ISSN: 2277-3754 ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT) Volume 4, Issue 12, July 2015 62 Abstract— In normal practice, restrained stresses in reinforced concrete (RC) structures due to temperature variation may be overcome by enforcing expansion joints every certain length limit, according to codes of practice. Nevertheless, expansion joints have their architecture and durability problems. In this paper, reinforced concrete flat slab systems of lengths much greater than the codes limits, if temperature effect is disregarded, are studied under dead and live loads, and thermal loads in order to examine the effect of temperature variation. The systems are modeled properly by accounting for material nonlinearity, particularly cracking. Different temperature gradients, uniform and nonlinear, are considered. The finite element method is employed for conducting the analysis by utilizing the finite element code ABAQUS, where different features of material nonlinearities are considered. The obtained results for the studied cases reveal that material modeling of reinforced concrete flat slab systems makes plays a major role in how these structures react to temperature variation. Cracking contributes to the release of significant portion of temperature restrain and in some cases this restrain is almost eliminated. The response of a system significantly deviates based on the temperature profile and the presence of gravity loads. Index— ABAQUS, finite element, flat slab, material nonlinearity, reinforced concrete, thermal loads. I. INTRODUCTION Due to the continuity of reinforced concrete flat slab and the interaction between the slab and the supporting columns with the assumption of rigid connections between slab and columns, the slab is not completely free to move under temperature variation. Hence additional stresses due to thermal loads, due to either uniform temperature variation or temperature gradients, will be produced in the slab and columns. In such condition, exterior and corner columns will be the most critical columns. The stiffness of different elements of the structural system plays a major role in the resulting stresses; for instance, cracks due to existing loadings relieves significant portion of such stresses. In order to release restrained stresses from temperature variation many designers use "rules of thumb" that set limits on the maximum length between building expansion joints. Although widely used, rules of thumb have the drawback that they do not account for the many variables which control volume changes in reinforced concrete buildings. Examples of the variables which affect the amount of thermally induced movement include the percentage of reinforcement, which limits the amount of movement and cracking in the concrete; the restraint provided at the foundation, which limits the movement of the lower stories; the geometry of the structure, which can cause stress concentrations to develop, especially at abrupt changes in plan or elevation; and provisions for insulation, cooling, and heating, which affect the ability of a building to dampen the severity of outside temperature changes [1]. Due to the complexity of the problem and the previous limitations for using expansion joints in addition to its bad appearance and difficulty of construction and maintenance, designers become interested in the design of buildings without expansion joints and take the effect of temperature variations and additional stresses into account during the design stage. The analysis of reinforced concrete flat slabs under temperature variation is a three dimensional problem but it may be assumed as a two dimensional problem since the temperature varies only within the slab thickness and can be assumed constant all over the slab length. In this study, the effect of thermal loads, in the presence of gravity loads, on the behavior of flat plate system utilizing the finite element method, is examined. The employed analysis accounts for material nonlinearity which has a significant effect, particularly cracking, on the structure response. Three models of flat plates are included in this paper under the names ST1, ST2 and ST3. The computer code ABAQUS is utilized to perform the finite element analysis. II. FLATE PLATE MODELS A. Model ST1 Geometry and Dimensions Fig. 1 shows the geometry of the slab ST1 which consists of one story flat plate of thickness 250mm resting on square columns of dimensions 500×500mm with clear height equal to 4.0m. In the x-direction there are seven spans, each span is equal to 8.0m from the center lines of columns, so the total slab length in the x-direction is equal to 56.5m. In the z-direction there are five spans, each span is equal to 8.0m from the center line of columns, so the total slab length in the z-direction is equal to 40.5m. Reinforcing Steel The slab is reinforced with a bottom and top steel mesh of bar diameter Φ10 mm every 150mm in the x- and z-directions. The slab is provided with additional top reinforcement at the intersection zones of column strips such that the total top reinforcement is equivalent to a bar of diameter Φ18 mm every 150mm in the x- and z-directions at these zones. The top and bottom concrete covers are equal to 25mm measured from the slab exterior fibers to the center of the rebar. The vertical and horizontal reinforcements of columns are shown in Fig. 2. Salah E. El-Metwally 1 , Hamed S. Askar 2 , Ahmed M. Yousef 3 , Essam H. El-Tayeb 4 Structural Engineering Department, Mansoura University, El-Mansoura, Egypt Analysis of RC Flat Slab System for Thermal Loads
12
Embed
Volume 4, Issue 12, July 2015 Analysis of RC Flat Slab ... 4/Issue 12/IJEIT1412201506_11.pdf · Volume 4, Issue 12, July 2015 62 Abstract— In normal practice, restrained stresses
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
ISSN: 2277-3754
ISO 9001:2008 Certified International Journal of Engineering and Innovative Technology (IJEIT)
Volume 4, Issue 12, July 2015
62
Abstract— In normal practice, restrained stresses in reinforced
concrete (RC) structures due to temperature variation may be
overcome by enforcing expansion joints every certain length limit,
according to codes of practice. Nevertheless, expansion joints
have their architecture and durability problems. In this paper,
reinforced concrete flat slab systems of lengths much greater than
the codes limits, if temperature effect is disregarded, are studied
under dead and live loads, and thermal loads in order to examine
the effect of temperature variation. The systems are modeled
properly by accounting for material nonlinearity, particularly
cracking. Different temperature gradients, uniform and
nonlinear, are considered. The finite element method is employed
for conducting the analysis by utilizing the finite element code
ABAQUS, where different features of material nonlinearities are
considered. The obtained results for the studied cases reveal that
material modeling of reinforced concrete flat slab systems makes
plays a major role in how these structures react to temperature
variation. Cracking contributes to the release of significant
portion of temperature restrain and in some cases this restrain is
almost eliminated. The response of a system significantly deviates
based on the temperature profile and the presence of gravity loads.
