The deck is the part of the bridge that wears out first and gets replaced most, so its design is as much about durability as about strength. A cast-in-place concrete deck on girders is typically 200–250 mm thick, spans transversely between girders at 2–4 m, and cantilevers past the outer girder to carry the barrier.

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AASHTO allows two design routes. The empirical method (9.7.2) relies on arching action between girders and simply prescribes isotropic reinforcement for decks within its limits; most highway decks qualify. The traditional (strip) method analyses a 1 m strip as a continuous beam for wheel loads and designs it as an RC slab; it is required for overhangs and for decks outside the empirical limits. Overhangs are designed for barrier collision loads as well as wheels, and usually govern the top transverse steel.

Precast full-depth deck panels with UHPC joints, and orthotropic steel decks on long-span and movable bridges, are the alternatives when speed or weight matters.

The design of a pretensioned girder is a negotiation between two moments in its life. At transfer, the girder is young, the concrete is weak, and the prestress is at its highest: the bottom must not crush and the top must not crack, so strands are harped (deflected upward at the ends) or debonded near the ends to reduce the moment there. At service, decades later, the losses have taken their share and the full dead and live load is on: the bottom fibre tension is checked under Service III against the allowable, and this check, not strength, usually sets the number of strands.

Then strength: flexural resistance with the strand at its stress at ultimate, shear by the modified compression field theory (sectional model with β and θ), interface shear between girder and deck, and the anchorage zone at the ends. Camber is estimated for transfer and for erection, because a girder that arrives too high or too low costs haunch concrete or grinding.

Section families, AASHTO I-girders, bulb-tees, NU, Florida FIB, Washington WF, Korean PSC I, trade web thickness, bottom-flange strand capacity and stability during handling. Spliced girders with post-tensioning push precast concrete past 60 m.

A steel I-girder is designed twice: once as a bare steel section carrying the wet deck, and once as a composite section carrying everything after. The first case often governs. During the deck pour the top flange is in compression, unbraced except at cross-frames, and lateral-torsional buckling is the check; on curved or skewed bridges the pour sequence itself must be analysed.

Composite in positive bending, the section is usually compact and reaches its plastic moment. In negative bending over piers the bottom flange is in compression and the deck is cracked, so the section is treated as steel plus rebar, often noncompact, and the web may need longitudinal stiffeners. Shear is carried by the web, with transverse stiffeners allowing tension-field action.

What actually cracks steel bridges is fatigue at details: cross-frame connection plates, cover-plate ends, out-of-plane distortion at web gaps. The design rule is to choose a good detail category first and calculate stress ranges second. Steel tub (box) girders add torsional stiffness for curves and need internal bracing to hold their shape until the deck closes the section.

A balanced-cantilever box is a structure whose stress state depends on when each piece was cast. Cantilever tendons in the top slab hold the arms up; after closure, continuity tendons in the bottom slab carry the sagging moment; and creep then redistributes the moments for years, moving them from the built-as-cantilever pattern toward the built-in-one-piece pattern. Time-dependent analysis, stage by stage, is not optional.

The section itself is checked for longitudinal bending and shear, for transverse bending of the top slab as a frame, for torsion and distortion when loads are eccentric, and for the thermal gradient that can put more tension in the bottom slab than the traffic does. Diaphragms at piers transfer reactions and anchor tendons; deviators in externally post-tensioned boxes turn the tendons.

Precast segmental boxes use match-cast epoxy joints with shear keys; because there is no reinforcement across the joint, no tension is allowed there under service load, and the joint's durability depends on the tendon grout.

A stay is a bundle of individually sheathed, greased, galvanised strands inside a high-density polyethylene pipe, anchored with wedges in a socket at each end; a main cable is thousands of parallel wires. Both are designed for tension, but their real problems are fatigue and vibration.

Stay fatigue comes from live-load stress ranges at the anchorage; codes limit the stay to about 45–50% of ultimate under service load and require fatigue testing of the anchorage system. Vibration comes from wind, from rain running down the sheath in a rivulet that changes the aerodynamic shape (rain–wind vibration), and from parametric excitation by deck motion; the answers are helical fillets on the pipe, dampers at the deck anchorage, and cross-ties. Every stay is designed to be replaced under traffic.

A stay's own weight makes it sag, so its effective stiffness is lower than a straight bar's, the Ernst modulus, and the catenary geometry decides unstressed length, which matters for fabrication and for the construction sequence.

A bridge that is strong enough can still be uncomfortable or ugly. AASHTO's optional live-load deflection limits are span/800 for vehicles and span/1000 where there are pedestrians; many owners make them mandatory. Deflection is computed with the full width acting together and all lanes loaded.

Camber is the upward curve built into a girder so that it arrives flat under dead load. For prestressed girders it grows with time as creep acts on the prestress; for steel girders it is cut into the web. Getting it wrong shows immediately in the ride and in the deck thickness.

Footbridges have their own problem: people walk at 1.6–2.4 Hz and bridges with a vertical frequency in that range, or a lateral frequency near 1 Hz, can be driven into resonance, London's Millennium Bridge (2000) closed two days after opening for that reason. Design either keeps the frequencies out of the band or adds tuned mass dampers.