Anchor Reinforcement

[EDub24]

Hi, question has popped up in the office regarding anchor reinforcement and the overstrength factor. When designing anchor reinforcement to restrain concrete breakout for seismic load combinations do you include the overstrength factor or just use the regular load combinations using rho? My thinking is that the overstrength factor is just for the anchor itself and anything below that (including the anchor reinforcement) uses the standard load combinations with rho as long as the reinforcement is properly developed on either side of the breakout cone. I'm interested in hearing people's opinion.

 

[TLHS]

Canadian code does it slightly differently, but basically my thinking has been that if you are using the reinforcing to replace a breakout mechanism or other concrete check you should design it to the same conceptual level of overstrength as you would for capacity or overstrength design of the rest of the anchorage system. That may mean replacing a concrete material factor with one more appropriate for steel, but the conceptual capacity should be there. Once you get into general section reinforcing that isn't part of the breakout mechanism, then you treat it like you would general member design.

Basically, try to match the intent of the code rather than whatever the specific wording may be.

 

[dreber]

In California, we want connections between steel and concrete under seismic load, to behave elastically, while the steel elements that are attached to the concrete are expected to behave ductily.

To that end, all elements of the "connection" are designed for overstrength. This includes the weld to the base plate, the base plate itself, the anchors to the concrete, the concrete, and any additional reinforcing that is relied upon to prevent concrete breakout.

 

[jittles]

Our approach is the same as dreber's - we treat the anchor reinforcement as part of the "connection" and design it for overstrength.

 

[EDub24]

Thanks for all opinions. I typically design for overstrength to be conservative but couldn't decide if it was actually required. I'll revise my opinion going forward.

 

[bones206]

From a building code perspective (ACI 318), anchor reinforcement is permitted to replace the concrete breakout capacity. So it follows that you would design the anchor reinforcement for the same loading that you would have used to evaluate the breakout capacity. The most common approach is to include an overstrength factor to the load, but there are other options available in the code where you wouldn't necessarily need to apply an overstrength factor to your anchor reinforcement design. It's a matter of consistently applying the chosen approach all the way through the connection.