Ask an engineer what a bridge has to carry and they will usually start with lorries. Ask what it has to carry every day, without fail, and the list begins with the bridge itself. Get the load cases wrong on a small span and the errors surface quickly: a slab that is short on shear at the support, a parapet fixing never designed for anything horizontal, a deck that bounces under a transit van and makes pedestrians stop and stare. None of that needs a software mistake. It just needs a load that nobody wrote down.
Start with the weight that never moves
Dead load — permanent actions, in code language — is everything that stays on the structure for its design life. On a typical concrete deck that means the slab or girders, the crossheads, the bearings, the abutments and the weight of the material itself. The unit weights are the easy part: reinforced concrete is normally taken at around 24 to 25 kN per cubic metre, structural steel at roughly 78.5 kN per cubic metre, and asphalt at something in the region of 23 to 24 kN per cubic metre depending on the mix and air voids.
The hard part is remembering everything. Self-weight rarely gets forgotten; the items bolted and poured on top of it often do.
Superimposed dead load is where the arguments start
Superimposed dead load covers the permanent things added after the structural frame. Kerbs and upstands. Waterproofing and its protection board. Parapets and vehicle restraint systems. Lighting columns, sign posts, drainage gullies and pipes, buried services, and on rail structures the track, ballast and walkway. Add a future surfacing allowance, because at some point somebody will resurface the deck and nobody will ask you first.
Each item is small. Together, on a short span with a light deck, they can be a meaningful share of the total load — and they sit in the same place, permanently, which is exactly what makes them dangerous to underestimate.
If you cannot name an item, you cannot weigh it. Write the schedule out before you open the model.
Then add everything that moves
Live load, or variable action, is everything the bridge carries that arrives and leaves. For road bridges, British practice historically used HA loading for ordinary traffic and HB loading for abnormal vehicles; current design to the Eurocodes uses a family of load models instead, from Load Model 1 (a uniformly distributed load plus tandem axle systems, placed to give the worst effect) through to special vehicles and crowd loading. Footways carry a nominal pedestrian pressure plus a notional single heavy load for local effects. Rail bridges carry their own train load models, which are their own discipline.
It helps to group variable actions before you start combining:
- Vertical traffic — distributed load plus heavy axles or tandems, moved along the deck to find the governing position.
- Pedestrian and crowd — nominal pressure on footways and footbridges, plus accidental or crowd cases where relevant.
- Horizontal traffic effects — traction and braking, centrifugal action on curves, and vehicle collision on piers and parapets.
- Environmental — wind, thermal range, snow, and for some sites water flow, debris or ice.
- Construction and accidental — erection loads, prestressing, temporary works, and the occasional impact from a plant strike.
Placement matters as much as magnitude. A distributed load sitting in the wrong bay can give a smaller bending moment than reality, and the wrong pattern of lanes can change which member is critical.
Dynamic allowance: why a rolling wheel hits harder than a parked one
Nothing about a moving vehicle is static. Suspension bounce, pavement roughness, a pothole at the expansion joint, the sudden arrival of an axle group — all of it means the structure sees more than the vehicle's weight. Codes deal with this by multiplying the static model by a dynamic factor, sometimes called the impact factor or dynamic amplification factor.
The important detail is that this factor is not a fixed number. It generally tapers with loaded length: modest for a long viaduct, substantial for a short span. It also varies with the check you are doing. The factor applied to a global bending check is not automatically the right one for a local slab assessment, a fatigue check or a bearing. Read the code clause you are actually working to rather than reusing a number from a previous job.
Short spans feel it most
A short deck has high natural frequency and relatively little mass, so an axle crossing it in a fraction of a second produces a response the structure cannot smooth out. A 6 m span might complete several cycles of vibration while the load is still arriving. Combine that with a live load that can be a large fraction of the total load, and short spans end up with the least forgiving combination of amplification and load ratio.
Load cases: the combinations that decide the design
Once you have the individual loads, you combine them. At ultimate limit state, a common Eurocode combination is 1.35 times permanent actions plus 1.5 times the leading variable action, with accompanying variable actions reduced by combination factors. At serviceability limit state you will usually check characteristic, frequent and quasi-permanent combinations, which is what deflection, crack width and movement calculations need.
The combination that governs is not always the one with the most traffic. A case with modest traffic but the full temperature range and a long, slow wind can govern an expansion joint or a bearing's movement capacity. Shear at a support, bending at midspan, bearing rotation, foundation bearing pressure and parapet fixing are often governed by five different cases on the same short span.
What to do on your next job
- Write out the permanent load schedule by hand, item by item, including future surfacing and services. Then compare it against the model.
- Place variable loads in more than one position. Check the reversal as well as the maximum.
- Apply the dynamic factor that matches your loaded length and your check — not the one you used last week.
- Check shear at the support and the local effect under an axle, not just the midspan moment.
- Run the serviceability combinations for deflection and movement, not just the ultimate ones.
- Ask whether the deck will feel right to walk on. Vibration comfort is a genuine design check, not a luxury.
Load cases are not a formality to be cleared before the interesting analysis begins — they are the analysis. Get them right and the rest of the design tends to behave. If the structure is anything beyond a straightforward assessment, have your load schedule reviewed by a competent bridge engineer before it becomes concrete and steel. It is far cheaper to find a missing 5 kN per square metre on paper.
Photo: Schwoaze / Pixabay
