Long-span bridges need a deck that combines high bending stiffness with relatively light weight. An orthotropic steel deck achieves this by using a flat steel plate that is stiffened with weld-on ribs and cross-stiffeners, creating an anisotropic deck whose stiffness is much greater in the longitudinal direction than in the transverse. This stiffened plate system gives a very high stiffness-to-weight ratio, allowing a thinner deck and enabling longer spans while keeping deflections and vibrations under control. When a concrete wearing surface is added, the deck often works in composite action with the girders, further enhancing overall stiffness and performance. Other deck types don’t match this combination: timber decks lack durability for long spans; precast concrete decks add substantial dead load and aren’t optimized for the stiffness-to-weight benefits of a stiffened steel plate; corrugated steel decks are lighter but provide less stiffness than an orthotropic plate, making them less suitable for very long spans.

In bridge design, different loading cases are used to capture how a structure reacts to traffic and other forces. For short spans, the alternate military loading represents the effect of a heavy two-axle vehicle arranged in tandem, which concentrates load in a way that often produces higher reactions and bending moments than standard traffic loads on those short spans. This is why it’s the governing loading for spans shorter than about 40 ft. The term tandem load describes the two axles carrying the vehicle’s weight in line, and the alternate military loading is simply another name for that same two-axle, tandem arrangement. The other options don’t capture this specific, concentrated, two-axle scenario: design lane load reflects typical highway traffic on longer spans, wind load is a lateral environmental load, and construction load is temporary during construction.

The main idea here is the use of steel tubes or pipes that are joined to vertical posts to form a railing, with a construction approach that makes customization straightforward. Steel tube railings are built from standard tubes or pipes that are connected to posts—often welded or bolted—so you can easily cut tubes to the needed lengths, adjust post spacing, and add or reconfigure sections to fit stairs, curves, or landings. This modular, tube-based design gives you flexibility in shape, height, and layout without specialized forms or heavy members, while still delivering solid strength and a clean, simple appearance. The other options don’t match this setup. Box beam guardrails use rigid rectangular beams rather than round tubes, which limits the easy customization you get with tubular connections. Thrie beam guardrails are a highway protection system designed for vehicle impact and long-span performance, not a modular railing built from tubes attached to vertical posts. The phrase “tubular rail system” is closely related, but the description points to steel tube railings as the specific, standard form of a tube-based railing that’s easy to tailor on site.

Bridge decks need a continuous, seamless barrier to keep water away from the concrete and embedded steel. A liquid-applied waterproofing membrane is the usual choice because it forms one monolithic film that bonds directly to the concrete, curing into a flexible layer that can crack-bridge small movements and conform to complex deck details. It can be sprayed or rolled to cover irregular shapes, around drains, and along edges, providing quick protection before the wearing surface goes on. This seamless coverage reduces leak paths that joints or seams in other systems can create. Sheet membranes rely on seams that must be perfectly bonded; if any seam leaks, water can penetrate. Mastic asphalt offers a durable, monolithic layer but requires hot, bulky application equipment and more intensive workmanship. Rubber joints aren’t a waterproofing layer itself—they are joints that must be detailed and sealed, not a primary deck waterproofing system.

Wood is the material that best fits here because in the 1800s most trusses and trestles were built from large timber members. Timber could be cut, shaped, and joined with simple tools and pins on site, making it practical and economical for frequent, long spans without sophisticated factories. Covered bridges were specifically designed with a roof and siding to protect these wooden members from rain, snow, and decay, dramatically prolonging the life of the structure. While stone and later steel or concrete were available, they weren’t the standard choice for typical 19th-century truss and trestle bridges, and the popularity of wood for covered bridges reflects its availability, workability, and the protective purpose of the coverings.