Seismic retrofitting of building structures in Manipur : A brief overview

Dr S S Mayengbam *

Manipur earthquake that occurred on early morning January 4 2016 :: Pix - Deepak Oinam

The January 4th 2016 earthquake inflicts considerable damage to the several building structures of different categories. Top priority structures which should retain their functionality immediately after an earthquake or other natural disaster include:
Governance buildings like District Magistrate's office and residence,
governance continuity offices like those of Disaster Management Agencies,
police and fire stations,
food stocking and distribution centres,
telecommunication facilities,
transportation system,
airport control towers,
pipelines;
Hospitals and health care centres;
Historic buildings and monuments;
Schools and Academic buildings;
Panchayat and post office;
and Critical industries involved in post earthquake management activities.

There is a need for such priority building structures in a high seismic risk state like Manipur to be Seismic Resilient. Seismic loss due to non-functioning, damage or collapse of such structure at the time of a seismic event will be substantial and the consequence for negligence in timely routine technical maintenance or monitoring will be disastrous.

Decision making authorities need to make a choice based on scientifically quantified comparisons between the cost of accepting the risk and that of systematic retrofitting or mitigation.

The buildings affected by earthquake may suffer both structural (beams, columns, reinforced concrete walls, etc) and non-structural (components other than structural) damages. Non-structural and architectural components get easily affected during an earthquake.

The architectural repairs like
Patching up of defects such as cracks and fall of plaster;
Repairing doors, windows, replacement of glass panes;
Checking and repairing electric conduits/wiring; etc.
do not restore the original structural strength of structural components in the building and any attempt to carry out only repairs to architectural/non-structural elements neglecting the required structural repairs may have serious implications on the safety of the building.

The damage would be more severe in the event of the building being shaken by the similar shock because original energy absorption capacity of the building would have been reduced.

Before taking up structural repairs and strengthening measures, it is necessary to conduct detailed seismic vulnerability assessment and quantify the structural and non-structural performance during expected site specific levels of future earthquakes.

Non-destructive testing techniques could be employed to determine the structural condition of the building. Based on the assessment results and documents like NDMA (2014), ASCE/SEI 41-06 (2007), SP-7 (2005), retrofitting is to be done on structural elements first which may include jacketing; strengthening using fibre reinforced polymers, changing the geometry, additional columns or beams, steel bracing, active or passive dampers; or base isolation.

Only after finishing the structural part, non-structural elements should be retrofitted.

For example, performance of brick masonry walls can be improved by grouting/ injecting the crack with epoxy resin (for fine cracks) or thick cement slurry, with stone chips for wider cracks; galvanised steel wire or chicken mesh can be used as mortar covered bands along wide cracks, and opening edges with enough bond length.

Many existing buildings do not meet the seismic strength requirements due to original structural inadequacies and/or material degradation with the course of time. Their earthquake resistance can be upgraded to the level of the present day codes by appropriate seismic retrofitting techniques.

Replacement of damaged buildings or existing unsafe buildings by reconstruction is generally avoided when: the cost is higher than that of strengthening or retrofitting; there is preservation of historical architecture; or when maintenance of functional social and cultural environment is necessary.

In general, the effort and technical input required to retrofit a building are much higher than that to make a new earthquake resistant building. According to the relevant Indian seismic codes, it is very much safe as well as cost-effective to construct earthquake resistant buildings at the initial stage itself.