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Performance of a fibre-reinforced polymer bridge deck under dynamic wheel loading
Composites: Part A 37 (2006) 1180–1188 www.elsevier.com/locate/compositesa
Performance of a fibre-reinforced polymer bridge deck under dynamic wheel loading Albert F. Daly*, John R. Cuninghame TRL Limited, Crowthorne House, Nine Mile Ride, Wokingham, Berks RG40 3GA, UK Received 27 April 2005; accepted 13 May 2005
Abstract The paper describes the research carried out by TRL Limited on behalf of the UK Highways Agency to examine the performance of fibrereinforced polymer (FRP) bridge decks under local wheel loading. The objective of the research was to produce a draft standard giving generic design requirements for technical approval of FRP deck systems. The project included the formulation of design guidelines for fatigue. Tests were carried out on a full-scale glass FRP bridge deck under static and dynamic wheel loading. The loads were imposed using the TRL Trafficking Test Facility, which replicates the effects of the wheel of a heavy goods vehicle. The deck was subjected to over 4.6 million cycles of a 4 tonne wheel load, equivalent to 30–40 years of service traffic. A simpler approval test is proposed, applying stress cycles to small sections of deck to simulate the passage of wheels. The paper includes a description of the FRP deck, the testing and a summary of the performance of the deck. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Glass fibres; B. Fatigue; D. Mechanical testing; E. Pultrusion; FRP bridge deck.
1. Introduction
deterioration and to increase load capacity without extensive and expensive bridge works.
1.1. Background Most modern short span bridges have concrete decks. In general, they are efficient and durable provided proper attention is paid to detailing and the standard of workmanship. Concrete is likely to be the most common deck material for some considerable time. However, some concrete bridge decks have suffered corrosion, due in part to the increasing use of de-icing salts. As there is unlikely to be an economically viable replacement for rock salt for de-icing, increasing interest is being shown in materials that are corrosion resistant. In addition, rapid growth in the volume and weight of heavy goods vehicles (HGVs) has led to serious problems and many older bridges no longer meet current design standards. There is, therefore, a need for methods of replacing bridge decks to deal with structural
* Corresponding author. Tel.: C44 1344 770449; fax: C44 1344 770355. E-mail address: [email protected] (A.F. Daly).
1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.05.040
1.2. Use of FRP in bridge decks The use of Fibre Reinforced Polymer (FRP) as a primary structural material is developing rapidly in the construction industry. FRP materials have considerable advantages in terms of weight, strength and corrosion resistance. They have been used for several decades in the aerospace, automobile and marine industries, where they have developed a good track record in a very adverse environmental conditions. However, the ‘product life cycle’ is much longer in civil engineering so uptake has been relatively slow, but FRP has been used in a number of bridges around the world. As production technology develops and design standards and guidelines become more generally available, these FRP materials will be used more widely to provide cost-effective alternatives to steel and concrete. Potential applications for FRP decks are new design, replacement of under-strength decks in existing bridges, and the provision of temporary running surfaces. In spite of the advantages, however, there is still a lack of information on the behaviour of FRP components in bridge
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applications. This has impeded the development of generally accepted guidelines for the design and application of bridge schemes. In response to this the UK Highways Agency commissioned TRL Limited to carry out research into the performance of FRP bridge decks with the objective of developing design guidelines. 1.3. Outline of research project The aim of the project was to examine the use of FRP in new and replacement bridge decks and produce draft design requirements. A desk study of current research and practice in bridge construction and other FRP applications was carried out to address issues such as design and detailing requirements, resistance to environmental effects, repair and maintenance. These issues have been dealt with in the proposed design guidelines which will be published by the Highways Agency in due course. Existing loading rules can be used for FRP decks and global static design is relatively straightforward provided material properties are known. Some design guides are available, but there are currently no fatigue design standards for FRP bridge decks. Local dynamic stresses at details potentially at risk of fatigue due to wheel loading may be difficult to calculate. There are likely to be high stress gradients and the stress cycle may be tensile, compressive or alternating. As these stresses are raised to the power of 10 to obtain fatigue life, it is important that they be calculated with sufficient accuracy. This is similar to the situation for steel orthotropic bridge decks, which are currently beyond the scope of the UK bridge design code [1]. Because of the high development costs, it is likely that most FRP decks will be produced as modular systems. The deck system tested was thought to be typical in that it consisted of top and bottom flanges supported by webs running transverse to the direction of traffic flow. The limited number of bridge deck systems that have been developed so far, most notably in the US [2] and Europe [3–5] are of this form. Therefore, it was decided to carry out full scale tests to study the performance of FRP decks under local wheel loads and provide data on which to base generic fatigue requirements for technical approval of FRP deck systems. Rolling wheel tests were carried out on a 4.0 m span model bridge; static and fatigue tests were carried out on small sections of deck in a standard testing machine.
Fig. 1. Roadway panel.
of configurations. The units have undercut grooves on the sides, which are used to connect them together using a rod with a ‘dog bone’ cross-section to provide mechanical interlock; the joints are also bonded. This Advanced Composite Construction System (ACCS) has been used in the Linksleader footbridge [3] at Aberfeldy in Scotland and a lifting bridge at Bonds Mill in Gloucestershire, England [4] which was designed to carry vehicles up to 40 tonnes gross weight. The ACCS deck was not designed to resist local wheel loading, so a new Roadway panel was designed and manufactured. This has a similar geometry to the lighter ACCS plank but with thicker sections to resist direct wheel loads. It was designed to be laid transversely on a sub-structure built-up from ACCS planks. The Roadway panel is shown in Fig. 1: further details of the design are given elsewhere [6]. 2.2. Model bridge A full-scale section of bridge deck was built at TRL. It spanned 4.0 m, was 2.12 m wide and had a total construction depth of 0.8 m: see Fig. 2. No surfacing was used (in practice, a thin anti-skid dressing would be sufficient). The deck was instrumented with electrical
2. Model bridge tests 2.1. Description of FRP bridge system The design of the deck used in the dynamic testing was carried out by Maunsell Ltd, UK who developed an FRP bridge deck system consisting of standard glass FRP cellular ‘planks’ and connectors that can be assembled in a variety
Fig. 2. Section through FRP deck.
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resistance strain gauges to determine the strain influence lines as the wheel moved along the deck. Displacement transducers were also fitted to measure deflection of the deck under load. 2.3. Rolling wheel test rig Rolling wheel tests were carried out in the TRL Trafficking Test Facility (TTF). The main features of the TTF are shown in the photograph in Fig. 3. It is similar to a conventional pavement test machine, but is smaller and much cheaper to operate. A wheel, either single, twin or ‘super-single’, rolls along a 3 m long track. The wheel load may be up to 5 tonnes and the load may be applied during both forward and reverse passes, or on the forward pass only. The wheel reaches a maximum speed of 20 km/h and can complete one pass per second (bi-directional), equivalent to 80,000 per day. The wheel, carriage and drive mechanism are set up as a spring-mass system operating at its resonant frequency. This greatly reduces the energy required and hence the cost of running the rig.
