Masonry Jambs Lateral Ties Required?

[jeffhed]

I have a masonry building where the jambs on each side of some roll up doors are 32" wide and each cell is reinforced with (2) #5 bars to increase my "d" for both positive and negative wind pressures. I typically design this as a wall segment and do not require ties around the jamb reinforcing. The plan reviewer has stated that it is best practice to install lateral ties enclosing the vertical jamb reinforcing as if it was a column. I have looked through all of our textbooks here and the NCMA Teks as well as ACI 530 and can't find anywhere where it states that ties would be required. The jamb vertical steel has been neglected in determining allowable axial stresses and the jamb element has been designed using p-delta effects. My axial stresses are 50% of allowable and my combined stress check works but is close to 1. Are ties really required or is it considered best practice by the reviewing firm that may be considered overkill by many others? The only document I can find that addresses jamb reinforcing is an old document Army TM-5-809-3 (from 1992). Design example 3 on page 6-7 through 6-10 shows a jamb reinforcing design example. The design example shows (4) #6 bars vertical but does not require any lateral ties. I have talked to an engineer that works for a masonry manufacturer that says ties are not very typical. I have also talked to another plan reviewer I know and he said he has seen ties but they are not necessarily on every project. So I have decided to take a poll here. Does masonry jamb reinforcing require lateral ties for confinement?

 

[mike20793]

Is the jamb in a shear wall or just an exterior load bearing wall? I assume you have 12 inch block if you have 2 bars per cell.

 

[jeffhed]

Mike,
The jamb is in an exterior wall and is also at the end of a shear wall. I would love a thicker wall unit, but I am limited to an 8" wall. It is tight but it does satisfy section 1.16 of ACI 530 for minimum bar cover and spacing between bars. However, if ties are really required, it may be too tight. I haven't checked that part yet. Mostly now I am just trying to determine if ties are really required. Do you specify them for jamb reinforcing? I am in a high seismic area, so typically any jamb reinforcing is at the end of a shear wall.

 

[EngineeringEric]

Do you need shear reinforcement? I have used ties for shear in masonry pilasters and beams before.

 

[jeffhed]

EngineeringEric,
No. Shear stresses are 6.75 psi, very low.

 

[EngineeringEric]

So then here a question, would hoops do anything if your wall was single bar per cell? what if you used a single #10? if it adds nothing, then it is not needed. But make sure you have enough horizontal reinf. for general CMU Design

 

[jeffhed]

EngineeringEric,
That's a good idea. If I have single bars that are larger then what would the ties do, not much in my opinion. I don't feel that they do much anyway but since I have specified two bars in each cell then the reviewer wants them confined like in a column. These walls are special reinforced masonry shear walls since I am in a high seismic area, so there is plenty of reinforcing in the wall.

 

[jeffhed]

The wall won't work with a single layer of bars, it is too tall. So I am stuck with two layers of reinforcing due to being confined to an 8" wall.

 

[jayrod12]

Is there really room in an 8" wall for 2 bars? I'd be inclined to say no. 8" - 3" of total sidewall thickness, then you want some space for the grout to flow between the block and bar and between adjacent bars. Seems pretty tight.

 

[jeffhed]

jayrod12,
You are right, it is tight, but you can do it and we have done it for years in the end cells of some highly loaded shear walls. 7.625"-2*1.5"(face shells) = 4.625" core remaining. ACI 530-11 section 1.16.3.5 requires a minimum of 1/2" of clear distance between the reinforcing and the face shell we typically require 1". 4.625" - 2" = 2.625" left. 2.625"-2*(5/8" bars) = 1.375". clear between #5 bars. Section 1.16.3.2 requires a minimum of the lesser of 1/2*bar diameter or 1-1/2". We have 1.375" so we have enough plus 3/8" of wiggle room. However, if I add ties, that uses up the 3/8" of wiggle room. Technically we still have some wiggle room remaining where we used 1" clear between the face shell and the rebar which would amount to 1" of room either way, but we don't generally like to go that tight. I still have yet to get a real answer to my question which is do others typically specify ties on reinforced jamb areas? It's not required by code and the reviewer is telling me that it is considered good practice (even though he has reviewed almost every one of my other masonry projects and this comment has never come up before) so is it considered good practice by others or just the reviewer?

 

[hokie66]

Sometimes, and this might apply in your case, I have used ties to maintain the position of the vertical bars. The masons won't like it, but then they won't get the bars in the right place without the ties, and maybe not with them.

 

[azcats]

Back when I was designing CMU, 2#5 in a cell was very common to make the jambs work. Not once do I remember utilizing ties around vertical wall reinforcing in cases like this.

So no, I would not consider it standard practice from my point of view.

 

[jeffhed]

hokie66,
But structurally speaking you didn't need put in the ties for confinement of the vertical steel? It was just to be sure you got the bars in the right place. That may not be a bad idea here, but I am still concerned about filling up the core with steel any more than I have to to make sure I can get the cells grouted.

 

[jeffhed]

azcats,
That's what we have done for years as well (2) #5 bars in the end cells of the walls at next to openings. This is the first time I have struggled with openings like this before. That's what made me want to do some more research before I pushed back with the plan reviewer.

 

[RPMG]

This is not a column or pilaster, and does not need to meet code requirements specific to columns or pilasters. Would confinement of 4"x16" be confined? However, I believe that you require #4 horizontal bars at 48" oc (minimum), and that they to be hooked around your jamb steel.

Also, you can spec fine grout and you only need 1/4" cover.

 

[hokie66]

jeffhed,
That is correct, in my case.

 

[jeffhed]

I got my response from the plan reviewer today. This was his response:

"The masonry code is very vague on definitions of columns, piers and pedestals and in many cases a member will meet the height limits of one but the width/length ratio of another and it does not address what are the reinforcing requirements if this is the case.

In our office and what is commonly seen up here is:

Closed ties are always provided if the vertical reinforcing in the pier is used to increase the compression strength or if the bending places high stresses into the tension bars.

We always provide ties in jamb columns on exterior walls at large openings (overhead doors, long windows etc.) subjected to out of plane wind/Fp forces."

Up here is Salt Lake City, Utah which is a high seismic region, higher so than where I am at but I am still in a high seismic region. If it is common up there, I wonder if what I am used to seeing down here is just less conservative design. Maybe with the height of the walls and only using an 8" wall ties wouldn't be a terrible idea. If it is common up there, maybe I am better off complying with the plan reviewers comment. If it came to a law suit for something, it would likely involve an engineer from that area who would probably have the same idea as far as the ties aren't required but it is considered good practice. After looking at the numbers again, no matter how I change the reinforcing in the cells, my wind column is compression controlled because of the lintel reactions being fairly large. Maybe that is a good reason to provide ties as well. Would have been nice if the walls were a bit shorter or I could use 10" block.

 

[jeffhed]

Update:
I just got off a couple of phone calls with a design engineer with a masonry provider as well as a design engineer with The Masonry Society. Both stated that ties are not required for jamb reinforcing and in fact they both discouraged the use of ties due to the constriction of the core and the possible voids that could result. The Masonry Society engineer told me he would not consider adding ties to these elements as best practice because of the potential for voids. He said he has seen so many voids in grout due to steel congestion especially at jambs that he said you need to think twice and be very careful when designing the reinforcing in those locations. I'll stick to my guns and see what the reviewers response is.