Journal of Bridge Engineering

After the 1989 Loma Prieta earthquake, California's Department of Transportation (Caltrans) began an ambitious program to screen, prioritize, and retrofit 2,000 seismically deficient California bridges. It was unfortunate that before this program was completed the Northridge, Calif., earthquake occurred. This paper summarizes how bridges with a post-1989 seismic retrofit performed during the 1994 Northridge earthquake. A description of typical retrofit details is given. A table provides data on all retrofitted bridges where ground motion exceeded 0.25g (where g= acceleration due to gravity) during the earthquake. Case studies of two retrofitted bridges that experienced ground motion of 0.50g are presented. There was no serious damage to any of these retrofitted bridges, although some were within 100 m of collapsed structures. Caltrans retrofits are designed to prevent collapse during the maximum credible earthquake. Bridge retrofit performance during the Northridge earthquake provides compelling evidence that these retrofits can achieve that goal.

Tension control bolts are a type of alternative design fastener, permitted by the Research Council on Structural Connections. The bolt has a splined end that extends beyond the threaded portion of the bolt and an annular groove between the threaded portion of the bolt and the splined end. The special wrench required to install these bolts has two coaxial chucks—an inner chuck that engages the splined end and an outer chuck that envelopes the nut. The two chucks turn opposite to one another to tighten the bolt. At some point, the torque developed by the friction between the nut and bolt threads and at the nut-washer interface equals the shear resistance of the bolt material at the annual groove. The splined end of the bolt then shears off at the groove. If the system has been properly manufactured and calibrated, bolt pretension is achieved at this point. Factors that control the pretension are material strength, thread conditions, the diameter of the splined end, and the surface conditions at the nut-washer-joint interface. The program reported here investigated the pretension of production tension control bolts as it varies from manufacturer to manufacturer and under different aging, weathering, and thread conditions.

Incremental launching is a competitive construction method for medium-span prestressed concrete bridges. Compared with the other techniques for in situ casting, in short bridges it is an alternative to the use of falseworks and reduces the cost of labor with the same investment. In longer bridges it is an alternative to self-launching centering and reduces investment with the same cost of labor. Compared with segmental precasting, it may reduce both investment and the cost of prestressing. The design of a launched bridge is affected by temporary stresses arising from its movement over fixed bearings. These stresses tend to increase the cost of materials, reducing the competitiveness of the construction method. Of the various devices used to reduce temporary stresses, adopting a launching nose has virtually become standard, permitting an effective control at competitive costs. The techniques of optimizing the nose-deck interaction are illustrated here through several graphs, and the analysis of special cases may be carried out by means of an ordinary spreadsheet.

This paper presents the investigation of the effect of dynamic loads on the dynamic amplification factors (DAFs) of an existing continuous bridge. A three-dimensional analytical model to idealize the vehicle and a finite element model to analyze the bridge have been developed. Experimental testing of this existing bridge has been conducted by the Ministry of Transportation, Quebec. Two vehicle models, having seven and 11 degrees of freedom with and/or without trailers, are considered in the analysis of the interaction between the bridge and vehicles. Based on International Standards Organization specifications, the road-surface roughness of the approach roadways and bridge deck is generated from the power-spectral density function for very good, good, and average roads. The solutions obtained from the present models are compared with the experimental results for different vehicle speeds. The DAFs are computed from the present finite element model for three different types of measurements (displacements, strains, and reactions). The numerical results show that the DAFs computed with displacements are less than those estimated from strains and reactions. It is concluded that current design codes tend to underestimate dynamic amplification factors, especially for long-span continuous bridges.

A simplified approach to the dynamic finite element modeling of composite girder-slab bridges is presented using the limiting case of a single beam element to represent the girder-slab cross section. Dynamic properties calculated with this simplified model are compared with experimental results obtained from an in situ composite girder bridge and with more detailed shell element model predictions. The simplified beam element model accurately calculates the mode shapes of the structure. This agreement, however, is dependent on accurate modeling of the piers and kinematic constraints between the bridge and piers and the appropriate representation of the bridge's torsional properties. The calculated resonant frequencies associated with these modes show some discrepancy when compared with the experimental results. This discrepancy is attributed to the inability of the single beam element model to simulate the three-dimensional boundary conditions found in the actual structure. The simple models provide approximations to the dynamic properties that are accurate enough to be useful in preliminary seismic scoping studies and in the simplified modeling of long, multispan bridges.