Unreinforced Pedestals?

[waytsh]

Two questions,

Would a pedestal with an aspect ratio of 0.5 and headed anchors that extend all the way to the bottom mat of the footing be considered unreinforced or could the anchors bolts be considered the longitudinal reinforcing?

Also, if the anchor bolts are failing for concrete breakout can shear friction between the pedestal and the steel base plate be considered in order to alleviate some of the load being applied to the bolts?

Thanks,

waytsh

 

[grandcasita]

I think it is best to consider anything less than ACI minimums to be unreinforced, since insufficient reinforcement can be worse than none at all.

How exactly would you evaluate the friction between the surfaces of a baseplate and concrete or grout with an indeterminate coefficient of friction? What is the nature of the failure, i.e., if the anchors are acting together (tied together by the baseplate) is the pedestal too narrow/small for the horizontal forces?

 

[waytsh]

I have seen other engineers use a coefficient of friction between the base plate and the concrete of 0.5. The short pedestal is fine, it is just the potential of having the anchor bolts breakout which concerns me. This may or may not happen because the shear plane for most of the bolts would actually cut down into the footing before it left the pedestal. Since this is in place I am trying to determine if any measures should be taken to reinforce the area around the pedestal. Although I'm not sure how exactly that would be done.

 

[twinnell]

Friction can be considered and the coefficient I have seen is 0.3. However, friction can not be considered if when resisting code earthquate loads (according to AISC).

 

[waytsh]

Good point about the earthquake loads. So would you agree then that this can be used to alleviate the horizontal thrust to the anchor bolts from DL+SL? (this is a rigid frame)

One other question. With this shallow unreinforced pedestal, would the anchor bolts be an acceptable form of reinforcement for shear friction between the pedestal and the footing if they had not been poured monolithically?

I would think they would serve the purpose of preventing the two pieces from separating as they slid across each other.

 

[twinnell]

Honestly, I can't say that I have ever heard of such a thing. Will the anchor rods provide some shear strength...yes. To analyze that could be difficult. Most modern articles don't even want you to take any shear in the anchor rod, but that is due to the fact that the base plate has to move some distance to engage the rod.

What are the pier dimensions? Loads?

 

[waytsh]

pier is 24" wide by 40" long by 12" high. It has (6) - 1.5" dia. anchor bolts with 6" spacing centered in it. The horizontal thrust is 61k with a corresponding vertical downward force of 118k. 3000 psi concrete. Anchor bolts are headed with an embedment of 44".

 

[jike]

If you want to use the anchor bolts to resist shear then make sure that the shear can be transfered out of the base plate into the bolts without slip in the oversized holes. A common method to do this is by providing a separate plate over each bolt with a standard hole and field welded to the base plate.

 

[waytsh]

I believe plate washers have been welded over the base plates. I guess the only outstanding question is whether the anchor bolts can be used to resist the shear at the pedestal/footing interface.