Anchor Bolts- ACI-318 App. D

[JStephen]

A question- in using Appendix D, you have the equation for breakout strength of a group of anchor bolts, which is based on the available area around the bolts. Suppose that area essentially includes the whole concrete surface, but the breakout strength still calculates as inadequate- what then?

It seems that additional rebar tying the failure surface into the concrete would increase the strength. Or including simultaneous downward loads. But neither of these is included in the methodology. Any ideas?

A related question- the Appendix is limited to 25" maximum embedment and 2" or so anchor bolts- what do you do when you go larger?

 

[WillisV]

JStephen - The answer to both of your questions is that using rebar to tie the breakout zone into the concrete is an acceptable method and can be used when you "go larger" than the limits. The anchor rods just need to be long enough and the reinforcing close enough to the anchor rods so that where the breakout plane crosses the reinforcing, it is fully developed above and below the breakout plane. This almost always requires hooks on the tops of the rebar.

This method is alloweb reference within Appendix D. See commentary RD.4.2.1 (in ACI-02). "For anchors exceeding the limitations of D.4.2.2, or for situations where geometric restrictions limit breakout capacity, or both, reinforcement oriented in teh directino of the load and proportioned to resist the total load within the breakout prism, and fully anchored on both sides of the breakout planes, may be provided instead of calculating breakout capacity."

 

[UcfSE]

Another way App D accounts for rebar is by allowing you a slightly larger resistance factor. This is for the case that rebar is perpindicular to the bolt, not lapped with it. It is limited, but then so was the research upon which App D is based.

You may also lap the rebar with the bolt, as WillisV states, provided you have enough room.

When you exceed the limits of embedment or bolt diameter, you will have to find another acceptable method for design.

 

[JStephen]

Thanks, that's exactly what I needed.