Removal Of CMU Wall

[spats]

I have been tasked with designing the removal of a load bearing CMU wall. The 8" wall is capped with an 8"x12" poured concrete tie beam, and there are 16"x16" pilasters in the wall at 20' on center. They would like to remove one section of wall between pilasters by retaining the tie beam, and through-bolting steel reinforcing channels each side of the tie beam. Problem is, since the pilaster flushes to one side of the wall, one of the channels is not able to bear on the pilaster. See attached detail.

I don't think I can get a strong enough seat under the unsupported channel. I figure if I through-bolt the channel to the tie beam at the pilaster with enough fasteners, then the channel's end reaction will be transferred to the pilaster through the tie beam. However, it bothers me that the thing I'm supporting (tie beam) is being used to support the reinforcing member itself. Anybody see a problem with this?

 

[mike20793]

How much load does the wall support? Can the tie beam span between the pilasters?

 

[structSU10]

Any way to run a channel or other shape down the face of the flush side of the pilaster? Even at partial depth, it could be designed long enough to transfer the shear you need into the pilaster if needed.

 

[spats]

The working dead + live is almost 1500 plf. The tie beam can't span. It's an older building for which drawings are not available. They chipped away the bottom of the tie beam at one location per my request, and found only 1#5 centered at 1 1/2" from the bottom. It couldn't possibly have proper stirrups as well.

 

[dhengr]

Spats:
Can the pilasters carry the new loading, or where they primarily for wall stability and wall thickness over length criteria (L/t or H/t limits)? What and where is the reinforcing in the pilaster and the conc. beam, for thru bolt clearance? On the flush side, I’d weld a length of the same channel, toes out and a couple courses long to the ends of the reinforcing channel. On the bottom end of this vert. channel, weld a 3/8" end plate which projects back into the pilaster a couple inches. Saw cut a horiz. kerf a couple inches deep in the side of the pilaster, shim the vert. and reinf’g. channel up and grout under the end plate. Maybe put a bottom plate under the conc. beam btwn. the two channels.

 

[KootK]

The through bolts don't bother me at all if they work by the numbers. You're using the channels for flexure where the concrete can't do the job and the concrete for bearing where the channels have little. It's a marriage made in heaven!

One thing that I like about structsu10's idea is that it would provide positive lateral attachment for the tie beam. Although, one would hope that there would be some rebar connecting the pilaster to the tie beam.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

I have too many questions:

Why is the grout needed under the channel at the pilaster? Is this because the concrete can't take the shear or bolt bearing loads? Or, is the grout used to create a better load path. If a fixed or continuous beam is assumed at the pilaster, can the channels continue the full width of the pilaster or further for the positive moment? If simply supported is assumed and the channels are only for moment reinforcement, do they need to extend over the pilaster?

Does the existing pilaster continue to the top of the tie beam? If yes, I assume everything works after removing 1/4 of the section at the top?

 

[spats]

I'll be doing a site visit to find out as much about the construction of the pilaster as possible. I want to make sure it's properly bonded together, and not just 8"x8" and 8"x16" blocks with a collar joint bonding it to a continuous 8" wall. See attached photo. The section to be removed is on the far right, and it's an interior wall. There's really no reasonable way to determine the pilaster reinforcing. Even if there are no vertical bars, it at least has to be filled solid (I will "ping" it to make sure). It ought to be able to work even unreinforced, even with some bending, being only 10'-8" tall. of course the unseen footing is also an unknown, but it doesn't take much of a footing to support 16 kips of so.

The tie beam has 1#5 at 1 1/2" from the bottom as mentioned, so I reasonably assume there is at least the same thing top. I'm through-bolting at 3" from the top and bottom, staggered at about 14" on center.

I would appreciate a few more readings on the channel that does not bear on the pilaster. I could give it a belt and suspenders as suggested, but I'd rather not if I don't have to.

Thanks for the help.

 

[spats]

wannabeSE, the grout is there to assure proper bearing. On second thought I'll probably bolt it down with a bearing plate for uplift as well. I'm not sure I understand the positive moment thing. At the near pilaster in the previous picture, I can bear all the way across. The far pilaster only protrudes 8" beyond the perpendicular wall, so I can only bear 8" there. As also shown in the picture and the detail, the pilaster stops at the bottom of the tie beam.

 

[KootK]

Based on the photo, it looks as thought you could provide a saddle over the tie beam to support the channels at the pilasters if you wished.

Maybe you could just install a wide flange on the inside of the tie-beam and structurally abandon the tie beam altogether. The trusses might be able handle that much eccentricity at the bearing connection.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[spats]

KootK,
I have structurally abandoned the tie beam altogether. the two channels carry all the load. The tie beam only needs to "span" about 14" between bolts.

The saddle might be a good idea if there isn't a truss directly above the pilaster (can't tell from the pic), but I would need to cut the wood nailer on top of the tie beam. The "saddle" could only be a plate across the top since the channels flush with the top of the tie beam. It also seems like it would be difficult to install (lots of welding), which I'd rather avoid. Still, I appreciate the thought.

 

[KootK]

I think that the saddles could dance around any truss position and be installed without any site welding. See the very crappy sketch below:

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.