In India, we are witnessing an unprecedented growth across different sectors in the construction industry – from high-end residentials to state-of-the-art commercial buildings, expansion of railway and metro railway networks, as well as modernization of existing railway infrastructure, expansion of airports to meet the ever-increasing demand of the aviation sector, as well as green field and brown field industrial projects. The extensive development across all these sectors is reshaping the construction landscape of the country.
The modern construction industry is replete with applications that connect steel elements to concrete structures. These can be heavy duty structural applications such as waler beams, mezzanine floors, structural glazing, canopy, elevator guide rails, tower cranes, steel jacketing as well as non-structural yet safety-critical applications such as equipment fixing, utility supports, cable tray bracket fixing, walkway fixing etc. And post-installed anchoring is extensively used for the purpose of fixing these elements to the concrete structures.

Figure 1 Typical applications in a building under construction

Figure 2 Typical applications for building finishings

Figure 3 Typical applications for tunnel structures

A post-installed anchor is a type of anchor that is installed in hardened concrete (i.e., "post" concreting) and is used to hold the element in place. Over a period of time, the technology has become more common, primarily due to the flexibility it renders during construction phase without compromising on the safety of the connection. Safety of a post-installed connection is a combination of three factors – it depends on the right selection of the anchor to meet the application requirement, proper design considering the different influencing parameters and eventually translating it at site through proper execution methodology.

However, in absence of adequate national standards in this regard over the past years, this has always lacked the right level of understanding and enforcement. While one may delve into the intricacy for some of the most critical connections, majority of them would be left to the wisdom and recommendation of the manufacturer. Also, we have seen examples of poor engineering judgements stemming from the lack of sufficient information and guidelines.

The Indian Standard IS 1946 – 2025 (Part 2) – “Design of Post-Installed Anchorage to Concrete - Code of Practice” by the Bureau of Indian Standards will address this apparent gap in the industry. With safety at its core, the standard outlines a comprehensive design framework for post-installed anchors to ensure the highest level of reliability and consistency. It also requires the anchors to go through an extensive and stringent test regime as a starting point.

Scope of the standard –

The standard covers the design of post-installed mechanical anchors and post-installed adhesive anchors in concrete when subjected to static, quasi-static and earthquake loads. The concrete that forms the base material shall be in conformity with IS 456 and IS 1343. The standard is not applicable for design under fire, fatigue, and blast loads. Specialist literature may be referred to for this purpose.

The standard is applicable only to anchors that have been tested and assessed according to the test protocol mentioned in IS 1946 Part 3 and IS 1946 Part 4, as a minimum. The assessment report (AR) of the anchor generated based on this assessment shall be referred to for various design parameters

An example of globally accepted AR is a European Technical Assessment (ETA) which is issued by an approval body (known as Technical Assessment Body in Europe). This is done as per the testing and assessment of post-installed anchors based on the test protocols of EOTA EAD 330499 ‘European Assessment Document — Bonded fasteners for use in concrete’ and EOTA EAD 330232 ‘European Assessment Document — Mechanical fasteners for use in concrete’.

Types of post-installed anchors as per IS 1946 (Part 2) –

The standard covers the different types of post-installed anchors as follows –

Figure 4 Types of post-installed anchors

Mechanical anchors typically transfer load to concrete through friction, keying, mechanical interlocking, or combination of any of these three working principles. Adhesive anchors consist of threaded anchor rods which are anchored in pre-drilled holes in hardened concrete by bonding the metal parts of the anchor to the sides of the drilled hole with an adhesive. The adhesives used comprise of chemical components formulated from organic polymers (like, epoxies, polyurethanes, etc.) or inorganic materials that hardenon curing.

Friction mechanism is the load-transfer mechanism typical of systems where expansion force is generated by a clip or a wedge pressed against the walls of the borehole during the installation process. Frictional resistance equilibrates the external tension force on anchors.

Mechanical interlock/keying defines the working principle where the load is transferred by means of a bearing surface between the anchor and the base material. Some post-installed anchors develop a mechanical interlock between the anchor and the base material.

Adhesive bond mechanism involves the transfer of the external load to the concrete base material via an adhesive bond. The forces are transferred from the anchor element to the mortar via mechanical interlocking and to the base material via a combination of micro-interlock and chemical adhesion between the mortar and the lateral surface of the borehole.

Figure 5 Working principle of post-installed anchors

The selection of the appropriate type of anchor shall be governed by multiple factors like condition of the existing concrete, area of application, type of load, magnitude of load, feasibility of application. For example, in poorly compacted concrete, for high tension load or for close to the edge applications, adhesive anchors may be preferred. For high shear dominated applications, mechanical anchors may be preferred. However, this is not restrictive in nature as to the use of the anchors. The selection may be made by the concerned structural engineer and confirmed by the manufacturer.

Anchor diameter and embedment –

The anchors under the provision of the given standard shall have a minimum diameter of 6 mm.

For mechanical anchors, the embedment depth of the anchor shall be at least 6 times the anchor diameter, but not less than 40 mm. For fastening statically indeterminate non-structural elements, minimum embedment depth of anchor shall be 30 mm.

For adhesive anchors, the maximum embedment depth for design of the anchor shall be 20 times the anchor element diameter. Any embedment depth in excess of 20 times diameter shall be ignored in design calculation.  The minimum embedment depth () shall be as follows:

For,[KB1] [2] [KB3] 

,     ;

,      ;

,     ;

,     ; and

,     .

