FRP Truss Bridge Fabrication And Response
S. B. Singh, Professor, and Himanshu Chawla, Research Scholar, Civil Engineering Department, BITS Pilani
Trusses are one of the most widely adopted structural system use in different applications such as bridges, roofs, cranes, aircrafts, and even robots. Trusses have triangular arrangement of members such as angles, channel sections, etc. Triangles provides strength to the structure through tension and compression. Truss structures are economical and efficient load carrying structural systems. Some examples of truss structures are shown in Figure 1.
During the Second World War, glass fibre reinforced polymer (GFRP) was first used to build aircrafts. Later in the late 1950s, carbon fibre was discovered and widely used in aviation, automobiles and sports industries. Since three decades, FRP has replaced the steel in bridges such as carbon fibre reinforced polymer (CFRP) tendons, CFRP reinforcement bars in deck slabs and GFRP railings . Like in bridges, FRP can replace steel members in truss bridges to overcome deficiencies of steel truss bridges. Some of the issues related to the earlier trends in construction of steel truss bridges  are as follows:
– Maintenance cost such as protection from rust, which is due to marine (salt spray) exposures and industrial environment (Figure 2)
– Steel truss bridges are very heavy, therefore transportation cost is high and assembly of elements is not easy
– Repairing cost is too high
– Regular painting is required for aesthetic point of view
– FRP resists a broad range of chemicals and is unaffected by moisture or immersion in water.
– It has high strength-to-weight ratio and stiffness-to-weight ratio
– FRP is a good insulator with low thermal conductivity
– Nonconductive and high dielectric capability
Salient Features of FRP Truss Bridge
– Top chords and diagonal members are angle section with stacking sequence of the laminae is shown in Figure 4(a).
– Use of random oriented chopped strands mat (CSM) is for uniform distribution of strength in all directions.
– Angle sections and gusset plate were fabricated from hand-layup process, while the box section was made by pultrusion process.
– All the structural elements of FRP bridge were connected with steel bolts and nuts.
– Deck slab is supported on FRP box sections (floor beams), while box sections are connected to truss through gusset plate, while plane trusses are connected with each other by top struts, which resists the out-of-plane bending of truss. (see Figure 3)
The structural elements of FRP truss bridge were made by hand layup process and pultrusion technique. FRP angle sections and plates were made by hand layup process. In order to make angles, first channel section was made by orientations of the fibres as shown in Figure 4. Then channels were cut from middle along the length of the channel to get two angle sections. Hand layup process for fabrication of channel is described below:
– Firstly, wooden moulds of channel section were made. A mixture of resin and hardener was prepared. Epoxy (Resin) of grade ‘Resin 691’ and hardener of grade ‘Reactive Polyamide 140’ was used for making the paste. Amount of hardener used was 8% of the weight of epoxy. The paste of resin and hardener is spread evenly above the mould using putty knife.
– A layer of fibres was wrapped around the mould above the layer of resin. Again, paste is spread over the fabrics (see Figure 5(a)) and the layer is compressed using roller to squeeze out the extra resin from fibre and to obtain uniformity as shown in Figure 5(b). Second layer of fibres was again placed on this layer and the same procedure was repeated, till all the layer were placed.
– Similarly, another FRP channel section was made on another mould. A polythene sheet was wrapped on each mould separately, for avoiding the contact of layers of both moulds.
– In order to compress the layers of web of channel section, both moulds were placed back-to-back (see Figure 5(a)) and the ends of moulds were connected together by bolts.
– Fibre layers in the flanges of the channel section were compressed by placing wooden plates over the flanges of moulds and weights were kept over the plates.
– Moulds and plates were removed after 24 hours. After that, specimens were cured under open atmosphere for 7 days, then it was further used in fabrication of truss bridge.
The performance of the FRP Bridge was predicted by test under static loading . Load was applied on the middle girder of the bridge and simply supports were provided under the end girders of the bridge as shown in Figure 3. Deflection of the top strut of bridge was measured using linear variable differential transducer (LVDT). Bridge was loaded till the buckling of top chord. Predicted load-deflection response is presented in the Figure 6.
