Opening in existing cmu - concrete lintel?

[bookowski]

I have an existing 8" cmu load bearing wall, I need to put a new opening ~6' wide. Typically I would use the detail with channels and squeeze the wall (or angles) but I need the finished product to be as close to flush as possible, the walls are not built out very much architecturally. The lintels would also need to be fireproofed.

I've been thinking about using a concrete beam instead of steel but have never done this, does anyone have experience with this? My thought is to place two channels above the future concrete beam w/through bolts frequently and squeeze the wall. I can then remove one course of block (or two if reqd.) below the channels, drop in bars, form up and pour. You wouldn't be able to completely pour it due to the channels/wall above but this could be grouted up with non-shrink after. The other option is to drop a precast beam in there and grout the gap on top. Once the concrete beam is set I can take out the channels and the wall below.

I also plan on inserting bars and grouting the cells adjacent to the new opening so I like the continuity of the rebar/grout on top of the opening as well as the sides.

This seems ok to me - it solves the fireproofing issue and leaves a flush/clean final product. Does anyone see any problems with it?

Thanks for your help

 

[sdz]

Sounds feasible. From your description it sounds like there is another storey above; is that so?
It also depends on loads and ensuring you get adequate bearing at supports.

 

[STATICPH]

What we have done very successfully is using 25mm rebar and small timber wedges as short props.
Then we knock out pockets in the brick work to install the props before we knock out the rest of the beam volume. Use plastic sheet between the soffit of your beam and the brick work that must be removed - otherwise it will stick. Once your beam volume is opened up install your rebar in the void. Place the form work and cast from the top, do the anti-shrink grouting to fill the gaps the next day. Do not remove your brick work under before the concrete is set (he he he).
We also used horizontal gum poles to bridge brick work and then remove everything before we install the RC beam.
However, sometime steel beams just work better.

 

[bookowski]

sdz - It is one story so just roof loading from above, nothing too crazy. I plan on threading rebar and grouting the adjacent cells so I should be ok with the compression on each side.

STATICPH - I like the sound of your method, it sounds essentially similar to what I am proposing. What is a 'gumpole', I haven't heard this term before. Also, I'm assuming that the timber props get cast into the beam is that correct?

 

[engineertipsman]

Why not a precast lintel, pour before and lift into place and grout? You could form it up on site to fit perfectly.

 

[bookowski]

If you look at the original post I mentioned precast also, my question is just if people have any experience with doing this in a new opening in an existing wall. I've always done steel (channels or angles) but the concrete option looks good to me (both precast and cip). I'm checking to see if anyone has had problems with this and how people handle the sequencing of slotting the wall and getting the concrete in.

 

[structuresguy]

We have done what you suggested before, by doing almost exactly the same thing using temporary angles across the opening. We have poured in place, don't think we have ever inserted a precast lintel. With vertical bars either side, you would need to notch out one face of the precast lintel at each end to slide it in over the vertical bars. Could be done I guess. You could then knock out the face of a block or two over the lintel, so that you can pump grout into it.

 

[Teguci]

Consider using two 4" wide precast lintels. Contractor breaks out the one side, places one lintel then breaks out the other side and finishes the project. We have been successful doing this in the past.

 

[a2mfk]

I've had contractors do pre-cast or cast-in-place lintels in CMU walls lots of times. A good CMU contractor knows how to do it, make sure you CYA on your drawings with shore existing structure as necessary, etc. The dirty secret of CMU and brick is that arching action allows you to punch holes in walls without doing much of anything (sure we have all seen it, right or wrong). Especially in a wall with only roof load, the loading is usually minimal anyway... I imagine little to no shoring is done in the field all the time in this situation, they just do it really fast

 

[msquared48]

If you can do this at an existing 8" bond beam with horizontal resteel, you may not even have to use the channels to install the concrete beam, considering arching action above the bond beam.

Other than that, seems workable to me.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[STATICPH]

Gum pole - trunk that was debarked and treated ready for stuctural use, otherwise not cut in planks

Yes, the "short props" remain in place, but it is thick rebar (25mm) that is wedged in with timber wedges on both heads (hold in place with nails)

 

[rscassar]

Have you established that every second core is grout-filled and reinforced? If not, I would go to site with a drill and stud finder and establish this.

I think your proposal has merit, I would install the channel to act as a temporary lintel and knock out an additional 16" either side of the opening for the lintel to bear-on. I would grout fill the lintel and adjacent cores from the top of the wall. I would specify a high slump, small aggregate size to help to grout self consolidate.

Because this is only one story and roof, does uplift become a consideration?

 

[ToadJones]

on small openings here is what I used to do back in my builder/carpenter days.
I'd go up a few courses and use an angle grinder and remove the mortar joint to a depth of about 3" on each side and a foot past the opening and slip in one leg of an angle on both sides. I never through bolted or "squeezed" anything. Then I just pulled one angle out at a time and grouted after putting the new beam in. I usually used precast lintels that you can pick up at most masonry supplies.
I would not recommend this method on large openings.

 

[bookowski]

Thanks for everyones input, it sounds like this is fairly common. I've done the detail in steel many times but never in concrete - I wanted to make sure I wasn't headed down a 1 way street. The concrete option seems better than steel in terms of long term durability for corrosion and also fire protection.

kikflip - Thanks for the sketch, it is basically what I was proposing. I haven't established the grouting/reinforcing of the wall yet, I don't currently have access as this is just a feasibility study. I think I'll be ok with uplift, if I go w/concrete it will be tied to the wall.

msquared48 - The opening is too close to the roof to count on arching (although I'm sure it still exists to some degree).