Clearance holes in base plate for anchors –
 
This is the gap in the base plate between the anchor and plate.
 
 

 
 = diameter of clearance hole in base plate
 = diameter of anchor
Hole clearance =
 

Figure 6 Hole clearance

The diameter of clearance hole () in the base plate shall not be larger than the values given below with the following exception that for an individual anchor or anchor group loaded in pure tension, a larger diameter of the clearance hole is acceptable if a suitable washer is used.

For,

               + 1;

     + 2; and

               + 3

Where,
d = da, if bolt bears against the base plate; and
d = dnom, if sleeve bears against the base plate.
 
Concrete Base Material
 

The concrete thicknesses required for post-installed mechanical anchors shall be at least 2, but not less than 120 mm. The concrete thicknesses required for post-installed adhesive anchors shall be at least equal to ( + ), but not less than 100 mm,shall be taken as 2 or 30 mm whichever is larger. Concrete shall be assumed to be cracked for design purpose. 

 
Permissible Anchor Group Configurations
 

The anchor group configurations illustrated below are valid for all loading directions and all edge distances. In case, the gap between anchor and plate is filled during installation, the shear force is taken up by all anchors. In case hole filling cannot be ensured at site, shear force is taken up by the front row of anchors.

Figure 7 Permissible anchor group configuration

Factors affecting the performance of post-installed anchors –

The performance of a post installed anchor is governed by multiple in-service and installation parameters.

The installation parameters include the following:

1.    concrete grade and type

2.    condition of the drilled hole surface

3.    diameter, position, and orientation of anchors (centre to centre and edge distance of anchors)

4.    hole clearance

5.    working and curing time of adhesive anchors

6.    applied torque (for mechanical expansion anchors)

In-service parameters include the following:

1.    environmental conditions (corrosion, effect of freeze & thaw, impact of chemicals, etc.),

2.    temperature of the base material

3.    load duration (significant impact for sustained loading).

4.    the condition of concrete - cracked or uncracked

Post-installed anchors experience a significant reduction in resistance when exposed to concrete cracks. Concrete members are designed to develop cracks over their service life up to 0.3 mm. As such, it becomes important that the anchors are assessed and designed for use in cracked concrete.

It is extremely important to understand the consideration of cracked concrete in the design aspect. Under the current limit state method, reinforced concrete members are designed as cracked sections with consideration for an allowable crack under serviceability limit state. There is a high probability that anchors installed in concrete will be located in a crack when the cracks develop. This is mainly because high tensile stresses are caused due to prestressing and loading of the anchor and the drill holes act as notches leading to stress concentration. The effect of cracking on anchor concrete breakout capacity is two-fold. First, the distribution of hoop stresses around the anchor gets modified to maintain equilibrium, which applies to all types of anchors. Consequently, the anchor load-displacement behaviour is significantly affected by the load acting on the anchor in any direction and the crack in which the anchor may be located. Earlier studies have indicated that the resistance of a post-installed anchor is reduced significantly (e.g. by 20 to 30%) when located in concrete cracks. The effect becomes more pronounced in case of earthquake. Hence, the general recommendation is to always consider cracked concrete for any design of post-installed anchors to avoid any safety concern.

Design consideration for post-installed anchors –

The design follows a limit state method

Figure 8 Principle of design for post-installed anchor

 
In the design of post installed anchors, the following failure modes are generally observed and considered –

Figure 9 Failure modes of post-installed anchor

Steel failure primarily occurs if the number of fixings are not adequate, or the diameter or strength of steel is insufficient. Pull-out failure mainly occurs when the bond between the concrete and the anchor fails. For adhesive anchors, it may occur due to slippage at the interface of the adhesive and concrete or adhesive and the anchor rod.  To obtain a higher concrete cone resistance, it is required to increase the volume of the engaged cone by placing the anchors farther from the edges or increase the spacing between anchors or the anchor embedment. Splitting failure occurs in thin low-reinforced members and for long embedment, resulting in a greatly reduced concrete section on the deeper end of the anchor itself.

As we know, anchors do not behave in isolation, it is important to check for the influence of adjacent anchors on the performance and thereby defining the resistance of the group. This takes into consideration the resistance of an individual anchor placed in concrete (function of the effective embedment depth and the grade of concrete), the idealized projected concrete cone area of an individual anchor with no influence of edge and spacing on the concrete cone strength of the anchor and the actual projected concrete cone area of the anchorage at the concrete surface, along with other influencing factors.

The concrete can also break out in pry-out far from the edge, especially with shallow embedment. The resistance regarding this failure can be improved by increasing the embedment depth or the volume of concrete engaged. A lateral concrete edge break-out occurs under shear load when the anchors are close to the edges. The resistance depends mainly on the concrete class, the concrete condition (cracked or uncracked) and the volume of concrete cone engaged by the anchor.

 
Seismic design of post-installed anchors –

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. Earthquakes do not just impact main structural components; they also threaten non-structural elements as well. Hence, securing them adequately becomes extremely important.

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.

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. 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. Stand-off fastening or other configurations where anchors may be subjected to shear load with lever arm shall not be permitted under seismic conditions. Base plates with grout layer shall not be permitted under seismic conditions.

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.