Masonry Wall Design for Out of Plane Loads at Openings 2

[SteelPE]

I have been asked to seriously consider designing a building with masonry bearing walls. A sample elevation is attached.

The required code is IBC 2015 with ACI 530-13. I am wondering how to design the piers in-between the windows. Without windows my wall has an ultimate moment of 3,038 +/- ft-lbs/ft at mid height. I have an ultimate moment of approximately 2,700 ft-lbs/ft at the head of the opening.

I am leaning towards designing the masonry above the opening to span between the head of the opening and the roof support. I would then have a bond beam at the head of the opening to put a point load on my pier (see attached diagram). I would then design the pier to design these loads (including any gravity loads calculated the same way).

This seems like a logical analysis, but I would just like the opinions of others to see how they would approach the design.

 

[EZBuilding]

I agree with your approach

 

[EZBuilding]

Hopefully you are considering at least a 12" masonry wall.

 

[SteelPE]

At least 12"??? Yes, I am actually considering using 2-#6 bars in each cell adjacent to the openings (3.5" in from each face). These bars would be continuous up to the roof. I have done some preliminary design with the NCMA software and it appears as if it will work (surprisingly).

 

[JLNJ]

I'm not sure I understand the approach.

The wall spans from floor to roof, right? It's just that a portion of the wall doesn't exist where there are windows, correct?

At the windows, you are seeing 8' of wind tributary width, but you only have 4' of wall to resist it. Above the windows, you have the full eight feet or trib and pier width. At the window lintel height you have a bond beam of sorts to distribute out the horizontal and vertical loading.

I think I would design the 4' pier between the windows to span full height and just place nominal reinforcing in the wall between window piers above the windows. This way you avoid the question of how the forces lap from the pier reinforcement to the wall reinforcement. If its a 12" wall, the pier reinforcing shouldn't be too crazy. I think the architect was quite kind to give you a 48" pier to work with. I hope he/she isn't trying to talk you into an 8" thick wall.

 

[KootK]

Nothing new but semi-official: Link

 

[SteelPE]

At the windows, you are seeing 8' of wind tributary width, but you only have 4' of wall to resist it. Above the windows, you have the full eight feet or trib and pier width. At the window lintel height you have a bond beam of sorts to distribute out the horizontal and vertical loading.

I think I would design the 4' pier between the windows to span full height and just place nominal reinforcing in the wall between window piers above the windows. This way you avoid the question of how the forces lap from the pier reinforcement to the wall reinforcement. If its a 12" wall, the pier reinforcing shouldn't be too crazy. I think the architect was quite kind to give you a 48" pier to work with. I hope he/she isn't trying to talk you into an 8" thick wall.

I think we are pretty much saying the same thing. At the window heads I plan on using the typical reinforcing for the wall. This will allow the wall to span from the head of the opening to the roof. The load from the window head will then be taken horizontally over to the two piers by the bond beam at the opening head. Then the pier will resist the loads from the opening (1080#) and the loads from the pier itself (108#/ft length).

I might just place bond beams at 10'-0" o.c. and then design the 4'-0" pier for a tributary width of 8'-0". This way I can more accurately capture the deflection of the pier and check it against the 0.007H requirement of ACI 530.

In regards to the wall thickness. No, they are not trying to convince me of using an 8" CMU wall. The project was originally suppose to be 8" precast. Now, the GC would like to make a change to CMU. Now that we are switching to CMU we are looking at using 12" (which I am still not happy about because placement of reinforcing is key). Sad part is shop drawing review of the foundation reinforcing and structural steel was completed two weeks ago. Now we have to start all over.

 

[oldrunner]

KootK pretty much provided you with how to do this. I know it was only 5 seconds between responses. Check other references on the Masonry Chronicles website:

http://cmacn.org/masonry-chronicles.htm

 

[SteelPE]

KootK,

That's a pretty good reference. I have my reservations about designing a 30' high 12" thick masonry wall with an ultimate wind load of 27psf, however, the design example shows a 12" was with an out of plane seismic load of 72psf and a height of 29 feet. I guess my reservations are unfounded. The tallest 12" masonry wall I have ever attempted was 29 feet with a similar 27 psf out of plane load. My design ended up looking similar to what was shown in the reference you provided, so I suppose I feel better about it now.

 

[jayrod12]

I don't know about where you guys are, but even at 12" block, it's a bit of a crapshoot regarding reinforcement placement in the blocks. They will put in what reinforcement we say, however actually expecting anything other than approximately centrally located reinforcing is a pipe dream.

They build a wall to the bond beam height and then drop in full lengths of reinforcing. How do we know, how can we ensure the reinforcing is set closer to the outside face?

We have had some contractors make their grout inspection holes large enough at the bottom to manipulate the bar, but even at that, you're hoping.

 

[XR250]

I don't know about where you guys are, but even at 12" block, it's a bit of a crapshoot regarding reinforcement placement in the blocks

I always call out bar positioners in tall walls - probably a pipe dream as well.

