Anchorage to top of brick masonry wall

[JAE]

Trying to attach a series of small stub columns onto the top of an existing, and fairly old, multy-wythe brick wall.
I think the wall is 3 or 4 wythes thick. Probably 80 to 100 years old.

There is tension uplift on the stub columns - probably 1000 to 2000 lbs but we have some control on this to minimize.

Hilti and others have data on anchors into the face of the wall but not the top of the wall. We called them and they confirmed that there is no testing for it.

One thought was to have the contractor drill or cut down into the top of the wall and install grouted pockets of grout with embedded/headed anchors.
The idea being that the grout plug would engage the brick around it and if we are deep enough we can then lock in the weight of the surrounding brick to provide the necessary hold-down weight.

A previous thread on this covered this similar topic: thread194-368784

Any thoughts on an approach? We are reluctant to cut down the side of the wall (into occupied tenant spaces) and fasten to the sides.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[WARose]

The approach sounds good.....but the trick is deciding how much to use for uplift resistance. If you spread out the uplift "cone" at all....you will be counting on tensile stresses between the brick and the grout. (Something some people aren't too comfortable with, and the code doesn't allow very high capacities of.)

 

[reubz]

The approach makes sense. I would be pretty conservative on the safety factor for tension / uplift for brittle bricks. I would also maybe add some reinforcing into the grout pocket to make it more ductile...

 

[dhengr]

JAE:
I think your options have a lot to do with the exact makeup and condition of the wall. Three vs. four wythes makes a big difference, and whether the wythes are bonded together, etc. will dictate what you might do. The condition of the brick and mortar will determine whether you can drill or dig out the center one or two wythes without busting up the whole wall. You better know more about the actual wall conditions before you get too far into this project. Partly, I fall back on my comments in that earlier thread. I assume this is a parapet, and you might consider excavating the middle one or two wythes and actually casting a reinforced bond beam 6, 8 or 10 courses deep. Then, support the two faces and tie btwn. the outer plywd. and pour the conc.

 

[JAE]

dhengr - probably what we will do in a similar way.
Since the uplift isn't too big, we think we are going to simply cast a 12" x 12" concrete cap on the wall top with only moderate drilled/adhesive anchorage to the brick below.
This size of cap is heavy enough to provide resistance to net uplift (0.6D + 0.6W) just by itself.
So that means any connection to the brick below is for lateral shear connectivity (i.e. the even smaller lateral loads from wind/seismic).



Check out Eng-Tips Forum's Policies here:
faq731-376

 

[bones206]

I'm just curious how do you justify the post-installed anchorage/dowels in terms of code compliance. Is there a code section that permits the use of anchors without having been specifically qualified for anchorage in the top of a brick wall? In other words, how do you certify the shear resistance of the anchors in this case?

 

[JAE]

bones....probably by the fact that the shear is less than 100 lbs. per anchor...

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[bones206]

Ok, fair enough. I was just wondering if there was a code loophole that could be employed in cases like this, when there is no applicable ICC-ESR available.

 

[LECT12]

Usually the mortar at these old multi-wythe walls have low tensile strength values. Aside from engaging enough brick mass to overcome any uplift, you also need to check the net tension in your mortar.

I've used helical pins to reinforce old masonry walls before. The product is called Helifix Dryfix. If you only have access on one side you can do a staggered install of horizontal and diagonal drilled pins at mortar joints so you can reinforce the joints and engage multiple wythes of brick. So if you do that, then all you need to do is engage enough brick mass to overcome uplift (with FS=1.5 against uplift). So for 1000lbf uplift you'd need ~10sf of brick area (assuming 4 wythe), so maybe a 3x3 or 4x4 patch of reinforced brick is adequate. Drill down with a HAS rod with HY-70 down 12 inches, pin the brick around/under, and count on that chunk of reinforced brick as a deadman against uplift.

Your idea of casting a concrete dead man is also a good idea. If you have room for that I think it's the cheapest option.

 

[JAE]

LECT12 - there is no net tension on the mortar as my cap beam overcomes any net uplift. Only shear on the anchors between the concrete cap and brick below.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

At 12"x12", you'll need to have a cap thing about 10' long, yes? If you fasten that to the wall anywhere in the middle, I think that you'll wind up with all of the load being delivered to those fastenings before it engages the full weight of the cap. There's no way that a 12" deep cap beam can compete with the wall in terms of stiffness. You may have a solution that's safe overall but risks local damage to the masonry.

 

[LECT12]

If the force is just a thousand pounds I think the wall is likely able to sustain that kind of load, assuming that the mortar is intact and in good condition. At the top of an old wall, the mortar might be questionable and I'd add a provision to locally reinforce the masonry, or at minimum, rout and repoint the mortar joints in area within several feet of the concrete dead man.

 

[JAE]

KootK - we'll have more closely spaced connections than 10 ft. so not a problem.


Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

KootK - we'll have more closely spaced connections than 10 ft. so not a problem.

The more closely spaced connections is the problem that I'm worried about. I'm envisioning all of your load traveling through the connection nearest the load source well before engaging the deadman mass further out.

 

[bones206]

To alleviate KootK's concern, you could forego anchors altogether and remove some interior-wythe bricks to form a keyway or individual shear keys. Although the code may require positive anchorage for seismic loads... and like dhengr said, the existing condition of the masonry may limit what can be done without compromising the integrity of the wall.

 

[dhengr]

JAE:
Koot’s concern is the relative stiffness or flexibility of the 12x12 beam vs. the brick wall, and in that respect, you might be better off with a 10x16 or 8 or 10x24” deep cast beam, to lessen the likelihood of too much uplift movement (bond failure or single course brick lift) right under the load. If it were fairly easy to do you might consider removing the inner brick wythes several coursed deep and then casting your beam atop that, since beam depth/stiffness seems to be an improvement. I’ve often wondered about the sanity of A.B’s. into the top cores of found. blks. or into brick walls, when with the heel of my hand, I can often bump the top bricks out of the wall, or with just a little more effort do the same with a top conc. blk. Put a little tension on a “J” bolt and tap the brick or blk. lightly with a hammer and you will lift the top couple bricks or blk. right out of the wall. But, it does seem to work more often than not, mostly because it never really sees the design load.

 

[KootK]

Kinda wacky but, given the small tension demand, maybe a localized concrete boot on the stub columns? Not sure if this is a high end penthouse or just a mechanical platform.

 

[JAE]

So multiple stub columns spaced close together - tied down to a concrete curb that is more than adequate for uplift resistance - i.e. the curb will never move upward - 0 deflection.
You have to move the curb up to engage any dowels into the brick below.

The spacing of the connections between concrete curb and brick wall below doesn't change that.

The structure is an elevated deck about 3 ft. off an existing roof.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[KootK]

Since the uplift isn't too big, we think we are going to simply cast a 12" x 12" concrete cap on the wall top with only moderate drilled/adhesive anchorage to the brick below.

tied down to a concrete curb that is more than adequate for uplift resistance - i.e. the curb will never move upward - 0 deflection.

If you've got this kick-ass, un-deflect-able beam in the mix, what's the point of the 12" x 12" cap thing? Just attach to the super curb.

 

[JAE]

The 12x12 is ballast - so that the block wall doesn't have to serve as Superman holding down his high flying Batman.

Check out Eng-Tips Forum's Policies here:
faq731-376