Concrete Pilaster Design

[2meterman]

Hello,

I am designing a concrete foundation for a steel pre-engineered building.

The spread footings at each column are located approximately 5 feet below finished grade.

A concrete pilaster (or short column) will be cast on top of the spread footing and then the steel structure will be bolted to the anchor rods that extend out of the top of the concrete pilaster.

The spacing of the anchor rods is very tight in the base plate provided by the steel building manufacturer. I need to constrain the concrete cone failure plane with rebar ties so that I can develop all of the tensile capacity of the anchor rods into vertical deformed bar that will then carry the load into the footing.

The footing really acts as a shoe to prevent uplift since the governing loads on this building are uplift forces from wind.

My question is where to find the required tie spacing that I need to restrain the anchor rods and vertical deformed bar that will transfer the load from the pilaster into the footing.

I have looked in ACI 318 but have not found anything explaining min or max spacing of the ties.

Does anyone know the place to look in ACI 318 for this information? Or better yet, can anyone reference me to a design guide or example for the constraint of the vertical deformed rebar as well as the development length.

Thanks for your help!

 

[hokie66]

I won't try to answer your question directly, as I don't use the ACI code, but would just point out that your forces on the pedestals will not be just uplift, there will be horizontal thrust as well.

 

[jike]

ACI 318-02

Tie Spacing: 7.10.5.2

Tie Spacing at Top of Pier: 7.10.5.6

As hokie66 said, the PEMB column will also have shear, so you need to check the size of the pier to resist shear. Add ties (stirrups) if needed for add'l resistance).

Make sure that you have enough overlap on anchor bolts and vertical steel to transfer the uplift load from bolts to rebar.

 

[vandede427]

another tip, the min vert reinf in concrete pedastals/piers/etc in a foundation is 0.5%, not the 1% like a column.

 

[steve1]

Take a look at Concrete International magazine for December 2008. There is an article entitled, "Increasing Anchor Capacity with Reinforcement". I belive that this will answer your questions.

 

[Den32]

Also need to design the pedestal for Moments!

One moment from the column base of the PEMB

Plus second moment from the shear x pedestal height (in your case 5 feet above footing)

 

[DaveAtkins]

All of the above comments are valid,

But to address your original question, I make anchor bolts very long--long enough to lap with the vertical bars in the pier and develop those vertical bars. When considering uplift, I believe the pier will fail as a flat plane across the entire area of the pier, because this will be a smaller concrete area than a pyramidal or cone failure will produce. And so, if the vertical bars in the pier are developed, they can prevent the pier from pulling apart. When I am unable to develop the bars because the pier is too short, I hook the top of each vertical bar.

DaveAtkins

 

[BAretired]

Hokie mentioned a horizontal thrust. How are you handling that? I usually find it economical to use hairpin bars around the anchor bolts and into the concrete slab. Some folks don't like that detail, but being five feet below grade, the footing has to be pretty large to accommodate the thrust.

BA