Seismic response analysis of a structure is a complex process. Depending on
the structural problem and the needed accuracy in solution assumptions are
made. In this paper, "Comparative Seismic Response of Multi-Hinged
Articulated Offshore Tower", the authors, Nazrul Islam, Syed Danish Hasan, Moonis Zaheer
and Khalid Moin, have analyzed offshore tower in the sea environment in deep water.
A multi-hinged tower was modelled on an inverted double pendulum and the
response was calculated under two earthquakes El Centro 1940 and Taft 1952, and
random sea wave action was also considered. In the numerical studies, the duration
of excitation was considered as 30 seconds for both the earthquakes, with a
starting time on the tower as 490 seconds. The authors conclude that the seismic
responses are predominant than the responses under random sea waves. The importance
of the earthquake consideration in the design of articulated offshore tower should
not be ignored.
Base isolation of structures is a technique where superstructure is
decoupled with the substructure with designed arrangement. This helps to control the
structural response, particularly, in seismic excitations. In this paper, "Effect of Variation in the
Damping of Bearings for Base Isolated Buildings", the authors, Poonam, Anil
Kumar, Saraswati Setia and V K Sehgal, have studied the effects of damping variation
in bearings of base isolated structure. A four-storey moment resisting RC frame
building was analyzed. Base isolation was considered with variation in damping in the
bearing from 5% to 40% for the study. For comparison of results, the building was
modelled as a fixed base system. Base shear, storey drift, period shift and acceleration
response were compared against fixed base results. The authors conclude that the rate
of response reduction is high up to 20% damping in bearing, and beyond 20% the
rate of response reduction is not significant. The authors have reported a
significant increase in the time period of the building as much as five times. If that happens
the earthquake forces developing in the building will be much less and the building
can be designed economically.
High rise buildings are heavy structures and need heavy foundations. Raft or
pile foundation is quite common. An improvement in using raft or pile foundation is
a combination of pile-raft support. The paper, "Study of Piled Raft Foundation
Using FEM", by the authors, V A Sawant and G S Kame, states that if the
superstructure load is carried by raft through contact with the soil and also by piles through
skin friction, then the substructure may become economical. In this analysis,
finite element formulation was used. The raft was modelled as a thick plate element
resting on Winkler foundation. Four noded iso-parametric elements with three degrees
of freedom per node, transverse displacements, and rotation about two axes
were considered with independent bilinear shape functions for displacement and
rotational degree of freedom. Piles are considered as bar elements with surrounding
soil modelling by Winkler's hypothesis. A parametric study was done to compare
the results. The comparison of results indicates that maximum deflection and
maximum moments reduce more considerably in pile raft foundation than in a raft foundation.
In the paper on "Reliability Analysis of Concrete Columns Reinforced
Internally with Glass Fiber Reinforced Polymer Reinforcements", the authors, A
Deiveegan and G Kumaran, report the development of a resistance model. Reliability
analysis predicts the probability of performance of structural members during its service
life under exposed environmental conditions. The authors discuss the development of
a resistance model for concrete column reinforced with fiber reinforced polymer.
The resistance model was used to calculate the probability of failure and reliability
index. The strength limit state functions were further developed for rectangular
shape columns with load eccentricity and reinforcement ratio. In the assumed
problem, the interaction diagram does not show the balance point. The results obtained
for two different columns have been presented.
The behavior of circular and square steel column filled with demolition
debris, fiber and granite rubber have been tested and its experimental strength
predicted by code formula. The effect of column dimension, shape and confinement in
concrete has also been studied. The authors, E K Mohanraj, S Kandasamy and R Malathy,
in their paper, "Strength of Steel Tubular Slender Columns Infilled with Concrete
Using Waste Materials", report that the test specimen started bending during the test,
and there was no change in longitudinal dimensions. The strength of column filled
with partial replacement of waste material is almost as that of plain concrete
filling, whereas partial replacement of granite as coarse aggregate shows a higher
value. The strength of fiber reinforced column is quite high. The shape of column has
no effect on the strength. The modified ACI/AS equation provides good predictions.
Self-compacting concrete is considered as a high performance concrete with
good deformability and segregation resistance. To produce this concrete the major
work requires its proper mix design and evaluation of its properties. Because of its
high fluidity, self-compacting and segregation resistance, it reduces honeycombing
in concrete. In the paper, "Development of Mix Design for High Strength
Self-Compacting Concrete Using Admixtures", by G Giri Prasad, G V Ramarao and M V Seshagiri
Rao, rice husk ash was used to replace partly the aggregates, as it has pozzolanic
properties. The authors conclude that the designed concrete satisfies the recommended
values, and the requirement for the cement is reduced. In a conventional concrete of M
50 and M 60 grade, addition of 5% Rice Husk Ash (RHA) shows better results, and
M 70 and M 80 grades of concrete requirements of RHA becomes 10%. In this
study, the best self-compacting properties were obtained with fly ash of 70% and RHA
of 30% and 53 grade cement with constant water cement ratio.
-- Satyendra P Gupta
Consulting Editor
