DEVELOPMENT OF LONG SPAN COMPOSITE TRUSS BRIDGES

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1 DEVELOPMENT OF LONG SPAN COMPOSITE TRUSS BRIDGES Seo Jin (Paul) Kim, Structural Engineer, Wagners Composite Fibre Technologies (CFT) Michael Kemp, General Manager, Wagners Composite Fibre Technologies (CFT) Abstract This paper presents an engineering development process for long span composite truss bridges. A brief background and history of developing composite truss is given firstly. Over the development process, intensive desktop and experiment studies were undertaken. The initially developed truss system was utilised for medium span bridge projects after a series of engineering validations. Followed by the first development, further improvements were requested for (1) costeffective manufacturing procedure and (2) structural performance for longer span applications. Alternative materials and other systems were considered and experimentally validated. Experimental details and results are reported accordingly. Introduction Regardless of the type of construction materials, bridge structures should have sufficient capabilities in both strength and serviceability for expected loading during the service life. For longer span bridge structures, in general, the serviceability (e.g. deflection and natural frequency) becomes more critical than the strength. Historically, many different types of truss systems have been developed for bridge structures as the truss system allows achieving longer span bridges with minimal capacities of each structural member. Wagners CFT pultruded sections are made of continuous glass fibre with vinyl ester resin matrix using a pultrusion procedure. The pultruded hollow section has high tensile strength (nominal 640 MPa) in fibre direction while relatively lower in stiffness (tension modulus of 36 GPa) compared to conventional construction materials (e.g. structural steel: tensile strength of MPa and 200 GPa in modulus). The utilising composite sections for bridge structure constructions, serviceability requirements govern the structural design due to such material characteristics. (Wagners 2014) This paper presents a series of development and validation details for long composite truss bridge constructions. Firstly, a brief of development procedure of Stage 1 composite truss system is presented. Details of specific considerations and steps of developments are provided accordingly. Following the first development, further improvements were requested for (1) cost-effectiveness over manufacturing procedure and (2) structural performance for longer span applications. Alternative materials and other systems were considered and experimentally validated. Experimental details and results are reported accordingly. Development composite truss system Stage 1 Initially, a simple truss system was developed for pedestrian bridges with WCFT composite sections. The truss consists of continuous top and bottom chords bolted by vertical and diagonal chords. WCFT 125 x 125 square hollow sections (SHS) and 250 x 125 bonded rectangular beam (BRB) sections were used (Wagners 2014). All joints were constructed with WCFT inserts and M24 grade 316

2 stainless steel threaded rods and nuts. The WCFT insert is an anti-crush plastic block to prevent crushing the hollow section at bolted joints and through gluing procedure achieves significant joint strength compared to noninsert joints (Kim and Kemp 2014). The developed truss system was selected for our two awarded medium length pedestrian bridges (i.e. 12m and 14m spans). During these applications, the system has been experimentally evaluated and service temperature and long-term loading were specifically considered (Kim and Kemp 2014). A set of joint strength tests were undertaken under ambient and elevated temperature (i.e. 80 C) for evaluating the design strength of joint. Two scaled truss specimens were also constructed with identical materials and the same manufacturing process of the constructed bridges and tested for proving the structural performance under ultimate loading condition (i.e. proof testing, AS/NZS ) and one million cycles of loading tests. Through a series of validations, the truss system demonstrated its performance successfully and satisfied requested criteria, i.e. strength as well as serviceability. Stage 2 The improvement of the truss system requested for (1) improving manufacturing procedure and (2) increase serviceability of the truss system for longer span bridge. Previously developed joint system utilised post-cure procedure for achieving high bond strength between the insert and pultrusion however it required additional time, labour and equipment. In order to eliminate postcure procedures, another type of glue which had higher transition temperature (Tg) and expected to have equivalent or better bond strength was selected. In order to improve serviceability of the truss system particularly for long span truss, groutfilled sections were considered for providing higher compressive strength and stiffness in compressive top chord (i.e. for simply supported truss). Utilising grout-filled composite hollow sections do not require large changes of the developed system but expected substantial strength and stiffness enhancement for compressive truss members. The benefits from grout-filling of composite hollow sections have been well recognised and abundant of engineering information can be found from open literature. Also a practical design guideline is available (AASHTO 2012). For the grout-filled section, the grout is filled completely inside and confined by the section. Owing to the deformation of compression chords, the composite action between the grout and section becomes more effective as the compressive stress increase and the transferred stress from the joint is to be taken by the whole section (i.e. both composite and grout section). Considering the excellent compressive strength of the grout, the insert was not installed for the grout filled joint. Two experimental studies was under taken for investigating structural performance of Stage 2 truss system and detailed information is given in the Experimental study section as below. Experimental study 1. Joint strength at elevated temperature Temperature related behaviours of composite material are not well established area but great care should be given for designing each structural element and joint in order to prevent premature failure. In general, the Tg of resin system of composite and glued joint shall be ensured not less than expected service temperature of bridge structure. According to the composite design standard (e.g. ASCE 2010), safety factors should be considered over 35 C service temperature and additional engineering assessment by tests should be undertaken for over 60 C. A range of temperature (i.e. from ambient to 80 C) was considered to ensure the integrity of the bolted joint with glued insert under tested temperatures. A typical double shear strength tests were utilised and all elevated temperature samples were tested inside of the environmental chamber (Figure 1).

