Bridge Systems and Load Distribution

Bridge Systems and Load Distribution

Reference lock: For an exam before April 2027, use the AASHTO LRFD Bridge Design Specifications, 8th edition (2017), the AASHTO PE Exam Collection, and the identified May 2018 errata. Do not substitute provisions from a later office edition.

Bridge Load Path

A wheel first loads the deck locally. The deck distributes that load transversely and longitudinally to girders or other primary members. Girders deliver reactions through bearings to piers or abutments, which carry force into foundations and ground. Diaphragms or cross-frames stabilize girders and participate in load distribution according to the actual system; they are not decorative lines on the framing plan.

A slab-on-girder bridge, box-girder bridge, truss, arch, or integral system distributes load differently. Select formulas only after identifying system, material, geometry, continuity, skew, and deck action.

Longitudinal Placement Versus Transverse Distribution

Longitudinally, place the AASHTO vehicle or lane load to maximize the requested effect using influence lines or the specified procedure. The position for positive moment differs from that for support shear or negative moment in a continuous bridge.

Transversely, a distribution factor converts the applicable multiple-lane or vehicle effect into demand for one girder, strip, or component under the stated AASHTO rules. Factors can depend on girder spacing, span, deck thickness, number of loaded lanes, stiffness, system type, continuity, and skew. Exterior girders, overhangs, or geometries outside empirical limits may require a lever rule or refined analysis.

Read what the factor already represents. If it includes multiple-presence behavior, do not apply a second multiple-presence factor. Do not apply a live-load girder distribution factor to permanent deck self-weight without authority; permanent loads are commonly distributed by their own tributary or structural model.

Worked Per-Girder Moment

A problem using the required AASHTO 8th-edition procedure supplies a moment distribution factor g = 0.62 for the relevant interior girder and truck effect. The longitudinal one-truck moment is M_truck = 420 kip-ft, before dynamic allowance. The problem gives IM = 0.33 for this eligible vehicular effect. Per-girder live-load moment is

M_LL+IM = g M_truck(1 + IM)

M_LL+IM = 0.62(420)(1.33) = 346.3 kip-ft.

The same girder has permanent moments M_DC = 180 kip-ft for structural components and M_DW = 40 kip-ft for wearing surface/utilities. For an illustrative combination, the problem provides factors 1.25DC + 1.50DW + 1.75(LL+IM). Then

M_u = 1.25(180) + 1.50(40) + 1.75(346.3)

M_u = 225 + 60 + 606.1 = 891.1 kip-ft.

The numerical factors are problem-given for this example. On the exam, select the exact AASHTO limit state, load factors, dynamic-allowance scope, and distribution rule. Do not apply IM again if the supplied live-load effect already includes it, and do not multiply by the number of lanes when g already represents the prescribed lane case.

Bridge Loads and Limit States

Keep load identities visible through the calculation: structural dead load, future wearing surface/utilities, vehicular live load and eligible dynamic allowance, pedestrian load, braking/traction, centrifugal action, wind, water, temperature, settlement, collision, and seismic effects as applicable. The AASHTO Strength, Service, Fatigue and Fracture, and Extreme Event families answer different questions and use different combinations or response limits. Do not move a factor from one family to another because it appears conservative.

Fatigue can be governed by stress range and a specific truck/load placement rather than the maximum Strength combination. Service checks can control cracking, deflection, rotation, or bearing movement. Construction stages can govern girder stability and deck stresses before the final composite system exists.

Bearings, Continuity, and Skew

A fixed bearing restrains specified translations; an expansion bearing permits specified movement while carrying other reactions. Real bearings also accommodate rotation within limits. Map longitudinal and transverse restraint at every support so braking, wind, thermal, and seismic forces have a complete path. Assuming every bearing is an ideal pin or roller can either omit a force path or create artificial restraint.

Continuity produces negative support moments and changes live-load placement. Skew can create unequal reactions, twisting, and different transverse distribution. Curvature introduces torsion and radial effects. Deck joints, integral abutments, and bearing replacement details alter movement and force paths.

2026 Bridge Workflow

1. Identify bridge type, span continuity, skew, deck, girders, diaphragms, bearings, and substructure.
2. Separate permanent-load distribution from vehicular transverse distribution.
3. Position each moving load longitudinally for the requested effect.
4. Apply the AASHTO 8th distribution rule only within its stated scope and limits.
5. Add dynamic allowance only to eligible components and exactly once.
6. Assemble the correct limit-state combination without merging load categories.
7. Trace reactions, movement, and lateral actions through bearings to foundations.
8. Check strength, service, fatigue, extreme-event, and construction behavior as required.

A correct single-girder moment is useful only when its distribution assumptions and downstream reaction path match the bridge that was actually modeled.