Bridge option for forest roads: A composite timber beam and concrete deck bridge

Transcription

1 Bridge option for forest roads: A composite timber beam and concrete deck bridge A.G. Lee* Forestry Tasmania Abstract Bridge options for forest roads are traditionally limited to either timber bridges or bridges in permanent materials. The options have widely different design lives and costs. A composite timber bridge with a reinforced concrete deck designed and constructed by Forestry Tasmania provides a viable third option. Introduction In the last five years, Forestry Tasmania (FT) has developed and constructed various composite timber bridges with a reinforced concrete deck. To provide composite action, shear connectors have been used to connect the timber stringers to the concrete deck which is cast on site. Both single- and twin-span structures of single- and two-lane configuration have been constructed up to a span of 13 m. The objective was to develop a bridge design that had the characteristics of a permanent bridge structure without the cost premium. The benchmark to date has been the standard all-timber bridge. A summary of the characteristics of each bridge type is shown in Table 1. Cost comparisons Based on the indicative construction costs in Table 1, it is also necessary to factor in future maintenance and demolition costs as well as the initial construction cost and replacement costs for each bridge option. To illustrate, a discounted present-value cost analysis Table 1. Bridge comparisons. Forestry Tasmania Timber bridge composite bridge Permanent bridge Design life 20 years 40 years estimated 80 years plus Construction cost $400/m 2 $600/m 2 $800/m 2 plus Construction One week Three weeks plus Depends on materials; for period deck cure time example, one day road prior to usage interruption for pre-cast deck planks and abutments Skill level for Moderate High High installation Maintenance High, depending on Low Low depending on bridge requirements usage replace deck materials planks and deck runners * allan.lee@forestrytas.com.au Tasforests Vol

2 Table 2. Typical bridge replacement costs. (FT = Forestry Tasmania) Life cycle Assumptions Cost ($) usage (yrs) Timber bridge complete replacement 400/m 2 20 Timber bridge replacement runners 100/m 5 Timber bridge replacement deck 100/m 2 10 Timber bridge superstructure replacement stringers 165/m 2 20 Timber bridge abutment replacement 3000 each 40 Timber bridge demolition 1000 each FT design composite concrete deck on timber bridge 600/m 2 40 FT design bridge demolition 2000 each Permanent bridge structure 800/m 2 80 Table 3. Discounted present-value (PV) cost of replacement for a typical timber bridge option single lane, 10 m overall length structure and 4.5 m overall width (deck area = 45 m 2 ) for an 80-year life benefit. PV ($) Discount Rate Year Actual ($) Complete timber replacement Replace runners 5-yearly Replace runners and deck 10-yearly Bridge demolition and complete timber superstructure replacement 20-yearly abutment replacement 40-yearly Total cost ($) Table 4. Discounted present-value (PV) cost for a typical Forestry Tasmania composite bridge option and permanent material bridge option single lane, 10 m overall length structure and 4.5 m overall width (deck area = 45 m 2 ) for an 80-year life benefit. PV ($) Discount Rate Year Actual ($) Forestry Tasmania composite concrete deck on timber bridge Complete replacement Bridge demolition Complete replacement Total cost ($) Permanent material bridge Complete replacement Total cost ($) Tasforests Vol

3 is performed using typical costs and a common bridge situation (Table 2). Table 3 provides total and discounted costs over time for a typical timber bridge. The total costs allow for four 20-year replacement cycles for the timber bridge to provide a total of 80 years life benefit. Table 4 provides total and discounted costs over time for a typical FT composite bridge option assuming two 40-year replacement cycles to again provide a total of 80 years life benefit. A valid cost comparison of each bridge option requires that costing to achieve the same benefit be performed. The permanent material bridge option in Table 4 assumes a design life of 80 years. The FT composite concrete deck on timber bridge option gives the lowest total presentvalue cost for discount rates up to 7%. This suggests that it is the most economic option based on a whole-of-life cost analysis. Actual costs using both contracted prices received from the tender box and day-labour construction have confirmed a characteristic construction cost of around $600/m 2. Design concept The basic premise of the FT composite concrete deck on timber bridge design is that the design life of the timber stringers is doubled from 20 years in a timber bridge to 40 years by placing a concrete deck over the stringers and keeping the stringers dry. This reduces the rate of rot and decay in the stringer, which is the most common cause of stringer failure. The design features are: 1. Placement of concrete deck over the stringers to shed the water (Photo 1a, 1b; Figure 1). Photo 1a. Completed composite concrete and timber bridge. Tasforests Vol