Fig. 4. Influence line for strain at top of web.
† ‘Global’: where the relevant loading is the total load due to vehicles on the span. An example is the cracking that occurred in the test, adjacent to the bearings. † ‘Local’: where cyclic stresses arise due to individual wheel loads and cracking occurs under, or close to, the wheel tracks. Cracking due to local loads is the most common form in lightweight steel decks and it is necessary to ensure that similar cracking does not arise in FRP decks.
Initial static tests were carried out to determine the strain levels in the running surface and to select an appropriate load for the dynamic test. Each series of tests consisted of placing the wheel at various positions on the deck and recording the strains. Transverse and longitudinal influence lines for strain could then be plotted and used to determine the load to be used in the dynamic test. Fig. 4 shows a typical influence line, giving the strain at the junction between the web and top flange of the roadway panel as the wheel traversed the deck (see insert in figure for location of the gauge). This was one of the locations indicated by analysis to be subject to high stress range due to traffic loading. The strain shown is in the direction of the traffic movement (i.e. longitudinally in the deck, transversely in the Roadway panel). A strain range of 1000 microstrain (tension) to K1290 microstrain (compression) was recorded. This was the highest range of strain recorded in the deck. The influence lines exhibited the expected strain reversal as the wheel rolled over the webs. Another critical point for fatigue was thought to be in the top flange, mid-way between two webs. Fig. 5 shows the influence line for longitudinal strain on the top surface of the second cell of the Roadway panel. Here, the peak strain recorded was K1090 microstrain (compression).
Fig. 3. FRP deck in test rig.
Fig. 5. Influence line for strain in top flange.
2.4. Test methodology The rolling wheel tests were carried out to study the behaviour of the FRP deck, in order to develop a method by which a client could ensure that an FRP deck proposed for use on a UK highway bridge would have an adequate fatigue life. Potential fatigue damage may be:
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3. Rolling wheel test results 3.1. General performance of the deck
Fig. 6. Transverse influence line for strain in top flange.
Fig. 6 shows the influence line for the strain as the transverse position of the wheel is changed. Again, the strain shown is in the direction of the traffic movement. Two plots are shown, at the locations of the maximum and minimum strain as the wheel swept from one end of the deck to the other. The strains recorded for this gauge ranged from 270 (tension) to K950 microstrain (compression). Influence lines were also recorded for deflection of the deck to indicate the global performance. The maximum deflection was just under 1 mm. Once the test load level was determined, the dynamic test was started, with the logging system set up to record maximum and minimum values of each gauge output every 10 or so cycles. At various rig cycle counts, a full sweep of strain and deflection measurements was recorded and retained to provide influence lines for strain and deflection. These were used to monitor any change in behaviour as the dynamic test progressed.
The dynamic test was continued for as long as possible, within the constraints of the project time-scale. In all, the deck sustained about 4.6 million load cycles. At various times, the test was stopped and close visual examination of the surface was carried out. Fibre-scope examination of the gauged area inside the Roadway panels did not indicate any damage. Subsequent examination of the surface (including sections and cores taken through the running surface) indicated that there was no visible deterioration and it was concluded that if this were a real structure there would be no cause for concern. A 500 mm section of the Roadway panel, taken from along the line of the wheel loading, was subsequently tested statically to failure (Specimen FS22-3 as reported in Table 1). Comparison with a similar test on a not-previously loaded section (Specimen FS22-12) indicated no significant different in strength. This confirmed the above conclusion, that the Roadway panel provided a robust structure capable of sustaining HGV traffic. 3.2. Damage in the sub-structure The only visible damage in the deck was in the substructure. After 247,400 load cycles, damage was noticed adjacent to one of the bearings. This consisted of cracks in the first and second webs of the ACCS plank and debonding of the hard-wood insert (used to prevent local distortion and buckling of the section at the bearing): see Fig. 7. At 256,000 load cycles, some debonding was noted in six of the eight supports, although cracks were only
Table 1 Static and fatigue test results Specimen
Top flange
Support type
Maximum static load (kN)
Fatigue test load (kN)
Fatigue test load/ ultimate
Endurance
FS22-1 FS22-2 FS22-3 FS22-5 FS22-5 second crack FS22-6 FS22-7 FS22-8 FS22-8 second crack FS22-9 FS22-9 second crack FS22-10 FS22-11 FS22-12
Thick Thick Thin Thick
Uniform Uniform Uniform Uniform
476 – 353 510
– 150 – 150
– 0.315 – 0.294
Thin Thick Thin
Uniform Ends Ends
373 473 –
100 80 80
0.268 0.169 0.244
Thin
Ends
343
60
0.175
Thin Thick Thin
Ends Ends Uniform
328 422 384
– – –
– – –
– 442,500 – 144,000 201,000 151,000 2,700,000 42,100 250,900 1,151,300 1,697,800 – – –
Endurance is number of cycles to a surface crack R50 mm long in the underside of the top flange. One cycle consists of one half sine wave load on both load pads 1 and 2. Specimen FS22-3 was cut from the panel tested in the rolling wheel rig (from directly under the wheel track). Specimens FS22-2 and FS22-8 (to first crack) were not included in the regression.
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Fig. 7. Cracking at support in FRP deck.
apparent at one support. The dynamic movement across these cracks was 1.1 mm. At load cycle 247,400, it was noted that some of the FRP angles holding the end diaphragms in place had become debonded from the ACCS webs, with relative movement of about 0.07 mm occurring. At the completion of the test, two other angles had also become detached. The only other significant damage was in the bonded joint between the ACCS webs and the top flange. This was first noticed because the two surfaces were grinding together and the adhesive was being ground into a fine white powder. Fig. 8 shows the joint (viewed from under the deck) at the conclusion of the test. The relative movement at this joint was about 0.5 mm.
