DEVELOPMENTS IN FRP RAILWAY BRIDGE APPLICATIONS

Transcription

1 DEVELOPMENTS IN FRP RAILWAY BRIDGE APPLICATIONS Lee CANNING Principal Engineer Sinclair Knight Mertz The Metro Building, 33 Trafford Road, Salford Quays, Manchester, UK. M5 3NN. Abstract Network Rail are the infrastructure owners and maintainers for approximately 40,000 bridges. A large proportion of the bridgestock comprises metallic railway bridges with timber decking carrying ballasted track. The substructure and metallic structural elements of this bridgestock are typically 100 years old with limited total capacity, no available additional construction depth, and low ballast depth. Timber decking typically has a serviceable life of up to 30 years and is known to worsen the durability of steelwork essentially due to moisture retention. Steel decking may be used to replace timber decking but has the associated issues of galvanic corrosion and transportation/access/installation of heavy steel panels. The development of lightweight, durable, FRP (fibre reinforced polymer) decking is discussed based on three case studies where timber decking on metallic railway bridges was replaced with FRP decking. The factors discussed include manufacturing and design methods, specification and testing, derailment capacity, installation methodology, and future developments. Keywords: analysis, bridges, design, FRP decking, installation, railway. 1. Introduction A significant proportion of the existing Network Rail bridgestock of 40,000 bridges comprises metallic bridges with timber decking. The timber decking traditionally is formed from 100mm (or 4 inch) thick timber planks spanning over cross -girders or between main girders (see Figure 1) and supporting ballasted track. Typically some form of waterproofing system was laid on top of the timber decking, although invariably such waterproofing fails after a short period of time, allowing the timber planks to rot and the adjacent steel to corrode. In this situation the practical life of the timber decking is typically a maximum of 30 years, often less. It is well known that timber increases the rate of corrosion of adjacent steel, particularly in the very severe railway bridge environment where local relative humidity and moisture content can be high. Furthermore, timber decking does not meet the required design standards for derailment load. The exact derailment load varies depending on the particular country (and also ballast depth for the particular bridge), but for the UK comprises a factored load of approximately 28 tonnes on a circular area of up to 350mm diameter (for 300mm ballast depth). Regardless of the above deficiencies, degraded and sub-standard timber railway decking has historically been replaced with timber decking (essentially like -for-like replacement) or in some cases steel plate decking. Replacement with timber decking, although of low capital cost and also lightweight, implies periodic replacement of the decking at approximately 25 year intervals with significant associated disruption to the railway (typically a 2 -day possession would be required). In addition, continued use of timber decking further deteriorates the supporting steel girders due to corrosion effects. Page 1 of 10

2 Where steel plate decking is used, this is typically up to 50mm thick where standard deflection limits and derailment capacity are required. In some instances, departure from the design standards is allowed and a lesser thickness may be used. This decision is often related to whether the supporting structural elements also have derailment capacity or not. It can be seen that if replacement decking fully compliant with the design standards is required, steel decking is heavy (typically four times heavier than traditional 100mm thick timber decking) and crane lifting limits become important, restricting the steel deck panel size. The existing capacity of the supporting structure may also become critical due to the increase in dead load. Replacement steel decking also has limited durability due to the very severe environment (effectively a buried steel element which experiences wet/dry cycling and various chemical solutions). Additionally, as the replacement decking is effectively a partially buried element, it is difficult to confirm its condition without non-destructive or partially destructive testing, or disruptive trial pits in the ballast from the track. The combination of the above factors has driven the need for alternative solutions for railway decking that is fully compliant with design standards, lightweight, and durable. An alternative material that has been used in recent years in heavy duty civil engineering applications, particularly bridges, is FRP composite. The critical factors in using this novel material and some initial applications are discussed below. 2. Design Requirements and Constraints It can be seen from the introduction that a significant factor in choosing the type of railway decking (and its weight/cost), regardless of material type, is whether derailment capacity is required or not. When derailment capacity is required this essentially drives the decking solution to being a thick solid section to provided the local punching shear resistance and global flexural capacity. When derailment capacity is not required, deflection limits (typically between span/600 to span/300 or 1.7mm to 3.3mm for a typical 1m decking span) become the governing factor and thinner or cellular lightweight structural forms can be used. This distinction is particularly important for materials of high specific cost (cost per unit weight) as a reduction in required material weight will directly reduce material costs. Therefore, for FRP composite materials which have a relatively high specific cost compared to steel or timber, the requirement for derailment capacity will have a significant effect on material cost. The replacement of railway decking has a number of technical and practical constraints: i. Construction depth. Typically the existing decking will comprise 100mm thick timber planks and the existing ballast depth over this cannot be reduced (and often is required to be incre ased to improve ballast depth and permanent way maintenance). ii. Decking weight. Existing metallic bridges often have limited structural capacity. Therefore, overall permanent load on the structure cannot be increased and a decrease in weight is preferred to maximise the live load capacity of the structure. Page 2 of 10

