Strengthening existing slab for punching shear
Strengthening existing slab for punching shear
(OP)
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?
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?






RE: Strengthening existing slab for punching shear
I suggest you look up seismic retrofit as the drift inthese tends to cause punching shear failure.
RE: Strengthening existing slab for punching shear
RE: Strengthening existing slab for punching shear
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.
RE: Strengthening existing slab for punching shear
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).
RE: Strengthening existing slab for punching shear
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.
RE: Strengthening existing slab for punching shear
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...
RE: Strengthening existing slab for punching shear
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
RE: Strengthening existing slab for punching shear
RE: Strengthening existing slab for punching shear
RE: Strengthening existing slab for punching shear
RE: Strengthening existing slab for punching shear
How will a topping help the shear capacity, even if is effectively bonded to the existing concrete?
RE: Strengthening existing slab for punching shear
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.
RE: Strengthening existing slab for punching shear
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.
RE: Strengthening existing slab for punching shear