Seismic Design Concept for Non-Structural Connections using Post-installed Anchors

Kamalika Kundu, Head – Codes and Technical Marketing, Hilti India Pvt. Limited

Introduction

As the seismic activity in and around India, has been steadily increasing, the government has taken the initiative to conduct several workshops and trainings to spread awareness on the subject. Seismic design of structures is essential in high seismic zones. It is more so for important buildings like hospitals, schools etc. In the recent past, several Indian Standards on seismic evaluation and design including our National Building Code have undergone changes to make them more robust. This is evident from the shift in our approach towards performance-based design. Our revised National Building Code 2016 recognizes the need for properly designing our primary structure as well as securing non-structural connections so that they can withstand the seismic forces. A book on earthquake protection of non-structural elements and connections in building titled - “Introduction to Earthquake Protection of Non-structural elements in buildings”, has also been published by Gujarat State Disaster Management[1] to highlight the damages that can be associated with failure of these structures and counter measures to be adopted. Despite these efforts, there is still little to no guidance on how to do seismic design of post-installed anchor connections for fixing secondary structures or service utilities.

During less severe earthquake, most damage to life and property is caused due to failure of non-structural connections like appendages to buildings, services and utilities. It is important to ensure that the service utilities continue to perform especially in structures like hospitals. To ensure this, it is important to use a post-installed anchor system that in qualified for use in seismic connections as well as to design it for seismic forces.

This paper discusses behavior of post-installed anchors under seismic loads and presents basic design concept for seismic conditions.

Codes and Standards

Examples of non-structural connections using post-installed anchors
Figure 1: Examples of non-structural connections using post-installed anchors
We have several standards like IS 1893, IS 13920, IS 4326 etc. that provide guidance on design of building to withstand earthquake forces and impact. The latest edition of IS 1893 – “Criteria for earthquake design of structures” and IS 13920 –“ Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces-Code of Practice (First Revision)”, emphasize of importance of connections but don’t provide details on how to design non-structural connections using post-installed anchors. Our National Building Code 2016 has also specifically included clauses to provide guidance on how to approach non-structural connection. Fig. 2 highlights some excerpts from NBC 2016 that are relevant for non-structural connections. NBC 2016 recognizes the fact that standards for design of non-structural connections using post-installed anchors are not available and thus recommend to use “specialist literatures”, where no standards are available.

Current approach

In absence of local standards and guidelines, our engineering community relies on their experience and engineering judgement for fixing non-structural components using post-installed systems. Some of the current practices are listed below:
  • Practice 1- No design or Follow the same approach as that for static conditions
    • Though, our codes and standards don’t provide guidance on how to design these connections, they do require these connections to be properly designed for seismic conditions. Therefore, the practice of designing for static conditions is incorrect.
  • Practice 2 - Applying addition safety factors on capacity of anchors suitable for static conditions
    • The anchor performance is significantly impacted by cyclic opening and closing of cracks that may occur during seismic conditions. Use of anchor that is not qualified for use under seismic conditions can lead to premature failure. No amount of over-strength designing through additional safety or load factors will compensate for this behavior. Therefore, the practice of applying additional safety factor on anchors approved for static conditions is also incorrect.
  • Practice 3 - Using anchors suitable for use under seismic conditions
    • The anchor capacity varies when used in group depending on spacing and edge distance. Just using an anchor suitable for use in seismic conditions is not sufficient to fulfil codal requirement. It is important to design the post-installed connection and not rely solely on load tables that contain values only for a single anchor
Excerpt from NBC 2016 on non-structural connections
Figure 2: Excerpt from NBC 2016 on non-structural connections

Correct design concept

The correct approach is to select an anchor that is suitable for use under seismic conditions and to design it as per current available literature or standards for given design conditions. If the post-installed anchor connection is not designed properly, it could result in improper load-displacement behavior or failure. Even a minor displacement of anchor can translate into large displacement of structural element which can hamper the performance of structure in the event of earthquake. Though, there are no national standards for design of post-installed connections, there are several international standards like ETAG[2] that can be referred to. The design provisions in these standards can be only used for anchors that have been tested and assessed for use under seismic conditions.

Qualification

The performance of post-installed anchor is measured in terms of load-displacement behavior. Functional behavior of these systems is thoroughly assessed under different load and use conditions. ETAG 001[2] – “Guideline for European Technical Approval of Metal Anchors for Use in Concrete” by European Organization for Technical Approvals (EOTA) is one such guideline that provides guidance on testing and assessment of post-installed anchor. Annex E of this guideline specifically covers tests for evaluating anchor performance under seismic conditions. Fig. 3 schematically compares the variation is approach for assessing anchors under static and seismic conditions.

ETAG 001[2] Annex E recommends pulsating tension test, alternating shear test etc. in cracked concrete for evaluating performance of anchors for earthquake loading. Based on assessment, ETAG 001[2] allows a category to be assigned to the anchor subject to its performance - C1 and C2; C2 being most stringent. Based on the assessment, an approval report (like ETA) is issued by an independent third party institute. EOTA’s Technical Report TR045[3] gives guidance on seismic design of post-installed anchors that have been qualified for use under seismic conditions as per ETAG 001[2] Annex E.

Design

EOTA’s Technical Report TR045[3] for design of anchors under seismic conditions provides three design options as shown in Fig. 4. This technical report links the anchor performance to the seismicity requirement as shown in Fig. 5. For detailed understanding on the design approach, please refer TR045[3].

Schematic comparison of assessment of post-installed anchor under static and seismic conditions
Figure 3: Schematic comparison of assessment of post-installed anchor under static and seismic conditions

Different design options as per TR045[3]
Figure 4: Different design options as per TR045[3]

Summary

Our revised codes and standards emphasize on giving due attention to non-structural connections, especially under seismic conditions. Currently, there are several incorrect practices being followed by engineering community due to lack of local standards or guidelines. The correct approach is to select an anchor that is suitable for use under seismic conditions and to design it as per current available literature or standards for given design conditions. In absence of national standards on the subject, international standards or guidelines like ETAG 001[2] and TR045[3] can be referred to.

Excerpt from TR045[3]
Figure 5: Excerpt from TR045[3]

References
  1. Murty, C. V. R., Goswami, R., Vijaynarayanan, A. R., Pradeep Kumar, R. and Mehta, V. V. (2013). “Introduction to Earthquake Protection of Non-structural elements in buildings,” Gujarat State Disaster Management Authority.
  2. ETAG 001, Guideline for European Technical Approval of Metal Anchors for Use in Concrete, European Organisation for Technical Approvals (EOTA), Brussels, Belgium, 2013.
  3. TR 045, Design of Metal Anchors for Use in Concrete Under Seismic Actions, European Organisation for Technical Approvals (EOTA), Brussels, Belgium, 2013.

NBM&CW February 2018

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