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






RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
The cost a beefy C8 is probably equal to the cost of many HSS8 tubes, so the material cost isn't a major factor. Whats the difference in weight of steel between the two?
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
@hokie66 I figured they would just move with the wall. The opening will just be for storefront doors. For smaller openings I figured you can get away with the C's on the inside. But my 10' openings with 5' above it, I can't figure another way to support out-of-plane forces.
Thanks for the responses.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
This is assuming that the typical wall reinforcement will provide sufficient strength for the vertical bending. I have found this to often be the case because most of the mass is removed at the opening.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
I believe the concern is bracing the wall for out of plane loads, right? Meaning in most framed openings you have full height jamb studs (aka king studs). It seems like when people have masonry openings, for some reason, they throw this concept out the window (no idea why). The lateral load is applied to the window or door and resisted by the edges of the masonry opening. Typically in new construction you have a masonry pier to resist the lateral from the lintel and sill. However for retrofit cases these piers do not exist. Either kickers at the header or a new frame which can transfer the lateral load to the roof/foundation will need to be installed. Unless you can show that some reasonable width of wall on each side of the opening can act as a jamb/pier and transfer the loads to the roof/foundation. What is reasonable? Well that is a good question. In any case for a large opening you would need to do something, so you're not crazy...
EIT
www.HowToEngineer.com
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
@RFreund That is what I was trying to get at. I was not sure if I was missing something. Thanks for the reassurance.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
I do not disagree with your assessment of the requirement for full height jamb reinforcement. It is absolutely required. I have just found that quite often the typical wall reinforcement, #4 @ 24" in this case, will provide the required strength. It is true that the opening will have wind load for the full tributary width but the seismic load will be much less than an equivalent width of solid wall because of the removed mass from the opening. In a 10' opening, you will be cutting 4 bars. That works out to 2 bars per jamb but with the reduced mass and likely over-strength in the original wall design you have a good shot.
You will need to determine the effective width of the jamb and see if it works. If it does not, then the jamb will need additional reinforcement full height of wall.
When the jamb needs reinforcing I do it the same way only with channels with legs let-in to the wall vertically. The lintel is then welded to the jambs.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Are you saying that you would have the contractor saw cut a vertical line from base of wall to top of wall and install your steel angle on each side to create the needed jam reinforcement?
I've often wondered about these applications and haven't really dealt with large opening applications...just really small stuff.
So for large openings, you may need to run a full height steel member on each side. If you aren't saw cutting a slot for an angle to penetrate into the wall, how are you folks doing this?
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
The jamb would be done in a similar way. After the channel is fitted into the wall, the holes in the channel are used as drill guides for drilling out the masonry.
The advantage of this system is that it allows all of the supplemental reinforcement to be installed before the opening is cut out. If you install the reinforcement after the opening is cut out you should really shore the masonry above the opening with needle beams.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Also what sort of connection between the channel jambs and the lintel do you use?
Thanks!
EIT
www.HowToEngineer.com
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
drilling the holes this way you get the best fit and tightest tolerance. it may not be 100% composite for out of plane bending but it is good enough IMHO. ;>)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Do you happen to have a detail you could post that shows both the jamb fix that goes from foundation to roof as well as the channel support beam? I'm really interested in seeing the jamb part. The channel is relatively simple in my mind, but I'm trying to wrap my mind around creating a new jamb out of steel that goes from foundation to roof.
Thanks.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
If you are using composite action of the channels (and I'm not as convinced as you that it works), why not just use plates for the jambs? You would avoid the vertical sawcuts.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Mike McCann, PE, SE (WA)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
It seems that this jamb is stronger than needed.
I use the channels so that I do not need to worry about compression buckling of the plate. With double channels, it isn't so much a composite design as a couple with the shear transfer done by the bolt through the CMU.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Cutting those slots for the channel legs all the way to the top of the wall sounds like unnecessary work.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
EIT
www.HowToEngineer.com
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
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.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
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.
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
Mike McCann, PE, SE (WA)
RE: New Opening in Existing CMU Wall (Out-of-plane Loads)
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.