Three-wyth, unreinforced, freestanding wall 1

[enginerding]

I have a client who is dealing with an existing mechanical enclosure without a roof constructed of three-wythe masonry. His client wants to punch 4' wide x 28" tall holes near the bottom of this wall for additional ventilation. Building is only 5 years old, but there are no drawings for this enclosure, and apparently ties have been cut between the owner and the architect...

One exploratory opening was made to determine the construction. Outer wythes are 4" utility size brick. Inner wythe is 4" CMU. No grout between wythes. No 4 dowels 16" long, 24" on center. 9 gage ladder reinforcement at 16" on center vertically. No other reinforcement.

Is there a way to reinforce and grout the collar joints after the fact? How much room do you need to grout? Is it possible to reinforce the 4" cmu?

Thanks,

 

[msquared48]

Why don't you propose that the hole be reinforced on each side with metal angles and thru-bolted with (6) 1/2" diameter A307 bolts. Could use angle 4 X 6 X 5/16, or not...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Not much joy in trying to reinforce an existing wall as you described it. You could install reinforcement, but it would require sawing slots for the reinforcement, then grouting the bars in. What are the plan dimensions of the enclosure? If not too large, it probably tries to span horizontally.

 

[concretemasonry]

How thick is the wall? How high is the walls (to determine the problem is flexure)? What are the length dimensions of the containment walls?

Is it a brick-CMU-brick wall configuration? If so, how wide are the areas for possible grouting? - 4" CMUs cannot the filled with grout from a practical standpoint the they are commonly filled with mortar if the dowels are somehow connected to the other vertical steel.

If it is what I imagine the voids between the brick and CMUs are not adequate for grouting according to codes, but there are alternate non-complying methods that will increase the lateral strength

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[dhengr]

Since this is a 4' wide by about 28" high ventilation opening, likely covered by a louver on the outside, why not make a structural steel frame which will be hidden on the outside, by the casing trim around the louver. This must act as a lintel, two jambs, and a base or bearing plate, bolted and grouted into the opening. Then the louver is installed and hides the steel from the outside.

 

[enginerding]

The wall is brick-cmu-brick. It is 11-1/2" wide, which gives 3/8" between wythes as I figure it. No vertical reinforcement besides the dowels going only 16" high. To be honest, I don't think this wall could meet code as it is being 12' tall with no roof. Then by reducing the moment capacity (resulting from gravity only) with two 4' wide holes, this is going to be a problem.

The dimensions of the enclosure are 20'4" x 30'-6".

I tried to check the wall spanning horizontally. The shorter wall worked this way if it was fully composite, but I cannot confirm that the void is packed with mortar or grout. The long wall does not check out even if it is fully composite. I didn't really get into checking with the wire joint reinforcement to help with that yet.

We floated the ideas of simply adding roof joists to restrain movement at the top of the wall (this is a 3-sided enclosure with the 4th side being a school building), but the owner didn't like that. At this point I might be running some sort of vertical steel (either internal or external) to add moment capacity to this cantilever wall...

 

[concretemasonry]

You have at worst 3 3-5/8" walls parallel that are 12' high. When you punch 2 48" wide holes in the wall near the base you are responsible for the future performance of the non-code wall after you sign the plans. Technically, the horizontal joint reinforcement does not transfer vertical loads between the wythes for composite action and the cost to verify if there is sufficient mortar between the wythes is not in the financial plans, there must be a compromise otherwise the risks outweigh benefits to continue.

Since the owner wants the holes and will not accept and open joist without a roof surface throw the concept of providing vertical reinforcement on the unexposed side, it does not sound like the owner realizes a roof surface over the joists is needed, since he is apparently interested in increasing circulation.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[hokie66]

Is the footing capable of resisting the lateral loading if you reinforce the wall as a cantilever? I like the approach of providing bracing for the top of the wall. This could be in the form of a lattice frame, or just girts, forming a box all around the enclosure.