<P> The subsequent design was led by Expedition Engineering assisted by Arup Materials, Balfour Beatty Regional Civil Engineering, Black and Veatch, Bridon, Cambridge University, Cleveland Bridge UK, Dorman Long Technology, Flint & Neill, Formfab, GCG, GERB, Imperial College, RWDI, Spence Associates, Speirs & Major, Stainton, and William Cook while White Young Green were project managers . </P> <P> English Partnerships appointed Flint & Neill Limited to carry out a category III independent check of the bridge design including loading, wind tunnel testing, and investigation of failure modes, a number of aspects of which fall outside current standards . The bridge has a 120 - year design life . </P> <P> The bridge is a dual, tied arch bridge or bowstring bridge . It has a pair of continuous, differently - sized structural steel arches with suspended precast concrete decking and one asymmetrically placed river pier . The tapering arches with a trapezoidal box section are fabricated from weathering steel plate . The arches both bifurcate within the spans to form a double rib over the river pier . A reflex piece between the two arches holds them together making the two arches one continuous curve . No other bridge is known to have quite the same design . </P> <P> The offset river pier is to accommodate water sports and leisure craft to one side . The river pier is supported by an 11.5 m square by 2.5 m thick pile cap on sixteen 1 m diameter hollow steel pipe piles . On the pile cap beneath the water line are four 3 m cylindrical concrete legs onto which are bolted and welded the four inclined grey steel legs visible above water . Riprap covers the river bed around the river pier for scour protection against the large flows when the Tees Barrage downstream discharges . Each of the two concrete riverside piers are supported on four 500 mm hollow steel piles and a pile cap . </P>

What type of bridge is the infinity bridge