Editing Bridge (section)

=== Traffic loading on long span bridges ===
[[File:Forth from above.jpg|thumb|Traffic on [[Forth Road Bridge]] in Scotland prior to its opening to general traffic; traffic has now been moved to the [[Queensferry Crossing]] (on left)]]
Most bridge standards are only applicable for short and medium spans<ref>{{Cite journal|last1=A.S|first1=Nowak|last2=M|first2=Lutomirska|last3=F.I|first3=Sheikh Ibrahim|date=2010|title=The development of live load for long span bridges|journal=Bridge Structures|volume=6|issue=1, 2|pages=73–79|doi=10.3233/BRS-2010-006|issn=1573-2487}}</ref> - for example, the Eurocode is only applicable for loaded lengths up to 200 m. Longer spans are dealt with on a case-by-case basis. It is generally accepted that the intensity of load reduces as span increases because the probability of many trucks being closely spaced and extremely heavy reduces as the number of trucks involved increases. It is also generally assumed that short spans are governed by a small number of trucks traveling at high speed, with an allowance for dynamics. Longer spans on the other hand, are governed by congested traffic and no allowance for dynamics is needed.

Calculating the loading due to congested traffic remains a challenge as there is a paucity of data on inter-vehicle gaps, both within-lane and inter-lane, in congested conditions. [[Weigh in motion|Weigh-in-Motion]] (WIM) systems provide data on inter-vehicle gaps but only operate well in free flowing traffic conditions. Some authors have used cameras to measure gaps and vehicle lengths in jammed situations and have inferred weights from lengths using WIM data.<ref>{{Cite journal|last1=Micu|first1=Elena Alexandra|last2=Obrien|first2=Eugene John|last3=Malekjafarian|first3=Abdollah|last4=Quilligan|first4=Michael|date=21 December 2018|title=Estimation of Extreme Load Effects on Long-Span Bridges Using Traffic Image Data|journal=The Baltic Journal of Road and Bridge Engineering|volume=13|issue=4|pages=429–446|doi=10.7250/bjrbe.2018-13.427|issn=1822-4288|doi-access=free|hdl=10344/7494|hdl-access=free}}</ref> Others have used [[microsimulation]] to generate typical clusters of vehicles on the bridge.<ref>{{Cite journal|last1=OBrien|first1=E. J.|last2=Hayrapetova|first2=A.|last3=Walsh|first3=C.|date=March 2012|title=The use of micro-simulation for congested traffic load modeling of medium- and long-span bridges|journal=Structure and Infrastructure Engineering|volume=8|issue=3|pages=269–276|doi=10.1080/15732471003640477|bibcode=2012SIEng...8..269O |issn=1573-2479|hdl=10197/3061|s2cid=54812838 |hdl-access=free}}</ref><ref>{{Cite journal|last1=Caprani|first1=Colin C.|last2=OBrien|first2=Eugene J.|last3=Lipari|first3=Alessandro|date=May 2016|title=Long-span bridge traffic loading based on multi-lane traffic micro-simulation|journal=Engineering Structures|volume=115|pages=207–219|doi=10.1016/j.engstruct.2016.01.045|bibcode=2016EngSt.115..207C }}</ref><ref>{{Cite journal|last1=OBrien|first1=Eugene J.|last2=Lipari|first2=Alessandro|last3=Caprani|first3=Colin C.|date=July 2015|title=Micro-simulation of single-lane traffic to identify critical loading conditions for long-span bridges|journal=Engineering Structures|volume=94|pages=137–148|doi=10.1016/j.engstruct.2015.02.019|bibcode=2015EngSt..94..137O |hdl=10197/6998|s2cid=56030686 |hdl-access=free}}</ref>