It is reasonable to expect that a bridge wide enough for a fertiliser truck can safely carry a fully laden fertiliser truck.
But who is liable if it doesn’t?
What are the minimum design standards and legal requirements for rural bridges?
These pages summarise some of the design requirements for rural bridges on private roads and farms.
Will your bridge comply?
‘Post’ Heavy Vehicle Bridge Limits signs on any bridges that do not have sufficient load-carrying capacity to safely support the normal range of heavy vehicles.
Only old bridges can be posted with a diminished load rating; new installations must be built to today’s Transit Bridge Manual codes. If a vehicle can fit on the bridge, it must be able to support the load.
Requirements for Rural Bridges
A bridge, whether temporary or permanent, is classified as a building under the Building Act (Section 8).
According to the Building Act (Section 17), all building work must comply with the New Zealand Building Code, regardless of whether a building consent is required.
The Building Code specifies the load requirements that different parts of a structure must be capable of withstanding, based on the Australian/New Zealand Standard AS/NZS 1170: Structural Design Actions.
AS/NZS 1170 refers to other publications for information necessary for designing certain structures, including bridges (AS/NZS 1170.0 C1.1(A)). In Australia, this is covered by the Australian Bridge Design Code, while in New Zealand, it is addressed by the NZ Transport Agency (formerly Transit NZ) Bridge Manual.
Appendix D of the Bridge Manual specifically addresses rural bridges and includes reduced deck loading and barrier requirements compared to highway bridges. The design loads are suited for the normal range of heavy vehicles (3.5 tonnes to 44 tonnes). Some relevant extracts are included on the following pages.
While AS/NZS 1170 does not specifically address heavy vehicle loading on structures and bridges, it does include provisions for light vehicles in car parks and similar applications, which might be relevant for bridges where heavy vehicles are excluded (e.g., by the narrow width of the bridge).
The Heavy Motor Vehicle Regulations require that signs be posted on bridges where the load-carrying capacity for heavy vehicles is limited.
Requirements for Rural Bridges
Extracts from the NZ Transport Agency Bridge Manual:
(These extracts apply to rural bridges that may occasionally be used by heavy motor vehicles, complying with the Heavy Motor Vehicle Regulations and the applicable Land Transport Rules, but excluding overweight or high-productivity vehicles. This means bridges that could be crossed by vehicles weighing between 3,500kg and 44,000kg. The full document is available on the NZTA website.)
The section copied above specifies that kerbs must be provided on a rural bridge as a minimum standard for edge protection. They must be designed to withstand a minimum horizontal ultimate load of 13.4 kN/m (1.37 tonnes per metre) applied at a height of 0.2 m above the deck. Other types of vehicle or traffic barriers are not commonly used on farm bridges, as design load actions for such barriers are significantly greater.
While this section indicates that pedestrian barriers may not be required on some remote bridges where the risk of accidental falls is very low, the Building Act and Code state that, in all other situations, a barrier must be provided where a person could fall vertically more than 1 metre from the deck. Barriers are also generally needed as part of fencing to prevent stock from entering waterways.
The wording of the Bridge Manual provisions for pedestrian barriers, copied above, is intended to comply with the requirements of the Building Act and Building Code – some of which are copied below:
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Rural Bridge Design Loads
The ultimate load factors and the load combination that usually governs the structural design of a rural bridge is as copied from the Bridge Manual below:
For example, the design ultimate load imposed on the deck of a 14m long rural bridge, including the usual dynamic Impact Factor ‘I’ of 1.3, is equivalent to a static load of 785kN (80 tonne). The design includes load factors to ensure that compliant vehicles up to 44 tonnes cross the bridge with a ‘margin of safety’.
The Bridge Manual design ultimate horizontal load that the top rail of the pedestrian barrier must be capable of resisting is 1.75kN/m x1.70 = 2.97kN/m (300kg per metre). This applies only where the barrier is ‘protected’ by a kerb from vehicle impact. Much greater loads would need to be designed for if the railing were part of a vehicle barrier.
For comparison, the equivalent load prescribed by AS/NZS 1170 for pedestrian areas in car parks is 1.5kN/m x 1.5 = 2.25kN/m (230kg per metre), 75% of the value for bridges.
In regard to the vehicle barriers on car parks and ramps, the horizontal ultimate design load to be applied to any 1.5m length of barrier is either 45kN (4.6 tonne, for vehicles less than 2,500kg: 30kN x 1.5 = 45kN – the same as for a Bridge Manual TL3 guardrail post) at 0.5m height or 60kN (6.1 tonne, 40kN/m, for vehicles more than 2,500kg and less than 10,000kg) at 1.0m height. This is more onerous than the horizontal load of 20kN (2 tonne) at 0.2m height that would be applied to a 1.5m length of kerb in designing to the requirements of the Bridge Manual Appendix D.
Timber kerbs designed to the Bridge Manual standard are often damaged by truck and tractor wheels. The QTS farm bridge integral concrete kerbs are designed for more than three times the Bridge Manual minimum requirement: that is, > 40kN/m (4.1 tonne per metre). This is similar to the AS/NZS 1170 requirement but applied at kerb level. They also prevent stock effluent from flowing over the sides of the deck and entering the stream.
Under these regulations, if the bridge has insufficient capacity, it is a requirement to:
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“erect and maintain near each end of the bridge a sign indicating any weight and speed limits so fixed in the form prescribed by Land Transport Rule: Traffic Control Devices 2004”
“In any prosecution for a breach of this regulation, it shall be sufficient evidence that the weight limits or speed limit with respect to any bridge had been fixed and were in force in accordance with this regulation, if the prosecution proves that at the time when the offence was committed the signs indicating weight or speed limits, or both, were in position near each end of the bridge, and that those limits had been fixed by the controlling authority not more than 12 months before the date of the alleged offence”.
Whereas this is generally applied to limit the liability of local authorities for accidents resulting from overloading of substandard bridges on roads, it could offer the same protection to owners of substandard bridges that may be used by employees and by others.
The design loads prescribed in the Bridge Manual – Appendix D are considered to be “100% of Class I”. A well‑maintained bridge built to comply with or exceed the requirements of the Bridge Manual – Appendix D has sufficient load carrying capacity and is not to be Posted with a heavy vehicle bridge limits sign.
Click here to look at the Full New Zealand Transport Agency Bridge Manual