what reference you are using to provide minimum steel in the masonry wall.  i use less than #5@16 on a regular basis in many walls.
why wouldn't you just design this as an unreinforced wall?  it seems that for this wall and these loading conditions, this would be feasible.
are you in a seismic area? you should have minimum steel in each direction if you are.
aci 530 does not require in minimum reinforcing for masonry walls unless it is in a seismic area or you have stack bond masonry.  so you only have to provide the reinforcing that is required for strength
i am not in a seismic area and i really dont have any load other than self weight of the wall, about 8ft above grade. it is not a load bearing wall, but i was under the impression that minimum steel was still required at least for crack control.  we typically provide horizontal wire reinf in the joints (durowal), but i was conserned abotu vertical requirements.
your horizontal reinforcement handles the crack control.
since you do not need vertical steel for strength and there is no concern for crack contol on such a short wall.
dick
see the msjc code (aci 530) for the requirements.  1.14.2.2.2.1
minimum vertical steel is a #4 bar every 120 inches.  #4 bars each side of openings and each side of movement joints.
that's the absolute minimum.
the .002ag doesn't come into play until sdc d.
as a practical minimum, i like to use verts at 48" just to hold the whole thing togeter.
unreinforced cmu used to be allowed with a lot of it in actual use, i don't know if it has changed in the current aci 530. if your foundation wall has no load except from unbalanced backfilling, i would solid fill the cells below grade and have some minimum horiz steel in the wall full height even if it is not required.
is this an exterior basement wall? how does it not have wind load?
he was a wise man who invented beer.
--plato
the requirement for .002ag should include the horizontal reinforcing, typically ladder or truss type every other course, 12 or 14 gage. this should increase your vertical spacing to 32" for a #5.