Section 2: Bridge Span Length
Anchor: #i1001720Overview
Bridge length depends on terrain, hydraulics, prescribed clearances, or aesthetics, and is controlled by economics and the capability of available structural systems.
Anchor: #i1001730Grade Separations
Prescribed clearances and header slopes govern highway grade separation span lengths. Sometimes aesthetic considerations may dictate longer-than-necessary spans to give the separation a more open look underneath. The larger prestressed concrete beams have become so economical that they are used where shorter spans would suffice.
Anchor: #i1001740Stream Crossings
Stream crossing span lengths usually have a main span that straddles the stream. For small streams, where hydraulic considerations are minimal, the bridge is often divided into equal spans for ease of construction. However, no bents or piers should be placed in erodable streambeds. River crossings almost always have a main span across the center of flood flow; the purpose being to discourage an accumulation of drift on the piers. There are no warrants for determining the length of this span; it is a matter of engineering judgment.
Anchor: #i1001750Marine Bridges
Intracoastal canal and international shipping lane main spans are subject to U.S. Coast Guard regulations. Approach spans are determined by economics and/or aesthetics. Navigable rivers as determined by the Federal Highway Administration (FHWA) are also subject to U.S. Coast Guard Regulations.