New Opening in Existing CMU Wall (Out-of-plane Loads)

[chamokinawan]

I am putting a 10'x10' opening in an existing 8" CMU wall, fully grouted, #4 @ 24"O.C. each way. I usually use tube jams to one face of the wall around the opening, similar to wood jams. (1) horizontal spanning across the opening and (2) vertical (1) on each side spanning near full height of the wall (in this case 15'). All are bolted to the wall throughout. The client freaked out mentioning that he sees places with just channels on the inside of the opening on the (3) sides. I know this detail is fine for vertical loads, but I cannot see how the engineers got away with out-of-plane seismic loads. I can't imagine how having channels only on the inside of the opening will resist out-of-plane loads. Is my design out of the ordinary?

 

[Ingenuity]

FRP applied to URM for out-of-plane loads are somewhat common...should work for RM depending on the kips, and how the FRP is anchored at the terminations.

 

[hokie66]

RFreund,
Yes, I would use jambs outside the wall to carry the out of plane load up and down. If this is not acceptable, some additional demolition would be required to provide more bars.

 

[Prestressed Guy]

If the client is okay with the legs out, that is the simplest and strongest solution for the jambs. I always suggest that as the first option but haven't always had the best luck selling it. The channels cut into the lintel and jamb is the fall back.

FRP would also be a great way to go as long as you can find qualified contractors. Much of my work is in the boondocks and it is much easier to find a contractor with a hot-saw than one that is trained in FRP.

 

[CBSE]

Do you guys use this same method with new large openings in concrete walls?

 

[msquared48]

For large openings, I never rely on any composite action, but the strength of additional internal steel framing to the wall (usually not within the wall though), whatever it may be.

Mike McCann, PE, SE (WA)

 

[Prestressed Guy]

I have used double channels as described above, FRP, external HSS framing and on at least one occasion that I can remember I did nothing at all, just cut the opening. It all depends on the situation at hand. In the case where nothing was needed, the opening was a 16'x12' opening in a 14' wall. The jambs did not need any supplemental reinforcement for out of plane bending because the loads did not increase appreciable (interior wall). The header did not need to be reinforced because it wasn't needed to carry the opening. There was a full-width non-bearing concrete wall above that acted like a deep beam and had no difficulty spanning from corner to corner.

I could have added all kinds of reinforcement and no one would question it. Instead I did a complete analysis of the situation which showed that the only thing needed was a saw. My fee was considerably higher than it would have been if I had just slapped some steel on the jambs and header but it was more than offset to the owner by the savings in construction.