PT Concrete Slab Gravity Support @ Shaft Walls

[KootK]

I'm working on a post tensioned concrete building with concrete shaft shear walls. The column spacing is pretty regular throughout.

At two locations, however, there are concrete stair/elevator shafts that essentially take the place of columns (i.e. there is a shaft wall where a column would have landed in the absence of the shaft).

Based on my judgment and the advice of my peers, I intend to treat the shaft wall as gravity support for the adjascent slab. Essentially it will be a big, funny shaped column for gravity purposes.

I have the following questions regarding this arrangement:

1) Does it seem reasonable to use the shaft walls in this way?

2) For modelling purposes, should I assume that the column is located at the centroid of the shaft wall section? This is complicated somewhat by the fact that my shear walls are C & L shaped sections.

3) Are there any special detailing concerns that I should be aware of?

Thanks,

Adam

 

[AUCE98]

Adam,

Concrete shear walls are used for gravity support in the majority; if not all structures they are found in. So to answer your first question I would say yes. I assume that you are designing a 2-way PT slab, what type of analysis program are you using for the design? In programs like ADAPT-PT, you would design the slab in what is considered “design strips” in both directions. The program analyzes these strips using the equivalent frame method. It would be difficult to say how to model your support conditions without knowing the total geometry; however, assuming the total stiffness of a C or L shaped shear wall in the equivalent frame method is not common. Typically only a portion of the wall is taken as the support width. If you are analyzing your floor in a FE program such as RAM Concept, you should be able to create the wall geometry in the model. In either case, you should model the wall supports as they will behave, based on the analysis you are performing. I would read “Design Fundamentals of Post-Tensioned Concrete Floors” published by PTI. It will provide you with a lot of information on the design and detailing of PT floors.

Best of Luck,

AUCE98