Concrete Masonry Bearing Walls 12" to 8" transition

[ingenii]

I have seen a joist manufacturer recommend stacking two courses of 12" concrete block (bond beam on top with lintel course below) on top of an 8" CMU bearing wall to obtain sufficient wall width to allow joists on either side of the wall to be butted together, in order to make the joist spacing uniform in every bay. Assuming the first two courses of 8" block below the 12" are grouted solid - does this arrangement cause any concern for anyone? The overhang on either side of the wall is only 2". I have checked shear assuming uniform bearing pressure at the joist bearing, although it would likely also work if triangular bearing is assumed.

 

[KootK]

This is the first that I've heard of it.

- I imagine that most of the load gets delivered near the interior of the joist seats.

- This becomes corbel-ish without tie reinforcing.

- For 2" over 16", it's probably ok as a corbel. Light-ish loads.

- Maybe the imbeds wind up inside of the 12" face shells such that the load actually is fully over the 8" wall even with the 2" overhang.

I'm not opposed to it but I'd not be excited about being the first person to do it. So I'd be seeking precedent.

 

[TRAK.Structural]

Agreed with Kootk about not wanting to be the guinea pig.

Will there be vertical bars at regular intervals in grouted cells that extend all the way up to the top of the 12" block?

 

[XR250]

Why not just beef up the embeds so they can cantilever 2"?

 

[KootK]

Why not just beef up the embeds so they can cantilever 2"?

I would be concerned that:

1) You'd load the face shells entirely and probably to very uniformly. Spalling potential.

2) You might pry imbedded anchor, or its grouted cell, right out of the block.

 

[dold]

I've seen this overhanging bond beam detail before in some retail strip malls near Seattle. 1970s-1980s construction. But they were wood parallel chord roof trusses spanning ~40'-50' so maybe they needed the extra bearing. And I believe it was an exterior wall with parapet - trusses on one side only. So I'd assume there was rebar continuing through from below. Unknown arrangement of ties. That's the only time I've ever seen the detail in the wild or on paper.

Why not just beef up the embeds so they can cantilever 2"?

SJI wants 1/2" clear from edge of embed to face of CMU in the anti-cantilever direction. OP are your joists steel or wood? I figure steel since you said joists.

 

[XR250]

I would be concerned that:

1) You'd load the face shells entirely and probably to very uniformly. Spalling potential.

2) You might pry imbedded anchor, or its grouted cell, right out of the block.

View attachment 6379

Makes sense. Thanks

 

[ingenii]

Thanks to everyone who commented here. I have used a similar detail in the past but only with wood trusses. I am considering using this for long span trusses with a higher end reaction. I have attached the detail I am considering. As the centroid of the reaction for LH joists is 2" from the end of the joist, the reaction still occurs over the 8" wall. The wall was "flared" to get close to the 6" minimum bearing for LH joists on masonry, though I can go less (down to 4") if the joists are specifically checked for the shorter bearing. Bearing pressure under the plate is less than 300 psi. I am specifying f'm of 2500 psi. I do have rebar continuing from below (there are three floors supported in this manner - the roof is independently supported). I have SDC B. My concern arose after reading test data for bearing failures that indicated that cracking between face shell and web commenced at 50% of ultimate load and that the number of courses of grout did not significantly improve the performance. That made me concerned I could "slice off" my face shell (even though I am far less than 50% of ultimate - but engineers are hardwired to obsess over those things).

 

[ingenii]

A few clarifications: the 1/2" SJI criteria is the maximum distance the bearing plate can be inset from the face of the CMU. There is no restriction regarding extension beyond the face of the CMU, but I agree that a long extension creates even higher stress on the face shell - which is what I want to avoid.

The LH joist spans are 30'. I am calling for LH instead of K joists to stiffen the floor against vibration as my slab is very thin due to floor-to-floor restrictions set by the architect.

As for belt and suspenders approach, I have been tossing the idea of #3 shear ties in my head - they won't keep the face shell from spalling but will keep the section intact in the event it does.

 

[dold]

I'd provide at least several layers of ties under the bearing at a minimum. A little mini-column in the 12" block. Just to keep the thing from blowing apart like you say. 300psi is still pretty high. Maybe look at an embedded grillage beam - WF or HSS maybe, stocky. That sort of creates a hinge for the 18-24" of wall at bearing though.

 

[ingenii]

I'd provide at least several layers of ties under the bearing at a minimum. A little mini-column in the 12" block. Just to keep the thing from blowing apart like you say. 300psi is still pretty high. Maybe look at an embedded grillage beam - WF or HSS maybe, stocky. That sort of creates a hinge for the 18-24" of wall at bearing though.

Thank you. The ties I was considering were to wrap the top and bottom bars in the 12" block section. The vertical bars are spaced too far apart and are also in the center of the wall, so tying them is of little benefit to my point of concern.

 

[jerseyshore]

I think I'd just offset the joists a few inches on one side of the wall instead of doing a weird detail. Seems easier than proving this works on paper and then hoping it gets installed and performs as planned.

 

[ingenii]

This is the first that I've heard of it.

- I imagine that most of the load gets delivered near the interior of the joist seats.

- This becomes corbel-ish without tie reinforcing.

- For 2" over 16", it's probably ok as a corbel. Light-ish loads.

- Maybe the imbeds wind up inside of the 12" face shells such that the load actually is fully over the 8" wall even with the 2" overhang.

I'm not opposed to it but I'd not be excited about being the first person to do it. So I'd be seeking precedent.

I managed to dig up the old detail I had originally followed. It called for 3 1/2" face shells for the 12" block but the thickest face shell I can find is only 2 1/2"..