4. Tests on small sections of deck The rolling wheel tests have the advantage that all parts of the deck are automatically included in the test, but the rate of testing is low (1 Hz) and the cost is relatively high for
Fig. 8. Deterioration of joint in sub-structure.
a routine test. Figs. 4–6 show that the influence lines for potential crack locations in the Roadway panel are short. This suggests that a test on a small section of deck, that simulates the effect of an individual wheel, should be sufficient to assess the deck details for fatigue, due to local loads. The UK bridge design code (British Standards Institution, 1980) gives a maximum wheel load for normal traffic of 30 kN on a contact patch of 200!200 mm: this provides a basis for a fatigue test load, with a suitable factor added. The objectives of the tests were to understand the behaviour of the Roadway panel under repeated local wheel loading, to define its fatigue strength and consider appropriate methods of estimating its fatigue life. A simple fatigue test could then be specified for checking and approving new FRP deck systems offered for use on UK highway bridges. The test sections were 600 mm wide, 115 mm deep and 500 mm long—sufficient to avoid edge effects, but small enough to fit a standard testing machine. Static tests were carried out using a single 200!200 mm pad, to determine the ultimate load capacity and mode of failure of the panel. Fig. 9 shows the test arrangement. For the dynamic and fatigue tests, a rig was devised to fit the testing machine in which two adjacent steel pads, 200!200 mm and faced with rubber, loaded the panel alternately, to simulate the passage of a single wheel. The arrangement is shown in Fig. 10. The loading and support conditions were developed to produce measured strains similar to those at the same locations on the full size deck. The cross-section of the Roadway panel was nominally symmetrical. It was found that the lengths supplied were dimensionally very consistent, but the top and bottom flanges were of different thickness, approximately 11 and 14 mm at their thinnest point midway between webs. This was not thought to be significant as both flanges contained the same amount of fibre, but the static tests showed that the ultimate capacity was reduced by around 23% if the thinner flange was uppermost. The support conditions were also found to be significant. The Roadway panels were designed to be uniformly supported by a deck structure consisting of thinner section ACCS panels, but in practice the top surface of the supporting deck was not quite flat. The connecting pieces above the ACCS planks forming the webs were slightly proud of the ACCS panels forming the top flange, so the Roadway panel was, to some extent, spanning the 600 mm space between webs. Supporting the 500 mm test sections along each end gave strains which were closer to those on the full size deck under the rolling wheel. In addition, ultimate static capacity was reduced by around 12%. This demonstrated that small differences in fit-up can have significant effects on load capacity; approval testing should simulate actual, rather than design conditions.
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Fig. 9. Static test arrangement.
Strain gauges were attached to some of the sections at the same locations as on the full size deck: see inserts in Figs. 4–6.
Acoustic emission (AE) was recorded during some of the static tests. Significant emissions were detected prior to failure and it appears that AE would offer a viable method of monitoring FRP bridge decks.
4.1. Test results 4.2. S–N curve from test results The results of the static and fatigue tests are summarised in Table 1. Fatigue cracks occurred in the top flange midway between webs and propagated longitudinally along the panel. The cracks extended through approximately 60% of the thickness of the flange. The cracks did not reduce the ultimate static capacity of the test panels under a wheel load, so their structural effect may be small. They would allow moisture to penetrate to the fibres so there could be long term effects on durability, but if appropriate materials are specified, any deterioration would take at least 40–50 years to occur. On a bridge, this type of crack could occur in each cell under each wheel track and propagate to the width of the wheel track, i.e. 500 mm or more. Cracks were also observed in the radius between web and flange, either at the start of the test, or after a small number of load cycles. It is thought that these cracks resulted from manufacture, possibly due to shrinkage. In one specimen sectioned and examined after testing, the cracks were in the resin only and did not extend through the outer layer of fibre during the fatigue test. They are not thought to be significant. Measured strain was not proportional to load, at higher loads. In addition, the most severe strain was not at the crack location. It is concluded that measured surface strain is not a satisfactory method of characterising the strength of FRP panels under high test loads.
Fatigue assessment of steel structures is usually based on stress at the potential crack location. A designer would calculate a stress spectrum due to traffic, then use an experimentally derived S–N curve and damage summation rule to estimate fatigue life. In most cases ‘nominal’ stress is sufficient, but for some details on steel orthotropic decks the local stress (which governs fatigue cracking) is different to the ‘nominal’ stress produced by a conventional FE analysis. This is why steel decks are outside the scope of
Fig. 10. Fatigue loading arrangement.
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the current design code, BS 5400: Part 10 [5]. FRP bridge decks are, if anything, more complex than a steel orthotropic deck due to the non-uniform material properties. Expressing fatigue strength in terms of load for the purposes of an approval test has the advantages of simplicity and reduced instrumentation costs. The test results would of course only apply to the component geometry tested, but at the current stage of development each new deck will need to be tested, so this is not a serious disadvantage. For fatigue due to local wheel loads, the relevant load is that due to the passage of a wheel over the detail being assessed. The S–N relationship can then be expressed as Lmax =Lu Z A K k log10 ðNL Þ where Lmax is the maximum load in a cycle (same as load range in this case). Lu is the ultimate load capacity under a simulated wheel load. NL is the number of cycles to failure. A is a constant. k is the slope of the regression line. In this equation, the fatigue strength of the specimens is expressed in terms of applied load divided by the ultimate static load. However, the mode of failure in fatigue tests may be different to that in static tests. There may not be a direct relationship between the dynamic loading causing fatigue damage and the static ultimate load. In that case, the ultimate static load capacity is merely a constant in the equation of the S–N curve. For the purposes of analysing the test results to produce an S–N curve for the panel, endurance (NL) was taken to be number of cycles to the first crack at least 50 mm long. At this length the crack could be detected reliably on a bridge. As the observed cracks had no structural effect, fatigue life in this case is not the same as the service life of the deck. Regression analysis was carried out between log10 (N) and (Lmax/Lu), with log10 (N) as the independent variable. There were too few test results to determine the most appropriate statistical method, so the procedure used to produce the S–N curves in BS 5400: Part 10 was used, with an allowance for the small number of results. Expressing the load as a fraction of ultimate static strength disguises the effect of the support conditions (uniform or ends only) and flange thickness, so all results were analysed together. Specimens FS22-2 and FS22-8 (to 1st crack) gave endurances very different to the others (see Fig. 11) and were not included in the analysis. The data and regression line are also shown in Fig. 11 and the rolling wheel test result is included for comparison. The predicted endurance of the Roadway panel in the rolling wheel test, assuming the result would lie on the mean regression line for the test specimens, is 12 million cycles. Hence, the wheel load was too low to give a 50% probability of
Fig. 11. Regression analysis of fatigue test results.
cracking if there is a cut-off at 10 million cycles. It should be noted that the wheel load in the test was 23% higher than the maximum permitted for normal vehicles on UK roads and that the endurance is to the appearance of non-structural cracks.
