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CHAPTER 5
FINITE ELEMENT ANALYSIS OF GFRP
COMPOSITE BRIDGE DECK PANELS
5.1 GENERAL
Bridge decks made of FRP have been widely studied and
increasingly used in highway bridges, both in new construction and
replacement of existing bridge decks. It is known that FRP composite
materials have a number of advantages including, high specific stiffness and
specific strength ratios, good fatigue behaviour, and corrosion resistance.
However, compared to traditional construction materials, such as steel,
timber, and concrete, GFRP materials have more complex material properties
and structures exhibit distinctive behaviours. Investigations of the behaviour
of FRP bridge decks were conducted through laboratory tests on FRP deck
components, and field tests on FRP bridges. Field tests were performed under
service loads which are significantly lower than failure loads.
Experimental investigations on the other hand are more suitable for
strength/capacity assessment studies through destructive testing. However,
such tests seldom consider the entire structure due to equipment limitations
and associated costs. Furthermore, parametric studies in experimental
procedures are time consuming and prohibitively expensive. Computer
simulations based on advanced methods, such as the FEM, are reliable and
cost effective alternatives in structural analysis for the study of structural
response and performance. FEM procedures have been successfully employed
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in research studying the performance of FRP bridge decks or their
components. The general purpose finite element software ANSYS or
ABAQUS can be used for the modeling and analysis of multicellular FRP
composite bridge deck panels with different cross sectional profiles and that
has many analytical capabilities, ranging from a simple, linear, static analysis
to a complex, nonlinear and transient dynamic analysis. In this study the finite
element software ANSYS is used for the modeling and analysis of
multicellular FRP bridge deck panels. A preliminary analysis was carried out
on models created using ANSYS by taking IRC class A loading, to optimize
the cross sectional profile that can be used for the fabrication of the
experimental models.
5.2 PERFORMANCE CRITERIA
From the literature review, it has been observed that the design of
GFRP bridge deck panels is driven by stiffness and hence maximum
deflection is the governing criteria in design. The loads imposed on the bridge
decks include dead load, which includes the self-weight and weight of future
surface wearing course, and the live load imposed in the form of wheel load.
These loads should be factored up suitably to account for impact and variation
in material properties. The deflection produced by this factored load must be
less than the limiting value of deflection. AASHTO has set up a deflection
limit of Span / 800 for FRP bridge deck panels.
5.3 IRC CLASS A LOADING
According the specifications given by the Indian Roads Congress
(IRC 6 - 2000), IRC class A loading is to be normally adopted on all roads on
which permanent bridges and culverts are constructed. The IRC class A train
of vehicles is shown in Figure 5.1.
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Figure 5.1 IRC class A train of vehicles (axle loads in tones, linear dimensions in m)
To obtain the maximum bending moment and shear force, the
maximum wheel load should be considered as shown in Figure 5.2. The
ground contact area for the maximum axle load of 114 kN as specified in IRC
6 - 2000 is 500 mm perpendicular to the direction of motion and 250 mm
parallel to the direction of motion. The minimum clearance to be ensured
between the outer edge of the wheel and the inner face of the kerb is 150 mm
for all carriage way widths. The width of a single lane carriage way is 3.75 m
and that of two lane carriage way is 7.5 m as per IRC 5 - 1998. The ground
contact area for the maximum axle load and the distances between the wheels
in both directions has been indicated in Figure 5.3.
Figure 5.2 IRC Class A loading
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All dimensions are in mm
Figure 5.3 Ground contact area for maximum Axle load of IRC Class A loading
5.4 SELECTION OF CROSS SECTIONAL PROFILES
Multi-cell box sections are commonly used in deck construction
because of their light weight, efficient geometry, and inherent stiffness in
flexure and torsion. Also, this type of deck has the advantage of being
relatively easy to build. It can either be assembled from individual box-beams
or manufactured as a complete section. Various cross sectional profiles of
multicellular bridge deck panels available in the literature were selected and
analyzed for IRC Class A wheel load using ANSYS, the standard FEA
software. The cross sections considered for analysis are shown in Figure 5.4.
The overall dimensions are arrived at based on the Indian Roads
Congress codes. The overall length of multicellular bridge deck panels were
kept equal to the carriage way width of single lane, 3750 mm. and the width
considered was 1000 mm.
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Model 1 Model 2
Model 3 Model 4
Figure 5.4 Cross sectional profiles considered for optimization
The depth and skin thickness of the cross section of bridge deck
panels were varied by trial and error basis. IRC class A loading was imposed
in the form of rectangular patch loads and the maximum deflection at the
center of each panel under the factored load was obtained using ANSYS.
Comparison of the deflection values for all the models is shown in Table 5.1.
A cross sectional profile of the fourth model is satisfied the deflection criteria
with minimum weight and is considered for further study. The analysis is on
the cross sectional profile of the fourth model with varying thicknesses of
flanges, webs and stiffeners as shown in Figure 5.5.