3 iii. Buildability/possessions. Maintenance and construction work on railway bridges is usually undertaken during possession, or temporary closure, of the railway. The maximum possession period commonly available in the UK is a maximum of 48 hours (often less), although 100 hour possessions may be available on less busy routes during holiday periods. Therefore, prefabrication of railway decking into large panels is useful. iv. Durability. Any alternative replacement railway decking system should be no worse in terms of durability compared to conventional steel or timber decking. 3. Structural FRP Composite Decking Options The combination of these constraints, together with a requirement for derailment capacity, limits the available options for the use of FRP composites. Structural options using commercially available FRP composite materials/systems can be summarised as: i. Solid CFRP-GFRP hybrid plate. ii. Solid quasi-isotropic GFRP plate. iii. Heavy duty GFRP grating/decking systems with bonded GFRP top plate. iv. Cellular GFRP decking systems. v. Moulded GFRP decking. vi. Sandwich GFRP decking systems. Examples of these structural options are shown in Figure 1. Figure 1 FRP decking types (cellular to left, heavy duty grating to middle, moulded to right) A technical, practical, and commercial review of these options was undertaken. Particular important aspects that are not immediately obvious include confirmation of bond quality (i.e. avoidance of blind bonding), sturdiness of the structural option (i.e. ability to withstand site handling, local damage, and loss of section due to ballast attrition, without significant affect on performance), availability of competing products, and specification compliance. Consideration of these aspects confirmed that the most suitable type of FRP decking system was heavy duty GFRP grating system with bonded GFRP top plate (and bottom plate if required for larger spans), shown in Figure 2. The positive features of this choice include: i. Visible confirmation of adequate bonding (avoidance of blind bonding). ii. Thick top/bottom plates provide adequate resistance against local damage and ballast attrition. iii. A minimum of three competing product suppliers in the UK. Page 3 of 10

4 iv. Use of standard pultruded products with internationally accepted product standards, such as EN [1], and well proven workmanship specifications. v. Minimum cost (compared to other options) and some documented proof of local load capacity similar to derailment loading. The importance of existing accepted product standards and workmanship specifications should not be overlooked considering the safety critical nature of this application, and also that inspection is difficult. The performance of other heavy duty GFRP decking systems (e.g. using moulded GFRP composite or sandwich systems), although having great potential, has been reported to be variable in the field for civil engineering applications [2]. Figure 2 Heavy duty GFRP grating (main bars in blue, transverse rods in white, top plate in red) 4. Analysis, Design, and Specification The general analysis and design basis for FRP railway decking is described below. The particular features and improvements made are described within each case study. The design of the heavy duty GFRP decking with bonded GFRP plates was undertaken using BD 37/01 [3] and the EuroComp [4] design guide in combination with proprietary design guides [5]. The main technical design criteria to meet were a factored design derailment point load of 280kN at FRP deck level, and deflection limits (typically span/600). Generally, FRP composites are not particularly efficient at resisting concentrated punching loads in comparison to their other properties, and this posed the main design challenge. The GFRP decking was analysed using a grillage analysis. Each longitudinal bar element was modelled individually (typically at 15-25mm spacing depending on exact product type), with transverse elements representing the top/bottom plates/rods at the location of each transverse rod (typically at 150mm spacing). The need to model each longitudinal bar is particularly important if a group of bars is modelled (e.g. a representative group at 150mm spacing) to give a square grillage, as is often used in analysis of slab or beam-and-slab) type bridges, initial failure of bars would be overlooked. This is critical for FRP composites which have a linear elastic response to failure with limited and uncertain residual capacity after failure. In this instance, first failure of a single bar would lead to progressive failure of other bars as load is shed onto stiffer elements. Current UK design standards allow some local damage to occur to decking elements in the event of derailment, as long as collapse of the decking system does not occur. Therefore, at the ultimate state event of derailment, debonding of the top/bottom GFRP plates (due to high longitudinal shear stress in the bonded joint) was allowed to occur. The remaining GFRP Page 4 of 10

