By: Javier Encinas, PE | January 9, 2017

The seismic design of anchor rods is included in the *ACI 318* code, originally in the *Appendix D* of the *ACI 318-11* and earlier, and now in the *Chapter 17* of the *ACI 318-14*. This document is an overview of the *ACI 318* seismic provisions that affect the design of anchor rods.

The anchor rods 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. The seismic provisions apply to anchors in structures assigned to Seismic Design Category (SDC) C, D, E, or F.

**What is the over-strength factor Ωo?**

Most seismic force resisting systems (SFRS) rely on the dissipation of energy through inelastic deformations. Some members or connections on the load path whose inelastic behavior may cause poor system performance may be deliberately affected by an over-strength factor Ωo** **to amplify the seismic load, and so force them to perform elastically. Per the AISC Seismic Provisions D2.6, the column bases belong to this kind of elements. Typical values of Ωo range from 1.0 to 3.0 for different structural systems.

**Seismic design of anchor rods in tension.**

ACI 17.2.3.4.1 waives the requirement to design for seismic tension “where the tensile component of the strength level earthquake force applied to a single anchor or group of anchors is equal to or less than 20 percent of the total factored anchor tensile force associated with the same load combination”.

If the anchor tension due to E is larger than 20% of the total factored tension, then the design should be performed using one of the options given in ACI 17.2.3.4.3, summarized below:

- This option ensures a ductile performance of the anchorage. Anchor requirements are imposed (ductile ratio > 1.2 non-ductile ratio), and the required anchor strength is E (Ωo = 1.0). Ductile steel elements are required with a stretch length of at least eight anchor diameters.
- It assumes a ductile yielding of the base plate. The anchorage should be designed for the tension force associated with the expected non-yielding strength (Ωo = 1.5). It should be noted that yielding of base plates is not recommended by AISC, as mentioned above.
- The anchor or group of anchors shall be designed for the maximum tension that can be transmitted to the anchors by a non-yielding attachment. Similar to option b. but with a non-yielding base plate.
- It requires the anchors to be designed for the maximum tension considering E increased by Ωo. This option assumes a non-yielding base plate and an elastic anchorage.

In addition. per ACI 17.2.3.4.4, a reduction factor of 0.75 shall be applied to the tensile strengths associated with concrete failure modes.

**Seismic design of anchor rods in shear.**

ACI 17.2.3.5.1 also waives the requirement to design for seismic shear “where the shear component of the strength level earthquake force applied to a single anchor or group of anchors is equal to or less than 20 percent of the total factored anchor shear force associated with the same load combination”.

If the anchor shear due to E is larger than 20% of the total factored shear, then the design should be performed using one of the options given in ACI 17.2.3.5.3, summarized below:

- It assumes a ductile yielding of the base plate. The anchorage should be designed for the shear force associated with the expected non-yielding strength (Ωo = 1.5). It should be noted that yielding of base plates is not recommended by AISC, as mentioned above.
- The anchor or group of anchors shall be designed for the maximum shear that can be transmitted to the anchors by a non-yielding attachment. Similar to option a. but with a non-yielding base plate.
- It requires the anchors to be designed for the maximum shear considering E increased by Ωo. This option assumes a non-yielding base plate and an elastic anchorage.

Unlike the design for tension, no reduction factor shall be applied to the shear strengths associated with concrete failure modes. It should be noted that the shear capacity of anchor rods is limited, therefore in practice the seismic shear is usually resisted by shear lugs instead.

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Best regards,

Javier Encinas, PE

ASDIP Structural Software

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