Bearing stress on concrete wall-slab junction 1

[hetgen]

Hi All,

Please see attached detail for Wall-Slab junction. The slab is supported on 40mm [~1.6"] bearing width which is equal to the thickness of the wall vertical reinforcement cover.

1. When calculating the bearing stress on on the support is it right to included the stress from the wall in addition to the stress from the slab reaction? see load-path on pic.

2. Considering the edge distances of the bearing area how do we evaluate the reduction of bearing capacity due to diagonal tensions as shown on the picture?

Thanks..

 

[BAretired]

Is the slab cast-in-place or precast?

BA

 

[hetgen]

Hi BA,

Yes, the slab is cast in place.

 

[BAretired]

1. Load from the wall above the slab will not contribute to the bearing stress as the fresh concrete will shrink, leaving a small gap between it and the wall.

2. If there is a reasonable amount of reinforcement running through the wall connecting the slabs, I would treat it the same as a monolithic pour. I would have been more concerned if the slabs had been precast with no ties across the wall.

BA

 

[hetgen]

Thanks for your though BA,

- This detail is to be used on a multi-story building, do you think axial shorting of the shear wall will be able to close the shrinkage gap on the lower floor and engage the load path in question?
- Yes there will be enough top and bottom slab reinforcement running through sleeves on the wall to achieve flexural continuity, but the shear has to be transferred through the bearing width as there is no aggregate interlock and only uncertain dowel action.

Sorry for not providing the added information on my original post.

 

[BAretired]

If the detail works like a monolithic pour which I think it will, the compressive stress in the wall just below the lowest slab will be nearly uniform across the wall thickness. With reinforcement extending through the wall as you describe, shear between slab and wall will be carried partially by bearing and partially by friction of the compression block of the slab against the vertical face of the wall.

BA

 

[asixth]

I wouldn't rely on the 40mm bearing width alone. I think you need reinforcement at the wall-slab junction to transfer the shear. Will also need this if you are relying on the wall as a shear wall to transfer the diaphram shear into the wall.

 

[hetgen]

Hi Asixth, the sleeves crossing the wall are to be larger than the slab reo diameter and it will be hard to justify the dowel action of the slab reo crossing the wall.

The building is located in non-seismic area; the lateral diaphragm force that we dealing with is only wind and can be transferred by friction on the bearing area.

Its expensive for us to provide threaded inserts with threaded slab reo as you indicated on this this thread

http://www.eng-tips.com/viewthread.cfm?qid=356331.

Are there any other method that we can employ?

Thanks a million.

 

[asixth]

The way I do it when a climbing wall form like a jump form is used to build walls ahead of the floor slabs is

1. Threaded inserts with rebate
2. Pullout bars with proprietary cast in rebate

Other than that it would conventionally forming and placing the walls followed by the slabs as you go up. As long as there is some anchored rebar across the interface it should work.

 

[BAretired]

I wonder if it would be practical to grout the holes before casting the slab.

BA

 

[hokie66]

I concur with asixth. Sleeves are not the way to go. While ferrules and threaded bars or proprietary reboxes may be considered expensive, they are a very minor cost to the project as a whole.

 

[BAretired]

You could use the conventional method of pouring one story of wall at a time and placing the slab continuous before the next wall pour.

BA

 

[hetgen]

Hi BA,

Beside further investigating the cost of threaded inserts as hokie noted above we are looking at using a cone shaped sleeves for each bar and pour the concrete on one side to minimize air-entrapment and add a small diameter [10mm] pull-out bars fully developed across the wall for easy of re-bending on-site.

If both options fail we will resort back to conventional method.

 

[hokie66]

In today's world, the 'conventional' method in high rise buildings is not one floor at a time. The cores are invariably advanced ahead of the floors.

 

[BAretired]

Another idea is to cast the walls ahead of the floors, leaving large enough openings for reinforcement so that concrete can be placed and compacted properly through the openings and around the steel.

BA

 

[a2mfk]

To your original question, see the attached textbook page, left side diagram. The concept is the same for the diagonal shear/tension stress in the wall in your situation. I would detail rebar along that failure plane parallel to the wall, unless you think the horizontal bars going through the wall will take care of this.

 

[hetgen]

Thank a2mfk.

Could you please post the name of the book that you referring from.