Terms commonly used in bridging
Permanent forestry bridges in New Zealand are normally used on arterial roads, and on some secondary roads where it is essential to maintain access. Bridges are used on forest roads when the site conditions or the river size preclude the installation of a culvert or other type of crossing.
Bridges have advantages and disadvantages when compared to other river crossing structures. Their strengths are that they can:
- Span steeply incised rivers, or those with high river banks, to improve crossing egress or eliminate fill over a culvert
- Provide unrestricted vehicle access across rivers that have large and variable water flow, unlike fords, battery culverts or drift decks
- Cross rivers with high bed load and debris potential, where a culvert would likely block
- Be used on a sensitive river bed, and banks that require minimal disturbance. Bridges can avoid or reduce river bed and bank disturbance during construction, and throughout its service life
- Cross high gradient, high energy river sections, where culverts are less suitable
- Reduce erosion, as the channel capacity is not restricted by structures within the river bed
- Be relatively low-maintenance in comparison to other crossing structures.
- Provide barrier-free fish passage
- Be used where other crossing types will be unacceptable.
Their disadvantages include the following:
- Are more expensive than other options, generally
- Require specialists to provide structural design
- Installation typically requires a crane for pile driving and beam placement
- Require enough beam clearance so that floating debris can pass underneath, otherwise they can fail
- May have gross vehicle mass restrictions. This can limit transporters and heavy logging equipment
- May require significant abutment scour protection and maintenance.
8.8.1 Types of bridges
There are a variety of bridges used for forestry purposes. Beam and deck construction are common. Beams are usually steel ‘I’, stressed concrete, steel truss, post-tensioned treated LVL or glue-laminated treated sawn lumber. Decks are prestressed concrete or timber. Shorter decks may be made from concrete slabs. Bridges typically cost more to construct than culverts or low-level crossings. In some instances, temporary portable bridges are used for short-term harvesting and transport access. These are designed for rapid construction and dismantling. Most consist of prefabricated combined beams and decking.
In some instances, temporary portable bridges are used for logging traffic for short-term harvesting and transport access. Bridges typically cost more to construct than culverts or low-level crossings.
Bridges come in many different structural forms. They also use many material types. The most common types and typical applications are as follows:
- Steel I Beam: These are the most common bridge type. They are cost-effective for spans up to 20 m. A timber or concrete deck may be used. However, a concrete deck has the advantage that it may be designed to act compositely with the beams to increase the bridge load capacity. Steel beam bridges will require corrosion protection and maintenance, and are less appropriate in high corrosion sites, such as coastal or geothermal areas
- Glulaminated timber beam: These are a cost-effective option for spans up to approximately 12 m. Properly treated timber bridges perform well in high corrosion zones. They can have timber or concrete decks
- Concrete beam: Various forms of stressed concrete beams, including I beams, Double-Tee and hollow-core beam configurations, are common. They are typically suitable for spans up to 25 m. Very durable, but because of the high weight, they require more extensive foundations
- Steel truss: Generally, forest steel truss bridges consist of a truss below deck level and a concrete slab deck. The traditional Bailey bridge is a steel truss bridge with the trusses above deck level, and the deck supported on transoms. Steel truss bridges are suitable for spans up to 30 m or longer
- Log bridge: Use of round log beams as a bridge can be a cost-effective option, especially if suitable hardwood trees are available in the forest or CCA treated power poles are locally available. Log bridges are generally used for short-term applications, such as during harvest, but if appropriately durable timber is used, and good attention is given to construction detailing, log bridges can provide permanent crossings.
8.8.2 Single span bridge design
Bridges require specialist engineering design, and construction supervision. Do not attempt design and construction unless you are a trained, suitably experienced and qualified engineer. Also, almost all bridges will require a building permit and code compliance certificate under the Building Act 2004 from the district council. Understand the NES-PF specific rules relating to bridges, as a resource consent will be required where it does not meet the permitted activity conditions. An experienced bridge design engineer will be able to advise on the most appropriate bridge type, and prepare a suitable design for forest bridges. The investigation, design, consent and construction process for a forest bridge can take a significant time period. The engineering design may call for a multi span bridge with piers, meaning that a resource consent is required. Bridges need to be planned and scheduled for construction well in advance of harvesting activity.
It is best to locate the bridge crossing site, if possible, at a narrow point on a straight and uniform reach of river, that has stable river banks at and upstream of the bridge site, to reduce scour of the abutments or bridge approaches. Both abutment protection and span length affect cost. Also, try to make the crossing is perpendicular to the river, which will reduce span length.
