Reinforcing for High Shear Stresses in Walls

[sybie99]

Hi Folks

I have a situation where a wall (forming part of an intake tower on a dam project) has areas of very high shear as a result of massive hydrostatic pressures on the wall. The back of the wall is "propped" by slab sections at certain intervals, so the high shear occurs where the wall and slabs join. The wall is very thick (2.3m) but still localised shear stresses are as high as 3Mpa. Now according to British Design Codes this can be dealt with as long as it does not exceed 0.8*?fcu or 5MPa, whichever is less.

I attach a image from the analysis model where it can be seen where the high stresses occur. The openings are for pipes. It can be seen that the highest stresses are at the edges of the slabs at the back of the wall, which makes sense.

My question is, how would you reinforce this wall against shear? My thinking is to first calculate the wall shear resistance, vc, as per the formula in BS8110 using the reinforcement provided in the wall. Then to provide shear links as per the formulas given for slabs or beams and placing shear links in all areas where the shear stress exceeds vc.

What would you guys suggest? Please keep your suggestions simple if possible, I would like to find a basic way of dealing with the shear stresses. I am not used to deal with high shear in walls, usually shear comes into play in beams and punching shear on slabs, pads and pilecaps. It may therefore be beneficial to see the wall as a vertical slab and reinforce accordingly.

 

[BAretired]

I have never been involved in such massive concrete walls, so please do not consider this a particularly informed comment.

I believe you are correct in your proposed approach, i.e. adding shear links in the highly stressed zones. With walls 2.3m thick, there should be no difficulty in developing the shear reinforcement at the ends. Straight bars with hooks could be used as they are easier to place than stirrups. They would be similar to shear studs used in flat slabs around columns.

BA

 

[JedClampett]

Are you sure you can't resolve the stresses by calling them stress concentrations? How big is your mesh?
Also, depending on the direction of your loads, I would be much more worried about the attachment of the slab to the wall. If the applied forces are in the negative Y direction, the wall could tear away for the slab.

 

[msquared48]

Looks like a similar situation in Naval Engineering where a section of plate dies into another. That is usually handled with either a hole at the intersection to control crack propagation, or a bulb of material to flare out the stresses if watertightness must be maintained.

That being said, can you flare the intersection of the slab to the wall with a rounded rather than perpendicular intersection at the slab?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[normm]

sybie99

I can only agree with the suggestions made by others about increasing the contact area between floor and wall and providing horizontal bar reinforcements. However I will also draw your attention to the fact that critical section for shear design is effective depth away from the support, and not at the support. So for a wall 2.3 m deep, the design shear can be considerably less.

A wall this thick will be very stiff in bending and will resist applied load not just in bending but also in arch action. I do not now what type of elements you have used to model the wall, but if you use shell elements instead of plate elements, you will see load transfer by axial stresses also rather than purely by shear stress.

 

[csd72]

normm makes a good point, it would be worthwhile seeing if you could apply the strut and tie analogy to this area then you may just end end up with a few additional straight bars at the face.

Personally I would forget looking at it as a stress and look at modelling it by hand as a punching shear reaction to the total force applied in that area.

 

[sybie99]

Thanks Guys, the strut and tie method seems like a good idea, then again. I could also use shear enhancement, as described in 3.4.5.8 of BS8110 for an shear zone closer than 2d to the face of the support, this will make quite some difference.