Concrete Bearing Wall Design

[GalileoG]

My very limited experience in concrete is showing, as I struggled to design a simple bearing wall today.

I have a bearing wall taking a line load at the top. This line load is imparted by the slab that the wall is supporting. My questions are:

1. Should I account for an eccentricity or can I assume that the load is imparted concentrically on the wall? If there is an eccentricity, what causes it? And how much should it be?

2. Should I take the wall as 1 meter strip and treat it as a column? If so, what is the procedure here as I have never seen or designed a column-type structural element with just one layer of longitudinal reinforcing (needed for tension on one side of the wall for the moment created by eccentricity)

3. I found an equation in CSA A23.3-04 code for a bearing wall, to the best of my recollection, the equation is: Pr = (2/3)(concrete compressive strength)(resistance factor of concrete)(alpha)(width)(1 - (effective length factor x unsupported height) / (32 x wall thickness)

I checked the code commentary but found no further reading on the above equation. When can one use this equation? It seems like this equation is for concentrically loaded walls as it does not account for moment. Assuming this is true, if the above equation gives me a resistance less than the applied load, do I just add a (phi steel)(area of steel)(yield stress) component for the compression steel to the equation until I get a Pr > Pf?

Thank you! I hope that made some sense?

Clansman

 

[engstructural]

The moment in the wall could be caused by the monolithic connection with the slab. Moment would be redistributed / transfered from the slab. If simple supported the load from the slab will act at 1/3 the bearing length giving a small eccentricity from the centre line.

 

[JAE]

We usually treat walls with moment and axial loads as columns - take a set strip width of wall and use the code requirements for columns....except we don't use the requirements for ties in the wall.

 

[ash060]

If you are using the ACI code you can use the empirical design formulas for walls. Because it sounds like you would meet the requirements for using it.

If not design as JAE suggests, but do not count on any steel to act as compression reinforcing or you will have to add ties.

The PCA Notes have a good example for a wall design using ACI chapter 10

 

[JAE]

Well, per ACI section 14.3.6 you can count on vertical reinforcement as compression reinforcement without ties as long as the steel area is less than 0.01 x Ag.