3 (a) Ambient temperature test (b) elevated temperature test Figure 1. Test set-up Table 1. Test specimens ID Temperature No. of tests Pultrusion # Thread rod Insert # A-01~10 Ambient (23 C) ~12 40 C ~12 60 C ~12 80 C 12 # Waners (2014) WCFT 125x125 SHS M20 EC P22 Details of test parameters are summarised in Table 1. All test samples were manufactured in accordance with Wagners CFT standard procedure and tested by 3rd party independent institute (i.e. Centre of Excellence in Engineered Fibre Composites, University of Southern Queensland). All test specimens showed consistent failure behaviour. The failure initially occurred at bonding interface between glued insert and pultrusion which was evidenced by the large cracking sound during the test prior to reaching the peak load. A typical bearing failure, i.e. slot hole failure (Figure 2), was formed when the load reached the peak and propagate with significant dropping of the load. Due to the limit in the space of presenting manuscript, only summary of test results is given in Table 2. The test results show that there is not a clear trend in the test strength over changing temperature. A slightly higher variation was observed at 40 C but not more than 10 % of coefficient of variation and less than 5 % for other tested temperatures (Figure 3). A comparison with the post-cure glue which developed in Stage 1 is also plotted in Figure 3. Overall, tested ambient system showed slight lower strength but such difference is within the test vitiations. Finally, the characteristic values of each joint have estimated in accordance with ASTM D7290.The estimated value is a statistically determined value representing the 80 % lower confidence bound on a 5th percentile value of a specific population. The estimated values can be used as reference strengths for design purposes. It should be noted that additional adjustment (e.g. environmental and load sharing factors) should be undertaken in accordance with design guideline before being used for the design.

4 (a) Over view - 40 C (b) Detailed view - 60 C Figure 2. Failure mode Table 2. Summary Test results Temperature 23 C 40 C 60 C 80 C Averaged tested strength (kn) Standard deviation (kn) Coefficient of variation (%) Characteristic value (kn) With post-cure glue (Stage 1) With ambient-cure glue (Stage 2) Figure 3. Test comparison Joint strength tests 2. Grout-filled truss system In order to validate the effectiveness of groutfilled section, Scaled truss tests were prepared. The truss consisted of three panels which have nominal dimensions of 1 m deep and 1.1 m long (i.e. total 3.3 m span 1 m deep specimen which measured at the centre line of BRB section) and only 250x125 BRB sections were used for all truss members (Figure 4). All joints were tightened by M24 grade 316 stainless steel threaded rods and nuts. Truss 1 was constructed as per WCFT standard manufacturing procedure. The grout-filled sections were utilised for continues top chords only. The top chords were predrilled and temporary plastic tubes were placed for forming bolt holes prior to grouting. All temporary tubes were removed before the assembly. The initial tests were undertaken after 7 days cure and presenting tests were undertaken 4 months after the grouting.

