Maximum Reinforcing - ShearWall Boundary

[slickdeals]

Folks,
I am having a building reviewer comment on a shear wall detail suggesting that the amount of reinforcing bars that I have at the end of a column exceeds the 0.08 Ag requirement for columns. He is suggesting that this is a requirement for placement of concrete and I am arguing that I am abiding by the requirements of Section 7.6 for spacing bars.

Have any of you had a similar experience? I believe he is taking the width of my boundary element and using it to compute the Ag.

Any comments?

 

[slickdeals]

I also noticed that in Section 14.4 in which walls are designed as compression members, it does not need to adhere to Section 10.9 which has the reinforcement limits.

 

[Lion06]

Do you need the boundary elements? Have you considered/tried getting it to work out with the distributed reinforcement along the length of the wall?

 

[slickdeals]

No, I can't get it to work with plain-old distributed reinforcing. I will need some good heavy reinforcing at the ends. And it is not seismic, I was using the term "boundary" to refer to my end steel.

 

[hokie66]

Consider constructability issues. I assume with that much reinforcement that this is a multi-storey building. The recommended maximum amount of reinforcement when you have to splice bars is 4%, which gives 8% through the splice area. If the design is controlled by compression, why not use higher strength concrete instead of so much reinforcement? Or maybe the wall has to be thicker or longer.

 

[slickdeals]

But what is the 4% based on? I was assuming that this is a column requirement to ensure that you don't have an over-reinforced column? For a wall, 4% Ag is a huge number.

 

[Ltdog]

I think 0.08Ag reinforcing is a huge number for the bundary of the wall

 

[hokie66]

slickdeals,
Yes, the 4% is a big number, based on columns. In your case, I assume it is a column or pilaster at the end of the wall built monolithically with the wall. It is nearly always more economical to use a larger column or higher strength concrete than so much reinforcement. But you were talking about 8%, I was arguing against so much.

 

[slickdeals]

The reinforcement I need is really from a tension-standpoint and not compression. I have about 6% reinforcement (strictly based on the the reinforcing area/boundary width*thickness). I am sure they build tons of tall buildings where you try to put all your steel at the end to maximize your lever arm. Just out of curiosity, why doesn't that limit apply to beams in case they are worried about congestion? Afterall, a wall is just a deep beam with flexure and axial load. I am following all spacing requirements per Section 7.6 of the ACI code.

I am not being argumentative here, but trying to see if there is any valid reason other than congestion mentioned in the commentary (which is specifically for columns)

 

[asixth]

Are you putting the reinforcement down one end of the wall to increase the lever arm and therefore the moment capacity. I agree that any reinforcing in a vertical element that exceeds 4% cross-sectional area (8% for laps) is a lot of reinforcement and therefore concrete placement becomes difficult. The bars won't be doing any work if the concrete can't be sufficiently compacted. By the sounds of it, you need to increase the wall length, width or have more walls that can take out the load.

 

[structuresguy]

Are you using 60ksi steel? If so, why not consider 75 ksi (or higher) rebar? That would alleviate some congestion.

 

[slickdeals]

No, its a shear wall. I have it at both ends. And to put things in perspective:
I have a 36' long wall, 24" thick and I have 30#11 at each end.

 

[structuresguy]

Can you make your boundary element longer (into the wall), to reduce the steel area ratio? I know your lever arm would decrease, but would you need to increase steel to compensate. Maybe you could give us a little more specifics as to the size of the boundery element, lenght of wall, number/size of bars, etc... I assume you are putting ties around the bars in the boundary, effectively forming an in-wall column.

I would agree with the others that 6% steel away from a splice is a lot of steel, and it could get very congested at the splices. Do you have any horizontal steel which intersects the shear wall in the splice zone? If so, can it fit correctly?

 

[KBVT]

Concrete placement into a column vs. a beam are two very different animals. Think about it, ensuring that the concrete in the beams can adequately flow around the rebar can usually be done with handheld vibration tools. In a column, you are usually talking about a 10-15 ft high form.

 

[ron9876]

I think the 8% is based on concrete placement and I think I read somewhere that larger amounts haven't been tested. I don't think that the column % applies to shearwall reinforcement (would be nice if the code addressed shearwalls since they are used so much). If you meet the placement requirements for clearance, etc. then you should be okay. For the most part it is a beam not a column.

I have used heavy steel in the ends of walls many times. I normally require staggered splices to help with concrete placement. For large buildings I think if the shearwall isn't maxed out at the bottom you are wasting money or creating too much interference with the floor plan because the requirements drop off quickly as you go up the building.

 

[slickdeals]

Yes, the commentary states that columns with as much as 17% have been tested with similar results (but a few caveats) and that the 8% is really a number of sake of economy and ease of placement.

 

[hokie66]

The splices are the problem. So If you stagger the splices, say half at each floor, you could probably make it work. The first lift would be the problem, where unless you stand full height starters, all the bars would have to be spliced at one level. You might also consider mechanical splices. They take up some room as well, but not as much as lapped splices. Bundling the bars would also give you more room.

This is also a good application for post-tensioning, but I don't know how that fits in with the rest of your building.

 

[Murali27]

We could use mechanical splices to improve buildability.
I have also come across with MMFX steel with 80 ksi strength. If possible, the size of boundary element could be adjusted to bring down the precentage of steel.

 

[KootK]

Hokie

I'm intrigued by your PT suggestion. Can you elaborate on the detailing at all? This is vertical PT? Would each strand have to go full height or could it be lapped somehow?

 

[hokie66]

Yes, vertical PT. You cast in corrugated circular ducts, then feed in the required strands just as you do in any bonded post-tensioning system. There might have to be a coupler at some stage depending on the height. Suggest contacting VSL for details. They know how to do it.