5. Estimated fatigue lives The objective of the fatigue testing was to enable the service life of a bridge deck to be estimated. Development of a suitable calculation method was beyond the scope of the project, so the cumulative damage method given in BS 5400 Part 10, Section 8.4 was used. Bridges are long life structures and the dynamic loads due to traffic are generally small compared to the ultimate load. The vehicles in the BS 5400 design traffic loading spectrum have a range of wheel loads, from 7.5 kN for lightly laden 2-axle vehicles (type 2R-L) to 40 kN for a small number of very heavy ‘special order’ vehicles. Therefore, a key part of the calculation is the treatment of low stress cycles, i.e. those corresponding to constant amplitude endurances greater than 107 cycles. Low stresses only become damaging after some fatigue damage has occurred; this is taken into account in the BS 5400 method by a change in the stress exponent (m) at 107 cycles. It is thought that there may be a constant amplitude fatigue limit for FRP components at a stress corresponding to an endurance of around 107 cycles, but it is not known how lower stresses should be treated. Adopting the BS 5400 method of changing the exponent from m to mC2 would have little effect on estimated fatigue life. The calculations for the Roadway panel were carried out in terms of wheel load/ultimate load rather than stress, for three cases: (i) Assuming all loads are equally damaging, i.e. the S–N curve is linear to zero load. (ii) Assuming that low loads only become damaging after high loads in the spectrum have caused some
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Table 2 Estimated fatigue lives S–N curve
Conditions
Load at 107 cycles
Estimated life (years)
% Damage due to HGVs O36t
Mean S–N curve from test data
All loads damaging Bi-linear S–N curve Cut-off at 107 cycles
0.098 0.098 0.098
16 55 O1000
0.1 0.4
‘Design’ S–N curve based on test data
All loads damaging Bi-linear S–N curve Cut-off at 107 cycles All loads damaging Bi-linear S–N curve Cut-off at 107 cycles
0.051 0.051 0.051 0.214 0.214 0.214
5 7 16 13 40 O1000
0.1 0.2 0.3 0.1 0.3
Notional S–N curve to give life of 40 years for bi-linear curve
Estimated life is years to a surface crack R50 mm long in the underside of the top flange. Traffic loading is BS 5400 Part 10, Table 11 with one million HGV per year.
damage, i.e. a bi-linear S–N curve. As no data are available, the S–N curve for endurances O107 cycles is arbitrary and intended only to test the effect of this type of treatment of low loads. The curve for NO107 cycles was set to midway between cases (i) and (iii). (iii) Assuming low loads never become damaging, i.e. there is a cut-off at 107 cycles, so the S–N curve becomes horizontal at this point. The calculations for these cases were repeated for several S–N curves. (i) The mean regression line for all the test results (‘mean’ curve). (ii) The lower 95% confidence limit of the test results (‘design’ curve). (iii) A notional S–N curve with the same slope as the test results, selected to give an estimated life of 40 years for the bi-linear case (‘40 year’ curve). Estimated lives are given in Table 2 for one million HGVs per year on an un-surfaced Roadway panel. Treating all load cycles as equally damaging (no change in slope in the S–N curve) is too severe, but equally, ignoring all cycles below the 107 cycle cut-off could be unsafe. For short lives, e.g. the ‘design’ curve, changing the slope of the S–N curve is of limited practical significance. However, lives based on the ‘mean’ curve show that the treatment of low loads can change the estimated life from 16 years to O1000 years. If the test results are considered to provide lower bound results for structural cracking, then a panel in service should remain free from structural cracks for around 55 years, based on the ‘mean’ bi-linear curve in Fig. 11. In some cases, fatigue life may be affected by the small number of ‘general’ and ‘special’ order vehicles, i.e. those with gross vehicle weight (GVW) greater than 44 tonnes, which have high wheel loads. The damage due to these vehicles is included in Table 2; it will be seen that these
vehicles are not significant for the FRP deck. This is encouraging as the load in the rolling wheel test (37 kN) was above the maximum for normal HGVs (30 kN). The result of the rolling wheel test is included in Fig. 11; it is close to the ‘design’ curve, indicating there was a 2.5% probability of cracks midway between webs. The rolling wheel test load was less than the load at 107 cycles on the mean S–N curve, so this type of cracking might never have occurred. On the basis of these results, it appears that nonstructural cracks could occur in a Roadway panel on a UK bridge within the 120 year notional design life normally used for UK highway bridges, unless the effects of wheel loads are reduced by asphaltic surfacing. Clearly, the effect of low loads (less than 10% of ultimate, corresponding to constant amplitude endurances of O107 cycles) is significant, and a meaningful estimate of fatigue life cannot be made without taking them into account. Further work is needed to determine how they should be treated in design. In the meantime, an approval test based on wheel loads occurring in normal design traffic, plus a suitable partial factor, should provide an interim solution.
6. Conclusions Dynamic testing of an FRP deck has shown that FRP components can provide a robust bridge solution complying with the general requirements of the UK design code and in particular, capable of resisting local wheel loads due to heavy vehicles for at least 30–40 years without structural damage. Non-structural cracks may occur before this, but their effect on durability would be long term. Careful attention is needed to prevent local damage in highly stressed regions of the supporting deck, such as web to flange connections and close to bearing supports.
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A simple fatigue design method, not requiring a damage summation, appears possible. This would be based on demonstrating that fatigue damage will not occur under design traffic loading. It would consist of fatigue tests on small sections of deck panel with test loads set to ensure that almost all traffic loading cycles are below the damage threshold. This avoids the problem of the lack of a method for calculating the effect of low loads in the spectrum and is not thought to be too severe a requirement at this stage of development of FRP decks. A procedure for fatigue assessment has been devised for incorporation into the draft Highways Agency Standard for FRP decks. It is considered important to minimise the testing requirement so as to encourage development of FRP bridge decks. The results obtained in this project suggest that a simple test simulating a single wheel load will be sufficient.
References [1] British Standards Institution BS 5400. BS 5400: Part 10: 1980. Steel, concrete and composite bridges Code of practice for fatigue. London, UK: British Standards Institution; 1980. [2] Cassity, P, Richards D, Gillespie J. Compositely acting FRP deck and girder system. Structural Engineering International, Vol. 12, No. 2, International Association for Bridge and Structural Engineering (IABSE), Zurich, Switzerland; 2002. [3] Daly, AF. Developments in modular FRP decking systems for highway bridges. Paper presented at one-day conference at Aston University: Bridges 2001: Old problems—new challenges; 17 Oct 2001. [4] Oliver A. Loaded questions New civil engineer. London: Thomas Telford; 1994. [5] Luke S. FRP deck for West Mill Bridge, UK. Proceedings of the 10th International conference and exhibition—structural faults and repair conference, London. Edinburgh: Engineering Technics Press; 2003. [6] Daly AF, Duckett WA. The design and testing of an FRP highway bridge deck. Journal of Research, Vol. 5, No. 3, Transport Research Laboratory, Crowthorne, UK; 2002.
