Bhavin Shah - Chief Executive Officer at VMS, Ahmedabad (India)
In this article, I am sharing my views over the transition of earthquake engineering experienced in India through my journey as a structural engineer. The technical details mentioned here are very brief and the relevant codes should be referred to gain sufficient understanding. The article shows the transition in earthquake engineering practices from 2001 to till today. Way forward is also suggested at the end of the article.
During my post-graduation studies, a five-storey residential apartment building was under construction close to my house. As a curious Structural Engineer, I would always try to peek into the on-going adjacent construction site and observe the reinforcement and member size details of the building. Just after I completed my post-graduation studies on the 18th of January 2001, a tragic earthquake of 7.7 magnitude struck our locality in Gujarat on the 26th of January 2001. During the earthquake, as I rushed out of my house to go at an open place, I could observe the same under construction residential apartment being tested on a real-time shake table. The building performed quite well during the earthquake; I still do remember shaking of the building in front of my eyes. The deflection at the top of the building was roughly around 150 mm (~5.9 in).
Immediately after this earthquake of significant magnitude in 2002, the process of revising building codes of the Indian Standards was initiated. There were several modifications done to the IS 1893 (Criteria for Earthquake Resistant Design of Structures) and there was a major shift in a few of the design processes thereafter. Some of the changes observed were a migration of analyzing a 3D model instead of a simplified 2D model, equivalent static method being adopted for regular buildings and requirement of dynamic analysis for irregular buildings, torsional eccentricity values were revised upwards in view of serious damages observed in buildings with irregular plans.
In 2007, the Steel Structures Code IS 800: 2007 was revised with a major change of shifting to the Limit State Design (LSD) Method from the older Working Stress Design (WSD) Method. Section 12 is introduced in the code for seismic considerations with the overall concept to provide more ductile detailing to steel beam-column joints at the location of plastic hinges; so that they could withstand higher intensity of earthquakes. These changes led to steel member sections to be either the same or heavier in a few situations, but the connection details became more stringent compared to the earlier code. The latest code still provides an alternative to use the WSD method when it is not feasible to apply the LSD; but the application of the latest WSD method is significantly different than the earlier code. This code defines specific details and performance criteria for joint rotation of different structural systems such as OCBF, SCBF, OMF and SMRF. Therefore, it is now required to perform non-linear analysis to demonstrate the adequacy of joint rotation. In my opinion, still in many parts of India, the code is not being followed in its entirety due to lack of training of engineers, non-availability of handbook and updated softwares, etc.
In 2013, IS 15988:2013 was published for seismic evaluation and strengthening of existing reinforced concrete buildings. It is mentioned in the code that it is intended to be used as a guideline. This standard describes a set of key steps and procedures for assessment of the expected seismic performance of existing building in the event of a design level earthquake and where found necessary, strengthening of existing structural systems and elements for improved seismic performance. The code details out different steps required for preliminary evaluation & for detailed evaluation. Thereafter, the standard describes seismic strengthening options and strategies at a general level. The code also introduces few new terminologies like acceptance criteria, knowledge factor, lateral load modification factor, modified material factor, etc.
In 2015, the IS 1893: Part 4 for Industrial Structures was revised. The code defines different damping coefficients based on construction materials and design basis level of MCE (major earthquake) and DBE (minor earthquake). Industrial Structures are classified into four categories and the effect of earthquake force should be considered simultaneously in three directions in case of irregular structures. The earthquake force need not be scaled based on the time period. P-Delta analysis has been made mandatory for all the structures and non-linear analysis is required to verify the collapse mechanism of the structures to ensure safety against collapse.
In 2016, IS 1893: Part 1 was revised; where the concept of DBE and MCE is eliminated and the damping value is defined as 5% irrespective of the construction material. Mathematical limits are introduced for irregularities in buildings in plan and vertical directions. For flat slab structures, punching shear failure is to be avoided and lateral drift at the roof has to be restricted to 0.10%. At least 2% structural shear wall should be provided in each principal direction for structures in seismic zones III, IV and V. Consideration of vertical acceleration and soil structure interaction is required for majority of the structures. New sections are introduced for the cracked moment of inertia of beam and columns and for properties of infill bricks for taking them under consideration for analytical modeling. As per code, the gravity columns must withstand the ultimate displacement without losing its vertical load carrying capacity.
In 2017, revised IS 13920 was introduced which has geometric constraints for member sizes. Minimum column dimensions were made 20db (db is the largest diameter of longitudinal reinforcement in the beam). For the structural systems other than Moment- Resisting Frames (MRF) such as flat slab, pre-cast/ post-tensioned structures; non-linear dynamic analysis and experimental evidence is required to demonstrate that the proposed structural system will have sufficient ductility as the MRF. The concrete capacity should be ignored while calculating the shear capacity of reinforced concrete beams. The concept of strong column-weak beam is introduced in this edition. The beam-column joints are to be checked for distortional shear when the aspect ratio is less than 0.40. In 2017, IS 16700 was published which defines criteria for structural safety of tall concrete buildings (height greater than 50m [164 ft] but less than or equal to 250m [820.2 ft]). This standard covers the selection of appropriate structural systems, geometric proportioning of the building, integrity of the structural system, resistance to wind and earthquake effects and other special considerations related to tall buildings.
The latest Indian Standard Seismic codes are moving towards building drift control philosophy. At present, for majority of the structures, we carry out first order elastic analysis. It is apparent that during extreme seismic events the structure will experience plastic deformations. From the above mentioned analysis, we are not aware of two major factors, firstly, reserved strength of the structure beyond elastic deformation and secondly, collapse mechanism of the structure. To have a better understanding of these aspects, non-linear analysis is required. The codes are also becoming increasingly complex as we are striving to match with the global practices.
I believe this is an even greater transition in our design practices as compared to the transition which was witnessed during 2002. In my view, we all structural engineers need to gear up for bridging the gap between the latest methods/ implementations suggested in the revised codes v/s our current industrial practices. For precise application of the codes, more communication is required between our structural engineering fraternity for resolving doubts/ queries, etc. and it is very much important to understand the overall philosophy of the code. At this juncture, authentication of software is also of paramount importance for the proper implementation of the updated codes. Structural Engineers should be aware of the limitations of the software being used while performing the design. The interface between different codes is another important area to look for. In any case, reinforcement detailing is very much important for resisting earthquake forces, so attention should be given in the drawings and also the same should be followed at site.
References: (Following Indian Standards may be referred for more details.)
IS 1893 (Part 1) : 2002 Criteria for earthquake resistant design of structures (Part-1 General provisions & buildings)
IS 800 : 2007 – General construction in steel – Code of practice
IS 15988 : 2013 – Seismic evaluation and strengthening of existing reinforced concrete buildings – Guidelines
IS 1893 (Part 4) : 2015 – Criteria for earthquake resistant design of structures (Part 4-Industrial structures including stack-like structures)
IS 1893 (Part 1) : 2016 - Criteria for earthquake resistant design of structures
IS 13920 : 2016 – Ductile design and detailing of reinforced concrete structures subjected to seismic forces – Code of practice
IS 16700 : 2017 – Criteria for structural safety of tall concrete buildings