Brick wall uplift connection

[CodeQuandary]

I have to look over engineering analyses of uplift of beams attached to the top of a brick wall. The wall is 12" wide multiwythe unreinforced solid brick, the uplift force is typically between about 300 and 1000#. I'm not an engineer but my physics background helps me follow the reasoning and calculations involved.

I have looked at many dozens of these, stamped by many different PE's. Far and away the most common method of attachment is a bolt or threaded rod embedded in the top of the wall 6-10" with Hilti HY70 or equivalent epoxy. In looking into this further, I've realized that the HY70 ratings are for mounting into the side of the wall, not the top, and require a 16" distance from edges, obviously not possible in the top of a 12" wall. Some other approaches include essentially the same but with anchoring cement in the hole, or threaded or mechanical expansion devices (typically with fairly shallow embedment, 2-4").

I've talked with engineers at Hilti, and they basically said they have no tests or reliable way of determining the best way to do this. I've done a fair bit of web research. I've talked with several experienced structural engineers. I have found no solid information, nor really anything that could justifiably be called "best practices".

The reason this came to a head was that one PE, undertaking an analysis of his own, concluded that the embedment of the 3/8" threaded rod required for the uplift had to be embedded 50". If you understand how these are installed, you'll know that's impossible, or at least practically so. And yet, he's actually the only engineer that didn't improperly apply the load tables (although there are still arguable flaws in his reasoning).

Now there are real problems with the issue to begin with: unreinforced brick walls are inherently suboptimal for uplift. And yet, the problem needs to be solved. So I'm looking for something that I can have some faith in, research towards that end, or any help towards a "best practices" solution. Thanks for any help.

 

[hokie66]

You would have to engage enough brickwork to resist the uplift force, factored appropriately. You can't depend on tension in the bed joints. Anchorages need to be built in, not drilled in. The so-called engineers who are depending on 6-10" of embedment are kidding themselves, and doing a disservice to the profession.

In addition to uplift, you should check bending of the wall. If this is in a high wind area, flexural strength of unreinforced masonry will typically be an issue.

 

[CodeQuandary]

Thanks Hokie!

Just to be clear: these are not freestanding walls, they are either the outside/end wall of a duplex or rowhouse/townhouse, or they are a party wall in such a structure. There is no option of "pre-existing" anchorage, these are retrofits. What I am looking at has happened over and over again, and will continue happening: I'm just trying to establish best practices. I suspect there is no optimal solution. OTOH, dozens of PE's have stamped drawings with the details I've described. The uplift is associated with wind loads, and as mentioned the per-connection loading is on the order of 300-1000#, so although it's not a high-design-wind area (90 mph), I don't think this pertains (that is, it's already factored into the load).

 

[hokie66]

I didn't think they were freestanding, but flexure can still be an issue. Presumably, some code change has dictated these retrofits, and what you described is just tokenism to 'do something' to comply.

 

[CodeQuandary]

No, this isn't tokenism, sorry if I didn't explain adequately. This is about installing Solar PV systems on existing roofs. They are going up by the thousands around the country, and yet I (and presumably many other interested parties also) can't find reliable information on what makes for a safe install re:attachment uplift in brick walls. Some installing companies pay for really complicated engineering, some get a much briefer analysis done, but in all cases it appears they rest on a bunch of untested assumptions and/or ill-applied testing data.

There is a lot of interest in facilitating these installations, for a number of reasons and from a variety of parties. And, as I said, they are going up in droves no matter what I find. The question is, is there a path to a reliable best-practices approach?

I'm also curious why you would say "you can't rely on tension in the bed joints": I realize they may not be in the same league as, say, reinforced concrete, but if you couldn't rely on them at all brick walls would be very delicate indeed, I think. So, how much can you rely on them? As a starting point I assume we start with a good-condition average-quality brick wall, though that's not very well-defined... or verifiable, anyway.

Hilti has an interesting example in some of their literature involving drilling the hole in at 22.5º, and then installing a bent anchor to make tensile/withdrawal strength much higher (that's for walls, still), but I'm unconvinced that's important: if there's enough going on wind-wise to approach the withdrawal limits, there are probably forces in all kind of directions that mimic that 22.5º effect. But I could be wrong. I'm just mentioning it to note another possible approach.

 

[hokie66]

If anyone tries to resist an uplift force of 1000 pounds by embedding an anchor 6-10" into the top of a brick wall, I do not resile from calling that tokenism, and stupid as well.

All brick walls are "very delicate indeed". Don't be deceived by the common saying "as solid as a brick wall". Brick walls are strong in compression, less so in bending, and useless in tension perpendicular to the bed joints. But that is just my opinion based on many years of experience using various codes...what does your code say about tension perpendicular to bed joints? If it is not allowed, that means that you have to engage enough brick to resist the uplift force, and you have to do that from the bottom of the engaged brick, not the top.

Hilti is just giving you information about pulling out an anchor from a substrate that stays in place, which is not a realistic assumption in the case of your brick walls.

 

[Ron]

Agree with hokie66's comments.

You don't say how the beam is taking and transferring load. Why can the beam not take all the uplift load and transfer to columns/filled pilasters, etc? Please describe the framing of the building.

Next, the engineer who concluded 50" of embedment was correct might have been onto something. The brick has mass. If you tie enough of that mass together, it has uplift resistance. 50 inches would be adequate to induce wedging (assuming a running bond in the brick). Maybe he was just trying to induce dangling weights on the bottom of the beam.

Give us a bit more info on the framing as there might be no reason to tie the brick to the beam for uplift resistance.

As hokie66 noted, lateral resistance of the brick is pretty low.

