Overstrength Applicability

[lexeng18]

Hello all,

First post. I am working on a project now that is a structure in a high seismic zone. PEMB structure so rigid frames in one direction and rod x bracing in the other. The PEMB will sit on piers that are on shallow spread footings.

The question has come up on whether or not the anchor reinforcement (vertical bars & ties) needs to be designed with overstrength factor included. To me ACI makes it clear that yes, the anchors themselves need to be designed with overstrength, but makes no mention of if that requirement also extends to the anchor reinforcement.

The pier cage is quickly becoming convoluted when I include overstrength, and so I am looking for a way out.

 

[JoshPlumSE]

Yup, I've struggled with this same issue before. My belief is that yes, the anchorage force you design for needs to be the overstrength force. So, that includes the reinforcement you add in to resist the anchorage force.

But, anchorage has become a lot trickier these last 15 years or so.... Now I see people encase the whole bottom of the column in a grade beam. Weld some of the longitudinal reinforcement directly to the column.

 

[BridgeSmith]

If there is plastic hinging or ductile failure expected anywhere in the structure, all other components in the load path expected to perform elastically must be designed for overstrength to ensure that the hinge or ductile failure occurs where it is supposed to.

 

[CCox]

Where I live, wind is by far the higher force. Even still, I can't make 2' Sq. plinths work anymore with even moderate PEMB reactions. I have gone entirely to using U-shaped vertical bars because I have to transfer the tension loads from the anchor bolts into the vertical reinforcement. This actually works well. You just need to make sure the vertical bars are close to the anchor bolts.

 

[EDub24]

I consider anchor reinforcemet part of of the anchorage and would include the overstrength factor.