WEIGHING IN: The spectacular appearance of a new cable stay crossing of the River Labe at Usti nad Labem in the Czech Republic owes more to form than optimum structural efficiency, according to the engineer supervising construction. While Prague architect Roman Koucky maintains the physical bulk of Mariansky bridge - in particular the pylon - was visually necessary, Mott MacDonald Praha director Jiri Petrac argues that this actually makes the 180m asymmetric, single pylon design only 40% efficient. Despite the cable fans, he says, the deck's 3.25m deep, 4.1m wide box section central spine behaves like a continuous girder and so is all but self-supporting. But none of this stopped virtually the whole town turning out for the £18M structure's opening last month. The landmark was commissioned by the City Council and Czech Department for Urban Development in 1993 and engineered in steel by Czech transport and civils consultant CityPlan. Construction started in 1996 with the bridge manufactured in sections and assembled on site by Ostrava-based contractor Hutni montaze.