Performance of a fibre-reinforced polymer bridge deck under dynamic wheel loading Albert F. Daly*, John R. Cuninghame TRL Limited, Crowthorne House, Nine Mile Ride, Wokingham, Berks RG40 3GA, UK Received 27 April 2005; accepted 13 May 2005
Abstract The paper describes the research carried out by TRL Limited on behalf of the UK Highways Agency to examine the performance of fibrereinforced polymer (FRP) bridge decks under local wheel loading. The objective of the research was to produce a draft standard giving generic design requirements for technical approval of FRP deck systems. The project included the formulation of design guidelines for fatigue. Tests were carried out on a full-scale glass FRP bridge deck under static and dynamic wheel loading. The loads were imposed using the TRL Trafficking Test Facility, which replicates the effects of the wheel of a heavy goods vehicle. The deck was subjected to over 4.6 million cycles of a 4 tonne wheel load, equivalent to 30–40 years of service traffic. A simpler approval test is proposed, applying stress cycles to small sections of deck to simulate the passage of wheels. The paper includes a description of the FRP deck, the testing and a summary of the performance of the deck. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Glass fibres; B. Fatigue; D. Mechanical testing; E. Pultrusion; FRP bridge deck.
1. Introduction
deterioration and to increase load capacity without extensive and expensive bridge works.
1.1. Background Most modern short span bridges have concrete decks. In general, they are efficient and durable provided proper attention is paid to detailing and the standard of workmanship. Concrete is likely to be the most common deck material for some considerable time. However, some concrete bridge decks have suffered corrosion, due in part to the increasing use of de-icing salts. As there is unlikely to be an economically viable replacement for rock salt for de-icing, increasing interest is being shown in materials that are corrosion resistant. In addition, rapid growth in the volume and weight of heavy goods vehicles (HGVs) has led to serious problems and many older bridges no longer meet current design standards. There is, therefore, a need for methods of replacing bridge decks to deal with structural
* Corresponding author. Tel.: C44 1344 770449; fax: C44 1344 770355. E-mail address: [email protected] (A.F. Daly).
1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.05.040
1.2. Use of FRP in bridge decks The use of Fibre Reinforced Polymer (FRP) as a primary structural material is developing rapidly in the construction industry. FRP materials have considerable advantages in terms of weight, strength and corrosion resistance. They have been used for several decades in the aerospace, automobile and marine industries, where they have developed a good track record in a very adverse environmental conditions. However, the ‘product life cycle’ is much longer in civil engineering so uptake has been relatively slow, but FRP has been used in a number of bridges around the world. As production technology develops and design standards and guidelines become more generally available, these FRP materials will be used more widely to provide cost-effective alternatives to steel and concrete. Potential applications for FRP decks are new design, replacement of under-strength decks in existing bridges, and the provision of temporary running surfaces. In spite of the advantages, however, there is still a lack of information on the behaviour of FRP components in bridge
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applications. This has impeded the development of generally accepted guidelines for the design and application of bridge schemes. In response to this the UK Highways Agency commissioned TRL Limited to carry out research into the performance of FRP bridge decks with the objective of developing design guidelines. 1.3. Outline of research project The aim of the project was to examine the use of FRP in new and replacement bridge decks and produce draft design requirements. A desk study of current research and practice in bridge construction and other FRP applications was carried out to address issues such as design and detailing requirements, resistance to environmental effects, repair and maintenance. These issues have been dealt with in the proposed design guidelines which will be published by the Highways Agency in due course. Existing loading rules can be used for FRP decks and global static design is relatively straightforward provided material properties are known. Some design guides are available, but there are currently no fatigue design standards for FRP bridge decks. Local dynamic stresses at details potentially at risk of fatigue due to wheel loading may be difficult to calculate. There are likely to be high stress gradients and the stress cycle may be tensile, compressive or alternating. As these stresses are raised to the power of 10 to obtain fatigue life, it is important that they be calculated with sufficient accuracy. This is similar to the situation for steel orthotropic bridge decks, which are currently beyond the scope of the UK bridge design code [1]. Because of the high development costs, it is likely that most FRP decks will be produced as modular systems. The deck system tested was thought to be typical in that it consisted of top and bottom flanges supported by webs running transverse to the direction of traffic flow. The limited number of bridge deck systems that have been developed so far, most notably in the US [2] and Europe [3–5] are of this form. Therefore, it was decided to carry out full scale tests to study the performance of FRP decks under local wheel loads and provide data on which to base generic fatigue requirements for technical approval of FRP deck systems. Rolling wheel tests were carried out on a 4.0 m span model bridge; static and fatigue tests were carried out on small sections of deck in a standard testing machine.
Fig. 1. Roadway panel.
of configurations. The units have undercut grooves on the sides, which are used to connect them together using a rod with a ‘dog bone’ cross-section to provide mechanical interlock; the joints are also bonded. This Advanced Composite Construction System (ACCS) has been used in the Linksleader footbridge [3] at Aberfeldy in Scotland and a lifting bridge at Bonds Mill in Gloucestershire, England [4] which was designed to carry vehicles up to 40 tonnes gross weight. The ACCS deck was not designed to resist local wheel loading, so a new Roadway panel was designed and manufactured. This has a similar geometry to the lighter ACCS plank but with thicker sections to resist direct wheel loads. It was designed to be laid transversely on a sub-structure built-up from ACCS planks. The Roadway panel is shown in Fig. 1: further details of the design are given elsewhere [6]. 2.2. Model bridge A full-scale section of bridge deck was built at TRL. It spanned 4.0 m, was 2.12 m wide and had a total construction depth of 0.8 m: see Fig. 2. No surfacing was used (in practice, a thin anti-skid dressing would be sufficient). The deck was instrumented with electrical
2. Model bridge tests 2.1. Description of FRP bridge system The design of the deck used in the dynamic testing was carried out by Maunsell Ltd, UK who developed an FRP bridge deck system consisting of standard glass FRP cellular ‘planks’ and connectors that can be assembled in a variety
Fig. 2. Section through FRP deck.
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resistance strain gauges to determine the strain influence lines as the wheel moved along the deck. Displacement transducers were also fitted to measure deflection of the deck under load. 2.3. Rolling wheel test rig Rolling wheel tests were carried out in the TRL Trafficking Test Facility (TTF). The main features of the TTF are shown in the photograph in Fig. 3. It is similar to a conventional pavement test machine, but is smaller and much cheaper to operate. A wheel, either single, twin or ‘super-single’, rolls along a 3 m long track. The wheel load may be up to 5 tonnes and the load may be applied during both forward and reverse passes, or on the forward pass only. The wheel reaches a maximum speed of 20 km/h and can complete one pass per second (bi-directional), equivalent to 80,000 per day. The wheel, carriage and drive mechanism are set up as a spring-mass system operating at its resonant frequency. This greatly reduces the energy required and hence the cost of running the rig.