 

[SteelPE]

I don't know about where you guys are, but even at 12" block, it's a bit of a crapshoot regarding reinforcement placement in the blocks. They will put in what reinforcement we say, however actually expecting anything other than approximately centrally located reinforcing is a pipe dream.

I agree. I expressed my warning to the architect, GC and the owner. I also expressed my displeasure with regards to masons as well. The GC is pushing for this and the owner is listening to them, so if it's possible to design it then we need to do it.

To combat the issues, I was thinking about providing a little more reinforcing in the jambs than necessary. The would help with regards to slight misplacement.

 

[phamENG]

If placement is critical, you could go as far as specifying open ended block. Mason places the bars and ties them off/builds a quick wood frame to support them, and then builds the wall. When they come to a reinforced cell, they use an open ended block so the can pass the bar through the side of the block instead of having to pick the block up 12 feet and lower it down. You can use bar positioners as frequently as you feel they are warranted.

Couple that with a robust inspection program and I think you can get some pretty good reliability.

 

[dhengr]

SteelPE:
I generally agree with JLNJ’s design approach (3FEB, 22:31) to this wall problem, it’s pretty clean and consistent for the masons to carry the piers up to the roof. One possible exception, I might try to get back to a std. more uniform reinforcing arrangement by about mid-height or the elev. of the bot. of the upper tier of PC-2 face material, that’s a good, and obvious change elev. You might want to see a more uniform vert. rebar spacing at the max. moment region. I’d have to think about how to do this a little more, and the primary reason would be to make life a little easier for the masons to do it right on the upper portion of the bldg. I would want the bond bm./lintel at the heads of the window openings to protect the opening corners, and to start to distribute the pier load back into the whole wall width. We know that will try to happen once you get a couple ft. above the openings anyway. Pay attention that you don’t kinda make a hinge line in the region just above the window openings. We used to make widow and door jambs a little huskier by alternating 8” and 16” lintel blocks laid on their sides for a more open core for rebar and grout. At the 8” conc. blk. we needed a blk. with one end shell removed. So, I agree with PhamENG’s 4FEB, 16:59 post too.

That’s what the GC wanted, so keep em honest by detailing it to get what you want, and inspect it seriously from the outset, to see that they are doing what you want. It seems that they keep telling us what they want, and how to do it, and we should stand behind whatever they want, stick our necks out a mile, and them live with their crappy workmanship too.

 

[SteelPE]

We have proposed back to the GC to lower the wall height slightly.

Currently top of foundation is 8" below top of slab. We are proposing to mover the top of foundation to top of slab.
We are also proposing lowering the back side of the building by 14". We can do this by adding another set of interior drains along the first interior bearing line from the back vs the second bearing line.

If we can get this though we can get the bearing height down to 29'-3" (It was above 31' along the back of the building). I have been advised that these items will be rejected, to which I told them that I would like to be released from the project if they are. This leaves me in a precarious position in terms of being paid for the work I have already complete (design done, shop drawing review done) but I would much rather lose out financially then to lose sleep at night.

Thanks to this forum I have confidence that I can design it right, I am just not comfortable with the execution. I live an hour from the site and can't spend too much time onsite.

 

[dhengr]

SteelPE:
Did your original contract say you had to design it and check shop drwgs., etc. several different ways before expecting to be paid for your original efforts? It seems to me that you should be paid for the complete design effort you have already done. And, since the GC wants the change, apparently because he doesn’t like the final price for the precast wall panels and their erection, he can share those cost savings by paying you for the second design effort, which he now wants. And, the Arch. and owner should back you up on this. A third party special inspector is part of the cost of this cheaper alternative. It seems all too common that the builder/GC b.s’s. the owner and Arch. about all he can do and gives a lowball price. Then, when things start to go south, it’s all the damn engineer’s fault for the actual cost. He could do it much better and much cheaper if the engineer didn’t bother to follow the codes and insist on a good sound engineered design and construction.

 

[KootK]

I have been advised that these items will be rejected, to which I told them that I would like to be released from the project if they are.

You'd take such an extreme step over a 5% difference in wall height? I'd personally be quite comfortable with all this done in 12" block.

 

[JLNJ]

12” block. No worries.

One argument against the full height pier is that you will probably want to distribute out the reaction out along the wall length at the top. This is especially true where wind suction is the controlling force and the wall (spanning parallel to the trusses) is connected to the roof only by deck welds and an expansion-bolted ledger angle. Those expansion bolts suffer horribly from prying of the ledger. You will want all the bolts and welds you can get working together.

If this is a gym at a school, also look for the architect’s use of “Soundblox”. (Slotted and chambered block). Its geometry will reduce the available space for rebar placement.

 

[bones206]

If you are worried about rebar placement, wouldn't that be where special inspections come into play?

btw not to go off on a tangent, but I seriously dislike soundblox. Maybe I should start the "I HATE soundblox!" thread... I wish architects would just stick with regular block and hang up some acoustic panels. So much easier (for me).

 

[IceNine]

Agree with those above. 30ft+ is common for 12" CMU in my area. We always specify rebar positioners be used, and have had good luck with rebar placement.

https://wirebond.com/category/rebar-positioners