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?

 

[EngineeringEric]

I have used channels for smaller openings. I think it is just a matter of what works... You have probably Checked if channels work since they are simpler, they didnt and you went to HSS. Your client is as most are, not an Engineer, they will balk at anything besides nothing (until your invoice arrives).

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?

 

[hokie66]

Jamb channels just inside the opening would just go along for the ride, without spanning up to the top. What goes in the opening? If it is just left open, the problem is lessened. The 5 ft of masonry about the opening would seem to be the main issue. If it will span horizontally, you would need only to address the jambs.

 

[chamokinawan]

@EngineeringEric I can use either C's or HSS's. I meant that what the client is used to seeing are C's on the underside of the lintel and the inside of the side walls (8" face of wall) as if they are shoring the wall. With this, I can't imagine it supporting out-of-plane forces.

@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.

 

[EngineeringEric]

I misunderstood the concern. I agree, a channel as a beam would not work. I have seen, but never done, two channels back to back sandwiching the CMU and through bolted through the system at a tight spacing. In concept i see it working, the details make me uneasy doing it however. Maybe this is what your client had in mind?

 

[Prestressed Guy]

I have used double 8" channels with the legs in that are placed into the grout joints to act as a lintel in a new opening for an existing CMU wall. The grout joints are cut back sufficiently to allow the legs of the channel to be fully embedded in the wall and then the channels are through-bolted to the wall with 8" extending past the opening. After the channels are installed, the wall below is removed.

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.

 

[RFreund]

It still seems the above responses are missing the point here...
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

http://www.HowToEngineer.com

 

[chamokinawan]

Thanks, guys.
@RFreund That is what I was trying to get at. I was not sure if I was missing something. Thanks for the reassurance.

 

[Prestressed Guy]

RFreund:
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.

 

[CBSE]

Haydenwse:

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?

 

[Prestressed Guy]

I use channels on both faces of the wall for the lintel and also for the jamb where needed not angles. Yes, you are correct, the contractor cuts a pair of grooves in the wall to except the channel flanges. The channel I try to use if possible is the MC8x8.5 because it has the narrowest flanges. Z for a pair is 13.9 which makes a pretty good beam. I once was on-site when a contractor was installing the lintels and he had a section of the channel about 6" long that he used as a gauge for cutting the kerf.

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.

 

[RFreund]

Haydenwse -> good information, thanks. Quick question -> what did you mean by "the holes in the channel are used as drill guides for drilling out the masonry"?
Also what sort of connection between the channel jambs and the lintel do you use?

Thanks!

EIT

http://www.HowToEngineer.com

 

[Prestressed Guy]

This is a two-fer! The channel is held in place with ½"ø through-bolts. I use two rows @16" o.c. staggered and have them match drilled to each channel. You put the channels in place on the wall nd use the holes as a drill guide to drill halfway through the wall. then you go to one side and run the bit all the way through to correct the minor error in hole location.

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. ;>)

 

[CBSE]

Haydenwse:

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.

 

[hokie66]

Haydenwse,

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.

 

[msquared48]

For a 10 foot opening in a 15 foot wall, I would expect a minimum of 4#5 jamb steel full height. As this is not present internally, it must be added externally - hence the HSS tube sections, or something similar, are required.


Mike McCann, PE, SE (WA)

 

[SAIL3]

as hokie66 pointed out, I also have doubts about the composite action....

 

[Prestressed Guy]

I am curious about the (4) #5 for jamb reinf. The OP said that the wall was solid grout with #4 @ 24 each way. If you have a 10' opening you are cutting 4 bars or 2 each side. So you are removing 0.6 sq.-in (counting the bar in the jamb which is replaced by the #5's). You are replacing that 0.6 sq.-in with 1.24 sq.-in. I am assuming that seismic controls so you have lower mass and lateral force with twice the reinf.

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.

 

[hokie66]

"A couple with the shear transfer done by the bolt strength through the CMU" sounds like composite action to me.

Cutting those slots for the channel legs all the way to the top of the wall sounds like unnecessary work.

 

[RFreund]

Hokkie, any suggestions? Just curious

EIT

http://www.HowToEngineer.com