Concrete breakout

ACI Committee 318 first approved the inclusion of Appendix D - Anchoring to Concrete in ACI 318/318R-02. It provided strength design requirements for anchorage to concrete that consider several potential failure modes such as steel strength, concrete breakout, anchor puUout, side-face blowout, and anchor pryout (shear) in accordance with the CCD Method. ACI 318-08 includes the following important enhancements to Appendix D ... 

Concrete Breakout Strength of a Group of Anchors in a Rectangular Pattern in Shear... 

In accordance with ACI 318 Appendix D, the concrete breakout strength of a group of anchors in a rectangular pattern in shear should be taken as the controlling value of the following ... 

The concrete breakout strength of the row of anchors farthest from the front edge perpendicular to the direction of force, if closed shear ties or other mechanisms are used to transfer the load from the row of anchors closest to the front edge to the row of anchors farthest from the front edge. (See Figure 3.12.)... 

The load transfer method outlined in this section is based on the requirements listed in ACI 318 Appendix D. In the petrochemical industry, the unreinforced concrete breakout strength in tension and shear is rarely sufficient to exceed the ultimate anchor strength. 

It is a construction preference to keep the anchors inside the pedestal and not extend them into the mat or footing. However, in some cases this may not be practical. If the anchor extends into the mat, the concrete breakout strength in the mat must be checked with the effective embedment depth measured from the top of the mat (Figure 3.4) assuming reinforcement is not adequately lapped to transfer the tension. 

Previous editions of ACI 318 recognized the beneficial effects of supplementary reinforcement across the potential concrete breakout cone when evaluating the strength of an anchor. In order to reduce some confusion about this reinforcement, ACI 318 now defines two types of reinforcement that can be used across a potential breakout cone supplementary reinforcement and anchor reinforcement. 

The advantage of using the STM for analyzing shear transfer and designing shear reinforcement for pedestal anchorages is the elimination of questionable assumptions related to the size and shape of the concrete breakout cone, the crack location (whether the shear cracks propagate from the middle of pedestals, front-row anchors, or back-row anchors), and the amount of shear reinforcement that is effective to restrain the concrete breakout cone. 

Calculate the concrete breakout strength of the plate shear lug in shear. (The method shown in Example 3 is from ACI 349-06 Section D. 11. 

Note The stress area is calculated using 45 degrees as opposed to the approximate 35 degrees used for the concrete breakout strength of anchors.)... 

Kc-factor(s) for determination of concrete breakout capacity in uncracked and cracked concrete, as applicable... 

The tension and the shear forces in the anchors are transfered to the longitudinal rebars and shear reinforcement, respectively, which will be designed as anchor reinforcement. Therefore, the concrete breakout strength in tension and shear (D.5.2 and D.6.2) is not checked. The concrete pryout strength in shear (D.6.3) is assumed OK by inspection because it is usuaiiy criticai for short and stiff anchors. 

Check concrete breakout strength of the shear lug in shear. 

Wey, E., Hayes, T., Naqvi, D. (2010), Concrete Breakout Strength in Tension for Vertical Vessel Anchorage in Octagon Pedestals, Proceedings of the Structures Congress, American Society of Civil Engineers Reston, VA. 

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