Introduction –

In India, we are sitting on a seismically active zone. As per the Vulnerability Atlas of India, almost 60% of our country’s landmass is earthquake-prone, and 78% of our population lives in moderate to high-risk zones. Post-installed anchors are widely used in the construction industry for fixing of steel structures to existing concrete members, ranging from heavy duty structural applications such as waler beams, mezzanine floors, structural glazing, canopy, elevator guide rails, tower cranes, steel jacketing to medium and light duty applications such as equipment fixing, utility supports, cable tray bracket fixing, walkway fixing etc. As such, it is extremely important to secure these connections to minimize the damage in the event of an earthquake.

The Indian Standard IS 1946 – 2025 (Part 2) – “Design of Post-Installed Anchorage to Concrete - Code of Practice” by Bureau of Indian Standards clearly addresses the requirement of seismic design of post-installed anchors in concrete.

Overview of seismic design requirement

One of the most important factors affecting the performance of post-installed anchors is the presence of concrete cracks at the anchor location. Cracks may run inclined to the axis of the component in case of shear or torsion, and they occur parallel to reinforcement as a result of transverse splitting stresses. It has been observed that when cracks form in a concrete member, there is a relatively high likelihood that they will intersect the anchor location either directly or tangentially. This occurs because higher tensile stresses exist around the anchor as a result of hoop stresses associated. Consequently, the anchor load-displacement behavior is significantly affected by the load acting on the anchor in any direction and the crack in which the anchor may be located. Studies have indicated that the resistance of a post-installed anchor is reduced significantly when located in concrete cracks. The effect of concrete cracks become more pronounced in the event of an earthquake, which requires an even more stringent assessment and design for anchors used in high seismic zones, considering the effects of pulsating tension, alternating shear and crack cycling (see Figure 1)

Figure 1 Effect of concrete crack on anchor performance in static and seismic conditions

The basic design requirement and provisions for static conditions shall also apply to design of anchor for seismic conditions. As a prerequisite for seismic design, the anchors shall be tested and evaluated as per seismic test provisions in addition to cracked assessment requirements.  In addition to the test protocols for assessment of performance under static conditions, the post-installed anchors are required to undergo the following tests, as mentioned in IS 1946 Part 3 and IS 1946 Part 4 –

1.    Tension tests in standard and high strength concrete in 0.8 mm crack width

2.    Shear test in large crack width (0.8 mm) in standard concrete

3.    Tests for pulsating tension load

4.    Tests for alternating shear load

5.    Crack cycling test with large crack width

Further, it shall be assumed that anchors do not dissipate energy by means of ductile hysteretic behavior and do not contribute to the overall ductile behavior of the structure.

In general, the gap between the base plate clearance hole and the anchor should be avoided in seismic conditions. If it is not possible to avoid this gap, then a reduction factor shall be applied to account for the effect of this gap on the behavior of the anchor. 

For all seismicity levels defined as per IS 1893 (Part 1), anchor shall be designed for seismic conditions. The design seismic force acting on the base plate shall be determined according to IS 1893 (Part 1) and other relevant standards.

When the seismic tension (or shear) component of the design force at the ultimate limit state applied to an individual anchor or an anchor group is equal to or less than 20 percent of the total design tensile (or shear) force, then seismic provisions need not apply for the verification of the tension (or shear) component acting on a single anchor or a group of anchors.

Seismic forces may be considered for non-structural components also. Reference to IS 1893 (Part 1) or IS 16700 may be made in this regard for calculation of seismic force on non-structural elements.

The effect of concrete cracks become more pronounced in the event of an earthquake, which requires an even more stringent assessment and design for anchors used in high seismic zones. This standard ensures that the connections are designed to perform safely and consistently even during seismic events.

Seismic design options

(i)             Capacity design – As per this method, an individual anchor or an anchor group shall be designed for the maximum tension and/or shear load that can be transmitted to the anchor based on either the development of a ductile yield mechanism in the attached element or the base plate taking into account strain hardening and the capacity of a non-yielding attached element (or structural element) (see Figure 2)

Figure 2 Seismic design options

(ii)            Elastic Design – As per this method, an individual anchor or an anchor group shall be designed for the maximum load obtained from the design load combinations (including seismic) corresponding to the limit state [refer IS 1893 (Part 1)] assuming an elastic behavior of the anchorage and of the structure. Furthermore, uncertainties in the model to derive seismic forces on the anchorage shall be taken into account.

(iii)           Design with Requirements on the Ductility of the Anchors – This method shall not be used for anchor connections of primary seismic members. This method may be used only for secondary/tertiary seismic member. As per this method, an individual anchor or an anchor group shall be designed for the maximum load obtained from the design load combinations (including seismic) corresponding to the limit state [refer IS 1893 (Part 1)]. The tension steel capacity of the anchor or anchor group shall be smaller than the tension capacity in concrete related failure modes. Elongation capacity of the anchors shall be such that it accommodates the deformation according to the seismic analysis of the connection.

Seismic design of anchors

The design strength in tension and shear shall be calculated and checked according to limit state of design for all applicable failure modes as per Table 1. The minimum strength shall govern for each load direction.

The characteristic strength for an anchor or an anchor group shall be determined for each failure type as follows:

=reduction factor to account for inertia effects due to any gap between anchor and base plate in shear; as given in the relevant AR

If  value is not available in AR then  may be assumed to be 1.0 in case of no hole clearance between anchor and base plate and 0.5 in case of connections with hole clearance () as given in IS 1946 Part 2.

= reduction factor to account for the influence of large cracks and scatter of load-displacement curves (see Table 2) and

=characteristic seismic resistance of a single anchor for a given failure mode and not influenced by adjacent anchors or edges of the concrete member determined as per Table 3.

It is to be noted that the design of anchors under earthquake loading for earthquake Zones III and above and for important building classes [as per NBC 2016 and IS 1893 (Part 1)] shall be based on an assessment under pulsating tension, crack cycling under constant tension load and alternating shear in cracked concrete at a crack width of 0.8 mm.

 
Displacements
 

The anchor displacement under tensile and shear load at limit state of displacement shall be limited to  and  value of the application to meet requirements regarding functionality and assumed support conditions. If anchor and attached elements are expected to be operational after an earthquake, then relevant anchor displacements shall be taken into account. If the anchor displacements  under tension loading and/or  under shear load provided in the relevant AR are higher than the corresponding required values  and/or , the design resistance may be reduced according to the following equations:

When a rigid support is assumed in the analysis the designer shall establish the limiting displacement compatible to the requirement for the structural behavior. The acceptable displacement associated to a rigid support condition is considered to be 3 mm.

If the connection such as façade element is designed to accommodate deformations (that is displacements or rotations) then it shall be demonstrated that the anchor can accommodate these deformations. The rotation of a connection  (see Figure 3) is given by:

Where,

   = displacement of the anchor (limited to 3 mm) when subjected to earthquake load; and

    = distance between the outermost row of anchors and the opposite edge of the base plate.

Key
1  Sleeve or debonding length
2  Anchor
3  Base plate (anchor plate)
4  Element

Figure 3 Example of rotation and displacement of a connection

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