Single Curtain Reinf. in 10" Shear Wall

[lutein]

ACI 318 allows the use of single layer wall reinforcing (single curtain) for walls less than or equal to 10" thick. anything thicker than 10" shall have (2) layers.

We did an inspection on a post-tension concrete slab on cast-in-place concrete shear walls structure. the concrete shear walls are the (2) stair cores at each end. they are 10" wall with single layer of reinforcing. we found diagonal cracks along the 2 faces of the shear wall cores from 1st to 5th floor consistently. these cracks were formed during construction on the 6th floor. no exterior envelope walls were constructed during that time, i.e. minimal wind.

In my opinion, these cracks are not due to wind given the heavy reinforcing in the walls and % of wind load the structure was seeing when the cracks formed. The structure is intended to be 16 stories tall, so at 6th floor construction with no envelope walls, the wall reinforcing, per our calculation, was way more than sufficient. Additionally, per our calculation, the overall design was in compliance to the code.

And that's what get us puzzled!

We suspect they are due to combined reasons:
1. the post-tension effect (precompression) of the slab, which caused forced displacement to the walls.
2. single layer of reinforcing which causes the wall to have 5" concrete until the stress hits the steel.
3. with early strength mix of PT slab, the wall strength may still be in 60% range, while the slab is being stressed.

Any input would be greatly appreciated. We are really puzzled by this phenomenon, and are wondering if any of you have seen this, that could share some insights.

Thanks.

 

[Teguci]

I'd say "rookie mistake", but I once did a survey of an old precast building with a similar condition and the building formed its own expansion joint right down the center by breaking the welded tee to tee joints and "walking" the inverted tee beam at the center.

Even if you allow(ed) for the PT shortening by using a pour joint, you still have to contend with the thermal and long term volume changes that will occur over the next 20 years. The shearwalls at the two ends will be having a "tug-of-war" over the slab (and center shear wall) and someone's going to lose. So far the two end walls have lost.

 

[csd72]

I agree with all the comments above about the posttensioning most likely being the root cause of the cracking though I think that the central reinforcement in a 10 inch wall is also a contributary factor.

Personally I would never design anything over 7 inches with only one layer of reinforcement not even a small precast panel let alone an insitu shear wall.

Just because the code allows it does not mean that it is right for every situation.

The problem is that the shrinkage stress of the concrete is over the whole width of the wall whereas the restraint from the reinforcement is only over the middle zone meaning that the cracks on the outer face are much wider than they would be with reinforcement at each face.

 

[ron9876]

I would say that there appear to be more walls than necessary in both directions. The core in the center will likely resist all lateral loads in the short dimension. The long walls as the ends will likely resist all lateeral loads in the long dimension plus account for any eccentricty. However when you pour the end walls of the stairs and the interior stair walls you have added too much stiffness in the short direction at the ends of the building. These walls are fighting against the PT and shrinkage and since the walls are lightly reinforced they are losing the battle.

When you add shearwalls at the end of a building like this they need to be able to deflect to allow the slab to shorten.

 

[hokie66]

ron,
The problem with that reasoning is that the central core does not affect the total slab shortening. Take out the central core, and provided the two end conditions are identical, the shortening affecting the end walls is the same. It is a difficult issue, caused by the overall floor plan design, and cannot be completely solved. Time is the friend of the structural engineer, but is the enemy of the builder.

 

[ron9876]

hokie66 it is shortening from the core to the endwalls. From a shortening point of view the center wall doesn't have an impact since the slab/layout looks the same on both sides (shortening to the center of the building with or without the center core wall).

When you restrain a slab out at the edge like that on a PT slab you are asking for problems. If they hadn't used the portion of the walls that is parallel with the long building dimension the walls would have been much more flexible laterally and they may have leaned over enough to relieve the stresses.

 

[lutein]

Found a very good article from attendance to a seminar by PTI. The speaker was Dr. Bijan O. Aalami, who is a very well known researcher in post tension slab. the article address the same crack pattern as described.

"Restraint Cracks and their mitigation in unbonded post-tensioned building structures"
Bijan A. Aalami, and Florian G. Barth

 

[hokie66]

ron,
We agree completely. The point I was trying to make is that there are many structures where there is no central core, due to site constraints and/or architectural design. Unless we can somehow educate architects about these issues, the problem won't go away.

In structures with cores at the extremities, PT slabs are often a poor choice of system. It is difficult to actually get the PT into the slab without cracking the walls. And then, the additional shortening due to the PT and the typical lack of slab crack control reinforcement in these systems makes them much more prone to objectionable cracking than conventionally reinforced systems.

All my experience is with bonded PT, and the problems would be much greater with unbonded.

 

[ron9876]

hokie66 we do agree. My point was that this particular problem probably could have been avoided with a different wall layout. You are certainly right about architects. Some often won't listen to logic.

Back to the original question I don't think ACI gives any guidance for reinforcement in a shearwall.

 

[Teguci]

For a P/T building especially, but all buildings should have a volume change calculation done. It is up to the structural engineer to inform the architect where he needs to place an expansion joint. Engineers have long known about the impact a P/T job has on volume change and have come up with numerous systems to account for it as follows:
- Delayed Pour Strip - a 3 to 5 ft wide strip of slab that is poured well after the P/T tensioning has been applied. Additionally, the tendons are pulled from the strip which reduces friction losses for the tendon vs a full length pull.
- Column sand pocket - Void form the base of the column with sand in order to create a hinge zone. Clean out and fill with concrete after tensioning. (I could see this being done for the walls as well).
- Column lengthening - drop the top of foundation elevation down in order to provide a more flexible column.
- Wall slip joints - Double koralath bearing pads (someone mentioned felt above?) and grout tubes for rebar where the slab meets the wall.
- Expansion joint - Double columns, double beams with a gap.
- Lateral bracing reconfiguration - In the above example, a posible fix would be to box the end stairs with columns/pilasters and infill with a nonstructural wall (in the length direction only) with a top slip joint and 1" side expansion joints

The critical locations where volume change is going to get you is foundation to 1st floor and last floor to roof (thermal concerns only).

In response to the post, I do not believe this is a single layer of rebar problem (item 2) or an early concrete strength problem (item 3 - this probably helped reduce cracking if anything) but a volume change problem (item 1)that would have occurred regardless of the rebar wall configuration.

For low seismic shearwalls, nothing stated hear would cause me to eliminate the option of using just one layer of rebar in a 10" or less thick wall.