4 Figure 1. Concrete deck on timber stringers. Photo 1b. Deck stringers only. Tasforests Vol

5 2. Placement of concrete abutment curtain wall with impact angle to prevent water infiltration into the ends of the stringers, which is the greatest area of rot in the stringer (Photo 2; Figure 2). 3. Polystyrene foam placed beneath the curtain wall to prevent cracking of the abutment wall after settlement of the abutment (Photo 2; Figure 2). 4. Overhanging kerb with drip edge and aluminium flashing draped over the outermost stringers to protect these from moisture (Photo 1a; Figure 1). Figure 2. Concrete abutment curtain wall. Photo 2. Completed abutment curtain wall. Tasforests Vol

6 5. Overhanging deck scuppers to keep deck run-off away from the outermost stringers (Photo 1a). 6. Timber greasing of end grains and installation of anti-fungal boron 'polesaver' rods for all timber components. 7. Stringers to be 'let in' to abutments either by cutting out of the abutment log or tapering of the stringer ends to prevent slabbing of the bottom of the stringer. 8. Provision of shear connectors between the tops of the stringers and concrete deck to provide composite action (Photo 3; Figure 3). This has resulted in an increased span capacity of two to three metres for a given stringer configuration (mid-span stringer diameter and number of stringers). 9. Shear connector detail to allow for stringer shrinkage and avoid punching of the shear connectors into the concrete deck (Figure 3). 10. Deck hold-down bolts to allow for stringer shrinkage (Photo 3; Figure 4). These design features are expected to increase the service life of the stringers from 20 to 40 years. There is enough evidence from existing structures in the field to support this expectation. The bridge crossing the Little Rapid River approximately 17.5 km from the start of the South Tayatea Road in the Circular Head District was built in This two-span structure has a concrete deck cast over four timber stringers on each span. Field inspections indicate that the stringers are dry and show very little deterioration for 23-yearold stringers. Timber bridges nearby of a similar age and working environment show a marked deterioration and are nearing the end of their useful life. The timber abutments of the bridge are still subject to ground moisture and therefore become the weak link in the structure although 40 years is a common design life for a crib abutment. Standard timber detailing to extend the life of the abutment is now the focus. Photo 3. Deck with shear connectors and hold-down bolts. Tasforests Vol

7 Shear connectors are added to provide composite action between the timber stringers and the concrete deck which is cast on site. Effectively, this results in the requirement for a smaller timber stringer for a given span. This is particularly important where the availability of long, large diameter stringers is restricted. Table 5 highlights the effect of composite and non-composite beam action using the design concept. Table 5. Span tables comparing composite and non-composite action. Mid-span Required span using Required span using beam diameter composite action non-composite action 400 mm up to 7.5 m up to 6.5 m 500 mm m m 600 mm m m 700 mm m m Figure 3. Shear connector detail. Figure 4. Deck hold-down detail. Tasforests Vol

8 Maintenance requirements Specific maintenance requirements for the composite concrete timber bridge are: Periodic tightening of the deck hold-down bolts as the timber stringers shrink. Cleaning of bridge scuppers and replacement of aluminium flashing if required. Replacement of anti-fungal boron rods in the abutments. The normal maintenance requirements of a timber bridge still apply. These may include: Periodic inspection of the bridge opening to remove flood obstructions and allow circulation of air to reduce shade and damp. Maintenance of the road approaches to minimise mud and run-off onto the bridge deck and adverse impact loading. Conclusion Experiences to date have indicated that the composite timber bridge with reinforced concrete deck is a viable bridge alternative in terms of mid-range cost and life compared with timber bridges and bridges in permanent materials. Design principles that minimise the rate of timber decay in the timber stringers by encasing the ends of the stringers in concrete and shedding water away from the stringers is expected to at least double the life of the stringers to an estimated design life of 40 years. Acknowledgements Ron Neve and Steve Smedley of Engineering Branch provided background information and drafting support. Structural analysis of the FT composite concrete deck on timber bridge was carried out by Peter Wood of Wood Brownley & Associates Civil Engineers under the technical direction of Ron Neve. Tasforests Tasforests Vol