Minimum Bearing of Steel Beam at CMU Wall

[C Hietbrink]

I have a modification to an existing building in which we are removing some precast hollow core planks and replacing it with a composite deck floor system supported by steel beams. I wanted to try and utilize the existing foundation stem walls for support of the beams. I have a 12" thick CMU wall which supports the ends of two of the hollow core planks back to back. We will be removing one of the planks leaving approximately 5-13/16" allowable bearing on the CMU wall for the new steel beam. Is there any code stipulation that specify the minimum bearing of the steel beam in this case. I was going to utilize a bent plate anchored into the face of the CMU wall as a bearing plate and weld the ends of the beam to the bearing plate. Any help would be greatly appreciated.

 

[DETstru]

Look at ACI 530. I don't have it on me right now but I believe they specify bearing capacity as something like f'm/3. Maybe f'm/4, I don't remember.

It seems low but that's the code.

 

[tony1851]

Just to throw confusion into the mix our (BS) code is similar, using fm x 0.35 for old (existing) walls and fm x 0.4 for newly-constructed walls -- not a million miles from your Code. fm x 0.25 would be safest.

 

[BAretired]

I don't like the bent plate idea.

If the steel beam bears on hollow block, provide a poured concrete bearing pad under the beam. Anchor the base plate in the pad and weld the beam to the plate.

If the steel beam bears on top of the existing bond beam, the base plate should bear on the concrete fill, not on the masonry face wall. Anchors should be drilled vertically down into the concrete fill.

If the beam reaction is large, filling cores with concrete below the bearing area may be considered.

BA

 

[concretemasonry]

The f'm is not necessarily a measure of the compressive strength of the concrete in the wall. The test for for determining f'm is a 2 block high HOLLOW prism that actually fails in a diagonal shear failure as a result of the shape of the CMU that is a series of 1" to 1" to 1-1/2" interconnected vertical plates.

I have used 8500 psi(net area)8x8x16 hollow CMUs used in a 2 block prism that tested at 4800 psi. - The mortar was 2200 psi and was not involved in the failure mechanism.

The value of the f'm is for masonry structural design were the design involves the design of the wall panels and not a myopic analysis of a small element of a structure. The strength of the concrete in a concrete block is at least 3000 psi and could be as high as 5000-8000 psi. The mortar is much weaker by design.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dik]

ConcreteMasonry:

Can you provide a design example of the method stipulated?

Thanks, Dik

 

[hokie66]

BA is correct. While hollow block may in some cases provide adequate bearing for line loading like the precast planks, steel beams are point loads, and they need to be supported on solid sections of the wall.

 

[C Hietbrink]

Thanks for all the information. The wall is fully grouted so the bearing plate would be supported by the fully grouted wall. The idea of the bent plate with the anchors installed through the face of the wall was to ensure that the anchors could meet all of the necessary edge distance and spacing requirements. I believe that the the allowable bearing of f'm x 0.25 is acceptable from the loads that I am dealing with. In addition to the minimum bearing from the masonry wall standpoint, I was curious if there was any requirements for the length of end bearing of the beam itself? Is the 5-13/16" enough length for the end of the beam to bear on the the bearing plate?