Page 4 - Open-Access-Nov-2019

TECHNICAL PAPER

seismic PeRfoRmAnce

AnALysis of

oUtRiggeR

stRUctURAL system

With DAmPeRs M. RAME GOwDA, ShIvIE R. P. GOvEAS

Abstract actions and plays important criteria in its structural design.
Growth in economy leads to development in cities at a much Earthquake ground motion can occur anywhere in the world and
higher rate and tall buildings are a direct effect of this growth. the risk involved with tall buildings under severe earthquake is
The cities with much higher seismic risk will have to develop new more and special attention should be provided in its analysis
innovative concepts of designing tall structures and an outrigger and design, as tall buildings often have thousands of occupants.
structural system is one of many such design concepts which The structure should be designed to perform well during an
have made helped in constructing tall buildings. Outrigger earthquake and ensure that the building remains serviceable
structural system is easy to construct and they provide high without causing major damage to the structure. Innovative
lateral stiffness. Further this outrigger system is provided with structural design methods have continuously been sought in
dampers to improve the efficiency of the structure. This work the design of tall structures with the intention of reducing the
presents the performance analysis of such outrigger structural structural response due to the lateral loads.
system with dampers. Models consisting of one and two
outriggers are developed and their performance is analyzed 1.1 Outrigger structure
based on their response to dynamic analysis (time history) An outrigger structural system is one of many design concept
carried out to the past historical earthquakes in the California used to reduce the structure’s response due to lateral loads.
region. This work has been carried out using the ETABS This design concept consists of reinforced concrete or steel
software. The parameters discussed are lateral displacements, braced core that is connected to the exterior columns by a
inter-story drifts, shear force and bending moment in the pair of stiff flexural horizontal cantilevers at certain story levels.
core for dynamic analysis. Also the performance of outrigger While the outrigger system is effective in providing lateral
structural system with dampers is analysed varying flexural flexural resistance to the structure but it does not increase its
rigidity ratio from 0.25 to 2. From the analysis it is found that shear stiffness and the core carry all the lateral shear forces. The
there is reduction in responses of the outrigger structure with outrigger acts as a stiff beam which connects the central core
dampers. with the exterior columns. When the lateral load acts on the
structure the column restrained outrigger resists the rotation of
Keywords: Outrigger, dampers, flexural rigidity ratio the core reducing the moments of the core than that if only the
free standing core alone resisted the loading. The resistance
1. INtRODUCtION offered by a free standing core wall system alone is adequate;

Tall buildings have always fascinated mankind. The construction however the drift increases approximately to the cube of the
of tall buildings is a response or impact due to the economic height of the building. This makes the core system inefficient
growth, increase in population and also limited land space. for very tall buildings as drift will increase with the increase
Tall buildings have been a prestige symbol for big cooperation in height of the structure. The outrigger system on the other
organization as well as for developed countries. They have hand will help to reduce this lateral drift and is very efficient for
also been a tourist attraction to many countries especially the tall buildings. Further to improve the seismic performance of
countries in the Middle East. In some countries due to the this outrigger system, dampers are introduced in the external
topographical restrictions for example in Rio de Janeiro and peripheral columns. These dampers act like hydraulic shock
Hong Kong, make the tall buildings as the feasible solution absorbers whenever seismic energy is transmitted through them,
for residential purposes. From a structural engineer’s point of dampers absorb a part of it and thus dampen out the motion of
view, a tall building may be defined as one that because of its the building.
height is affected by the lateral forces due to wind or earthquake

16 The IndIan ConCreTe Journal | november 2019