Index— ABAQUS, finite element, flat slab, material
nonlinearity, reinforced concrete, thermal loads.
I. INTRODUCTION
Due to the continuity of reinforced concrete flat slab and
the interaction between the slab and the supporting columns
with the assumption of rigid connections between slab and
columns, the slab is not completely free to move under
temperature variation. Hence additional stresses due to
thermal loads, due to either uniform temperature variation or
temperature gradients, will be produced in the slab and
columns. In such condition, exterior and corner columns will
be the most critical columns. The stiffness of different
elements of the structural system plays a major role in the
resulting stresses; for instance, cracks due to existing loadings
relieves significant portion of such stresses.
In order to release restrained stresses from temperature
variation many designers use "rules of thumb" that set limits
on the maximum length between building expansion joints.
Although widely used, rules of thumb have the drawback that
they do not account for the many variables which control
volume changes in reinforced concrete buildings. Examples
of the variables which affect the amount of thermally induced
movement include the percentage of reinforcement, which
limits the amount of movement and cracking in the concrete;
the restraint provided at the foundation, which limits the
movement of the lower stories; the geometry of the structure,
which can cause stress concentrations to develop, especially
at abrupt changes in plan or elevation; and provisions for
insulation, cooling, and heating, which affect the ability of a
building to dampen the severity of outside temperature
changes [1].
Due to the complexity of the problem and the previous
limitations for using expansion joints in addition to its bad
appearance and difficulty of construction and maintenance,
designers become interested in the design of buildings
without expansion joints and take the effect of temperature
variations and additional stresses into account during the
design stage.
The analysis of reinforced concrete flat slabs under
temperature variation is a three dimensional problem but it
may be assumed as a two dimensional problem since the
temperature varies only within the slab thickness and can be
assumed constant all over the slab length. In this study, the
effect of thermal loads, in the presence of gravity loads, on the
behavior of flat plate system utilizing the finite element
method, is examined. The employed analysis accounts for
material nonlinearity which has a significant effect,
particularly cracking, on the structure response. Three models
of flat plates are included in this paper under the names ST1,
ST2 and ST3. The computer code ABAQUS is utilized to
perform the finite element analysis.
II. FLATE PLATE MODELS
A. Model ST1
Geometry and Dimensions
Fig. 1 shows the geometry of the slab ST1 which consists of
one story flat plate of thickness 250mm resting on square
columns of dimensions 500×500mm with clear height equal
to 4.0m. In the x-direction there are seven spans, each span is
equal to 8.0m from the center lines of columns, so the total
slab length in the x-direction is equal to 56.5m. In the
z-direction there are five spans, each span is equal to 8.0m
from the center line of columns, so the total slab length in the
z-direction is equal to 40.5m.
Reinforcing Steel
The slab is reinforced with a bottom and top steel mesh of
bar diameter Φ10 mm every 150mm in the x- and z-directions.
The slab is provided with additional top reinforcement at the
intersection zones of column strips such that the total top
reinforcement is equivalent to a bar of diameter Φ18 mm
every 150mm in the x- and z-directions at these zones. The
top and bottom concrete covers are equal to 25mm measured
from the slab exterior fibers to the center of the rebar. The
vertical and horizontal reinforcements of columns are shown