CMU arching/thrust; deep lintels

[rholder98]

The caveat to using arching action is having not only enough wall height over the opening, but also having enough wall on either side of the opening to resist the resulting thrust. Can you resist this thrust with a tension tie--say, the steel in the lintel--instead of relying on the jambs? This is what I understand dik to say in thread507-48233, which really piqued my curiosity. This would create a couple, with the tension of the steel and the compression in the masonry at the top of the arch. Such a condition would allow for shallower lintels in openings near the end of the wall.

That leads to another concept, though. I've asked around this office, and looked on this site, and the general consensus seems to be that few people will design lintels more than 3 or 4 courses deep (2 or 3 max here). Could you not use the couple concept to create a deeper lintel, that does not necessarily have to be fully grouted? That is, say you've got your two-course lintel over the opening, you build the wall on up with vertical bars at 24/32/etc., and place another 2-course lintel 6 or 8 feet up, grouted solid. It seems to me that that would give you the same action as a typical flexural member, with less self weight. What am I missing? I don't think I'm creating new concepts, it's just that nobody at my office is very fluent in masonry design (it's all done by charts and spreadsheets--I'm not sure anyone here can even calculate it by hand). On the other hand, there could be an obvious reason this won't work. I just wanted to throw it out there for discussion.

 

[haynewp]

I am not sure you can get the required development length of the lintel bars if you are using them as a significant tension tie near the end of a wall. The thrust will try to rip the final 8" (or whatever you have left between the edge of the opening and the end of the wall) off of the lintel bar.

You could use a couple concept but you have to transfer the shear flow through the bed joints, normally there is face shell bedding only and there is also usually the wind/seismic shear that is coming through the bed joints down the wall. You really should look at combining those shears if you used that concept.

 

[jike]

I generally do not consider arching unless I am trying to prove an existing condition works. I also would not consider arching if the wall is stack bond or there are openings or control joints in or near the area resisting thrust.

 

[haynewp]

I use arching when I can, but I always use standard lintels without any special coupling action going on. The masonry construction is pretty crappy where I live, so I try to keep things simple.



{Cracks open another cold, cold beer}

 

[dik]

In addition to using a horizontal tie, I've treated the lintel as a propped cantilever too; both work. I don't think so much as it transferring shear, but transferring compression forces. Although for critical areas, I usually grout cores, the wall behaves pretty much the same even if the voids aren't filled.

Another thing to consider is that with arching action, there is a higher concentration of force at the ends of the arch and the masonry has to be checked for this additional compressive force.

Dik