Column base plates - ACI question 1

[cessna98j]

Greetings, my question is in regards to column base plates on grout leveling pads. We do a lot of industrial structures and have always used grout leveling pads underneath column base plates, typically 1-2" thick, on top of the concrete foundation (4000 psi typ.) I just noticed that when doing anchor bolt design using ACI 318-02, Appendix D, there is a 0.8 reduction factor (ACI D.6.1.3) to the shear strength of the anchors as calculated by ACI D.6.1.2, when using a built-up grout pad. There is no further commentary on this reduction, but I'm trying to find out what exactly defines a "built-up" grout pad. Could this be just a 1 or 2 inch leveling pad or do they perhaps mean something like a built up support of some kind where most of the anchor is embedded in grout. I just wanted to know how some of you interpret this reduction, or if any of you use it in conjunction with grout leveling pads.

 

[Taro]

I interpret the reduction to apply to any grout pad (the attachment is "built up" with grout so that it is not flush with the surface of the concrete).

 

[miecz]

PCA Notes on ACI 318-02 explains that the nominal shear strength values "must be reduced by 20% to account for the flexural stresses developedin the anchor if the grout pad fractures upon application of the shear load."

It would seem that a 20% modifier would apply to a 1-2 inch leveling pad. Put another way, 20% wouldn't cut it for, say, a 4 inch "fractured" pad.

 

[WillisV]

The 0.8 reduction in ACI is actually aimed at the 1 or 2 inch type leveling grout pad that you are referring to.

This factor is a misguided (IMHO) attempt to take into account the bending that can be introduced into the anchor rods in situations where the anchor rods are called upon to transmit shear from the baseplate to the foundation be reducing the shear capacity of the anchors.

A more technically rigorous method is outlined in AISC Design Guide 1 (2nd edit) to calculate the combined bending and shear stresses in the anchor rods. Note however that in general AISC recommends against using the anchor rods to transmit more than very small amounts of shear - preferring embedment or shear lugs.

 

[WillisV]

Also note that AISC specifically discounts the 0.8 factor when using the combined stress method in their design guide.