CMU questions 1

[mark519]

I have (2) questions regarding CMU...

1. What is the best way to attach WF beams to the FACE of a CMU stair shaft wall? I was considering a cast-in-place concrete beam with embed plates built in the plane of the CMU wall, and attaching the WF to the embed plates with a standard full depth connection.

2. In the same building, I have a 50' tall 12" CMU wall with no return walls. The wall spans vertically about 16'-18' from floor to floor. Are masonry anchors attached to steel framing considered sufficient to brace the wall? Or do I need to attach the CMU wall to the steel framing with a more secure connection such as a Hilti anchor?

Thanks

 

[asixth]

1. Depending on the magnitude of the loads, if it is light roof framing I would face fix a plate with hilti anchors to the wall (corefilled and reinforced) and cleat from that. More substantial loading such as floor loading I would provide a blockout in the wall and bear the WF on the CMU.

2. If the connection requires any significant fire-rating than I would provide mechanical hilti anchors.

 

[mark519]

asixth, thanks for the response.

1. I wanted to give the contractor the flexibility to build the entire CMU wall shaft prior to erecting the steel. Therefore I do not want to provide a blockout in the wall. Another structural engineer on the job has recommended the cast-in-place beam option. I have never done this before, so I wanted to get a second opinion. Do you see any issues with that approach?

2. The connection does not require any fire-rating. The wall was originally intended to be a metal stud wall, but I asked for it to be CMU because I need it for a shear wall. Do the masonry anchors alone brace the wall at each floor?

 

[JAE]

To me forming a pocket in a beam is very easy for a mason and still allows the wall to be built without waiting for the beam installation (assuming the beam doesn't brace the wall)

Plus you avoid the eccentric load on the wall when attaching to the face of the wall.

 

[DST148]

1. (a) Agree with asixth. But rather than using a wall-plate, I would suggest all-bolted-seated connection using an angle. The angle could be fastened to masonry wall using drilled-in anchors and the beam fastened to angle using bolts. (b) Regardless of the magnitude of the loads, I prefer structural members bearing directly on the masonry. Assuming 8" thick wall, I would provide a partial block-out on the wall, a minimum of 4" in width. That leaves 3 5/8" for masonry which would provide about 2 hours of FRR to the beam sides. The bearing plate may be shop welded to the beam and fastened to the wall using off-center drilled-in anchors to get the required edge distance. Masonry can then be built around the block out.
2. Per ACI 530-05: Connectors shall be designed to transfer horizontal design forces acting either perpendicular or parallel to the wall, but not less than 200 lbs per lineal foot. The maximum spacing between connectors shall be 4 feet. To avoid closely spaced connectors, you can always provide a masonry beam on top of the wall which can span horizontally greater distances.

 

[mark519]

Everyone,
Thank you for the responses. I think these are all good solutions. If in fact I still go with the cast-in-place beam built within the plane of the wall (primarily because another engineer prefers that method), do you believe that is acceptable? We would run continuous vertical bars from the CMU wall below, through the conrete beam at the floor, and into the CMU wall above.

Again, I have always used a solution similar to what you have all suggested, but in this instance I am curious if the concrete beam option would pose any unforseeable problems.

 

[DST148]

Of course, the concrete beam is acceptable. But why bring in another trade when things can be managed with masonry.

 

[RFreund]

Agree with the comments above.
As far as bracing the wall with the steel beam connection. You may need a continuous horizontal bond beam spanning between steel beams depending on the spacing of the steel beams. The wall would span vertically to this bond beam.

EIT

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