Editing Bridge (section)

== Analysis and design ==
[[File:Burbank Ave I-5 overpass under construction in Burbank, California.JPG|thumb|A highway [[overpass]] over construction on [[Interstate 5 in California|Interstate 5]] in [[Burbank, California]], in 2021]]
Unlike buildings whose design is led by architects, bridges are usually designed by engineers. This follows from the importance of the engineering requirements; namely spanning the obstacle and having the durability to survive, with minimal maintenance, in an aggressive outdoor environment.<ref name=":2" /> Bridges are first analysed; the bending moment and shear force distributions are calculated due to the applied loads. For this, the [[finite element method]] is the most popular. The analysis can be one-, two-, or three-dimensional. For the majority of bridges, a two-dimensional plate model (often with stiffening beams) is sufficient or an upstand finite element model.<ref>{{Cite journal|last1=O'Brien|first1=E.J|last2=Keogh|first2=D.L|date=December 1998|title=Upstand finite element analysis of slab bridges|journal=Computers & Structures|volume=69|issue=6|pages=671–683|doi=10.1016/S0045-7949(98)00148-5|hdl=10197/4054|hdl-access=free}}</ref> On completion of the analysis, the bridge is designed to resist the applied bending moments and shear forces, section sizes are selected with sufficient capacity to resist the stresses. Many bridges are made of [[prestressed concrete]] which has good durability properties, either by pre-tensioning of beams prior to installation or post-tensioning on site.

In most countries, bridges, like other structures, are designed according to [[Limit state design|Load and Resistance Factor Design]] (LRFD) principles. In simple terms, this means that the load is factored up by a factor greater than unity, while the resistance or capacity of the structure is factored down, by a factor less than unity. The effect of the factored load (stress, bending moment) should be less than the factored resistance to that effect. Both of these factors allow for uncertainty and are greater when the uncertainty is greater.