Contribution of reinforcement in concrete breakout strength. 1

[FrankUAE]

Are there any guidelines in ACI for calculating the tension resistance provided by the reinforcement in a foundation?
I have a group of anchors located closer than 1.5hef from all four edges of my foundation plinth, as such, the concrete cone cannot develop fully, and the embedment depth has to be reduced. This will result in a smaller concrete breakout area and a lower concrete breakout strength.
Can I calculate the tensile forces (by using T = AxFy)provided by the plinth reinforcement and add this to the concrete breakout strength to increase the capacity? Is there guidance on this in ACI 318?

 

[CDLD]

Refer to Pip STE05121 for a guide on designing anchor reinforcement according to ACI.

If breakout strength is insufficient you can rely on the pedestal bars by effectively “lapping” them with the anchor bolts. You need to provide sufficient development length above and below the breakout cone to preclude a breakout.

 

[AskTooMuch]

Of course, otherwise, most of the concrete pedestal supporting steel column would fail because they're just an inch or so bigger than the base plate.

A good guidance would be Design of Anchorage for Petrochemical Facilities.

 

[icebloom]

In the ACI anchorage to concrete is in section 17. Reinforcement can be relied upon to preclude the breakout in tension provided it is spaced close enough to the anchors (code has guidance on placement of the rebar). The reinforcement must be developed adequately either side of the assumed failure cone to sustain the tensile force. The concrete capacity and reinforcement capacity is not additive - you either rely on concrete cone capacity or reinforcement capacity alone. If your cone is small due to small edge distances the best option is to increase your anchor embedment depth, which gives you more flexibility in providing reinforcement to take the tension. This typically will be in the form of U bars - the ACI has diagrams showing this. If you can't fit U bars then hooked bars are your best alternative, or some kind of reinforcement terminators maybe. If you have got any shear forces then you need to also consider the combined shear-tension equations in the code.

Note the anchor types for tension typically are headed anchors, J bolts etc (or post fixed epoxy anchors) in order to mobilise the cone resistance. A simple threaded bolt for example, cast in, will give minimal to no tensile capacity as you have no mechanical means of anchorage and are relying only on bond strength of the concrete to the anchor, and concrete quality can be poor in the vicinity of the threads.