5 decking system, comprising the main bars and transverse rods only, were then checked for capacity (although still taking advantage of the additional load dispersal through the GFRP top plate thickness). Following a train derailment, the GFRP decking would need to be inspected to determine whether strengthening or replacement of particular deck panels is required due to local damage. The material properties of the individual elements of the GFRP decking (main bars, transverse rods, top/bottom plates, and adhesive) are shown in Table 1. Other limitations of commercially available products, such as depth and spacing of bars, are also noted. The material properties were incorporated into a generic materials and workmanship specification. The specification also included items such as bonding procedure, chemical resistance, compliance testing, and acceptable defects based on EN Property/Element Main bars Transverse rods Top/bottom plates Adhesive Elastic modulus, (x) 1-10 GPa 10 (y) Tensile strength, (x) 12 MPa 70 (y) Compressive strength, MPa (x) 135 (y) - Flexural strength, (x) - MPa 125 (y) In-plane shear strength, MPa Out-of-plane shear (estimated) 12 strength, MPa Comment Max depth Min spacing Max thickness Epoxy or 63.5mm 150mm 25.4mm polyurethane Min spacing 21.5mm Table 1 Material properties of GFRP railway decking (x is parallel to pultrusion direction, y is perpendicular to pultrusion direction) 5. Case Studies Particular details of the GFRP decking, including benefits and technical limitations, are now described in three case studies. 5.1 Calder Viaduct Calder Viaduct is situated on the Carnforth North Junction to Carlisle South Junction (via Barrow) in Calder, just south of Sellafield station (Figure 3) in the UK. The viaduct comprises three simply supported square spans. The two side spans consist of half through type edge girders and one half through type central girder with cross girders and longitudinal timber decking. The middle, main span comprises half through type edge girders supporting cross girders and longitudinal structural timber decking. All girders are constructed in early steel manufactured in The central span permanently bridges the River Calder and the revetments beneath the outer spans carry occasional storm and high water. The abutments are constructed from sandstone blockwork, brickwork and reinforced concrete bearing plinths. The intermediate piers are constructed from brickwork with sandstone bearing plinths, on piled foundations. Page 5 of 10

6 Figure 3 Calder Viaduct The main constraints for the re-decking scheme were the use of a 100 hour main blockade with supporting 12 hour possessions, limits to construction depth due to the existing substandard ballast depth (in some areas less than 150mm) and adjacent longitudinal timber railway bridge preventing track lift at one end of the structure, and the requirement for minimal dead load due to the capacity of the existing structural members to be retained. In addition, the design requirement for derailment load capacity to the new decking further limited the structural options. Due to the existing track and cross-girder levels throughout the viaduct, two supporting methods were developed for the GFRP decking system to achieve the required minimum ballast depth of 150mm (require to allow tamping of the ballast without damaging the decking). The first comprised the GFRP deck spanning continuously over the existing crossgirders (at approximately 914mm spacing) with bolted connections th rough the GFRP deck and cross-girder top flanges. The second comprised the GFRP deck supported on new steelwork, itself supported on the existing cross-girders, to form a level surface no higher than the existing cross-girder top flanges (Figure 4). The de sign concept was such that all structural bonded connections were made in the factory under controlled conditions and minimum time pressure, whereas all connections made on site were either non-structural bonding (to seal joints) or simple bolted details. Figure 4 Underslung girder and FRP deck arrangement to maximise ballast depth Page 6 of 10

7 At all bolted connections between the GFRP deck and cross-girders, the GFRP deck was made solid by the use of bonded infill pieces in the open area. The solid area was sufficiently large to provide some tolerance for site fixing. This detail was developed to generally provide a robust connection detail and in particular to maximise the bolt bearing area under longitudinal and transverse load effects. In the centre span, a flexible joining detail was developed to again allow tolerance on site for positioning of the FRP deck panels. Using the previously described analysis method, the GFRP decking comprised 57.2mm (2.25 inches) depth by 15mm width main bars at 21.5mm spacing, transverse rods at 150mm spacing, with a bonded 19.0mm (0.75 inches) depth GFRP top plate (no bottom plate was required). Ballast boards comprised bolted GFRP angle and plate. The total depth of the GFRP decking was 80mm (allowing for toleran ce and adhesive bond thickness), an improvement on the existing 100mm depth timber decking, and allowed the ballast depth to be improved. A 100 hour possession was planned for replacing the decking at Calder Viaduct with additional short possessions before and after the main possession for preparatory and finishing works. To minimise the installation time for the replacement FRP decking system, the following methods were used: i. Maximise GFRP deck panel size based on track-mounted crane capacity, ii. Additional supporting elements detailed to allow partial installation from the underside outside of possession. GFRP deck panels were limited to approximately 2 Tonnes weight (a panel area of approximately 3m width by 4m length) to allow installation by rail-mounted crane from the adjacent track. The installation of the GFRP decking panels is shown in Figures 5 and 6. Figure 5 Craning in of GFRP deck one track Figure 6 Installed GFRP deck panels to panel 5.2 Stansty Bridge Stansty Bridge carries the railway over a single carriageway road, near Wrexham in the UK (Figure 7). The bridge has a single span of approximately 8m, and comprises early riveted steel half-through girders with steel cross-girders and timber decking supporting ballasted track. Inspection and assessment of the structure had identified that steel plate strengthening work was required to the main girders, together with replacement of the deteriorated timber Page 7 of 10