5 Figure 4. Test specimens Truss 1 and Truss 2 Table 3. Truss test Section / Joint Chord Top Bottom Brace Vertical Truss 1 250x125 BRB / Grout-filled 250x125 BRB / Glued insert Truss 2 250x125 BRB / Glued insert As a counterpart control specimen, additional non-grouted truss (denoted by Truss 2 in Table 3) was prepared by replacing groutfilled top chords with normal BRBs with glued inserts after testing Truss 1. A detailed test program is given in Table 3. For comparison purpose, the truss was only loaded up to 300kN with a point loading at the top of vertical chord. In order to even load distribution over the contact face, 10 mm rubber pad was inserted between the truss and a 15 mm thick spreading steel plate. Under the each end vertical chord, a simple pin support (i.e. support free to rotate span direction) was simulated with a 10 mm thick steel plate and a 50 mm thick steel roller. A hydraulic jack was manually operated for both tests. The load-displacement responses of each test are presented in Figure 5. Both trusses deformed linearly within the tested load range. Minor slippage was observed at lower loading range although all trusses were preloaded several times prior to the tests (up to 250 kn). The relative stiffness change of test trusses has been accessed by comparing the slope of load-displacement responses. The slope of Truss 1 shows clearly stiffer than that of Truss 2. The displacement of Truss 1 at 300 kn shows approximately 23 % lower than that of Truss 2. For more accurate comparison, the slopes of each test are estimated at the load range between 100 and 200 kn (Figure 6). The Truss 1 has approximately 31 % higher than Truss 2. The enhancement of serviceability owing to grout-filling is clearly demonstrated compared to the counterpart test result. In addition, the additional weight of grout expects to contribute the mass of truss bridge and shorten natural frequency, especially for long light-weight composite bridges.

6 (a) Overall (b) Details Figure 5 Load-displacements responses Conclusions and remarks The engineering development procedure of composite truss bridge was presented. A brief description about the initial development (i.e. Stage 1) including background and practical applications was given firstly followed by the further development details from the previous version to current version (i.e. Stage 2). The background of two main improvements and their experimental details were reported. The results of each experiment were summarised and then compared with the counterpart test results. References ASCE (2010) Pre-Standard for Load and Resistance Factor Design (LRFD) of Pultruded Fiber Reinforced Polymer (FRP) Structures (Final). American Society of Civil Engineers, Reston, VA, USA ASTM D7078/D7078M (2012) Standard Test Method for Shear Properties of Composite Materials by V-Notched Rail Shear Method, American Society of Testing and Materials, PA, USA ASTM D (Reapproved 2011) Standard Practice for Evaluating Material Property Characteristic Values for Polymeric Composites for Civil Engineering Structural Applications, American Society of Testing and Materials, PA, USA Kim, S.J. and Kemp, M. (2014) Engineering Validation of Fibre Composite Truss Bridge: Clewley Park Bridges, Toowoomba, Australia, Proceeding Sixth Australian Small Bridge Conference, Sydney, May 2014 Wagners (2014) Product Guide, Wagners Composite Fibre Technologies (CFT), (Accessed 26 August 2014) AS/NZS (2002) Structural Design Actions, Part 0 General Principles, Standard Australia, Sydney, Australia AASHTO (2012) LRFD Guide Specifications for Design of Concrete-Filled FRP Tubes, American Association of State Highway and Transportation Officials, Washington, D.C., USA

7 Author Biography Seo Jin (Paul) Kim Postal Address: 339 Anzac Avenue Toowoomba QLD, Australia Seo Jin (Paul) Kim is a structural engineer specialised in fibre reinforced polymer (FRP) composites applications. He has acquired his engineering competence from his career challenging over past 18 years. Since he received his first degree, he has maintained his career in various engineering practices, such as engineering consulting, contractor, manufacturer as well as academia. He experienced many projects including structural design, strengthening or rehabilitating existing structures with externally bonded FRP composites and fibre composite structures for mining, oil and gas, civil and building industries as a structural engineer. Michael Kemp Postal Address: 339 Anzac Avenue Toowoomba QLD, Australia Michael.Kemp@wagner.com.au For the Last 10 years, Michael Kemp has specialised in the application of structural fibreglass components to civil engineering applications and is credited with project managing the design and installation of the first composite fibre road bridge in Australia (Grafton NSW) and the first in Queensland (Blackbutt - Daguilar Highway). Additionally Michael has acted as the General Manager of Wagners CFT for the last 8 years where he has lead the product development, design and fabrication of innovative products for Civil industry.