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?
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
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
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
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
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
I have a 36' long wall, 24" thick and I have 30#11 at each end.
RE: Maximum Reinforcing - ShearWall Boundary
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
RE: Maximum Reinforcing - ShearWall Boundary
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
RE: Maximum Reinforcing - ShearWall Boundary
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
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
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
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
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
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
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
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
BA
RE: Maximum Reinforcing - ShearWall Boundary
RE: Maximum Reinforcing - ShearWall Boundary
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
BA
RE: Maximum Reinforcing - ShearWall Boundary
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
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.