Fig. 4. Influence line for strain at top of web.
† ‘Global’: where the relevant loading is the total load due to vehicles on the span. An example is the cracking that occurred in the test, adjacent to the bearings. † ‘Local’: where cyclic stresses arise due to individual wheel loads and cracking occurs under, or close to, the wheel tracks. Cracking due to local loads is the most common form in lightweight steel decks and it is necessary to ensure that similar cracking does not arise in FRP decks.
Initial static tests were carried out to determine the strain levels in the running surface and to select an appropriate load for the dynamic test. Each series of tests consisted of placing the wheel at various positions on the deck and recording the strains. Transverse and longitudinal influence lines for strain could then be plotted and used to determine the load to be used in the dynamic test. Fig. 4 shows a typical influence line, giving the strain at the junction between the web and top flange of the roadway panel as the wheel traversed the deck (see insert in figure for location of the gauge). This was one of the locations indicated by analysis to be subject to high stress range due to traffic loading. The strain shown is in the direction of the traffic movement (i.e. longitudinally in the deck, transversely in the Roadway panel). A strain range of 1000 microstrain (tension) to K1290 microstrain (compression) was recorded. This was the highest range of strain recorded in the deck. The influence lines exhibited the expected strain reversal as the wheel rolled over the webs. Another critical point for fatigue was thought to be in the top flange, mid-way between two webs. Fig. 5 shows the influence line for longitudinal strain on the top surface of the second cell of the Roadway panel. Here, the peak strain recorded was K1090 microstrain (compression).
Fig. 3. FRP deck in test rig.
Fig. 5. Influence line for strain in top flange.
2.4. Test methodology The rolling wheel tests were carried out to study the behaviour of the FRP deck, in order to develop a method by which a client could ensure that an FRP deck proposed for use on a UK highway bridge would have an adequate fatigue life. Potential fatigue damage may be:
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3. Rolling wheel test results 3.1. General performance of the deck
Fig. 6. Transverse influence line for strain in top flange.
Fig. 6 shows the influence line for the strain as the transverse position of the wheel is changed. Again, the strain shown is in the direction of the traffic movement. Two plots are shown, at the locations of the maximum and minimum strain as the wheel swept from one end of the deck to the other. The strains recorded for this gauge ranged from 270 (tension) to K950 microstrain (compression). Influence lines were also recorded for deflection of the deck to indicate the global performance. The maximum deflection was just under 1 mm. Once the test load level was determined, the dynamic test was started, with the logging system set up to record maximum and minimum values of each gauge output every 10 or so cycles. At various rig cycle counts, a full sweep of strain and deflection measurements was recorded and retained to provide influence lines for strain and deflection. These were used to monitor any change in behaviour as the dynamic test progressed.
The dynamic test was continued for as long as possible, within the constraints of the project time-scale. In all, the deck sustained about 4.6 million load cycles. At various times, the test was stopped and close visual examination of the surface was carried out. Fibre-scope examination of the gauged area inside the Roadway panels did not indicate any damage. Subsequent examination of the surface (including sections and cores taken through the running surface) indicated that there was no visible deterioration and it was concluded that if this were a real structure there would be no cause for concern. A 500 mm section of the Roadway panel, taken from along the line of the wheel loading, was subsequently tested statically to failure (Specimen FS22-3 as reported in Table 1). Comparison with a similar test on a not-previously loaded section (Specimen FS22-12) indicated no significant different in strength. This confirmed the above conclusion, that the Roadway panel provided a robust structure capable of sustaining HGV traffic. 3.2. Damage in the sub-structure The only visible damage in the deck was in the substructure. After 247,400 load cycles, damage was noticed adjacent to one of the bearings. This consisted of cracks in the first and second webs of the ACCS plank and debonding of the hard-wood insert (used to prevent local distortion and buckling of the section at the bearing): see Fig. 7. At 256,000 load cycles, some debonding was noted in six of the eight supports, although cracks were only
Table 1 Static and fatigue test results Specimen
Top flange
Support type
Maximum static load (kN)
Fatigue test load (kN)
Fatigue test load/ ultimate
Endurance
FS22-1 FS22-2 FS22-3 FS22-5 FS22-5 second crack FS22-6 FS22-7 FS22-8 FS22-8 second crack FS22-9 FS22-9 second crack FS22-10 FS22-11 FS22-12
Thick Thick Thin Thick
Uniform Uniform Uniform Uniform
476 – 353 510
– 150 – 150
– 0.315 – 0.294
Thin Thick Thin
Uniform Ends Ends
373 473 –
100 80 80
0.268 0.169 0.244
Thin
Ends
343
60
0.175
Thin Thick Thin
Ends Ends Uniform
328 422 384
– – –
– – –
– 442,500 – 144,000 201,000 151,000 2,700,000 42,100 250,900 1,151,300 1,697,800 – – –
Endurance is number of cycles to a surface crack R50 mm long in the underside of the top flange. One cycle consists of one half sine wave load on both load pads 1 and 2. Specimen FS22-3 was cut from the panel tested in the rolling wheel rig (from directly under the wheel track). Specimens FS22-2 and FS22-8 (to first crack) were not included in the regression.
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Fig. 7. Cracking at support in FRP deck.
apparent at one support. The dynamic movement across these cracks was 1.1 mm. At load cycle 247,400, it was noted that some of the FRP angles holding the end diaphragms in place had become debonded from the ACCS webs, with relative movement of about 0.07 mm occurring. At the completion of the test, two other angles had also become detached. The only other significant damage was in the bonded joint between the ACCS webs and the top flange. This was first noticed because the two surfaces were grinding together and the adhesive was being ground into a fine white powder. Fig. 8 shows the joint (viewed from under the deck) at the conclusion of the test. The relative movement at this joint was about 0.5 mm.
