Masonry to Steel attachment 1

[SteelPE]

I have an existing building where the owner would like to install some new openings in the perimeter of the building. Code is IBC2015/ASCE7-10.

The existing building is 50’s to 60’s with riveted clear span gable roof trusses supported by perimeter columns and infill 8” CMU. The existing trusses/column connection contains a knee brace from the column to the roof truss (I’m thinking/assuming these knee braces resist lateral loads).

The owner would like to install 2 new overhead doors in the gable end of the building. Due to existing window locations, we think it’s better to remove the existing wall in its entirety and reinstall a new 8” CMU.

The issue I have in regards to attaching the new wall to the existing truss (see attached sketch). I have two options I usually default to in similar situations

1) Come up with an angle/expansion bolt detail to attach the truss to the new CMU wall
2) Install nelson studs along the bottom chord of the truss and grout the nelson studs into a bond beam.

Both options are not great. Does anyone have a better detail for this situation?

The existing wall has wire ties at 10" o.c. +/- attaching the masonry to the existing truss (shown in the sketch).

At this point, CMU is the direction we have not plans to get away from the CMU so please do not offer a solution that uses other building materials.

 

[XR250]

I run into this when replacing residential CMU foundation walls.
I hang anchor bolts form the existing sill plate (weld studs to the truss in your case)and then have then break out one face of the CMU top course so it can be slid in.
Then they grout the top course with low slump mortar. Not sure how well that gets filled but it looks good when they are done!

 

[SteelPE]

@SteelPE - I'm not buying the argument that you don't need to worry about the load path. If you take decent-sized section of wall down, put it back up, and design the wall restraining connection element, consideration of the stabilizing load path seems well within the standard of care. Maybe the judge and jury would disagree with me, I don't know.

OK, so, you don't agree with the methodology. In accordance with the exception of 403.4 of the IEBC, I am not increasing or decreasing the lateral load in any member nor am I reducing the capacity of the existing structural system therefore, I do not need to make any modifications to the existing structure.

I understand the concern as I have the same concerns. However,adding lateral bracing perpendicular to the wall is going to be a major task, a task my competitor down the road will not take (he will just use the code allowed exemption), so if he can use it, why can't I?

 

[phamENG]

Here's another thought, Steel - you're modifying the wall due to large openings that make the old wall worthless to try to keep. How will that modify your wind load path? Will the "jambs" of these doors present out of plane point loads to the bottom chord where a uniform load was present before? That could alter the loads in the braces. By how much, not sure, but may be worth checking. Don't just add braces arbitrarily, but as you say you are yourself concerned it would probably be prudent to at least verify their role in the load path. The doors may not be big enough to make that matter - just presenting ideas.

 

[JLNJ]

SteelPE

I totally agree with you that you should take all the liberties you can and use the Existing Building Code provisions to the max. And I agree completely about keeping up with the competition. We have had many of these discussions recently on a multi-story project here at my place of employ.

If there was no load path before and something happens to the wall or truss in its "new" state, I think you would have a hard time convincing the jury that ignoring the top of wall restraint reaction into whatever stabilizes it is within the expected "engineering standard of care". The load path through the wall has definitely changed (as phamENG alludes to above). Maybe the full wall acted more as a defacto cantilever in its previous life. Maybe it's short and will still act that way even if the truss doesn't provide any real restraint. Or maybe you can force the wall to load into the truss exactly as it was (or all components within 10%). Or make the top restraint continuous so it smooths out the concentrating effect of the openings.

 

[KootK]

Maybe the full wall acted more as a defacto cantilever in its previous life.

This was one of the possibilities rolling round in my head as well. Unfortunately, if you demo and replace the wall, it may be tough to reconstitute that same moment connection at the base. Another possibility is, depending on building length and opening layout, maybe the top of the wall was getting by spanning horizontally. This too might be compromised with some large garage doors going in.

Or maybe you can force the wall to load into the truss exactly as it was

Even this is a stretch for me frankly. The code doesn't say this but, as a matter of professional ethics, I consider the situation to be this:

1) If I'm not appreciably changing forces and load paths, I can forgo investigating things that I have no reason to seriously question.

2) If I see something that I know for a fact doesn't come close to working by the numbers, I ought to address it regardless of the code "out".

If a couple of 2-1/2" angles spanning the width of the building is the only thing stabilizing the gable end wall joint, I'd consider that a #2. With the arrival of two big overhead doors, I'd also have to imagine that the potential for things bumping into the jambs will increase substantially.

@SteelPE: What is the nature of the system that laterally braces the bottom chord of the truss? Horizontal trussing at the ceiling level?

@SteelPE: for the record, what is the out of plane bracing at the top of the block wall? I'm getting the impression that there isn't any but don't want to get to far down that path until you've confirmed that.

Additionally, can you confirm that this end frame does indeed have knee bracing like the interior frame?

 

[KootK]

Another possibility is, depending on building length and opening layout, maybe the top of the wall was getting by spanning horizontally.

This would be unlikely, I suppose, given that the corners of the building masonry would be interrupted by the end moment frame there.

 

[SteelPE]

I don't recall seeing any bracing for the truss bottom chord, hence why I am a bit worried about it. As I read the provisions of the IEBC I think you could justify leaving it as is. I understand about concentrating the loads, but if you take a 20' long beam loaded uniformly the worst loading condition for bending moment is a uniform load (vs concentrating point loads at given spots).

Can I justify the wall acting as a cantilever... no, but could you justify URM acting as a cantilever, I doubt that too. Certainly an 8" wall with #6 bars at 40" o.c. is better that what is there now?

Can I confirm what is happening with the end frame.... No, I can't, as everything is buried in the wall. When I started the job my main concern was getting the loads from the building into the end of the wall. Then I realized every frame line could be considered a LFR system.

 

[KootK]

Are these end-walls only 20' long with two overhead doors?

 

[r13]

Do you have the freedom to add masonry columns, that acting in conjunction with bond beams?

 

[bones206]

I had assumed the truss bottom chord was originally designed to take the lateral reaction at the top of the wall since the load would have been delivered through the wire ties. Maybe that was a bad assumption.

 

[SteelPE]

The wall is approximately 64' long (as are the trusses). The trusses are spaced at 12'-0" o.c. and are gable trusses. This building is 60+ years old, and is in pretty good shape considering all of the reinforcing that is being proposed.