Pedestrian Bridge And Footbridge Design

A pedestrian bridge carries far less load than a road bridge, and it is often the harder of the two to get right.

The reason is weight. A footbridge is light and slender, people walk across it at a steady rhythm, and a light, slender structure being pushed at a steady rhythm is exactly how vibration problems start. Strength is rarely what decides a footbridge. How it feels underfoot usually is.

The standard it is designed to

Pedestrian and cycle bridges in Australia are designed to the AS 5100 series, the Australian Standard for bridge design.

That brings in a 100-year design life, crowd loading across the full deck, a maintenance or emergency vehicle where one could get onto the bridge, wind, thermal movement, and impact from vehicles where the bridge crosses a road. It is a step up from building design, and it is deliberate, because a bridge is public infrastructure that people use without ever thinking about it.

Why vibration governs

Walking puts a rhythmic force into whatever you walk on. Vertically, people step at around two paces a second. Sideways, the body sways at half that rate as weight shifts from one foot to the other.

If the bridge has a natural frequency close to either of those, the footsteps feed energy into it and the movement grows. The best-known example is the Millennium Bridge in London, which swayed sideways so much on its opening weekend in 2000 that it was closed within days. Pedestrians began to walk in step with the sway, which made it worse. It reopened in 2002 after dampers were fitted.

So the design checks natural frequencies in both directions, and where they sit in the sensitive range, it either stiffens the deck, adds mass, or fits damping. The same physics, at smaller scale, sits behind [floor vibration and footfall in apartments](/blog/floor-vibration-and-footfall-in-apartments).

Choosing the structure

The span and the site usually choose the structure.

Short spans suit a simple steel or concrete beam, often a pair of steel girders with a concrete or composite deck.

Medium spans suit a steel truss, which gives depth and stiffness for little weight, and lets the deck sit inside the truss so the overall depth below the walking surface stays shallow.

Longer or architectural spans go to arches, cable-stayed or suspension forms, where the engineering and the appearance are worked out together. The forms are discussed in [five famous bridges, and what engineers learn from each](/blog/five-most-famous-bridges).

Construction access is part of the choice. A bridge over a live road or rail line has to be lifted in during a short possession, often at night, which pushes the design toward prefabricated spans that arrive in one piece and drop onto bearings already waiting for them.

Bearings, joints and movement

A steel bridge grows and shrinks with temperature. Over a full year in Sydney, a long steel span moves measurably.

That movement has to go somewhere. Bearings at the supports let the deck slide or rotate, and an expansion joint at the end lets the walking surface move without cracking or opening a trip hazard. The bearings and joints are the parts of a bridge that wear, so they are designed to be inspected and replaced without lifting the whole span.

Crossing roads and rail

Over a road, the bridge needs clearance above the highest vehicle permitted, protection for the piers against vehicle impact, and usually protection screens along the edges so nothing can be thrown or dropped onto the traffic below.

Over a rail corridor, the clearances are larger, the electrical safety requirements apply, and construction is controlled by the rail operator.

Either way, the asset owner has to accept the design before construction, and that process takes its own time. The road side of it is set out in [Transport for NSW engineering assessment reports](/blog/transport-nsw-engineering-assessment-reports).

Access and ramps

A footbridge that people in wheelchairs or with prams cannot use has failed at its job. Access ramps are designed to AS 1428.1, which limits ramp gradients to no steeper than 1 in 14, with landings at set intervals.

At typical road clearances, that means the ramps are often longer than the bridge itself, and they can take more structure and more land than the main span. Lifts are the alternative where space is tight, with their own maintenance cost.

Durability

A bridge is outdoors, wet, and expected to last a century. Steel needs a coating system chosen for that exposure and for a long time between repaints, and the details need to shed water rather than hold it. The coating choice is covered in [corrosion protection for structural steel](/blog/corrosion-protection-for-structural-steel). Concrete decks need cover and crack control for the same reason.

How we approach it

We design small to medium pedestrian and vehicular bridges in steel and reinforced concrete, with the vibration check done early, because it is the check most likely to change the section sizes.

A footbridge should feel like solid ground to the person crossing it. If they notice the bridge at all, something in the design was missed.

ACSES provides structural engineering, including bridge design, across Sydney and NSW. Talk to us about a project.

Building Relationships Beyond Structures