Concrete columns Supporting PT slab drop cap retrofit

[jeffhed]

I was hired to perform a structural evaluation of the existing construction on a 6 building condo development. the development is a subgrade garage with concrete columns and PT slab above supporting up to two stories of wood framed construction. Originally it was only the distress report, but as my investigation revealed considerable construction defects, the owner then asked me to perform structural calculations on the existing structures to verify they were engineered correctly. Everything was OK, until I got to investigating the punching shear capacity of the PT slab at the columns. I hadn't noticed it before, but the concrete columns supporting the PT slab were only 12" square....with NO drop caps. As a result the punching shear force is about 35% higher than the punching shear capacity. Now my question, I need to attach a drop cap to the concrete column, but am unsure of the best way. With the connection force being around 70k on some columns, I don't think I can attach the retrofitted drop cap to the concrete column without a million bolts. I was thinking of fabricating bent plates that would sandwich around the concrete columns and then welding them together, thus providing a direct path to the foundation. I would like to hear if anyone else has any better/more cost effective ideas. I was looking into FRP, but couldn't find any examples of FRP and attaching drop caps on the internet.

 

[hokie66]

Make the columns bigger if you can.

 

[jeffhed]

Hokie66, thay was also an option we considered. However, the columns currently stick out 6" into the parking spaces, adding an extra 6" - 9" to the sides of the column (which is about what I have calculated I will need depending on the column) would ruin the parking space. We can't lose any parking spaces because there are currently two spaces per unit without any extras.

 

[rowingengineer]

couldn't you add the new dimension along the length of the car park so new column would be 12"x36"?

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[hokie66]

RE's solution is what I meant. Having recently reviewed another engineer's proposal for solving a punching shear issue, I am convinced that making the columns larger is the best approach. Installing steel brackets is cumbersome, involves a lot of head scratching about consistent deflections, and is likely impossible for such a small existing column.

 

[jeffhed]

hokie66,
I misunderstood your first post and it has been clarified by rowingengineer's post. In the review you recently did, how was the new column portion connected to the exiting column? I am somewhere in the order of 70k deficiency on some columns, seems like i would still be stuck with a lot of dowels to tie them together.

 

[dcarr82775]

Are the PT slabs supporting parking or something else? Not sure what the floor area tributary to the column is, but LL reduction could get you some of the way.

Do you have enough headroom to retrofit on a capital? Anything on a 12" column is going to be tough

 

[jeffhed]

dcarr82775,
The PT slabs are supporting wood framed living space above, some 2 story some single story. Live load reduction has helped out some. i was even able to reduce the roof live load a little bit. There is enough room to retrofit a capital or drop cap, which is what I was hoping to get some ideas on here. The 12" columns do make it tough, which is one reason why we were looking at surrounding the concrete columns with steel columns and providing steel cap and base plates to increase the punching shear.

 

[hokie66]

jeffhed,
I don't think the connection of the new adjacent columns to the existing needs to be cumbersome. Some nominal dowels...or even a gap between them. As long as the new column can take its share of the load and the total shear perimeter is increased as required, I would be comfortable with this solution. You could use steel columns, but fire rating issues may make the concrete more attractive. Either way, non-shrink grout should be used to ensure complete bearing.