The Cuscatlan Bridge in El Salvador

Abstract

THIS bridge is the largest in Central America, and has been erected across the Lempa River, to complete another link in the Pan-American Highway (Earthquake Notes, 14, Nos. 3 and 4 ; June 1943). The main suspension span of 820 ft. is carried by cables of open construction chosen to fit the unusual erection conditions. The bridge is near the base of an extinct volcano, and test borings indicated that the foundations and anchorages would be in volcanic materials consisting of cemented ash and loose boulders or stones. These materials lie in very irregular strata and pockets. Since the bridge may be subjected to earthquake shocks, special care was necessary to support the assumed earthquake forces in the design of the concrete approaches. A study of this feature indicated that multiple-span rigid -frame structures were required. A factor in the design of structures built to resist earthquakes is the natural period of vibration. Resonance must be avoided by making this different from the supposed destructive earthquake frequencies likely to arise in the district. In this case periods of from 0.5 sec. to 1.5 sec. were avoided. The natural periods of the transverse bents of the Lempa River Bridge concrete approaches is less than 0.5 sec. and that of the longitudinal frames is of the order of 0.25 sec. A factor contributing to the safety of the longitudinal frames against resonance is the varying heights of the columns, each of which has a natural period of its own different from the others. Tunnel anchorages have been used. The principal mass of the bridge is contained in the main-span trusses and the floor, and this mass would produce the greatest transverse earthquake force on the towers at the level of the trusses. The cables have a small mass but transmit heavy loads to the towers. The earthquake forces on the towers from the cables would be small, and the conservative design of the bridge was not affected by the assumed earthquake forces.