8 decking. The span of the decking between the cross-girders was 1220mm (greater than that for Calder Viaduct). The design requirements and constraints were similar to those for Calder Viaduct (derailment capacity was required together with deflection limited to span/600 at the serviceability limit state). Preliminary analysis of the GFRP decking showed that a GFRP bottom plate in addition to a top plate would be provide a sufficiently stiff and strong decking system. Detailed analysis confirmed this arrangement, with the GFRP decking comprising 63.5mm (2.5 inches) depth by 15mm width main bars at 21.5mm spacing, transverse rods at 150mm spacing, with bonded 12.7mm (0.5 inches) depth GFRP top and bottom plates. The detailed analysis also showed that, assuming a construction depth limit of 100mm (the same as for timber decking), a number of limit states were critical at this span (in particular flexural stress in the main bars at ULS derailment load, and deflection/bond stress limits at SLS). Therefore, using the previously described analysis methods and design guidance, and currently available GFRP products, this span represents the technical limit on GFRP railway decking where derailment capacity is required with a maximum allowable construction depth of 100mm. After completion of the design, the bridge owner decided to replace the whole bridge with a modern structure, and therefore the GFRP decking was not installed. Figure 7 Stansty Bridge 5.3 Rubha Glas Viaduct Rubha Glas Viaduct carries the West Highland railway line near Stirling in the UK. The bridge is located on a mountainside with difficult access, has two spans each of approximately 8m, and comprises steel plate half-through edge girders with steel cross-girders and timber decking. Inspection and assessment of the bridge had shown that strengthening works were required to the central pier and edge girders, together with replacement of the deteriorated timber decking. The span of the decking between the cross-girders was 1015mm (greater than that for Calder Viaduct). The design requirements and constraints were similar to those for Calder Viaduct (derailment capacity was required together with deflection limited to span/600 at the serviceability limit state). Page 8 of 10

9 Analysis of the GFRP decking confirmed the GFRP decking comprising 63.5mm (2.5 inches) depth by 15mm width main bars at 21.5mm spacing, transverse rods at 150mm spacing, with bonded 19.0mm (0.75 inches) depth GFRP top plate. A non-continuous 19.0mm thick bonded GFRP bottom plate was also used to provide additional transverse strength and stiffness between lifting points. To minimise the amount of site drilling for holding down bolts, bespoke heavy duty fixings were designed to attach the GFRP deck panels to the cross-girders. The fixing comprised a clamping system using tension control bolts to resist uplift forces of approximately 30kN at each fixing with co-existant transverse traction/braking forces. However, pre-tensioned bolts cannot generally be used in FRP composites due to low through-thickness stiffness and creep effects. To overcome this, bonded steel bearing tubes were fabricated within the GFRP deck section in the factory to directly resist permanent clamping forces. A 50 hour possession of the railway was used to removed the existing track and ballast, replace a number of cross-girders, replace the timber decking with GFRP decking panels, and reinstate the track and ballast (Figures 8 and 9). Due to difficult access at the site, a small road-rail vehicle was used, which limited the weight of the GFRP deck panels that could be lifted. Therefore, GFRP deck panels of approximately 4.5m width by 2m length, with a weight of less than 1.5T were used. A minimum of 4 no. clamp fixings per deck panel (1 no. at each corner) were required to be tightened prior to opening of the railway to ensure adequate stability under temporary speed restriction, however, the remaining fixings (4 no. per deck panel) could be tightened outside of the possession to minimise work within possession. Figure 8 - (GFRP deck panel in factory to left, installed GFRP deck panels to right) Figure 9 Completed redecking and strengthening works to Rubha Glas Viaduct Page 9 of 10

10 6. Acknowledgements The author would like to acknowledge Network Rail for allowing use of the photographs in this paper. The views expressed in this paper are solely those of the authors and not necessarily those of SKM or Network Rail. 7. References [1] BS EN 13706, Reinforced Plastic Composites, British Standards Institute, [2] Transportation Research Board, Field Inspection Of In-Service FRP Bridge Decks, Washington D.C., [3] DMRB (Design Manual For Roads and Bridges), BD 37/01 Loads For Highway Bridges, [4] EuroComp Structural Design of Polymer Composites, Published by E and FN Spon, ISBN [5] Strongwell Design Guide, Rev.0502, Page 10 of 10