Removal of Load Bearing Wall

[ali07]

We have a situation where arch. need to remove exist. load bearing masonry wall supporting roof structure. Roof is in step there with about 3 ft level difference at wall location. Joist and beams are resting over this bearing plates. We are proposing new columns but my question is how can i keep that 3 ft block masonry above the low roof or either we can suggest to remove that infill with stud.

 

[msquared48]

Depending on the span of the opening, I would be worried about excessive deflection of the CMU wall section above the new beam line. Kinda depends what you can use to limit the deflection.

If the new beam is to be wood, you may consider removing the masonry and using a lighter wood infill for the fascade.

Mike McCann
MMC Engineering

 

[apsix]

The better, simpler and probably cheaper solution would be to remove the masonry completely. Temporary props to the roof each side of the wall, remove wall and rebuild in steel or timber.

 

[hokie66]

Agree with apsix. And remember to provide lateral capacity in the new structure to resist the force now taken by the masonry wall.

 

[ali07]

Steel joists and beam are connected to that 12" CMU Wall. It sounds better to remove that 3ft wall after providing shoring on each side and introducing new supporting beam and columns.

Just for curiosity, is there any practically way to hold the wall and introducing the new beam to support it, i can't visualize how to insert new beam in load bearing cmu.

 

[hokie66]

Depending on the geometry, it may be possible to support the top section of the wall by inserting "needles" supported on each side by temporary props. Then you could remove the lower section of the wall, install a new support system below the wall, wedge between the new support and the wall to pretension the new support so there would be neglible deflection, grout the interface, and remove the props. A lot of work, probably best to proceed with "Plan A" as recommended by apsix.

 

[NKT]

"Just for curiosity, is there any practically way to hold the wall and introducing the new beam to support it, i can't visualize how to insert new beam in load bearing cmu."(ali07)

I've seen this done with steel channel sections bolted through the wall before the masonry below the channels is removed. The channels are sized to act as a beam supporting the load that was supported by the bearing wall.

-NKT

 

[hokie66]

NKT's approach differs from the one I proposed in that the channels will deflect as the wall below is removed. It is usually desirable to preload the beam to prevent this deflection.

 

[NKT]

hokie66:

How about drilling the holes in a deflected-shape pattern?

The construction sequence might be something like this:
1. Drill the hole at one end and install the channel with one bolt.
2. Lift the channel to an angle such that the center is at its calculated deflected position under dead load.
3. Drill and install the center bolt.
4. Jack the other end of the channel into position, drill, and bolt.
5. Drill and install remaining bolts with epoxy to fill gaps.

 

[hokie66]

That concept is the same as mine, that is prestressing the beam to prevent deflection of the wall. But there would be a lot of force on that center bolt when you jack the channels, and I would also worry that the channels would buckle, as lateral bracing would be only at the one end and in the middle. The jack would be working at the end of a cantilever and not jacking near the shear center.

 

[ali07]

NKT

Does your proposal work for 30 ft span. We recemmend this technique for small span lintels but what about 30ft or 40ft span.

Thanks

 

[geoffdale]

What about shear transfer? Is the wall a shear wall? We
have used the channel system described above, but for something like 8-10' openings, not 30'-40'. For that span, you're talking a fair size beam, I would think.

 

[NKT]

Spans were not mentioned in the question, and I assumed relatively short spans. One way around this would be to jack the channel up incrementally along its length to get the required deflected shape (and corresponding load on the wall). The jacking points and loads would be dictated by the length the channel can span without buckling.

This is sounding less and less practical, but I still think it's an interesting idea.