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Maximum Reinforcing - ShearWall Boundary

Maximum Reinforcing - ShearWall Boundary

Maximum Reinforcing - ShearWall Boundary

(OP)
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?

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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.

RE: Maximum Reinforcing - ShearWall Boundary

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?

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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.

RE: Maximum Reinforcing - ShearWall Boundary

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.  

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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.

RE: Maximum Reinforcing - ShearWall Boundary

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

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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)

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

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

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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.

RE: Maximum Reinforcing - ShearWall Boundary

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?   

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
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.

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

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.



 

RE: Maximum Reinforcing - ShearWall Boundary

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?

RE: Maximum Reinforcing - ShearWall Boundary

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.

RE: Maximum Reinforcing - ShearWall Boundary

That sounds cool hokie.  Thanks for the extra info.

RE: Maximum Reinforcing - ShearWall Boundary

Post tensioned shear walls seam ludicris, your making the concrete work much harder all the time in order to relieve the tension from something thats only going to happen 5% of the time (number made up). I'd imagine that these PT walls are for extremely light buildings with huge wind loads.

How tall is your building slickdeals? and what is the tributary wind area on the walls? We're doing our first 15 story concrete building and I'm getting a 12" thick shear wall to work that's 54' deep x 154' high with a 150' wide trib- 40 psf wind load. This seems thin but we have nothing to check this against. The only thing I'm worried about is buckling but I did stiffen the ends with 3 foot flanges.

RE: Maximum Reinforcing - ShearWall Boundary

54' with 3' flanges seems excessive for a 15 story. Is it the only wall? That size building would normally use the elevator core plus stair towers (if needed). If buckling is a problem (check to see if it is a problem)you can add a column, maybe 16"x16", at the ends.

RE: Maximum Reinforcing - ShearWall Boundary

Well the building is 700 ft long x 54' deep. We have an expansion joint in the middle so each side of the building has 3 of these 54' deep shear walls. The middle shear wall is taking 140' x 154' area of wind.


 

RE: Maximum Reinforcing - ShearWall Boundary

(OP)
A 54' deep concrete wall is a monster. The behavior will mainly be in shear (deep beam behavior). It won't have much of a flexural behavior.

RE: Maximum Reinforcing - ShearWall Boundary

So these are coupled walls right? Will distribute based on relative stiffness assuming they are symmetric. Are you getting much stress in the walls? That is a total of 324' of wall. Seems excessive.

Again elevator and stair towers will typically work for that height of building. 700' is certainly long but there are probably several stair towers. A swag would be 3-12" walls with about 20"x20" elements at the ends that are about 20'-25' long for each side of the expansion joint.

RE: Maximum Reinforcing - ShearWall Boundary

I'm getting just about 0 tension at the ends of the walls for the wind load cases.

If I use the stairwells, then I would suspect huge overturning and tensile forces at the boundary elements. Here's a quick calc...

Height=154'
Tributary wind length =140'
Wind Load=40psf

Wind Resultant = 140'x154'x40psf=862kips
Overturning moment = 862kips x 154'/2 =66374 kf
(assuming wind resultant at mid height)

Now if I use a 54' wide wall the force couple is 66374/54' =1229 kips

Now if I use a 20' wall the force couple is 66374/20'=3318 kips

These are huge uplift and tensile forces

To counteract them I use 0.6 of the dead load:

7" NW concrete Slabs @ 15 stories and 27' trib on the shear wall gives (7/12)0.145x27'x54"x15stories=1849kips on the shear wall

Weight of shear wall= 54x154x1x0.145=1205k

Total Dead load at base = 1205+1849=3054kips

I'll use half of this load for each side of the shear wall = 3054/2=1527kips

Factor = 0.6D = 1527 x 0.6 =916kips


So  W+0.6D

= -1229 + 916 = -312 kips for 54 wall
= -3318 + 916 = -2402 kips for 20' wall (Thats a lot of piles)and a lot of reinforcing.

Am I doing something wrong?




 
 

RE: Maximum Reinforcing - ShearWall Boundary

Forty psf seems very high for a design wind pressure.  

 

BA

RE: Maximum Reinforcing - ShearWall Boundary

Have you allowed for the variation of the wind speed profile with height above the natural ground level. 40 psf would be achievable in hurricane/cyclonic areas.

RE: Maximum Reinforcing - ShearWall Boundary

Do you have a pair of shear walls at the expansion joint? If so, then do you have 4 walls for each 350' of the building?

Given symmetry and stiff diaphragms it seems like your effective tributary area could be more like 90' for each wall.
 

RE: Maximum Reinforcing - ShearWall Boundary

Once again, we are all guessing.  Provide a floor plan, even if it is only a rough sketch.

BA

RE: Maximum Reinforcing - ShearWall Boundary

40 psf was minimum requirement per this RFP.

In my model I accounted for a varying wind profile as f(height) but not for the simple calc.

For both sections, there are 3 shear walls each, 2 at the ends and 1 in the middle which has the largest trib. I'll post a plan when I get to work, I need to double check these loads, I'm going off of my boss's word that it's 40 psf wind load but even he can be wrong a lot. The seismic load is up there too which is controling the lateral in the long direction. Do you guys use shear walls to resist the long direction lateral? There seems like so much length of resistance in the slab that it's not even necessary to bother with the long direction.


 

RE: Maximum Reinforcing - ShearWall Boundary

Pilecaps will be designed based on P/A +- M/S not the simplified approach that was referenced.

Zero tension at the ends of the walls should tell you everything that you need to know about the amount of walls. Typically there is a problem with reinforcement congestion that has to be detailed away.

Lateral loads will distribute based on relative stiffness and placement not on tributary area.

Shearwalls are designed as columns with moment and axial load not the approach that was referenced. Some people also use P/A +- M/S with reinforcing resisting tension stresses.

I recommend that you do some research into designing this type of building before proceeding any further.

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