Strengthening existing slab for punching shear

[asixth]

Hi guys,

I have an existing slab that is changing from a carpark usage to retail. I have done a few numbers for punching shear with the the higher design loads and believe that the structure needs to be strengthened.

Has anyone got some good reference material on how to strengthen the slab. I am thinking of either a bonded topping or shear collars. I have heard alot about both of these methods but have not been able to find any literature.

Also, what are the steps I need to take in order to ensure a bonding topping will work for me in punching shear?

 

[csd72]

I am not surprised that the punching shear is insufficient as this is usually the critical factor in these slabs.

I suggest you look up seismic retrofit as the drift inthese tends to cause punching shear failure.

 

[Ron]

I'm confused. You're going FROM a carpark usage TO a retail use and your slab loads are going up? Where is the load coming from? Are you picking up new lateral load and diaphragm?

 

[lkjh345]

Ron:

Seems counter intutive from everyday experience, but it is true.

Parking garages are 40 psf Live Load per IBC Table 1607.1, Item 17. (I think it used to be 50 psf in previous editions).

Retail Stores are either 100 psf (First Floor) or 75 psf (Upper Floors) Live Load per Table 1607.1 Item 38.

 

[kslee1000]

Enlarge, or add brackets to, the column directly under the slab, if no existing capital is present. (Sounds like the "shear collar" idea you mentioned.)

I have serious doubt on how effective the bonded topping would be. Because no matter what you do (roughen the surface, add rebars, bonding agent..), a horiz. plane weak in shear has formed (theoretically possible, looking for trouble in reality, my personal opinion only).

 

[InDepth]

There are several appoaches
1) Drill and dowel reinforcement into the column and the slab and create a drop cap / capitol (not a drop panel, unless you plan on reducing moments in the slab and thus midspan deflections). Think about a good concrete mix design that doesn't shrink excessively.

2) Create a steel collar that is doweled into the column. Grout the void created between the plate and the slab.

3) Pour underslab beams (greater difficulty and greater calcs on strain compatibility and preload)

4) Place steel beam between columns


Note: My experience is that you may also have deflection issues with the new loading....may want to check it.

 

[youngstructural]

kslee1000 is right about the separation of the cover slab... You need to be looking at adding to the underside, so that any existing material presses down onto new.

Collar or drop slab from below is the way to go. Sorry but I don't know of any literature; I just design these as if the old and new portions are separate, and have always been very confident that they will combine strengths as I am adding below.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[PEinVA]

We do a lot of shear collars, as we do a lot of renovation work.

The collars are designed to carry almost all the slab load because you won't really get any sharing unless the slab fails to some degree.

We typically use angles spanning between vertical stiffeners attached to a face plate and which has post-installed anchors to transfer shear to the column.

Then we design the angle to transfer the shear in a uniform line load between stiffeners as a beam.

Pretty straight forward, not sure I've ever seen any literature on it.

The

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[civilperson]

Topping will work for punching shear but the horizontal crack may effect beam shear. Roughen and add "moose milk" bonding liquid for topping addition.

 

[kslee1000]

One suggestion on construction support. Since the deflection due to selfweight of the slab is already there, and would stay there, provide shoring only required to take out the construction live load imposed on the slab. That would minimize the stress changes in the interface between the old-new structures.

 

[ali07]

Does your flexural reinforcement works for double the laoding.

 

[apsix]

For the design codes that I'm familiar with the punching shear capacity is based on the effective depth.

How will a topping help the shear capacity, even if is effectively bonded to the existing concrete?

 

[hokie66]

I agree with others that topping is not the answer. You need to work from below. Can you just increase the size of the columns? Maybe not if the space below is to remain a carpark.

RCraine's advice on design of collars is good. Take the whole load on the steel and its anchors, as there are unsurmountable compatibility issues in sharing the load.

 

[asixth]

Thanks, I am having trouble with flexural reinforcement as well so I was going to provide top steel in the new bonded layer to help with the moment over the supports and possibly look at using carbon fibre reinforcement to strengthing the slab for moment midspan.

Can somebody please explain the applications and use of collars to me. I have only heard about them but I do not know what they are and how to attach the to the slab.

 

[hokie66]

A collar (in my terminology, at least) is just a steel bracket or corbel, probably fabricated in two pieces and bolted together around the column. Depending on how accurately you can locate the column reinforcing, you may have to predrill all holes in the column and fabricate the brackets to suit. Attachments to the slab are nominal, but you have to ensure complete bearing between the slab and bracket.