Permanent bridges must pass at least a 1-in-50-year flood event (2% AEP), although it is best to allow for a 1% AEP for many bridges. This means that there is a 1% chance of the design flood happening in any year. Allow at least 1 m of freeboard above the calculated maximum water level, to ensure floating debris does not damage the structure, or design the bridge to allow for overtopping. Avoid bridge design that places structural foundations on soil susceptible to erosion or structural failure. Bridges are long life structures so factor in potential natural channel adjustment changes over the bridge’s design life.
The design of the road approaches can limit sediment entering the river, and minimise the potential for aggregate being tracked onto the bridge. Include in the design raising the bridge deck slightly or lowering the road approaches, to direct stormwater away from the river, if possible. Design to divert road surface water off the bridge approaches, as close as practicable to the bridge, and ideally within 10 m. Use stormwater and sediment control measures such as berms, cut-outs, ditches and culverts, flumes and sediment traps. Use clean gravel on road approaches where the existing road surface could create a sedimentation problem.
Bridge design requires the consideration of several important factors. These include:
- Waterway clearance: Because of the high value of bridges and the long life, they are typically designed to have enough waterway capacity to pass a 100-year (1%AEP) flood flow with additional clearance under the beams to enable passage of floating debris
- Deck width: Single or two lane, and of sufficient width for large equipment such as haulers and skidders
- Deck load capacity: Logging truck capacity or additional capacity for haulers and/or off-highway stem trucks or overweight vehicles
- Period required: For short-term harvesting requirements, re-locatable temporary bridge options are economic. Steel beam bridges and steel truss bridges, with precast concrete or timber decks, can be easily dismantled and shifted to new sites
- Durability: In high corrosion sites, or where long-term high traffic use is proposed, more durable materials that are less subject to fatigue are likely to be more economic.
The NES-PF has specific regulations for single span bridges. Always refer to the NES-PF before design. For example, bridges must not decrease the bankfull channel width or restrict flow width by more than 10%.
Other reference material includes the NZTA bridge reference manual, see www.nzta.govt.nz/resources/bridge-manual/bridge-manual.html. Note that Appendix D, Lightly Trafficked Rural Bridges, may not be relevant due to logging truck use, and the potential for significant overloads caused by haulers on transporters, for example, crossing them.
8.8.3 Single span bridge construction
The normal river crossing tenets of constructing in suitable weather, checking for any fish spawning timing constraints under the NES-PF, limiting earthwork disturbance to the immediate construction site and minimising the need for machinery to operate in flowing water apply.
Additional specific bridge construction requirements include:
- Construct foundations onto non-erosion prone material, preferably rock. If this is not possible, then build to below the maximum level of potential erosion, or provide an acceptable alternate engineering solution
- Bridge abutments or footings should be on natural ground. This ensures that the length of the bridge is wider than the river channel, and provides a good bed for the bridge
- Protect the abutments, footings and approaches. If necessary, armour with concrete, rip rap, gabions, wing walls and other deflectors
- Wet or curing concrete must not be in contact with flowing water. Cement is a contaminant, and is toxic to invertebrates and fish. When pouring concrete where it could enter the river, the water channel will need to be temporarily diverted
- Elevated sediment discharge levels may occur during construction, but must not occur for more than eight consecutive hours
- Check regularly during and on completion of construction. If the work does not meet the design plan and standards, then initiate corrective actions
- At the end of construction, all excess equipment and materials must be removed from the river bed within five working days.
8.8.4 Prefabricated concrete slab bridges
Prefabricated concrete slab bridges can be effective long or short-term solutions to crossing waterways. When short-term access is required for harvesting operations, portable or removable bridges may be an economic and environmental alternative to consider.
Portable bridges provide infrastructure flexibility. If used over several sites, the initial capital cost of the bridge can be spread over all sites. They should be simple, rugged and lightweight. This makes them easy to transport from site to site. Spans of up to 25 m can be bridged with prefabricated beam structures. Environmentally, there is minimal waterway disturbance and the waterway is easily restored to its original condition. Portable bridges are designed for rapid construction and dismantling. Most consist of prefabricated beams and decking that is transported to the site and erected over the waterway on temporary foundations. Often temporary bridges can be erected using equipment typically available on forest sites. The foundations need not be as substantial as permanent bridges because temporary bridges are in place for a short term, where long-term settlement and ground movement risks are less of an issue.
All temporary bridges must have a building consent and code compliance certificate since they are subject to the Building Act.