Page 11 - Open-Access-Nov-2019

TECHNICAL PAPER

Design of seismic

stRengthening

techniqUe foR soft-

stoRy Rc fRAmes

Using bUckLing-

RestRAineD bRAces BIRENDRA K. KARAIYA,

DIPTI RANJAN SAhOO, AShOK GuPTA

Abstract is dissipated through the inelastic hysteretic response of
ground story columns only. Due to the limited lateral (flexural or
This study is focused on the seismic strengthening of reinforced shear) strength and plastic rotation capacity of these columns,
concrete (RC) frames with soft-stories using buckling-restrained
braces (BRBs) in order to control the severe damages or many OGS RC frame buildings have completely collapsed in
[2-3]
complete collapse of these frames. A design procedure the past earthquakes (e.g., ). The seismic performance of
has been proposed to determine the BRB sizes required these deficient structures can be improved by using effective
for the target design base shear of the study frames. The strengthening techniques.
seismic performance of the strengthened frames is evaluated Buckling-restrained braces (BRBs) are capable of yielding in both
numerically using a computer software SAP2000. The modelling [4]
technique adopted for BRBs and frame members are discussed. tension and compression under reversed cyclic loading . Figure
Linear modal analyses are carried out to determine the mode 1(a) shows a typical BRB with different components, hysteretic
shapes and time periods of the strengthened frames. Nonlinear response, and idealized backbone curve. BRBs exhibit nearly
static and dynamic analyses are carried out to investigate the symmetric and stable hysteresis along with excellent low cycle
[5]
lateral strength, failure mechanism, interstory drift response fatigue potential . Many early investigations revealed the
and residual drift response of the study frames. The seismic importance of achieving de-bonding between metallic core and
performance of the strengthened frames is also compared the surrounding buckling-restraining arrangement for stable
6-8
with those of the unstrengthened frames. The installation of and balance hysteretic response [e.g., ]. Figures 1(b) and (c)
BRBs significantly improved seismic performance of 4-story and show the hysteretic response and back-bone curve of axial
8-story RC frames with soft story. However, the lateral load- force-displacement of a typica BRB. The main purpose of the
resisting potential of BRBs could not be fully utilized in the concrete-filled tube is to prevent the buckling of the steel core
20-story RC frame with soft ground story. carrying the axial force. Steel core-concrete interface usually
consists of a slip surface to allow the relative axial deformations
Keywords: Displacement ductility; Energy dissipation; between the steel core and the tube infill. The slip surface is
Lateral strength Reinforced concrete frame; Seismic design; achieved by placing a low friction material between the infill
Strengthening material and the steel core. The transversal expansion of the
brace under compressive loads due to Poisson’s effect should
1. INtRODUCtION be accommodated providing a gap between the brace and
In many countries, reinforced concrete (RC) frames with open the encasing material. Recently, BRBs with short yielding core
ground story (OGS) are preferred in the building construction segments with detachable casings [9-10] and steel restraining
[11]
as they facilitate for many architectural and recreational elements have shown the stable hysteretic response and
[1]
activities, such as, parking, garages, and storage . The the better displacement ductility. The effects of type of beam-
increased flexibility at the ground story level results in the drift column joints and column sizes on the seismic performance of
concentration under lateral loading condition. Very often, this steel braced frames with BRBs have been investigated in order
effect is not considered in the design of masonry-infilled RC to control the development of soft-story mechanism and the
[12]
frames. In the event of earthquakes, the input seismic energy excessive residual drift response .
[13]

The IndIan ConCreTe Journal | november 2019 23