New Opening in CMU Wall - Shear Friction?

[RMC123]

I have an existing CMU wall which is made of 8" block and assumed to be unreinforced. The wall is 17' tall and we want to create an opening that is 11' wide by 12' tall. I want to use 10" channels on each side of the block above the new opening. There will be through bolts connecting the the channels through the CMU. I need to determine the required size and spacing of the bolt steel. Has anybody used shear friction to carry the wall load to the channels? The idea is to leave a 3/4" gap between the channel and the CMU wall to be filled with grout, which would mean the block would have to slip out from between the channels, which would mean that the through bolts would have to elongate first.

Additionally, we do not want steel columns on either side of the opening. Has there been any consensus on using through bolts in bearing as an end reaction? If the shear friction concept above works for the span, could it also work for the end reaction?

 

[jayrod12]

Why hold the channels off and then grout it in? I would not be relying on friction as the method of load transfer.

Either put a plate on the bottom of the wall between the two channels to pick up the weight directly, or transfer the load from the wall to the channels through the bolts, then back through the bolts to the wall beyond the opening. I don't beleive there's much in terms of published capacities for through-bolts in masonry, especially ungrouted. Can't you just have them grout it full as required to promote load transfer?

 

[JLNJ]

I've seen it done successfully this way, but I'm not sure we really understand what's going on with these kinds of lintels.

To get a decent amount of shear friction, you need a rough surface, which the back of a steel channel is not, and you need a decent pretension force in the bolt, which is not really possible in hollow block. More likely you have bearing on the bolts and bending in the bolts inside the hollow CMU. I don't know of any literature out there which addresses this kind of bolt bending/bearing action. When I have to do something like this I use conservative assumptions and big bolts and channels.

More likely yet, is that the unreinforced CMU arches and/or spans on its own across the opening, without really activating the channels in bending. If think about how much the channels would flex for a given span, the wall probably would need to crack to deflect that much.

Some people stitch a plate across the bottom to keep the chunks from falling out. That's probably a good idea. Some put vertical channels either side of the opening to form a steel pilaster of sorts and bear the lintel angles on it.

For smaller openings, I rather like the detail where toed-in angles are installed first from one side and then the other in the mortar joint. This way you get at last some direct bearing at the ends.

The manner in which the wall/lintel acts depends on the load, the span, the height above the lintel, and where the control joints are located.

 

[GC_Hopi]

I think in practice you get shear on the bolts and arching action between the bolts. In that spirit, the bolt spacing is critical and smaller diameter at a tighter spacing is better.

But agreed, the angles have a simpler load path with direct bearing.

 

[sticksandtriangles]

I have always used 2" of bolt bearing at each end of the through bolt for masonry pinch beam designs (grouted solid).

https://www.eng-tips.com/viewthread.cfm?qid=434595

We get laughed at for our bolt spacing, but it makes me feel comfortable in the lack of published values.

S&T -

http://www.re-tug.com

 

[dik]

As GC notes... I think it is the arching action between bolts that causes this method to work. I would not rely on shear friction. I've use this approach for temporary 'needle beams' for supporting walls while cutting out masonry below and installing a steel W section lintel below. I've also use permanent steel C sections and welded them to permanent vertical C sections at the ends.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[RMC123]

For reference, the load is only about 500 plf unfactored. That is Dead_roof, Dead_block, Live_roof. There is 64" of block above the opening.

jayrod12: I'm not sure what you mean by "have them grout it full as required to promote load transfer"

JLNG: I've also seen it done successfully but now I am trying to justify it with numbers. As far as friction, PCI (7th ed.) Table 5.3.1 has a μ of 0.7 for steel-to-concrete, I would use a value of 0.6 to take it conservative; we can also score the block horizontally with a grinder to increase that friction. To get the bolts tight we would place them on the end joint of a "stretcher block" so that there is a web immediately adjacent on either side of the through bolt. I'm not opposed to the stitch plate, but then my question that remains is bolt bearing at the end reaction.

GC_Hopi: The double angle is obviously much easier to install and alleviates a lot of concerns since it has direct bearing. The photo you attached looks like a wide opening. Any concern there with temperature movement of the steel? I've always been told to keep angle lintels under 8' span. Also, are the bolts in the photo simply to connect the two angles? If we are okay with cracking of block joints at the end, would an 11' wide opening be an acceptable length for double angles?

sticksandtriangles: So are you saying that even with a fully grouted core, you only count on 2" of bolt bearing because the remainder of the bolt will not actually be in contact due to bending? If so, what are your thoughts on filling the drilled hole with epoxy before setting the bolt so that it is in full contact?

 

[GC_Hopi]

Any concern there with temperature movement of the steel?

Expansion and contraction should be considered any time you have different materials. In this case the angle in on the conditioned side of the CMU and IMHO this is not a concern as long as the CMU has appropriately spacing joints.

Also, are the bolts in the photo simply to connect the two angles?

Photo is of a single angle (angle on interior side of the wall only) so the bolt are used to brace the angle thereby reducing the unbraced length.

If we are okay with cracking of block joints at the end, would an 11' wide opening be an acceptable length for double angles?

This is your decision as the design professional. Run some numbers for strength, deflection and thermal expansion to see what you get then make an informed engineering judgement.

 

[dik]

I've used them for 10', with no ill effects, including deflection or cracking... these were L8x4s... They were installed on each side and then the CMU below were removed.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[RMC123]

GC_Hopi: A few questions about your photo...How wide is your opening? Are those through bolts, or did you have them grout the header row of CMU to fasten into solid block?

dik: Appreciate that response!

Getting into the fine details a little bit...in order to work around the root radius at the inside corner of a steel angle, do you have the contractor chamfer the bottom edge of the block so the face of block and inside vertical face of angle are in contact? Or, leave a gap (the width of the radius) between the angle and the block?

 

[sticksandtriangles]

sticksandtriangles: So are you saying that even with a fully grouted core, you only count on 2" of bolt bearing because the remainder of the bolt will not actually be in contact due to bending? If so, what are your thoughts on filling the drilled hole with epoxy before setting the bolt so that it is in full contact?

That's right, 2" of bolt bearing even in fully grouted construction; thought being either bolt bending or local crushing of the CMU/grout near the exterior of the CMU reduces the effective length bolt bearing.

I'm not sure the epoxy makes me feel any better about the local crushing concern or the bolt bending.

S&T -

http://www.re-tug.com

 

[dik]

If looking at adhesive anchors, I'd use proper Hilti anchors (or equivalent)... not just a matter of filling a 'drilled hole'.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[GC_Hopi]

RMC123: Existing CMU with a few openings for windows. Lintel is a single steel angle at the interior side of the wall with post-installed adhesive anchors. Not shown on the photo is the non shrink grout to be dry packed at the lintel bearing. You can estimate the opening width off the 8 inch CMU in the photo. Its like a 2 ft and 8 ft opening.

Higher loading application with a through bolt and double angle.