19-01-2013, 11:25 AM
Structural Design of Multi-story Residential Building for in Salem , India
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INTRODUCTION
The structural analysis and design aspects of a four story reinforced – concrete building,
designed and built in Salem, Tamilnadu State, India, is described herein. The design was
referred to the second author for proof checking. The building is an apartment complex
proposed by the Tamil Nadu Housing Board . This is a reinforced concrete framed
building. This was done at the PSG College of Technology, Coimbatore, India, as a part
of senior thesis design at WPI, in Worcester MA, 01609 at the India Project Centre at
PSG College of Technology, Coimbatore, India, for a period of 7 weeks as a residential
student project. The students names are Frederic Carrie, Abraham Pinales and Antonio
Durate. This was accomplished as their project centre in PSG College of Technology,
Coimbatore, India, during the year 1998. This was part of the degree requirements for
BSCE in Civil and Environmental Engineering at WPI. WPI sent 20 students to the India
Projects program at PSG College of Technology, Coimbatore, under the sponsorship of
Dr. S. Rajasekaran, Chair, CEE Department at PSG College of Technology. Professor S.
Rajasekaran served as the on campus advisor, while Professor P. Jayachandran, served as
an off-campus advisor at WPI.
DESIGN SPECIFICATIONS
The three dimensional view of the building is shown in Fig.1 and the plan view is given
in Fig.2. The geometrical properties of the structure included a maximum length
spanning 48.27 meters, and a maximum width of 31.60 meters. The building comprised
of 2.9 meter floors and spanned 11.6 meters above grade. A wall beam was provided 0.75
meters below grade to support the earth pressure against supporting columns above the
footings of the structure. At the roof level, a mechanical room was provided for the
elevator of the complex. ( 1metre = 3.281 feet )
Each typical floor consisted of 8 apartments. The slab of the bath room was depressed
approximately 0.6 meters and was accounted for in the beam design around the toilet
rooms. A balcony was attached to each apartment, and the loading was accounted for in
the floor beam design . The Indian code BIS 456- 1978 was used using the concrete mix
with 15 MPa and 415 MPa reinforcing steel. The surrounding conditions indicated a low
– seismic, and a strong soil layer boundary with a strength of 150 KN/sq.m. As indicated,
the height to width ratio of the structure did not exceed 2, therefore a wind analysis was
not required according to the Indian Code. Due to the monsoon season, the Indian Code
included a specific loading for the roof of the structure.
SIX STEPS IN THE ANALYSIS
The following six steps are used in the Finite element analysis of any structure.
1. Idealize the structure into a number of elements
2. Develop the element stiffness matrix using constitutive law.
3. Assemble the element stiffness to form global stiffness matrix using compatibility
and equilibrium.
4. Apply necessary boundary conditions
5. Solve the equations [K]{r}={R} where [K] is the structural stiffness matrix, {r} –
generalized displacements and {R} – generalized forces.
6. Knowing the displacements solve for elemental stresses.
The four story-building complex included a 3 dimensional mesh arrangement consisting
of 2212 nodes with total of 6 degrees of freedom at each node. This model required a
total of 13272 equilibrium equations to be solved in the form of a matrix system, ad in
order to compute bending moments and displacements. Fortunately, the building contains
a symmetry line, which permitted the analysis of half of the structure , therefore reducing
the mesh arrangement to 1106 nodes and the number of equations was reduced to a total
of 6636 equations which were solved using STAAD-III. The renumbering of nodes is
automatically done so as to reduce the bandwidth. Reducing the number of equations and
reducing the bandwidth of the matrix system to be solved, which alternatively required
less time from cpu point of view.
RC-DESIGN SUITE
RC –Design suite is a reinforced concrete design program that has numerous applications
for the design of concrete structures. It contains modules for the design of beams,
columns , footings and slabs. For this project the students utilized this program only for
the design of floor slab and combined footings. This program will also be used in the
partial design of the raft footing. Design of beams and columns are carried out in
STAAD- III package itself. A clear cover of 20 mm is adopted for the slab. For each
column load the individual footing is designed. However, many of these individual
footings overlapped each other and had to be redesigned as combined footings. RC
Design – suite provided an effective way of designing these combined footings through
its footing design module. The footing design module of the program provides the
footing length, effective depth and reinforcement requirements. The raft footing was
designed in the area under the lift duct of the building. Some criteria dictated that the
support to the elevator shaft should be watertight. RC-Design Suite was used to engineer
the design of the raft. Four column loads of 562 kN, 425 kN, 832 kN and 498 kN are
being transmitted to the four corners of the raft of the lift well. The design of the raft
footing was modelled adopting an inverted slab-beam-column approach. To prevent
deterioration in the footing, associated with the position of the water table, it was decided
that a raft footing would be beneficiary since its depth would be considerably less than a
conventional shallow or combined – footing. The thickness of the raft was 0.6 m .