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Comparative Seismic Response
of Multi-Hinged Articulated Offshore Tower
-- Nazrul Islam, Syed Danish Hasan,
M Moonis Zaheer and Khalid Moin
The response of offshore structures to seismic sea environment in deep water conditions is a complex subject.
In such a harsh environment, special offshore structures called compliant offshore structures are
economically more attractive than conventional fixed offshore structures. Articulated towers are among the compliant
offshore structures that freely oscillate along with the waves and current, as they are designed to have lower
natural frequency than that of ocean waves. The present study deals with the comparative seismic compliance of a
multi-hinged articulated tower with two different seismic sea environments. For this purpose, time histories of
El Centro 1940 and Taft 1952 earthquakes are used and the results are compared with each other, as well as
with mild random sea state only. The tower is modeled as an inverted pendulum connected to the sea bed through
a universal joint with a concentrated mass at the top, having double rotational degree of freedom. It is
equipped with up-righting buoyancy chamber near the Still Water Level (SWL), which provides the restoring
moment against the destabilizing moment. The nonlinear equation of motion is derived using Lagrange's equation
and the solution is obtained by Newmark-b time marching scheme. Results are expressed in the form of
time-history of deck displacement, and the instantaneous bending moment, shear force and axial force are plotted
for comparative study. © 2009 IUP. All Rights Reserved.
Effect of Variation in the Damping of Bearings
for Base Isolated Buildings
-- Poonam, Anil Kumar, Saraswati Setia and
V K Sehgal
Base isolation is a modern earthquake-resisting technique in
which a superstructure is decoupled from foundation with the help of some isolation device to effectively reduce the transfer of ground motion.
Base isolation reduces strength and ductility demand on the
superstructure, resulting in lower dependence on post-yield behavior. The earthquake forces do not get directly transferred to the superstructure in
the case of base isolated structures, as they do in
the case of fixed base structures. Variation in damping of bearings
can further control the magnitude of earthquake forces transferred to the superstructure. An effort has
been made to study the effect of variation in the damping of bearing on
the response of two base isolated RC frames of different
configurations against earthquake forces. Damping in bearings is varied from 5%
to 40% and the response of the structures is compared in terms of period shift, base shear, storey drift
and acceleration-response with equivalent fixed base frames. Response spectrum analysis of these frames
is carried out with Manual STAAD Pro (2005). The response of base isolated frames
showed a significant reduction in various parameters as compared with
fixed base frame. It is found that the rate of
response reduction is high up to 20% damping in bearing. Beyond 20%
damping, the rate of response reduction is less. © 2009 IUP. All Rights Reserved.
Study of Piled Raft Foundation Using FEM
-- V A Sawant and G S Kame
This paper presents an improved solution algorithm based on Finite Element Method
(FEM) to analyze piled raft foundation. Piles are modelled as bar elements with soil springs. Finite element analysis of raft
is based on the classical theory of thick plates resting on Winkler foundation that accounts for the
transverse shear deformation of the plate. Four node-isoparametric rectangular elements with three degrees of
freedom per node are considered for the development of finite element formulation. Independent bilinear
shape functions are assumed for displacement and rotational degrees of freedom. The effect of raft thickness,
soil modulus and load pattern on the response is considered. Significant improvement in the settlements
and moments in the raft is observed.
© 2009 IUP. All Rights Reserved.
Reliability Analysis of Concrete Columns
Reinforced Internally with Glass Fiber Reinforced
Polymer Reinforcements
-- A Deiveegan and G Kumaran
This paper presents the development of a resistance model for concrete columns reinforced with
Fiber Reinforced Polymer (FRP) reinforcements. The resistance model is used to calculate the probability
of failure and reliability index of concrete columns reinforced with FRP reinforcements. The strength
limit state functions are developed for reinforced rectangular columns depending on the cross section
size, reinforcement ratio, and load eccentricity. For a known eccentricity condition, solutions for the
corresponding force-moment strength in the interaction diagram are developed. Sensitivity analysis is performed to
determine the design variables that have the highest influence on the reliability index. The First Order
Reliability Method (FORM) is employed to calibrate the resistance factor for a broad range of design variables.
The study shows that the introduction of FRP reinforcements in place of conventional steel
reinforcements improves reliability somewhat because the strength of FRP reinforcements has a lower coefficient of
variation than steel or concrete. However, the brittle nature of FRP reinforcements necessitates a reliability index
that is greater than that generally implied in flexural structural components. This leads to a resistance
factor that is slightly lower than currently accepted for reinforced concrete sections in flexure. © 2009 IUP. All Rights Reserved.
Strength of Steel Tubular Slender Columns
Infilled with Concrete Using Waste Materials
-- E K Mohanraj, S Kandasamy and R Malathy
The behavior of circular and square Concrete-Filled Steel Tubular (CFST) sections with various
concrete (partial replacement of coarse aggregate
with granite, rubber, construction and demolition
debris, and fiber) strengths under axial load is presented. The effects of steel tube dimensions,
shapes, and the confinement of concrete are examined. Measured column strengths are compared with the values predicted by
Eurocode 4 (EC4), Australian Standards, and American
Codes. 12 specimens were tested with a strength of
concrete as 20 MPa and a D/t ratio 25.0 and 41.2. The columns were 75 mm in
diameter, 70 mm in square, and
900 mm in length. All the three codes predicted lower values when
compared to experimental results. Modified ACI/AS gives the best estimation for CFST sections with rubber, fiber and
C and D debris concrete.
© 2009 IUP. All Rights Reserved.
Development of Mix Design for High Strength
Self-Compacting Concrete Using Admixtures
-- G Giri Prasad, G V Ramarao and
M V Seshagiri Rao
Self-Compacting Concrete (SCC) is ushering in a revolution in concrete technology. Not only can the
placement and compaction of concrete become literally silent, since no vibrators are needed, but the rate of
placement can also be increased manifold. The most difficult to place forms or molds are filled in completely by
SCC. There is an in built assurance of uniform placement and fully consolidated concrete when SCC is used at
site. This ensures high durability since air voids and other flows are likely to be absent in SCC. An attempt has
been made to develop a mix design for SCC for different grades of concrete, viz., M50 to M80, using the
industrial by products—Fly ash and Rice Husk Ash (RHA)—and investigate its flow and strength properties in
comparison with conventional concrete. The proposed mix design procedure coincided with the mix design
guidelines given by EFNARC and others. The flow properties such as passing ability, filling ability, segregation
resistance and compaction factor are checked by conducting various tests. The compressive strength of concrete at
the age of 3, 7 and 28 days and also the split tensile strength and flexural strength for 28 days are reported.
© 2009 IUP. All Rights Reserved.
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