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?

 

[KootK]

That sounds cool hokie. Thanks for the extra info.

 

[Stazz]

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.

 

[ron9876]

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.

 

[Stazz]

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.

 

[slickdeals]

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.

 

[ron9876]

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.

 

[Stazz]

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?

 

[BAretired]

Forty psf seems very high for a design wind pressure.



BA

 

[asixth]

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.

 

[JLNJ]

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.

 

[BAretired]

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

BA

 

[Stazz]

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.

 

[ron9876]

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.