Another Lintel in Existing CMU Question

[bookowski]

New opening in existing cmu wall (about 5.5' in lightly loaded bearing wall suppt roof), I want to use the commonly used detail of sawcutting a slot and insterting an angle and then repeating this on the other side. After the block is removed we'll stitch weld a closure plate on the bottom. This procedure seems fairly common but I have a few questions:

- Do the sawcut corner need to be chamfered to get the angle to fit in (i.e. to fit the radius of the angle toe)

- What is the end condition? Bearing plates? Grout?

- How do you ensure that the angle is actually tight to the underside of block? It seems like the sawcut cut won't be straight and a good portion of the block won't actually sit on the block. Any way to remedy this?

Thanks for the input in advance.

 

[BAretired]

You would have to chamfer the corners if you want to have the inside face of the angle flush with the block.

The bearing condition is probably just steel against block unless the stress becomes too high...then you might want to grout.

I don't think you would know whether or not the block is bearing tight to the angle because you cannot see it. Again, grout is probably the answer, possibly requiring a few holes through the face shell of the masonry.

BA

 

[rscassar]

Do the sawcut corner need to be chamfered to get the angle to fit in (i.e. to fit the radius of the angle toe)

I wouldn't be concerned with it unless you want the angle to be flush with the CMU wall.

What is the end condition? Bearing plates? Grout?

I just extend the angle and additional 16" past the opening and provide a chemical anchor at the end of the angle through the face of the CMU wall.

How do you ensure that the angle is actually tight to the underside of block? It seems like the sawcut cut won't be straight and a good portion of the block won't actually sit on the block. Any way to remedy this?

The builder should be able to perform a very straight cut, particularly if they cut along the bed joint. The CMU wall will crack in an arching pattern over the opening and will bear on the angle as the angle deforms.

 

[bookowski]

I would prefer to not see cracking over the lintel, this is a new window in an existing home and I am trying to minimize the collateral repairs. Does anyone do anything to minimize/prevent this?

 

[msquared48]

Allow the grout in the new bond beam ... of sorts ... to cure for at least 7 days before the block below the lintel is removed, and put in temporary wood shoring in the opening for an additional three weeks after the brick is removed. Then install the window.

Just go around to some masonry construction projects and look at what the masons do for openings of this type. You will learn a lot.

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

 

[msquared48]

To make this even better, prior to pouring the bond beam, I would remove one block at a time, inserting the plate with two neldon studs welded to it, between the lower legs of the two angles and weld it off, sequentially doing so with all the blocks below the angles to be removed. After this, pour the grout into the bond beam and shore for 28 days. Then remove the shoring, the rest of the blocks and install the window. Composite action and bond beam achieved.

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

 

[bookowski]

msquared - my angles are the permanent lintel, there is no beam being poured below. From what you describe it sounds like you are talking about using the angles as temporary support and pouring a beam below right?

 

[BAretired]

I think there are better ways of installing a lintel on a 5.5' span than the double angle method described here. This is not a method I would choose.

BA

 

[msquared48]

bookowski:

Yes... and No...

The angles are temporary supports, yes, and more...

And No... With the Nelson Studs welded to the top of the flat plate and inserted between the horizontal legs of the angles, with the plate welded to the bottom leg of the angles, the grout is ABOVE the angles and plate, with the Nelson Studs providing the composite action so that the angles serve as the lintel beam steel.

This is probably an overkill for such a short lintel, but would be very applicable for a longer, more highly loaded span.

As BA says though, there are other ways. For example, if there is suffficient arching action over the lintel, if the upper cut is made at the bottom of an existing bond beam, and if there is sufficient steel and deop[th of the bond beam for the load, no additional reinforcing would be needed to remove the block and install the window.

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

 

[bookowski]

BARetired - Could you please expand up the better ways to do this.

Msquared - got it, I don't think that's necessary on such a short span, but I like the idea. thanks.

I can't count on arching action, the opening is only a couple of courses below the roof and is close to a corner.

 

[BAretired]

1. Use steel plate each side of wall, bolted through masonry at 16" centers. Remove masonry in opening. Grout cores in each jamb to floor. Add plates on jambs and head to finish.

2. Provide needle beams normal to wall just above opening supported on temporary shoring. Remove masonry in opening. Grout cores in jamb blocks. Place W8x10 with 1/4" x 7" plate bottom (assuming 8" wall). Shim between beam and masonry. Add 2" x 8" solid block each side flush with wall to finish.

For an opening as small as 5'-6", many contractors would omit the needle beams and take a chance that the wall would hang together, particularly if the clear height above the opening was adequate to permit arching action.

BA

 

[bookowski]

BA - These options sound more complicated rather than simpler. Also, I can't shore needle beams as this is the top floor of a 10 story building and I also can't leave bolts in place on the exterior. Thanks though.

The original question was - assuming that double angles are used do people typically see problems with cracking above the lintel until the angles are fully engaged.

 

[BAretired]

What will be the clear height of masonry above the opening?

BA

 

[msquared48]

He said two courses below the roof BA...

That being said, there must be a bond beam at one of those courses for shear transfer from the roof diaphragm.

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

 

[bookowski]

About two courses below where the joists sit is correct. The roof is 5" slab on deck on light gage joists, there should be a bond beam up there but I don't want to count on it.

The loading is relatively light, I'm not worried about the final condition for strength/serviceability, almost anything will work - I'm just trying to minimze collateral disruption with the installation. This is an existing apartment and I'd like to minimize cracking above/around the new opening.

I wanted to do a concrete lintel but they are steering me towards steel. The double angles seem cheapest/easiest I was just wondering what people do about the end conditions and grouting between the angle/block (see original post).

 

[OHIOMatt]

One option that I have rarely seen used, but seems to work relatively well. This method was shown to me a foreman when I was working for a construction company in the summers during my college years.

Use (2) 8" channels as the lintels.

To install them, you identify the area just above were the new opening will be. You then break the face shells of the block on one side. So for this example you would break out an 8" high by 6'-6" +- area. Continue with the removal to include a portion of the webs, to the point were you can slide the channel in. Install a bearing plate and shims under the channel at the new jambs to assume some of the load of the wall. Repeat procedure on the opposite side.

Create the new opening. If appearnece is a concren, weld a plate to the bottom of the channels and fill with soaps.

We always did this without any shoring. Obviously you would want to make sure there is no heavy concentrated load directly above you.

 

[BAretired]

I like OHIOMatt's suggestion, but would prefer to see joists within the 5'-6" span temporarily supported.

BA

 

[OHIOMatt]

Thanks BA.

When I was working for the construction company, I was just entering into my engineering education. I wish that I would have had more technical knowledge when I was exposed to things like this. These guys knew about arching action and took full and sometimes more advantage of it.

So, I agree with you in terms of shoring. As a design professional I would never do half the things that I saw them do.

 

[bookowski]

Thanks for everyone's input but it's gone a little off track from the original question. Does anyone use the basic two angles, toes turned in detail and if so:
- do you do any grouting to ensure the angles are in full contact with the cmu and how do you get good access to grout?
- what do you do with the end conditions, just grout under the angles at the bearing location?

 

[BAretired]

I have never used the double angle solution, so cannot respond intelligently to your question. I suspect it will be difficult to avoid cracking.

BA