Boston Manor Viaduct opened to traffic in 1965 and was constructed as part of the expansion of the motorway network in the United Kingdom.It consisted of 21 spans.The four spans nearest to Heathrow airport were constructed of reinforced concrete beams and slabs.The remaining spans are all of composite steel/concrete deck design.There are 17 spans: fourteen steel plate girder beams supporting cross girders with composite concrete slab whilst the remaining three spans use a pair of steel trusses in place of the four plate girder beams.