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How are freeway bridges built?


After calculating the anticiPAted traffic for the bridge, cement/reinforced- with- rebar stanchions are sPAced over the freeway to accommodate the bridge. An 'off-ramp' from the freeway to the bridge is constructed, as is an 'on-ramp' to the subsequent road. Cement/rebar slabs are built and lifted with cranes to form the platform, and voila! Drive carefully.

Although the bridge deck/roadway is almost always a concrete slab, the structure that holds up the bridge deck can be reinforced concrete, structural steel, or a combination of steel and concrete.

The foundation under the bridge structure is usually a combination of reinforced concrete piers and/or footings, along with built-up soil embankments. Reinforced concrete refers to concrete with steel reinforcing bars (rebars) within the concrete to provide structural strength and to reduce cracking in the concrete. The selection of concrete and soil embankment elements used in the foundation depends on the size, shape, and elevation of the roadway(s) that are crossing at the bridge, as well as the strength of the existing soil at the site. Once the foundation and bridge piers are in place, one or more beam-like structures are placed on top of the foundation to create the bridge sPAn.

The primary bridge structure must sPAn a certain distance across whatever is below the bridge (a lower roadway, or creek, or railroad track, etc.). The material selected for the main sPAn(s) can be reinforced concrete or steel. On a typical interstate highway, you can look up at a series of bridges and you will often see example of both. Either concrete beams can be monolithic rectangular tubes that are poured in place with wet concrete poured into forms built on site, or they can be pre-cast concrete beams that are poured at a factory and then trucked to the site and placed by cranes. In both cases, reinforcement bars are placed within the beams to provide tensile strength (since concrete is very brittle and is weak in tension).

Alternatively, the beams can be prefabricated at a factory out of steel sections or plates and then trucked to the site. Smaller steel bridges can be constructed out of deep I-beams (technically the pre-manufactured beams are called wide-flange sections and are shape like an "I" in cross section.) For larger bridges, the steel beams are more usually fabricated out of steel plates to form large hollow rectangular tubes, which are trucked to the site in sections.

Finally, on top of the structural beams is a concrete road deck slab, about 12" deep, which is structurally attached to the beams. The road deck slab has an extensive network of rebar embedded within the concrete to provide tensile strength to the slab. Engineers spend a lot of time working on the details of these slabs to ensure that the concrete is strong, consistent, high-density, and comPActed well to avoid cracks that allow water to get to the rebar and corrode it over time. After the concrete cures, it is typically scored with grooves to improve traction in wet/icy weather and exPAnsion joints are filled with caulk to try to keep water away from the rebars.

To finalize the construction of the bridge, railings, drainage details, sidewalks, and signage are added.

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