Journal of Bridge Engineering

Achieving larger maximum spans by inventing new bridge systems has always been a fascinating intellectual challenge. Several concepts of modified cable-supported bridges have been proposed in the past that promise to surpass the more traditional cable-supported bridges in terms of maximum span length. Some of these concepts are recalled in this paper. An alternative concept derived from the classical cable-stayed bridge system is presented. Instead of vertical pylons, pairs of inclined pylon legs, spreading out longitudinally and connected at the top by horizontal ties, are used. The merits and shortcomings of such a system are discussed. Based on a comparative analysis of forces, quantities, and costs, it is concluded that the alternative concept presented in this paper not only allows the achievement of larger maximum spans, but also can lead to an economically advantageous design—even within the span-length range of the classical cable-stayed bridge system.

Increases in legal-load limits as well as lack of maintenance has rendered a large number of bridges in the United States deficient and in need of rehabilitation or strengthening. Many of these bridges are classified as deficient because their load-carrying capacity is inadequate for today's traffic. This paper deals with two methods of strengthening understrength bridges: posttensioning of the positive moment regions of bridge stringers and the addition of superimposed trusses to the stringers at the piers. These strengthening methods have been verified experimentally and analytically through a number of Iowa Department of Transportation (Iowa DOT) research projects. In most continuous-span bridges, the desired stress reduction in the stringers can be achieved by posttensioning only the positive moment regions. However, in certain situations, superimposed truss systems are needed at the piers to obtain the desired stress reduction in the negative moment regions. This paper presents the development of a design methodology for use by practicing engineers in designing a strengthening system (posttensioning in positive moment regions and/or superimposed trusses in negative moment regions) for continuous-span composite bridges.

A general refined analysis procedure for predicting the vehicular live load distribution on simply supported bridge girders is described. Load and Resistance Factor Design (LRFD) distribution factor formulas for flexure are carefully reviewed for modern prestressed concrete bridges made of I-girders or spread box girders with larger span-to-depth ratios. LRFD's results are compared with those obtained from the refined analysis method. Design recommendations are offered.

A limited field testing program is described in which pretension was measured in 44 large-diameter (25.4 and 28.6 mm diameter) A490 bolts installed by construction crews into slip-critical connections in a multistory steel-framed building. The bolts were installed with direct tension indicators into connections that varied from simple bracing connections to large column splices. The field test results showed that the minimum specified pretension was not achieved in a significant number of bolts with long grip lengths (greater than 178 mm or 7 in.), while bolts with shorter grip lengths were adequately tensioned. A combination of factors, including greater difficulty in snugging the plies in the connections with the longer bolts and the very high pretension forces that the large-diameter, high-strength bolts required to develop specified pretension stresses, are thought to have caused the low pretension stresses measured in the longer bolts.