Two Span Cont. Masonry Lintel 1

[Ryan H]

I have a proposed 7ft wide basement window condition in an existing 8" masonry wall. I am told the tributary first floor load is 6ft and there's 8ft tall first floor block wall above this proposed opening. without looking too deep into it, I assumed the 8"x8" x 7ft precast lintel wouldn't work, no arching action. the arch suggested framing out the opening with 2x6 pressure treated for window attachment and using the middle mullion as load bearing for a midspan support. Is it possible to design a masonry lintel as two-span cont.? I know there will be negative moment over the center span. I also would need to check the Fv of the 2x6 p.t. nailer as well as the proposed (2)2x6 mullion. Just wanted to hear the thoughts on this proposed idea.

 

[KootK]

1) A masonry lintel can be two span, just as a concrete beam can. However, as with a concrete beam, this usually implies reinforcing placed near the top of the beam which is not how your typical, simple masonry lintel is usually constructed.

2) Can you use the first floor block wall as an arching element to shed some load? Obviously, that won't help with your 6' tributary load.

3) The thing that you're calling the nailer is just a 2x6 laid flat, right? At best, I'd think that good for forming the CMU lintel in a temporary works kind of way.

4) Some codes prohibit the use of wood to support masonry. The usual concerns are durability and stiffness compatibility.

 

[Ryan H]

kootK,
1) A masonry lintel can be two span, just as a concrete beam can. However, as with a concrete beam, this usually implies reinforcing placed near the top of the beam which is not how your typical, simple masonry lintel is usually constructed.

​

correct, I would need to have both top and bottom

2) Can you use the first floor block wall as an arching element to shed some load? Obviously, that won't help with your 6' tributary load.

I am not sure, I imagine the first floor pockets into the masonry wall.​

3) The thing that you're calling the nailer is just a 2x6 laid flat, right? At best, I'd think that good for forming the CMU lintel in a temporary works kind of way.

I think the lintel would come as a precast element. maybe (2)4x8's which may be hard to reinforce. so perhaps an 8x8 lintel with (2) #4 top and bottom. they would just need to shore the wall to get everything out to install the new lintel in one shot​

4) Some codes prohibit the use of wood to support masonry. The usual concerns are durability and stiffness compatibility.

my fear was the center reaction on the mullion. the 2x6 laid flat nailer plate would be on all sides of the opening so the load bearing mullion would be propped in the middle no bracing both x or y.​

 

[XR250]

Might be easier to clear span using an I-beam with a plate.
Also, could you use a 12" or 16" tall pre-cast lintel?

 

[RHTPE]

the arch suggested framing out the opening with 2x6 pressure treated for window attachment and using the middle mullion as load bearing for a midspan support.

Of course he/she would.
I would NEVER utilize wood as a major support of a concrete lintel in a concrete wall opening.
Rot happens - maintenance not so much.

Ralph
Structures Consulting
Northeast USA

 

[masonrygeek]

Check out NCMA's concrete masonry lintel guide for some design tables and more:

https://ncma.org/wp-content/uploads/2019/02/TR91B-Lintel-2004-2012-UPDATED.pdf.

 

[StrEng007]

Have you considered installing a steel flitch plate or channel to the inside face of the CMU?

Calculate the plate or channel to take the full gravity loading and provide fully grouted cells and post-installed anchors securing the plate to the face of the wall. Make sure to resolve the vertical reaction load at each end of the plate. Once that is installed, you could saw cut and remove the existing block below the plate and create a new opening. Make sure you have the capacity to resolve the end reactions or provide an explicit member to carry this to the foundation.

Not sure if you'll have retained soil at some point below the window. Make sure you have a load path to resolve that horizontal load.