Table 5.1 Deflection values for various models
Model Deflection (in mm) Model – 1 6.50Model – 2 5.64Model – 3 2.49Model – 4 2.34
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Model -5
Model -6
Model -7
Figure 5.5 Cross sectional profiles with flange, web and stiffener thicknesses
Table 5.2 Deflection values for Optimized models
Model Deflection (in mm) Model – 5 3.34Model – 6 2.84Model – 7 2.34
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5.5 SIZE OF THE EXPERIMENTAL MODEL
The optimized cross section consists of a 3-cell section with
additional stiffeners connecting the web to the top flange it. The thickness of
the top flange, bottom flange and the exterior webs are kept as 60 mm. The
thickness of additional stiffeners is kept as 45 mm. The experimental models
used in this investigation are a 1:3 scale model of a 3.75m bridge
superstructure. The dimensions of the prototype and one-third scaled model of
the bridge deck panel are given in Table 5.3 and depicted in Figure 5.6.
Figure 5.6 Cross sectional profile of one - third scaled model
Table 5.3 GFRP Bridge Deck Panel Dimensions
Parameter Prototype (in mm) Model (in mm) Length 3750 1250Width 1000 333.33Depth 450 150Flange and outer web thickness 60 20Inner web thickness 45 15Additional stiffeners 45 15
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5.6 ANALYSIS OF GFRP COMPOSITE BRIDGE DECK PANEL
The GFRP bridge deck panel having the dimensions as specified
above was analyzed by assigning the orthotropic material properties
corresponding to the composites composed of the following materials.
E-Glass fibres in the form of CSM and ISO
E-Glass fibres in the form of WR and ISO
E-Glass fibres in the form of WR and ER
The followings are notations for the six multi-cellular GFRP
composite bridge are considered for analytical purpose and they are tested
analytical using ANSYS as stated below
1. CSIS1A - CSM and ISO under flexural loading condition
2. CSIS2A - CSM and ISO under shear loading condition
3. WRIS1A - WR and ISO under flexural loading condition
4. WRIS2A - WR and ISO under shear loading condition
5. WRER1A - WR and ER under flexural loading condition
6. WRER2A - WR and ER under Shear loading condition
The static analysis of multicellular GFRP composite bridge deck
panel of size 1250 mm × 333.33 mm × 150 mm was carried out using
ANSYS, the standard finite element software. SOLID45 brick elements were
used to model the bridge deck panel. SOLID45 element is defined by eight
nodes having three degrees of freedom (translations in x, y and z-directions)
at each node with orthotropic material properties. Orthotropic material
directions correspond to the element coordinate directions. This element has
plasticity, creep, swelling, stress stiffening, large deflection and large strain
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capabilities. The geometry, node locations and the coordinate system for
SOLID45 element are shown in Figure 5.7.
The bridge deck panel was assumed to be simply supported over
two opposite edges. Analysis is carried out for long edges simply supported
and short edges are supported simply as shown in Figure 5.8. The boundary
conditions were simulated by arresting the three translational degrees of
freedom in x, y and z directions at one end (hinge support) and two
translational degrees of freedom in y and z directions at the other end (roller
support).
Figure 5.7 8 noded solid 45 elements
The load was uniformly distributed over two rectangular patch
areas of 166.67 mm × 83.33 mm up-to ultimate load on bridge deck panel in
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the form of equivalent nodal forces. Figure 5.8 shows the geometry model of
GFRP bridge deck panel. Figure 5.9 shows the corresponding FE model.
Figure 5.8 Geometry model of bridge deck panel
Figure 5.9 Finite element model with patch loads
The deflected shape of the deck panel under the load is shown in
Figure 5.10 and the deflection contour of the bridge deck panel is shown in
Figure 5.11 for WRIS2A and WRIS1A. Figure 5.12 shows the deflection
contour of GFRP bridge deck panel made out of WRER2A and WRER1A in
the case of two long edges and two short edges simply supported condition.
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Figure 5.10 Deflected shape of the GFRP bridge deck panel (WRIS2A and WRIS1A)
Figure 5.11 Deflection contour of the GFRP bridge deck panel (WRIS2A and WRIS1A)
Figure 5.12 Deflection contour of the GFRP bridge deck panel (WRER2A and WRER1A)
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The maximum deflection and ultimate load carrying capacity of
three different models under flexure (short span hinged) and shear (long span
hinged) conditions are tabulated in Table 5.4. From the calculated values the
maximum bending stress is low when compared to that of the maximum
deflection.
Table 5.4 Ultimate Load and Maximum deflection from ANSYS
Models UltimateLoad, (in
kN)
MaximumDeflection,
(in mm)
Maximum bendingStress
(in MPa)
Flexure CSIS1A 199.5 2.23 48.5WRIS1A 248.8 2.56 31.4WRER1A 264.2 2.34 27.5
ShearCSIS2A 138.9 0.33 51.7WRIS2A 184.5 0.44 34.2WRER2A 246.8 0.38 28.9
5.7 CONCLUDING REMARKS
The best cross section is arrived at based on the mathematical
model of GFRP bridge deck developed by using ANSYS. Since bending
stress is low, the deflection is considered as a parameter for further studies.
The experimental observations are mainly included the measurement of
deflections which will indirectly indicates the strength / stiffness of the
member.