EVALUATION OF SEISMIC PARAMETERS FOR AN EARTH- QUAKE ANALYSISOlli T. Ravaska 1 and Panu A. Tolla 2 ABSTRACT The Final Engineering Design for a cable-stayed 1347 m long bridge with 17 spans over the Cam river in Hai Phong, Vietnam, was made by a design team from the Finnish National Road Administration (Finnra) in 1997-1998. Subsoil on the site is compos ed of soft alluvial and flood plain depo sits down to a depth of 30 metres, so that the only feasible foundations would be large diameter piles. One requirement for the bridge design was that an earthquake with a magnitude of M = 6.0, which may occur about 20 km from the bridge, should be taken into consideration. The special soil parameters needed for the earthquake analyses were the velocity of the shear wave, the dynamic shear modulus and the damping ratio. Because direct measurements on the site were not possible, seismic soil parameters for earthquake analyses were determined as based on laboratory tests on samples from the bridge site and on empirical correlations between geotechnical proper- ties, SPT results and dynamic parameters. Laboratory tests pe rformed in order to determine dynamic pa- rameters were the resonant column test and the bender element test. INTRODUCTION Evaluation of seismic parameters for earthquake analysis as discussed in this paper deals with the Binh Bridge Project which includes the design and construction of a 17-span bridge about 1.3 km long over the Cam river in Hai Phong located on the coast of the Red River delta area in northern Vietnam, Figure 1. The bridge will be cable-stayed with a steel and concrete composite superstructure which is continuous from one abutment to another. The main spans are 100+260+100 m long and the approach spa ns on both sides of the cable spans have a length of 60 m and abutment spans of 50 m . The total length is 1347 m and the effective width of the deck is 22.5 m. The pylons are comprised of modified H- type concrete towers with inc lined legs. The height of the towers is 101.6 m. The bridge will be founded on s teel pipe piles the maximum penetration of which is 35-45 m. Hai Phong Province is situ- ated at the south-west margin of the Caledonia-Katazia fold belt and next to the northern margin of the Red River Ce- nozoic Rift basin. The area has been affected by move- ments of the fault in the basin. Two seismic source zones are at a distance of 20-25 km from the city, with the potential ofcausing earthquakes with an intensity of 7 (MSK-64 scale) at the epicenter and intensities in the range 6-7 in the Hai Phong city area. Therefore the Institute of Geophysics at the Figure 1 : The Binh Bridge, modelled by Consulting Kortes Ltd. 1 Professor of Soil Mechanics and Foundation Engineering, Helsinki University of Technology, Dept. of Civil and Envi- ronmental Engineering, P.O. Box 2100, FIN-02015 HUT, Finland 2 Geotechnical Engineer, M.Sc. (Civ.Eng.), Finnish National Road Administration, Consulting Unit, P .O. Box 157, Hel- sinki, Finland
