March 3, 2021

anchor-bolt-seismic-design

The seismic design of anchor rods is included in the Chapter 17 of the ACI 318-19. This document is an overview of the ACI 318 provisions for anchor bolt seismic design.

Anchor rods are usually subject 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 ACI seismic provisions apply to anchors in structures assigned to Seismic Design Category (SDC) C, D, E, or F.

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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, columns and base plates belong to this kind of elements. Typical values of Ωo range from 1.0 to 3.0 for different structural systems.

Tension anchor bolt seismic design

ACI 17.10.5.1 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 17.10.5.3, summarized below:

a. 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.

b. 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.

c. The anchor or group of anchors shall be designed for the maximum tension that can be transmitted to the anchors by a crushing attachment. This option is not applicable to steel base plates.

d. 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 17.10.5.4, a reduction factor of 0.75 shall be applied to the tensile strengths associated with concrete failure modes. The following flowchart shows schematically the options listed above.

seismic-tension-anchors-flowchart

Shear anchor bolt seismic design

ACI 17.10.6.1 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 17.10.6.3, summarized below:

a. 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.

b. The anchor or group of anchors shall be designed for the maximum shear that can be transmitted to the anchors by a crushing attachment. This option is not applicable to steel base plates.

c. 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 non-yielding 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 high seismic shears are usually resisted by shear lugs instead.

seismic-shear-anchors-flowchart

Specifying seismic parameters in ASDIP STEEL

ASDIP STEEL includes the design of base plates and anchor rods. The ACI anchorage seismic provisions are fully implemented in the software. The image below shows the Seismic Provisions dialog box, where the values of Sds and Ωo can be specified. The software provides a link to the US Geological Survey website with valuable information on the US seismic design maps.

In the lower portion the user can select the options for seismic anchors in tension and in shear, as described above. As a guidance, ASDIP STEEL shows the corresponding seismic loads and a brief explanation for each option.

anchor-bolt-seismic-parameters

Takeaway

The design of anchor rods in seismic areas has to comply with the ACI 318 anchorage seismic provisions, which are complex and cumbersome. ASDIP STEEL includes the seismic design of base plates and anchor rods, with multiple options to optimize the design easily.

Detailed information is available about this structural engineering software by visiting ASDIP STEEL. For engineering background see the blog post Anchor Bolt Design – The Complex ACI Provisions. For our collection of blog posts about base plate and anchorage design please visit Anchor Rods Design.

You are invited to download the Free 15-day Software Trial, or go ahead and Place your Order. Best regards,

Javier Encinas, PE

ASDIP Structural Software

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