MASONRY WALL JAMBS

[CTSeng]

When designing a masonry wall jamb at a door or window opening can you use the effective width in compression as defined in ACI530 (least of c/c bar spacing, six time wall thickness, or 72”) or should it logically be half that?

Does anyone else find that this tends to be a weak link in a masonry wall system particularly if the openings are large and the walls are tall? I’ve been using concrete columns or 12” thick jambs often on large overhead doors.

 

[DaveAtkins]

I think you could prove that the effective width is 6t by using the horizontal joint reinforcement to transfer the load from the opening back into the wall. However, I typically reinforce the first one, two or three cores at opening jambs for the additional wind load coming off the opening. Remember that the reinforcing in the first core adjacent to the opening is ineffective because it will stop below the lintel bearing.

DaveAtkins

 

[CTSeng]

Thanks Dave, but
All to often the 6t is superceded by tighter reinf. spacing, generally 48" oc, cutting the effective compressive width to 28". It's been difficult to explain to architects that the maximum opening size must be less than the typical bar spacing because of the reduction of nearly half the wall section spanning vertically.

Yeah, I specify to grout and reinf. multiple jamb cores where it works but still have difficulty with large openings.

I should mention that most of my work is in the high wind areas of south florida. We use the hollow precast lintels with notched bearings to allow the reinforcment to pass thru the first core.

 

[DaveAtkins]

I have a couple of ideas. First, try using two bars in each core, to increase "d". Second, try using the ultimate strength method in the IBC. This will almost surely give you better results.

DaveAtkins

 

[AlohaBob]

ACI 2.1.7.1 Page C-18 Effective bearing is BP width + 4t. See also Figure 2.1-15 on CC-29. This is for running bond.

The c.c. spacing between bars or 72" should be referring to the maximum effective wall width per bar. That means, the maximum effective flange width for compression corresponding to a bar at yeild, for beam action.

These are two diffent issues.

The effective jamb width should be the BP + 4t. This should be the maximum design width also for bending for wind jamb, unless the jamb as mentioned earlier is broken or discontinuous at the lintel bearing.