Editing Bridge (section)

== Bridge traffic loading ==
While the response of a bridge to the applied loading is well understood, the applied traffic loading itself is still the subject of research.<ref name="engineer. 2015">{{Cite book |title=Bridge deck analysis|last1=OBrien|first1=Eugene J. |last2=Keogh |first2=Damien L. |last3=O'Connor |first3=Alan |date=2015 |publisher=CRC Press |isbn=978-1-4822-2723-9 |oclc=897489682}}</ref> This is a statistical problem as loading is highly variable, particularly for road bridges. Load Effects in bridges (stresses, bending moments) are designed for using the principles of [[Limit state design|Load and Resistance Factor Design]]. Before factoring to allow for uncertainty, the load effect is generally considered to be the maximum characteristic value in a specified [[return period]]. Notably, in Europe, it is the maximum value expected in 1000 years.

Bridge standards generally include a load model, deemed to represent the characteristic maximum load to be expected in the return period. In the past, these load models were agreed by standard drafting committees of experts but today, this situation is changing. It is now possible to measure the components of bridge traffic load, to weigh trucks, using [[Weigh in motion|weigh-in-motion]] (WIM) technologies. With extensive WIM databases, it is possible to calculate the maximum expected load effect in the specified return period. This is an active area of research, addressing issues of opposing direction lanes,<ref>{{Cite journal |last1=Enright |first1=Bernard |last2=O'Brien |first2=Eugene J. |date=December 2013 |title=Monte Carlo simulation of extreme traffic loading on short and medium span bridges |journal=Structure and Infrastructure Engineering |volume=9 |issue=12 |pages=1267–1282 |doi=10.1080/15732479.2012.688753 |bibcode=2013SIEng...9.1267E |issn=1573-2479 |hdl=10197/4868 |s2cid=10042252 |hdl-access=free}}</ref><ref>{{Cite journal |last1=Caprani |first1=Colin C. |last2=OBrien |first2=Eugene J. |date=March 2010 |title=The use of predictive likelihood to estimate the distribution of extreme bridge traffic load effect |journal=Structural Safety |volume=32 |issue=2 |pages=138–144 |doi=10.1016/j.strusafe.2009.09.001 |hdl=10197/2329 |s2cid=44049002 |hdl-access=free}}</ref> side-by-side (same direction) lanes,<ref>{{Cite journal |last1=OBrien |first1=Eugene J. |last2=Enright |first2=Bernard |date=July 2011 |title=Modeling same-direction two-lane traffic for bridge loading |journal=Structural Safety |volume=33 |issue=4–5 |pages=296–304 |doi=10.1016/j.strusafe.2011.04.004 |hdl=10197/3062 |s2cid=53475878 |url=https://arrow.dit.ie/engschcivart/41 |hdl-access=free}}</ref><ref>{{Cite journal |last1=OBrien |first1=Eugene J. |last2=Leahy |first2=Cathal |last3=Enright |first3=Bernard |last4=Caprani |first4=Colin C. |date=30 September 2016 |title=Validation of scenario modelling for bridge loading |journal=The Baltic Journal of Road and Bridge Engineering |volume=11 |issue=3 |pages=233–241 |doi=10.3846/bjrbe.2016.27 |issn=1822-427X |hdl=10197/9252 |url=https://bjrbe-journals.rtu.lv/article/view/bjrbe.2016.27|doi-access=free|hdl-access=free }}</ref> traffic growth,<ref>{{Cite journal |last1=OBrien |first1=E.J. |last2=Bordallo-Ruiz |first2=A. |last3=Enright |first3=B. |date=September 2014 |title=Lifetime maximum load effects on short-span bridges subject to growing traffic volumes |journal=Structural Safety |volume=50 |pages=113–122 |doi=10.1016/j.strusafe.2014.05.005 |hdl=10197/7069 |s2cid=59945573 |hdl-access=free}}</ref> permit/non-permit vehicles<ref>{{Cite journal |last1=Enright |first1=Bernard |last2=OBrien |first2=Eugene J. |last3=Leahy |first3=Cathal |date=December 2016 |title=Identifying and modelling permit trucks for bridge loading |journal=Proceedings of the Institution of Civil Engineers - Bridge Engineering |volume=169 |issue=4 |pages=235–244 |doi=10.1680/bren.14.00031 |issn=1478-4637 |hdl=10197/9246 |hdl-access=free}}</ref> and long-span bridges (see below). Rather than repeat this complex process every time a bridge is to be designed, standards authorities specify simplified notional load models, notably HL-93,<ref>{{Cite web |url=https://engineeringcivil.org/articles/bridge/hl-93-aashto-vehicular-live-loading-truck-tandem-design-lane-load/ |title=HL-93 AASHTO Vehicular Live Loading {{!}} Truck {{!}} Tandem {{!}} Design Lane Load|date=17 August 2016|website=EngineeringCivil.org|access-date=2019-03-15}}</ref><ref>{{Cite journal |last1=Leahy |first1=Cathal |last2=OBrien |first2=Eugene J. |last3=Enright |first3=Bernard |last4=Hajializadeh |first4=Donya |date=October 2015 |title=Review of HL-93 Bridge Traffic Load Model Using an Extensive WIM Database |journal=Journal of Bridge Engineering |volume=20 |issue=10 |doi=10.1061/(ASCE)BE.1943-5592.0000729 |issn=1084-0702 |hdl=10197/7068 |s2cid=53503763 |hdl-access=free}}</ref> intended to give the same load effects as the characteristic maximum values. The [[Eurocode]] is an example of a standard for bridge traffic loading that was developed in this way.<ref>{{Cite journal |last1=O'Connor |first1=Alan |last2=Jacob |first2=Bernard |last3=O'Brien |first3=Eugène |last4=Prat |first4=Michel |date=June 2001 |title=Report of Current Studies Performed on Normal Load Model of EC1: Part 2. Traffic Loads on Bridges |journal=Revue Française de Génie Civil |volume=5 |issue=4 |pages=411–433 |doi=10.1080/12795119.2001.9692315 |s2cid=111112374 |issn=1279-5119 |hdl=10197/4024 |hdl-access=free}}</ref>

=== 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>