Anchor Design in Seismic Conditions

The necessity of designing anchors to withstand not just static but also seismic loads, which included clarifying certain common misconceptions arising from gaps in the current IS codes that result in the use of some form of engineering judgment. These misconceptions can lead to using post-installed anchors in a way that is not suited for either the application, the loads, or both, causing failure of the connection, damage to the structure, and a loss of life. Our recent webinar covered this and discussed how following the Holy Trinity of qualification parameters, testing process, and design parameters ensures a properly engineered post-installed anchor connection in concrete. In this newsletter, we will outline both the performance categories and the ideal design approach involved in properly designing an anchor to withstand seismic activity.

Also Read: Understand the Basics of Anchor Design Under Seismic Conditions

Seismic Performance Categories & Testing Processes

Before covering the design approach, let us briefly examine the testing process that splits the anchor performance under seismic conditions into two sub-categories – C1 & C2. According to ETAG 001: Annex E, anchors tested to the first category, C1, must successfully sustain pulsating tension and alternating shear loads that are 50% of the total design load when set in 0.5mm wide cracks. The evaluation parameter is the residual load capacity of the anchor. As illustrated in Table 1 Below, this category is best suited to non-structural, non-critical applications that do not jeopardize occupant safety.

Category C2 is more stringent of the two; here, the anchor must sustain reference tension and shear tests in low and high strength concrete on top of sustaining the tests required by Category C1. Moreover, the anchor must perform under intense cyclical opening and closing of crack widths up to 0.8mm with applied loading at 85% of the total design loads. An additional evaluation parameter, on top of the residual load capacity, is that the anchor displacement must be less than 7mm.

However, the prerequisite to conducting the tests is that the anchor must first be approved for cracked concrete (with 0.3mm crack width), otherwise, it stands no chance of safely functioning under the cyclic opening and closing of cracks under Category C2. This Category completely alters the performance of the anchor and anchor group compared to a condition with static loads

Limit State Design and Conceptual Design Approaches

Simple qualification of anchor in the appropriate seismic category is NOT enough to guarantee safety under seismic conditions: designing these anchors according to the limit state design concept is equally important.

To amplify the characteristic to design loads, use is made of relevant load combinations obtained from the codes and the values obtained from calculations. However, the baseplate’s behaviour in relation to the loads applied can vary depending on the base material’s behaviour. Thus, there are three conceptual design approaches available for engineers –

(i) Capacity design; (ii) Elastic design, and (iii) Ductile anchor.
Approach (ii) is the only one that allows brittle failure in seismic design and this design option assumes no energy dissipation in the entire structure and that each component can stay within its elastic range under seismic conditions. Although a more conservative approach, it accounts for the brittle nature of concrete, which will mostly fail before either the anchor (Ductile Anchor) or the member, typically steel, attached to the baseplate (Capacity Design). This option amplifies the loads to correctly reflect the seismic loads acting upon the structure.

A Correct Approach Leads to a Correct Design

Following this approach allows comparison of the design loads with the design resistance and results in an engineered approach to designing post-installed anchors in concrete. Hilti’s Profis Anchor software allows engineers to resolve these calculations rapidly with great ease and flexibility. Our technical team is trained to provide you with guidance on designing and selecting the right anchors according to the right connections under the right loading conditions so no matter where your post-installed connection sits, you can be assured that your structure will have the highest level of safety we offer.
Read our next article on this topic, where we will cover the failure modes and associated parameters that reduce resistance of an anchor.

For further information, please contact a Hilti Field Engineer or write to us at: TeamEngineeringSupport.IN@hilti.com