 

[CodeQuandary]

So these are pre-existing rowhouses, like you see by the mile in many cities. The roof framing is typically not up to modern code, so it's thorny for installers to do anything that relies on it. Much more common is to span the party walls, it leaves the roof alone. I'm not sure what else you were asking about Ron: I'm not trying to solve a single building problem here, I'm trying to figure out if there's a reasonable best practices for many such buildings, and what the pertinent parameters/limitations are.

The 50" embedment suggestion I suspect still fails b/c it involved epoxying the rod in place: as pointed out, if one goes that way I think one has to have some form of an anchor plate or something on the other end, and the epoxy doesn't really do much...? I also don't think it's possible to drill a 50" 3/8 hole in a 12" brick wall (many times in a row), blow out all the dust, and get epoxy and a rod down there. And I'm not convinced that 50" of hammer drilling into a wall isn't more destructive than helpful, but I could be wrong there, with nuances of wall condition being the important elements there I think.

As mentioned: this is happening in great numbers in many cities, as far as I can tell. I have the idea of trying to develop a best practices approach that gets around a crazy variety of engineering assumptions that come across my desk, which I then have to try to make sense of. I'm pretty sure the installers would welcome such a thing, if it were workable in the field.

In most cases, the loads might be reduced somewhat by using more connectors, though you run out of space eventually. Still, if one could develop a system that was reliable to 200 or 300# (but not 1000#) that might be better than nothing...

 

[hokie66]

One idea is to go down into the room below, install a plate on the face of the wall with drilled in anchors. You could go down as far as required to mobilise enough mass to resist the uplift. That would require some disruption to occupants, as well as possibly some need to reinstate interior finishes, but hey, to make an omelette, you have to break some eggs.

 

[dhengr]

CodeQuandary:
In what capacity are you reviewing this problem? Are you a building official approving plans and permits, or a PV panel supplier wanting to build more, or what? You guys would be in hog heaven if you could just eliminate the engineer completely, put the engineer completely out of business; and every imaginable condition in the world was covered by a prescriptive code paragraph and detail. The engineers who are showing 6-10" A.B. embedment are harkening back to the IRC min. 7" A.B. embedment in the top of a found. wall and not thinking any further. The BO’s approving this are not complying with the intent of the code, even if they can recite every verse verbatim. And, they are both being negligent. You’ve heard some of the potential problems from a couple mighty knowledgeable and experienced fellows, but you’re not hearing what they are saying. You need a good engineer involved in this type of work. Someone with some knowledge of brick masonry, and of these old buildings and masonry walls, has to look at every wall, and maybe every 10' of every wall due to condition issues, and at the construction details and general conditions of that wall, to make a judgement about the possible solutions. You are correct to question a 6 or 10" long A.B. into the top of the wall, however they are installed. That just won’t work for much of anything, in those old brick walls, you’ll just lift or push 2 or 3 courses of brick right out of the wall. And, you better be darn sure to keep the water out of whatever you do too.

I think the 36 - 48" long rod sounds about right, the engineer has to check this. I’d drill the hole and remove a few bricks at the bottom of the hole. I’d install a 3/8"x6"x12 or 16" plate on the bottom of the rod so that some brick above was really engaged. Now, you can blow out the drilled hole for epoxy too, to engage more brick, and lock the rod in the wall. The brick above to steel plate, at about 45̊ from the ends of the pl. will now act as a reaction to the uplift. Then do a good job of grouting around the stl. pl., nut and washer, and replace the bricks. Again, the engineer on the job has to make a judgement on these types of things, and they may very from wall to wall, for any number of reasons. If the wall can’t tolerate the drilling, you probably have bigger problems than the rod length, but be careful. Many of these are beautiful old buildings, but because of the way they were built, you can’t remake them to today’s codes on the cheap.

 

[CodeQuandary]

Hokie & dhengr: thanks for your thoughts. I'm a building official. I'm not trying to put any engineers out of business, very very few that submit plans get beyond the 7-10" embedment... so should I just ok those b/c they're the ones that signed the sheet?

You all might want to see what's being down out there now -- I've never, ever heard of residential going in like you're describing. I realize it might be a very thorough approach, and if we can do some testing (I'm hoping to set some up) maybe it will be the only thing that can work, but I also notice that engineers (like everyone) are occasionally inclined towards CYA conclusions: play it safe, after all they don't have to build it. But if they spec something that can't be built, then I'm being negligent ok'ing it.

Dhengr, don't presume to know what I'm hearing just because I'm pushing back. No one in the room is using any test data, so everyone is making it up, unless you can show me otherwise. I've never heard of a PV system mounted in the manner that is mostly done (10" or less embedment) being torn off a roof... even though we've had some exceptionally high winds in my city just in the last year (and we have a lot of PV).

I'm sorry that I'm in a bad mood, but I was just trying to get a jump on the week's work and got started going over about 20 applications, and maybe one passed muster: all the others stunk. One very experienced engineer (big local reputation) didn't include any combination loads. Another basically didn't include any loads, just wrote a letter saying "I approve" except that he didn't reference the roof framing, only the PV racking system. It's this craptastic stuff that I am trying to change. By "best practices" it doesn't have to be a prescriptive approach, it might instead be guidance to get an engineer to do a better job.

As someone who's done a lot of building, I am cautious about suggesting drilling 4' into a 12" wide brick wall: the more you hammer drill into a brick wall, the more you tear it apart. But honestly, what we need is test data. I've got questions in to the Brick Institute, and am trying to figure out other ways to keep this real. I do appreciate people's thoughts. Sorry for a bit of a 'tude.

 

[hokie66]

Don't worry. We do understand your position. Where I am, the authorities don't care, the installers just do what they want with these solar panels on roofs of houses. Don't get me started on solar panels...a big boondoggle. Without massive subsidies by government, the economies don't make any sense. But that is a different matter from your problems.