Thru Bolts in CMU Wall

[TonyES]

I've searched many previous threads on this subject but unable to find a solution. I want to put a bolt through a CMU wall (a grouted and an ungrouted case) and a plate and nut on the outside face. It is to hold a cable that will be in tension. Does anyone know how to put a capacity to this application? This connection is done a lot with siesmic retrofits but how do we get the capacity of the cmu course receiving this force? My thought is to have the plate be 8" x 8" and centered so it's tributary to not just one course. I can size the plate and the wall as a hole just fine but how much bearing can the face of a cmu wall support? Thanks

 

[MiketheEngineer]

Pretty much depends on how good the CMU and grout are??

 

[concretemasonry]

In that case, just assume the normal strength block and mortar, since most block are far above ASTM minimums and mortar is just a material in a wall that does not have a great effect on the wall strength, but rebar (if any) will help and grouting empty core has little effect.

A masonry wall is a structural element that distributes concentrated loads and defies a microscopic analysis of the individual since all standards are based on the "masonry strength" (ACI 530/others).

The key is to distribute the applied load to the wall itself. It does make a difference whether the wall thickness (6",8",10",12", 14" or 16")

Dick

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

 

[TonyES]

The lack of information does make it difficult to properly understand as well as Engineer anything. I don't have and won't have any idea where they might place this connection so I'm going to have to find the worst case condition and assume. It is actually a double CMU wall so two 8 inch walls - but I don't know if there is a space in between so I will have to assume just a regular 8" unreinforced wall.

 

[dhengr]

Tony:
You really have to define your problem in more detail before you can even begin to try to put a number on it. And, bearing btwn. the washer plate and a face shell is the least of your problems. You seem to know very little about the actual wall materials, except that they are 8" CMUs. The double wall is probably ineffective unless you can find a way to make them act together. The wall might fail in bending in a vert. direction, as a beam/column of some width or in a horiz. direction as a beam of some height; or maybe as a two-way system ultimately. You might just pull a few blocks out of the wall in bond/shear in the mortar joints. But, you seem not to even know where the load will be applied. The client can’t ask this kind of question, and expect a meaningful answer, without allowing you to investigate and determine the pertinent parameters of the problem.

Remember a horizontal cable with a turnbuckle, or some such, applying a load perpendicular to the wall can apply very large concentrated loading. You can apply a substantial load just by tightening the turnbuckle or your nuts on the washer plates. And, the magnitude of the load increases quickly as you try to pull the sag out of the cable. If someone or something can apply a vertical load (gravity load) out in the middle of the cable, it exerts a very large horiz. load on your washer plate. You should take a look at some cable design literature to get a handle on the magnitude of these loads.

 

[amecENG]

Tony:
I usually assume that the backing plate is centered on a hollow core and treat this as a punching shear problem. The thickness of the concrete is the wall thickness of the CMU block and the shear perimeter is the backing plate.