CMU beam bearings fracturing 4

[OhioPEeng]

Fellow practitioners of engineering:

A few times now I have encountered masonry bearings fracturing, the face of the wall that is toward the beam span, affecting the face shell of the immediate block or two or three under the beam. It usually a longer span beam during construction. My only explanation is that thermal contraction as pulled on what is a very rigid connection - welded to a bearing plate affixed to wall with embedded studs in grouted cells.

Has anyone dealt with this and resolved this from happening again? I am considering using a slotted hole and finger tight nut on a threaded post welded to the plate that will allow the beam bottom flange to slide in and out of the span. And possibly even greasing the interface.

 

[driftLimiter]

Pictures, sketch?

 

[phamENG]

Yes, differential movement between the CMU and the steel can cause these. It can also be a bearing failure if the jamb isn't properly reinforced, grouted, and cured before being loaded.

Why use a steel beam? In a masonry building, it's best to use masonry lintels if you can. For retrofits steel is often a good choice, but a reinforced masonry beam is my go to for CMU bearing walls in new construction.

Here are some good details for lots of scenarios: Link

 

[OhioPEeng]

The buildings we do are steel beam and joist roof on block bearing walls. So I end up with some 40 to 50 foot span beams at times and even further on the joists. Oddly enough, I've not had the 100' or longer DLH joist do this to bearings.

It's beams and it's roof so they're lightly loaded. Not enough to fail the block even if it were hollow. These are grouted blocks that are popping apart toward the span of the beam as if it were shrinking.

Since you brought it up pham, I do use masonry beam lintels over punched openings and wherever they work, up to 32" in depth at times. But only with mason contractor buy in. Some them still prefer to throw a steel beam up and keep going rather than shoring openings.

 

[dhengr]

OhioPEeng:
Try setting the edge of the brg. pl. back an inch or so, or the face shell thk., from the face of the conc. blk. Rake this joint, and/or don’t let them grout the thickness of the brg. pl. immediately under the beam flg. You may be getting some relative expansion and contraction or you may be getting some beam deflection and concentrated bearing on the face shell of the conc. blk. And, you don’t want any concentrated brg. stresses on the face shell, it should be back in the grouted cell and just spread out into the whole conc. blk. Plus what Pham said.

 

[HTURKAK]

Common practice in my zone is , the use of PADSTONE for clay bricks and RC lintels for CMU to limit the bearing stress .

Pls have a look to The following doc .







I cannot give you the formula for success, but I can give you the formula for failure..It is: Try to please everybody.

 

[jayrod12]

I'm not sure what code you're designing to, however where I am, there's a pretty severe bearing stress penalty if the bearing plate isn't specifically detailed to have the load delivered at the centre, like a 50% reduction. For standard looking bearing plates with no special detailing, we can use around 250 psi bearing stress. That's really not that much.

Also, if there's been no provision for allowing movement at either end, then it's more likely thermal than bearing stresses causing issues, but if there's been provisions made to allow movement (as there should normally be) then it's more likely bearing than thermal.

 

[manstrom]

Allowable bearing stress for masonry is pretty garbage. I think it is 1/3 f'm (or it used to be). 500 psi is less than a Southern Pine plate. Also, the stress may be delivered more as a triangle / trapezoid than a uniform rectangular pressure.

On top of that, you will have some thermal pull/push from the beam. I've seen this create issues at concrete as well as CMU.

 

[Prestressed Guy]

I agree with Manstrom on the stress at the face of the wall. I had a precast column which fractured off a 12" high by full width of the column face chunk of the column right down to the ties. The reason for the spall was that the plant had not put the 3/4" chamfer on the top of the corbel. All of the other columns had the chamfer and did not suffer the problem.
This problem with edge loading is worse with CMU because the face shell is much weaker than the grouted core. I like to detail my bearing plate embeds so that they are held back from the edge 1.5" so that they are not over the face shell and detail them so that they stand 1/4" above the top of CMU to prevent any load going directly to the shell.

 

[OhioPEeng]

It is almost certain that it's not a load related bearing failure since it's happening with partial dead load (joists and deck only) and almost no live load. It is a thermal issue and I'm hoping someone has some experience with what that detailing may be. There is precedent in bridge construction or steel that is permanently exposed to the elements - with the teflon pad and various bearings available for those applications.

But a detail that is practical and inexpensive to provide some protection for these random failures when temperature extremes happen during construction is not something I've seen. In my first post, I attempted to describe something that would function like those teflon bearings, without actually going that route.

Now that I'm thinking about it, the walls with very repetitive framing probably don't experience this issue because the wall just moves to compensate enough to avoid rupturing the bearings. What I am observing happens at singular beam locations and at wall corners where there is no give. Maybe it's worth a slide bearing in those instances only.

 

[Prestressed Guy]

In order for the bearing failures to be thermally induced, the walls at both bearings would need to be restrained for lateral displacement independent of the roof joists attached to the bearings.

If this is a single bay building with two bearing walls that support a roof joist, between them, they are restrained laterally by the truss so there would not be any thermal stresses in the bearings.
If this is a multi-bay building with three or more bearing walls that support roof joists between them, the interior walls with joists going in both directions are restrained laterally by both of the trusses so any thermal stresses would be taken straight to the bearings.
Which do you have? For interior bearing walls, I make sure that I have a horizontal release mechanism such as slotting holes in the seat and weld studs on the bearing. For the interior walls you need someplace for thermal volume change to go.

 

[OhioPEeng]

I'm leaning toward slotting holes in the beam bottom flange and welded studs on the bearing plate with finger tight nuts.

 

[EZBuilding]

I have had this occur to me with a small span beam - corridor of a hotel. In my case, we surmised that the crack occurred due to the heat introduced to the beam during the welding process.

 

[JLNJ]

I have had this happen as well at an interior corridor corner at a school. Maybe I needed a wall control joint closer to the corner to allow the wall to "give" a little without cracking at the beam base plate bearing.