4. Tests on small sections of deck The rolling wheel tests have the advantage that all parts of the deck are automatically included in the test, but the rate of testing is low (1 Hz) and the cost is relatively high for
Fig. 8. Deterioration of joint in sub-structure.
a routine test. Figs. 4–6 show that the influence lines for potential crack locations in the Roadway panel are short. This suggests that a test on a small section of deck, that simulates the effect of an individual wheel, should be sufficient to assess the deck details for fatigue, due to local loads. The UK bridge design code (British Standards Institution, 1980) gives a maximum wheel load for normal traffic of 30 kN on a contact patch of 200!200 mm: this provides a basis for a fatigue test load, with a suitable factor added. The objectives of the tests were to understand the behaviour of the Roadway panel under repeated local wheel loading, to define its fatigue strength and consider appropriate methods of estimating its fatigue life. A simple fatigue test could then be specified for checking and approving new FRP deck systems offered for use on UK highway bridges. The test sections were 600 mm wide, 115 mm deep and 500 mm long—sufficient to avoid edge effects, but small enough to fit a standard testing machine. Static tests were carried out using a single 200!200 mm pad, to determine the ultimate load capacity and mode of failure of the panel. Fig. 9 shows the test arrangement. For the dynamic and fatigue tests, a rig was devised to fit the testing machine in which two adjacent steel pads, 200!200 mm and faced with rubber, loaded the panel alternately, to simulate the passage of a single wheel. The arrangement is shown in Fig. 10. The loading and support conditions were developed to produce measured strains similar to those at the same locations on the full size deck. The cross-section of the Roadway panel was nominally symmetrical. It was found that the lengths supplied were dimensionally very consistent, but the top and bottom flanges were of different thickness, approximately 11 and 14 mm at their thinnest point midway between webs. This was not thought to be significant as both flanges contained the same amount of fibre, but the static tests showed that the ultimate capacity was reduced by around 23% if the thinner flange was uppermost. The support conditions were also found to be significant. The Roadway panels were designed to be uniformly supported by a deck structure consisting of thinner section ACCS panels, but in practice the top surface of the supporting deck was not quite flat. The connecting pieces above the ACCS planks forming the webs were slightly proud of the ACCS panels forming the top flange, so the Roadway panel was, to some extent, spanning the 600 mm space between webs. Supporting the 500 mm test sections along each end gave strains which were closer to those on the full size deck under the rolling wheel. In addition, ultimate static capacity was reduced by around 12%. This demonstrated that small differences in fit-up can have significant effects on load capacity; approval testing should simulate actual, rather than design conditions.
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Fig. 9. Static test arrangement.
Strain gauges were attached to some of the sections at the same locations as on the full size deck: see inserts in Figs. 4–6.
Acoustic emission (AE) was recorded during some of the static tests. Significant emissions were detected prior to failure and it appears that AE would offer a viable method of monitoring FRP bridge decks.
4.1. Test results 4.2. S–N curve from test results The results of the static and fatigue tests are summarised in Table 1. Fatigue cracks occurred in the top flange midway between webs and propagated longitudinally along the panel. The cracks extended through approximately 60% of the thickness of the flange. The cracks did not reduce the ultimate static capacity of the test panels under a wheel load, so their structural effect may be small. They would allow moisture to penetrate to the fibres so there could be long term effects on durability, but if appropriate materials are specified, any deterioration would take at least 40–50 years to occur. On a bridge, this type of crack could occur in each cell under each wheel track and propagate to the width of the wheel track, i.e. 500 mm or more. Cracks were also observed in the radius between web and flange, either at the start of the test, or after a small number of load cycles. It is thought that these cracks resulted from manufacture, possibly due to shrinkage. In one specimen sectioned and examined after testing, the cracks were in the resin only and did not extend through the outer layer of fibre during the fatigue test. They are not thought to be significant. Measured strain was not proportional to load, at higher loads. In addition, the most severe strain was not at the crack location. It is concluded that measured surface strain is not a satisfactory method of characterising the strength of FRP panels under high test loads.
Fatigue assessment of steel structures is usually based on stress at the potential crack location. A designer would calculate a stress spectrum due to traffic, then use an experimentally derived S–N curve and damage summation rule to estimate fatigue life. In most cases ‘nominal’ stress is sufficient, but for some details on steel orthotropic decks the local stress (which governs fatigue cracking) is different to the ‘nominal’ stress produced by a conventional FE analysis. This is why steel decks are outside the scope of
Fig. 10. Fatigue loading arrangement.
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the current design code, BS 5400: Part 10 [5]. FRP bridge decks are, if anything, more complex than a steel orthotropic deck due to the non-uniform material properties. Expressing fatigue strength in terms of load for the purposes of an approval test has the advantages of simplicity and reduced instrumentation costs. The test results would of course only apply to the component geometry tested, but at the current stage of development each new deck will need to be tested, so this is not a serious disadvantage. For fatigue due to local wheel loads, the relevant load is that due to the passage of a wheel over the detail being assessed. The S–N relationship can then be expressed as Lmax =Lu Z A K k log10 ðNL Þ where Lmax is the maximum load in a cycle (same as load range in this case). Lu is the ultimate load capacity under a simulated wheel load. NL is the number of cycles to failure. A is a constant. k is the slope of the regression line. In this equation, the fatigue strength of the specimens is expressed in terms of applied load divided by the ultimate static load. However, the mode of failure in fatigue tests may be different to that in static tests. There may not be a direct relationship between the dynamic loading causing fatigue damage and the static ultimate load. In that case, the ultimate static load capacity is merely a constant in the equation of the S–N curve. For the purposes of analysing the test results to produce an S–N curve for the panel, endurance (NL) was taken to be number of cycles to the first crack at least 50 mm long. At this length the crack could be detected reliably on a bridge. As the observed cracks had no structural effect, fatigue life in this case is not the same as the service life of the deck. Regression analysis was carried out between log10 (N) and (Lmax/Lu), with log10 (N) as the independent variable. There were too few test results to determine the most appropriate statistical method, so the procedure used to produce the S–N curves in BS 5400: Part 10 was used, with an allowance for the small number of results. Expressing the load as a fraction of ultimate static strength disguises the effect of the support conditions (uniform or ends only) and flange thickness, so all results were analysed together. Specimens FS22-2 and FS22-8 (to 1st crack) gave endurances very different to the others (see Fig. 11) and were not included in the analysis. The data and regression line are also shown in Fig. 11 and the rolling wheel test result is included for comparison. The predicted endurance of the Roadway panel in the rolling wheel test, assuming the result would lie on the mean regression line for the test specimens, is 12 million cycles. Hence, the wheel load was too low to give a 50% probability of
Fig. 11. Regression analysis of fatigue test results.
cracking if there is a cut-off at 10 million cycles. It should be noted that the wheel load in the test was 23% higher than the maximum permitted for normal vehicles on UK roads and that the endurance is to the appearance of non-structural cracks.