FRP truss bridge is modelled in the finite element software such as ABAQUS 6.13 . Beams and stiffening elements are modelled with linear shell element with reduced integration, i.e., S4R. Load is applied through two bearing plates similar to experimental test. Bearing plates are modelled with shell discrete rigid object (i.e., undeformable object). Bearing plates are restrained to displace in transverse direction and allowed to translate in vertical direction, while restrained to rotate about vertical axis. Interaction between bearing plate and the beam is given ‘Surface-to-surface interaction’. The contact property in tangential direction (i.e., parallel to interaction) is given penalty with friction coefficient 0.3, while in normal direction (i.e., perpendicular to the interaction), contact is assigned by hard contact. Structural elements of the bridge were connected using fasteners. Truss bridge modelled in ABAQUS is shown in Figure 7.
Model is first analysed by linear buckling analysis, to determine the elastic critical buckling load using ‘Buckle’ step under linear perturbation module. Later, post-buckling response of the beam is determined for the first mode shape of beam. Imperfection of 0.1% of depth of beam is imparted in this analysis.
The load versus deflection responses of bridge obtained from experimental investigation and using ABAQUS is presented in the Figure 8. In this figure, ‘EXPT’ denotes experimental response and ‘WVH’ denotes ABAQUS response without vertical and horizontal stiffening elements. The results obtained from ABAQUS are in good agreement with experimental results.
In order to restrain the buckling of top chord and lateral deflection of the truss, top chords of both trusses are connected together with a horizontal strut as shown in Figure 10. Lateral buckling of the truss was restrained but the chord buckled locally. The load versus deflection response of the truss with and without horizontal strut is shown in Figure 11. It is observed that flexural strength of the truss bridge increases with addition of horizontal strut.
In another bridge, a vertical member is provided under the top chord of both truss. One end of the vertical member is connected with top chord and other end is connected with gusset plate. The deflected shape of the truss obtained from ABAQUS is shown in Figure 12. It is observed that buckling of the top chord in the plane of truss is restrained. However, bending of truss member in lateral direction occurs. The comparison of responses of the bridge with and without vertical members is shown in Figure 13. In this figure, ‘TV’ denotes truss with vertical member. It is noticed that there is little increase in the stiffness of the bridge with addition of vertical member.
In this section, the effect of addition of horizontal and vertical struts on the flexural response of the bridge is studied. The flexural strength of bridge obtained is more than bridge stiffened with horizontal or vertical members only. Response of bridge (THV) is nearly equals to response of bridge with horizontal strut (TH) (see Figure 14) while lateral displacement of the bridge is restrained (see Figure 15).
In this section, effect of addition of number of layers in top chord on the flexural strength is determined. Number of unidirectional glass fabric layers in 0o (longitudinal axis) is increased to enhance the buckling load of the bridge. Table 1 presents the percentage increase in the ultimate load carrying capacity with increase in the thickness and addition of layers.
The ultimate load carrying capacity of the bridge is predicted by increasing the high stiffness layers (i.e., carbon layers) in the top chord. Increase in the load carrying capacity with addition of high stiffness layers is presented in Table 2.
In this study, experimental and analytical responses of the FRP truss bridge fabricated at structural testing centre of BITS Pilani is predicted. Flexural strength is determined with addition of horizontal, vertical struts and fibre layers in top chord. Based on this study following conclusions can be made:
– FRP truss bridges are suitable for light loadings or light weight vehicles such as foot-over bridges, walkways, etc. FRP truss bridges can replace traditional steel bridges.
– Increasing the thickness of struts increases the load carrying capacity of FRP truss bridges.
– Connection of top chord by horizontal strut are more effective than connection by vertical strut.
– Chords and struts of FRP truss bridges made by CFRP, while the floor beams made by GFRP box section, is economical and efficient for heavy loading
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