Bridge Loading Basics: What You Need To Know
30 July 2020
The good news is a bridge can be designed to cater for any load. But deciding what your load requirements are, isn’t always obvious.
Knowing what you want to use your bridge for will help determine what load you need:
- Is it for pedestrians and cyclists only, or a fully-laden truck and trailer?
- If it’s the latter, will all the axles be on the bridge at once? Or will a short span (length) mean your bridge doesn’t need to hold the full gross weight?
- How many vehicle movements are there likely to be each day?
All of these factors have an impact on your bridge loading requirements. Here’s a run-down of the standard bridge loads you can choose from:
Pedestrian & cyclist
These loads are typically ‘5kPa’. This allows for approximately 500kg per m² (the equivalent of five people weighing 100kg each standing in 1m² area together). These bridges are suitable for pedestrians, cyclists and small maintenance vehicles.
Stock and light vehicles
Light vehicle loading is commonly a 1 tonne axle load. It will hold stock, cars, motorbikes and utes which generally weigh 2-3 tonnes.
0.85HN
Previously known as a ‘class 1’ bridge loading, this option is often used for residential and farm access and is suitable for rural roads which have low traffic volumes. These bridges can service up to 15 homes (but after that a heavier HN-HO-72 load rating is required). Speed is limited to 70km/hr. It is suitable for all road legal vehicles such as fire engines, fertiliser trucks and milk tankers. If the vehicle doesn’t require an overweight permit, then it should be good to cross. Note: this bridge loading isn’t appropriate for forestry loads or heavy weight vehicles crossing the bridge in high volumes.
Full HN-HO-72
This load capacity is often found on the NZTA highway network where there’s a high volume of traffic and high speeds (i.e. 100km/hr). It’s also predominately used in the forestry sector, construction sites and large residential subdivisions. This loading is suitable for HPMV (high productivity motor vehicles).
Custom
Custom bridge loading can be designed for large items of forestry equipment such as haulers and earthmoving equipment such as scrapers or articulated dump trucks. No matter how heavy something is, we can build a bridge to support it.
How Does Load Affect Cost?
Most people assume a lighter load will mean a bridge is cheaper to build. But sometimes it costs only a little more to get a heavier design-capacity bridge, meaning overall you’ll get better value for money and a future-proofed asset.
In general, your site investigation, design, resource and building consent, fabrication and installation costs will be relatively the same – the cost of materials associated with different design loads is the only variable. In many cases it’s worth opting for a heavier design load and getting more bang for your buck!
Unsure What Load You Need?
Tell us what you need your bridge for, and we’ll take care of all the calculations and figure out the best bridge loading option for you. Phone our experienced team on 0800 222 189 or email us.