5. Estimated fatigue lives The objective of the fatigue testing was to enable the service life of a bridge deck to be estimated. Development of a suitable calculation method was beyond the scope of the project, so the cumulative damage method given in BS 5400 Part 10, Section 8.4 was used. Bridges are long life structures and the dynamic loads due to traffic are generally small compared to the ultimate load. The vehicles in the BS 5400 design traffic loading spectrum have a range of wheel loads, from 7.5 kN for lightly laden 2-axle vehicles (type 2R-L) to 40 kN for a small number of very heavy ‘special order’ vehicles. Therefore, a key part of the calculation is the treatment of low stress cycles, i.e. those corresponding to constant amplitude endurances greater than 107 cycles. Low stresses only become damaging after some fatigue damage has occurred; this is taken into account in the BS 5400 method by a change in the stress exponent (m) at 107 cycles. It is thought that there may be a constant amplitude fatigue limit for FRP components at a stress corresponding to an endurance of around 107 cycles, but it is not known how lower stresses should be treated. Adopting the BS 5400 method of changing the exponent from m to mC2 would have little effect on estimated fatigue life. The calculations for the Roadway panel were carried out in terms of wheel load/ultimate load rather than stress, for three cases: (i) Assuming all loads are equally damaging, i.e. the S–N curve is linear to zero load. (ii) Assuming that low loads only become damaging after high loads in the spectrum have caused some
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Table 2 Estimated fatigue lives S–N curve
Conditions
Load at 107 cycles
Estimated life (years)
% Damage due to HGVs O36t
Mean S–N curve from test data
All loads damaging Bi-linear S–N curve Cut-off at 107 cycles
0.098 0.098 0.098
16 55 O1000
0.1 0.4
‘Design’ S–N curve based on test data
All loads damaging Bi-linear S–N curve Cut-off at 107 cycles All loads damaging Bi-linear S–N curve Cut-off at 107 cycles
0.051 0.051 0.051 0.214 0.214 0.214
5 7 16 13 40 O1000
0.1 0.2 0.3 0.1 0.3
Notional S–N curve to give life of 40 years for bi-linear curve
Estimated life is years to a surface crack R50 mm long in the underside of the top flange. Traffic loading is BS 5400 Part 10, Table 11 with one million HGV per year.
damage, i.e. a bi-linear S–N curve. As no data are available, the S–N curve for endurances O107 cycles is arbitrary and intended only to test the effect of this type of treatment of low loads. The curve for NO107 cycles was set to midway between cases (i) and (iii). (iii) Assuming low loads never become damaging, i.e. there is a cut-off at 107 cycles, so the S–N curve becomes horizontal at this point. The calculations for these cases were repeated for several S–N curves. (i) The mean regression line for all the test results (‘mean’ curve). (ii) The lower 95% confidence limit of the test results (‘design’ curve). (iii) A notional S–N curve with the same slope as the test results, selected to give an estimated life of 40 years for the bi-linear case (‘40 year’ curve). Estimated lives are given in Table 2 for one million HGVs per year on an un-surfaced Roadway panel. Treating all load cycles as equally damaging (no change in slope in the S–N curve) is too severe, but equally, ignoring all cycles below the 107 cycle cut-off could be unsafe. For short lives, e.g. the ‘design’ curve, changing the slope of the S–N curve is of limited practical significance. However, lives based on the ‘mean’ curve show that the treatment of low loads can change the estimated life from 16 years to O1000 years. If the test results are considered to provide lower bound results for structural cracking, then a panel in service should remain free from structural cracks for around 55 years, based on the ‘mean’ bi-linear curve in Fig. 11. In some cases, fatigue life may be affected by the small number of ‘general’ and ‘special’ order vehicles, i.e. those with gross vehicle weight (GVW) greater than 44 tonnes, which have high wheel loads. The damage due to these vehicles is included in Table 2; it will be seen that these
vehicles are not significant for the FRP deck. This is encouraging as the load in the rolling wheel test (37 kN) was above the maximum for normal HGVs (30 kN). The result of the rolling wheel test is included in Fig. 11; it is close to the ‘design’ curve, indicating there was a 2.5% probability of cracks midway between webs. The rolling wheel test load was less than the load at 107 cycles on the mean S–N curve, so this type of cracking might never have occurred. On the basis of these results, it appears that nonstructural cracks could occur in a Roadway panel on a UK bridge within the 120 year notional design life normally used for UK highway bridges, unless the effects of wheel loads are reduced by asphaltic surfacing. Clearly, the effect of low loads (less than 10% of ultimate, corresponding to constant amplitude endurances of O107 cycles) is significant, and a meaningful estimate of fatigue life cannot be made without taking them into account. Further work is needed to determine how they should be treated in design. In the meantime, an approval test based on wheel loads occurring in normal design traffic, plus a suitable partial factor, should provide an interim solution.
6. Conclusions Dynamic testing of an FRP deck has shown that FRP components can provide a robust bridge solution complying with the general requirements of the UK design code and in particular, capable of resisting local wheel loads due to heavy vehicles for at least 30–40 years without structural damage. Non-structural cracks may occur before this, but their effect on durability would be long term. Careful attention is needed to prevent local damage in highly stressed regions of the supporting deck, such as web to flange connections and close to bearing supports.
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A simple fatigue design method, not requiring a damage summation, appears possible. This would be based on demonstrating that fatigue damage will not occur under design traffic loading. It would consist of fatigue tests on small sections of deck panel with test loads set to ensure that almost all traffic loading cycles are below the damage threshold. This avoids the problem of the lack of a method for calculating the effect of low loads in the spectrum and is not thought to be too severe a requirement at this stage of development of FRP decks. A procedure for fatigue assessment has been devised for incorporation into the draft Highways Agency Standard for FRP decks. It is considered important to minimise the testing requirement so as to encourage development of FRP bridge decks. The results obtained in this project suggest that a simple test simulating a single wheel load will be sufficient.
References [1] British Standards Institution BS 5400. BS 5400: Part 10: 1980. Steel, concrete and composite bridges Code of practice for fatigue. London, UK: British Standards Institution; 1980. [2] Cassity, P, Richards D, Gillespie J. Compositely acting FRP deck and girder system. Structural Engineering International, Vol. 12, No. 2, International Association for Bridge and Structural Engineering (IABSE), Zurich, Switzerland; 2002. [3] Daly, AF. Developments in modular FRP decking systems for highway bridges. Paper presented at one-day conference at Aston University: Bridges 2001: Old problems—new challenges; 17 Oct 2001. [4] Oliver A. Loaded questions New civil engineer. London: Thomas Telford; 1994. [5] Luke S. FRP deck for West Mill Bridge, UK. Proceedings of the 10th International conference and exhibition—structural faults and repair conference, London. Edinburgh: Engineering Technics Press; 2003. [6] Daly AF, Duckett WA. The design and testing of an FRP highway bridge deck. Journal of Research, Vol. 5, No. 3, Transport Research Laboratory, Crowthorne, UK; 2002.