Masonry Wall Design Question

[INBCPE]

I have a 14 foot 4" concrete panel curtain wall system on a single story building for which the owner wants to increase one exposure's fire rating by 4 hours (BOCA requires a heaftier rating due to the building's proximity to another building on his property). He didn't bite on UL Des 490, so I think masonry, 8" block, on the interior side of the panels.

I can build him a wall with a 4 hour rating, but I'm questioning the idea of putting it right up against the panel wall system. Since the panels are supported by steel members, can ties be used between the two walls without overloading the steel? The vertical load of the new wall will be carried by the foundation. I was thinking of making the new wall self supporting with no ties to the curtain wall, almost like a partition, but with a 1" gap between the walls.

I'm used to basic footing, slab, floor, and load-bearing wall design. This one is throwing me a little. You make a partition wall out of 8" block in the middle of this building and you wouldn't blink. Put it up against a panel system curtain wall and suddenly the horn sounds off.

Any help or direction would be appreciated.

 

[JAE]

With the vertical load carried by a separate foundation, the new CMU wall (I presume its placed exterior to the existing concrete panels) could conceivably be tied back into the existing panels - let's see what issues come up:

1. Wind load should not be an issue as the existing connections between the conc. panels and the steel frame were designed to take the original wind load and by adding the new wall, you are not increasing that load so you should be OK (assuming that the original designer did it properly).

2. The CMU will be on new footings and be "young" block which means some shrinkage and vertical drop in the wall over time. The ties should be designed to slip vertically so as to avoid placing any stress on the panels themselves.

3. If you are in a seismic zone, the issue in item 1 above may be a problem as now you are adding extra mass with the load path thru the original connections that may not meet current seismic requirements on connection design. One solution would be to drill open holes through the existing concrete and extend rod connections from the new CMU, through the concrete panels, and directly into the steel frame. The holes could be drilled large enough to allow differentical movement yet still support the wall laterally. If the distance gets too great, rods may not work (for the compression case) and you would use a stiffer element but the idea remains.

 

[INBCPE]

JAE,

The CMU wall is on the interior of the panel. Wind loading should not be an issue.

Good point about shrinkage.

Being on the inside, I could tie the wall directly to the steel via rod, no? And as long as my tie-rods support the seismic moment (which is pretty low), I should be Ok, correct? So in lieu of the tie-backs to the panels, I can go to the steel columns.

Another question: since my wall weight is less than the design load of the slab, can't I lay the block on the slab?

Thanks.

 

[JAE]

Ooops! Didn't see your word "interior" above.

If the new wall is on the inside, then I would agree you could take it up to the structure above - and somehow create a connection that would laterally brace the top of the masonry along its length. If that is some kind of steel brackets, rods, angles, whatever, so long as you are spacing them at a distance small enough not to overload the existing steel.

But for fire protection, if your new wall on the inside extends up to and under the existing steel frame, don't you have that section along the beam where the fire rating is still the same?

As far as the slab goes, the wall would be a line load of higher concentration than the uniform design load in psf. You need to check the concentrated line load to verify that you aren't overstressing the slab - but yes, I would think generally it would work.

 

[INBCPE]

JAE,

Thanks for responding. I appreciate the direction immensely.

My wall would extend up to the underside of the roof (13'2" elev), and the beam running parallel to my wall would be protected. I would grout the gap at the ceiling per the appropriate UL standard.

Along that line of thought, though, I would have two existing beams extending perpendicular through my new wall into the open space of the building. I can't see that being a problem, because most of UL design cases have similar scenarios. Just a matter of convincing the building code official.

I've also rethought this whole design. I want to do CMUs for only the first 10 feet vertically. At that point, I can switch to UL Des 490 for the rest of the run up to the underside of the ceiling. This will save me the expense of putting a huge lintel up over the roll up door.