Shear Strength of Brick 1

[anchorengineer]

I am working with a contractor who needs to remove the limestone veneer on the lower portion of a building built around 1950. He is asking me to calculate what is required to hold up the brick veneer above the limestone veneer.
This is what I'm working with:
1. The building is a 9 story steel framed building.
2. The walls are cinder block with limestone and solid brick veneer. The limestone extends from the sidewalk to just below the windows of the second floor and then is brick up to the roof.
3. The cinder block walls bear on the floor slab and are directly above the steel fascia beam.
4. There are no relief angles in the building veneer. Instead, there is a course of brick that is "toed" into the cinder block at least every 16 courses acting as a ledger angle.
5. There is one course of brick between the limestone and the first "toed" course of brick.
6. The limestone is continuous below the brick and there are no brick "piers" in the veneer that could transfer the veneer load to the foundation.
7. The condition is temporary as the limestone is being replaced by new limestone.
8. There are no joints in the masonry.
9. There is a toed in course of brick at every floor slab.


I want to calculate the shear capacity of the brick and see if any structural members are actually needed to support the veneer (i.e. steel angles anchored to the cinder block) during construction and if the toed bricks acting as ledger angles are sufficient to support the weight of the veneer above.

My instinct says that the toed in brick is enough to support the weight of the veneer above it without a structural support. There are openings in the cinder clock which show the toed in brick extends past the outer shell of the cinder block. The bond strength between the mortar and the brick of the last toed course and the course of brick below is probably enough to hold it up although I would add a steel angle below it anyway in case they become loose during the demolition of the limestone. I've not had to rely on the shear or bending capacity of a single brick before which is why I'm seeking assistance. Any thoughts would be greatly appreciated.
Thanks!

 

[anchorengineer]

And yes I understand that relief joints should have been used but that is not what I'm being tasked with ;o)

 

[jayrod12]

I've always taken the capacity of brick to be next to zero in anything but compression. Call me conservative but I've seen way to many bricks with cracks all the way through

 

[tony1851]

....

 

[anchorengineer]

@ Tony1851: Funny sketch
Yeah, I've already told the contractor that he'd be installing temporary relief angles at each floor to resist the veneer load. I know I'll get some resistance from the toed bricks (similar to corbels) and even the mortar bond but nothing I can calculate with certainty and more importantly stand behind in court!

 

[dhengr]

Anchorengineer:
You are working with the contractor, for him, not against him, and you have given him an onerous task if you expect him to remove several courses of brick at all 9 levels for some temp. shelf angles. There must be a better way to skin this cat. Get him to assume the responsibility for you saving him a great deal of money and effort up to the 80-90' level; of course, do this safely and with good sound engineering judgement. You won’t find this in any of our nice new codes, but common sense and good sound engineering judgement and experience still works despite the codes. Look through the SCPI Tech Notes for some guidance and approx. strength ranges, same thing with NCMA TEK notes. Don’t ask what is the shear cap’y. of the brick; rather ask what cap’y. you need and then test some bricks to show that you have many times that cap’y. Is the brick and the entire wall in good shape? Is the collar jnt. filled with mortar, so you might get some bonding there or is this really a self supporting veneer? Are there any metal ties btwn. the brick and conc. blk? Doesn’t every fl. slab and its steel spandrel beam carry the masonry for that fl. level? Have you given up too quickly without really assuming much undue risk? And, the contractor should pay you for your efforts. You shouldn’t stick your neck out a mile to save him a buck, and you shouldn’t do something which is unsafe for the general public, or the bldg. owner. But again, if these walls have worked for 60 years, I’d see if there isn’t another way.

The way I see it... Brick-n-mortar at about 140#/cf = 45-50#/face sq.ft., per single wythe. A header course (toed-in brick) every 16 courses (3.5' vert.) and on top of every floor slab, means two or three header courses per fl. Doesn’t ever floor slab and steel spandrel beam carry the masonry wall (brick & conc. blk.) for that story? I would be a bit surprised if they assumed brick 80-90' high supported itself full height, you should investigate this. There is also conc. blk. backup wall for the limestone being removed, right? How is it stressed if it picks up some of the load. Then... 50#/sq.ft. by 3.5' = 175#/l.ft.; divided by 3 header bricks per ft. = 58#/brick; with brick area = 2.25"x3.75" = 8.4 sq.inches. Thus, 58#/8.4 = 7psi in shear. I’ll bet you can test some bricks to shear at many times that value. The brick may not transmit that 58# load as a cantilever, in bending, but I would think it would carry the load in shear, if it is well tied into a good sound brick wythe and good conc. blk. backup. This might require some investigation work for wall soundness and how the header course is tied into the conc. blk. backup wall, etc. Also, consider having a testing lab test a section of that brick wall for actual shear strength so you have something to hang your hat on. They can do this with very little damage to the wall, if it’s a sound wall. The contractor should go along with this. I would guess that this testing will cost much less than 9 levels of shelf angles.

I would install the temp. shelf angle just below the 2nd fl. windows, and monitor it for any movement. Maybe monitor some other locations on the wall too. The testing lab can help you determine some of these things. The lengths of these angles and their support system should be related to the window spacing, so you can take some advantage of the brick arching action, and so they can be removed one at a time as limestone replacement progresses. You should also discuss the staging of the entire job, so that the brick doesn’t sit with temp. support for months on end. The contractor needs your help, and you can make a good client out of him without undo risk. He shouldn’t be allowed to push you into that position.

 

[tony1851]

Has the whole of the limestone veneer got to be replaced in one operation? Can it be replaced in short lengths (say 3 or 4 ft) working section by section horizontally along the building. That way it might be possible to utilize the arching effect of the brickwork.

 

[hokie66]

Why is the limestone being replaced? If because it has been overstressed and is showing distress, I wouldn't be trying to carry the load of the brick in shear.

 

[anchorengineer]

Thank you all for your replies:
This is a contractor that I work for quite regularly. Of course they are only bringing me into this project days before they are scheduled to remove the limestone (for aesthetic reasons only). My intentions are:
1. I am going to calculate the shear capacity of the brick and make up the difference with the steel ledger angle.
2. I was not going to have them remove a course of brick for a ledger angle but rather place an angle against the outside brick and fish plate it through the walls. The inside of the building has been gutted so the plates are bearing directly against the cinder block.3
3. I was going to install crack monitors to make sure that was no movement as a precaution.
4. I will suggest testing the brick to them and sequencing as well. Unfortunately one contractor is doing the demo and another is doing the new veneer install which makes sequencing and coordination a pain in the a$$. To these guys time is money and they just need to start.
5. Logic says that the veneer weight is being transferred at each floor and into the fascia beam. I may only call for the angles at the bottom 2-3 floors as they would experience the greatest load transfer from the veneer to the toed in bricks (This is of course assuming there is any).

The collar joint appears fully grouted and I am certain that there will be some bonding action with the mortar. If this was a newer building I would have more confidence in this bond but I need to be conservative here since there are other buildings nearby that are being demolished (vibration) and I just don't know what I've got.

I welcome more comments if you've got them.

 

[anchorengineer]

FYI - I just saw the outside of the building on Google maps. It clearly shows the building façade and I must have missed a course of toed in bricks while I had my head stuck out a window. The building obscured by scaffolding with debris netting. I can clearly see they are every 24" vertically. A colleague that specializes in building envelopes said he has seen this a few times and that they usually coincide with the backup block walls.
I will still design an angle below the course just above where the limestone is removed (it stops about 1/4 into the 2nd floor) and will still recommend some crack monitors just in case.
Thanks again for the discussion.