By: Javier Encinas, PE | February 13, 2019
The design of anchor rods has become complex and cumbersome with the development of the ACI 318 anchorage provisions, originally in the Appendix D of the ACI 318-11 and earlier, and now in the Chapter 17 of the ACI 318-14. This blog post is an overview of the required checks in the design of anchor rods.
Multiple checks need to be done for several failure modes, which in turn depend on the geometry of the concrete support. The anchor rods in a typical column are usually subjected to a combination of tension and shear forces. The ACI treats separately tension and shear, and then it combines both effects in an interaction diagram.
Design of anchor rods for tension
The main purpose of the anchor rods is to resist tension. The design of anchor rods for tension forces implies checking the limit states of steel strength, concrete breakout, pullout, and side-face blowout. The calculation of the breakout is particularly important since a concrete failure would be non-ductile, and therefore should be avoided. Anchor reinforcement may be provided in order to avoid a breakout failure, and in this case the tension is resisted completely by the rebars.
For a more detailed discussion of the different tension failure modes please see the blog post Anchor Rod Design – The Complex ACI Provisions (Part 1).
Design of anchor rods for shear
The lateral forces acting on a structure will produce a horizontal reaction at the foundation level. For steel frames supported on base plates, a small horizontal force can be resisted by the friction between the plate and the underlying concrete. However, as the reaction increases, the friction may not be high enough to counteract the sliding force, and the plate will tend to slide transferring the shear force to the anchor rods.
The anchor rods should be checked for the limit states of steel strength, concrete breakout, and concrete pryout. As with tension rods, a concrete breakout failure should be avoided.
For a more detailed discussion of the different shear failure modes please see the blog post Anchor Rod Design – The Complex ACI Provisions (Part 2).
Seismic design of anchor rods
The ACI seismic provisions apply to anchors in structures assigned to Seismic Design Category (SDC) C, D, E, or F. Seismic design for tension is waived if the anchor tension due to E is larger than 20% of the total factored tension. Likewise, seismic design for shear is waived if the anchor shear due to E is larger than 20% of the total factored shear. Otherwise, the design should ensure either a ductile performance, or the anchor rods should be designed for the maximum tension considering E increased by Ωo.
For a more detailed discussion of the seismic design of anchor rods please see the blog post Anchor Rod Design: Overview of the ACI Seismic Provisions.
The design of anchor rods may be difficult and time-consuming. It involves checking for different failure modes due to tension, shear, and seismic forces. ASDIP Steel includes the design of base plates and anchor rods per the latest AISC and ACI provisions. This tool allows the design to be completed and optimized within minutes.
Javier Encinas, PE
ASDIP Structural Software