21-09-2013, 01:01 PM
Use of FRP Composite Materials in Seismic Retrofitting of Structures
Abstract
Seismic Retrofitting for reduction of vulnerability of a structure is a relatively new concept in
India. India was not considered to be a seismically active country unless recently some
earthquakes like one in Latur (93) and Bhuj Earthquake (2001) among the major ones has
happened. The code was revised and a full survey was done to include most of the regions in
earthquake activity in 2002 with introduction of IS 1893-2002. With the new codes the need
for seismic retrofitting for the existing and the new buildings has arisen. In this paper we
explore how FRP composites have been very effective in seismic retrofitting of structures.
We explore a full scale building test done at CPRI, Bangalore for studying the effectiveness
as a retrofit. We also present a Case Study of a building being retrofitted by using FRP
Composites and other methods to comply it with the new codes and new usage in New Delhi.
Introduction
Seismic retrofitting is the modification of existing structures to make them more resistant
to seismic activity, ground motion, or soil failure due to earthquakes. With better
understanding of seismic demand on structures and with our recent experiences with large
earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. In
India after the many earthquakes the Codes were revisited as more than 50 % of the Indian
land has become seismically active which was not taken care of in the previous codes.
The new code developed i.e. IS 1893 (Part 1) 2002 assigns four levels of seismicity i.e it
divides active zones into 4 Zones namely Zone 2, 3, 4 , 5. Higher Zone means high activity.
The specifications given in the design code (IS 1893: 2002) are not based on detailed
assessment of maximum ground acceleration in each zone using a deterministic or
probabilistic approach. Instead, each zone factor represents the effective period peak ground
accelerations that may be generated during the maximum considered earthquake ground
motion in that zone. Many buildings constructed before this code are not adequate and hence
fall in the danger zone.
FRP as a technology
A Fiber Reinforced Polymer (FRP) composite is defined as a polymer (plastic) matrix,
either thermo set or thermoplastic, that is reinforced (combined) with a fibre or other
reinforcing material with a sufficient aspect ratio(length to thickness) to provide a
discernable reinforcing function in one or more directions. FRP composites are different
from traditional construction materials such as steel or aluminum.
Testings for seismic Retrofitting by FRP:
Reactor Safety Divison of Bhabha Atomic Research Centre focuses on designs of structures
which can withstand earthquakes and do not jeopardise the human life in india.
There are many relatively new technologies developed for seismic Retrofitting which are
based on “Response control”. These techniques includes providing additional damping
using dampers (Elasto-plastic dampers, friction dampers, tuned mass and tuned liquid
dampers, visco-elastic dampers, lead extrusion dampers etc.) and techniques such as base
isolation which are introduced to take care of seismic control.
Testing of the Full scale Model of the RCC structure.
Reactor Safety Division (RSD), BARC along with Central Power Research Institute (CPRI)
Bangalore conducted this test along with the involvement of many other institutions. The
structure constructed for testing was a replica of a substructure of an existing office building
at BARC. Pushover tests were carried out on the structure with the tower testing facility.
The structure under consideration was tested till failure. Fig shows the structure being tested
at the tower testing facility at CPRI Bangalore.
Experimental Results
The pushover curves as obtained for as built and retrofitted structure are shown in Figs 4 and 5
respectively. The curves were plotted till large damage occurred at various locations and no
significant lateral load resistance remained in the structure.
As seen from the plots, the original structure could resist a peak base shear of around 900 kN. During
the test, the structure displayed various failure modes and at the end of the test, the structure could not
resist any significant lateral load. However, the structure after being repaired and retrofitted with the
FRPC could be restored up to almost 90% of its original state and the retrofitted structure could resist
a peak base shear of around 800 kN. Till date, almost all the tests on FRPC retrofitting have been
conducted at component level or small scale structural level. The results of this test could therefore
very well prove the efficacy of the retrofit system using FRPC.
Conclusions:
Many techniques for Seismic retrofitting of the existing or newly constructed structures have
been considered and the use of FRP for these applications were found to be effective.
Failure modes and test results of retrofitted structure in CPRI, Bangalore revealed that while
performing retrofitting design, one needs to understand the behavior of the structure in both
linear as well as nonlinear range by analysis. Identify the change in the failure modes and
locations and if required the retrofitting scheme has to be appropriately modified. Thus, the
retrofitting design is an iterative procedure. Another very important aspect of repair and
retrofitting is the workmanship. Surface preparation by removal of old loose concrete,
pouring of new concrete and its bonding with old concrete, rounding off sharp corners etc are
few important aspects. Also, proper care should be taken to bond the FRP sheets and
laminates to the original structure.