overstrength and rebar IN the footing itself bolt attachment load vs rebar footing design

[Materofact]

for metal buildings for instance, it is common to use the overstrength factor to avoid ductility requirements, etc in the ACI. overstrength is applied in order to design the bolt size and depth (also, there is a common design method to "ignore concrete breakout in tension and/or shear" by providing steel(stirrups for instance)in the footing that will take the entirety of those overstrengthed loads). QUESTION:

is it right to assume that these steel stirrups IN the footing itself ALSO need to be designed withthe overstrength factor.. seems yes, to be "safe" but there is a discussion happening in the office.

 

[BridgeSmith]

Avoiding brittle failure scenarios by conservatively estimating the strength of the planned ductile failure points is the basis of overstrength provisions.

The overstrength factor is used to ensure members that are supposed to remain in the elastic range actually do, and that any plastic hinging, etc. occurs elsewhere in the structure. I can't address how common it is in building design, but we use it regularly to capacity-protect bridge substructure components, such as drilled shafts, so that we can be confident that a visible feature such as a column or pier cap fails first (in a ductile manner) in a large seismic event.

I would think that all components in a footing would need to be designed for the overstrength to ensure that a failure occurs elsewhere, so that it's 1) ductile and 2) visible.



Rod Smith, P.E., The artist formerly known as HotRod10

 

[HTURKAK]

The overstrength factor is applied for the elements which are expected to suffer less damage and will not fail during seismic event. Literally,if failure will occur, should be with the elements not designed with the overstrength factor .

The overstrength factor is applied for the following cases,

- (Section 12.3.3.3) Design of elements supporting discontinuous wall or frames, only to systems with horizontal
irregularity Type 4 of Table 12.3-1, or vertical irregularity Type 4 of Table 12.3-2;
- (Section 12.10.2.1) Design of collector elements in SDC C, D, E, or F
- (Section 12.2.5.2) Foundations of cantilever columns systems (Section 12.2.5.2).
For other cases , pls look ACI 318 and AISC material specifications and requirements for non building structures anchorage.


This is copy and paste of ACI 318 -14 section;
(17.5.2.9 Where anchor reinforcement is either developed in accordance with Chapter 25 on both sides of the breakout
surface, or encloses the anchor and is developed beyond the breakout surface, the design strength of the anchor
reinforcement shall be permitted to be used instead of the concrete breakout strength in determining ϕVn. A strength
reduction factor of 0.75 shall be used in the design of the anchor reinforcement )

 

[bobbyboucher]

It depends what you are designing, and what code, and on your engineering judgement.

If just a bolted connection (say designing for seismic anchorage using ASCE 7 chapter 13), then no need to follow overstrength through to the concrete elements. Overstrength is only required to ensure the bolt does not fail, but the supporting structure can be designed elastically.

If it is a connection from a lateral force resisting system, then see HTURKAK's response regarding where ASCE 7 requires elements to be designed for overstrength.

Conceptually, one might say that the foundation elements should be designed for overstrength since they are the last element of the lateral force resisting system, and are not supposed to go inelastic, and should therefore be designed for EQ loads including overstrength. I tend to agree with this approach. It's also something that OSHPD in the CBC 1616A.1.16 covers as an modification to ASCE 7, although is not necessarily required by other codes.