Journal of Bridge Engineering

This paper explains how a major engineer, Christian Menn, conceived of his design for the 1998 Sunniberg Bridge in Switzerland. Menn had given a brief outline of his approach in an earlier paper, and this paper works out the consequences of his ideas by putting them into numerical form and interpreting those results. This paper brings out the close connection between aesthetic choices and structural performance and also shows how an apparently complex form can be broken down into simple elements to justify the initial conceptual design. There is little published writing about the creative process of bridge engineering and nothing in detail in English on the Sunniberg Bridge.

During the structural analysis of cable-stayed bridges, some specific problems arise that are not common in other types of bridges. One of these problem is the derivation of an optimal sequence for the tensioning of the stay cables. This paper describes a novel solution to this problem, the unit force method. The method takes into account all relevant effects for the design of cable-stayed bridges, including construction sequence, second-order theory, large displacements, cable sag and time-dependent effects, such as creep and shrinkage or relaxation of prestressing tendons. Information about the implementation of this method into a computer program is given, and an example of a practical application of this method concludes this paper. The method is not restricted to the design of cable-stayed bridges and may well be used for other structural applications in the future.

Long-span, prestressed concrete, box-girder bridges are haunched and have a span-to-depth ratio of 15 to 20 at the piers. This leads to slender webs, particularly for bridges built with high performance concrete. For girders with sloped webs and constant bottom slab width, the web plate is normally warped, which leads to web curvature in the direction of the principal compressive stresses. It is first shown that buckling is not critical as long as the web is uncracked. But, if the webs have shear cracks, the slenderness ratio of the diagonal compression struts can be very high so that the moments and stability of the curved struts need to be studied. It is shown that the tensile forces in the stirrups—determined according to the truss analogy—will counteract the lateral deformations of the slender compression struts. The procedure, which was developed for the design of the Confederation Bridge in Eastern Canada, will be illustrated by applying it to the slender webs of that bridge.

Modern highway bridges are often subject to tight geometric restrictions and, in many cases, must be built in curved alignment. These bridges may have a cross section in the form of a multiple steel box girder composite with a concrete deck slab. This type of cross section is one of the most suitable for resisting the torsional, distortional, and warping effects induced by the bridge’s curvature. Current design practice in North America does not specifically deal with shear distribution in horizontally curved composite multiple steel box girder bridges. In this paper an extensive parametric study, using an experimentally calibrated finite-element model, is presented, in which simply supported straight and curved prototype bridges are analyzed to determine their shear distribution characteristics under dead load and under AASHTO live loadings. The parameters considered in this study are span length, number of steel boxes, number of traffic lanes, bridge aspect ratio, degree of curvature, and number and stiffness of cross bracings and of top-chord systems. Results from tests on five box girder bridge models verify the finite-element model. Based on the results from the parametric study simple empirical formulas for maximum shears (reactions) are developed that are suitable for the design office. A comparison is made with AASHTO and CHBDC formulas for straight bridges. An illustrative example of the design is presented.

Heat curving is extensively used for fabricating structural steel girders for bridges. Current American Association of State Highway and Transportation Officials (AASHTO) specifications limit usage to Grade 345 steel $(F_y=50\mathrm{ksi})\text{,}$ ruling out Grade 485 $(F_y=70\mathrm{ksi})$ high performance steel (HPS). This paper presents results of a three-dimensional finite-element analysis to assess the applicability of existing AASHTO provisions for HPS 485W sections. The finite-element package NASTRAN was used to conduct the analysis and the model calibrated against experimental data obtained from full-scale tests conducted previously by U.S. Steel Corporation. Comparisons include curvatures, lateral deformations, and residual stresses. The calibrated model was used to predict the performance of an identical HPS girder subjected to the same heat/cool cycles. The three-dimensional analysis predicted smaller curvatures as compared with Grade 250 $(F_y=36\mathrm{ksi})$ or Grade 345 $(F_y=50\mathrm{ksi})$ steel. Comparable curvatures could be obtained by using higher temperatures.

The Land Transport Authority of Singapore has a continuing program of highway bridge upgrading for refurbishing and strengthening bridges to allow for increasing vehicle traffic and increasing axle loads. One subject of this program has been a short-span bridge taking a busy main road across a coastal inlet near a major port facility. Experiment-based structural assessments of the bridge were conducted before and after upgrading works including strengthening. Each assessment exercise comprised three separate components: (1) a strain and acceleration monitoring exercise lasting approximately one month; (2) a full-scale dynamic test carried out in a single day without closing the bridge; and (3) a finite-element model updating exercise to identify structural parameters and mechanisms. This paper presents the dynamic testing and the modal analysis used to identify the vibration properties and the quantification of the effectiveness of the upgrading through the subsequent model updating. Before and after upgrade, similar sets of vibration modes were identified, resembling those of an orthotropic plate with relatively weak transverse bending stiffness. Conversion of bearings from nominal simple supports to nominal full fixity was shown via model updating to be the principal cause of natural frequency increases of up to 50%. The utility of the combined experimental and analytical process in direct identification of structural properties has been proven, and the procedure can be applied to other structures and their capacity assessments.

The most popular type of bridge in service today is the concrete deck on steel-girder composite bridge. A finite-element model is built to analyze the superstructure of this type of bridge under working load conditions. The deflections along a test bridge are computed by using this method; the results obtained are close to the experimental data. The concrete deck of the bridge is analyzed using nonlinear finite elements, of which the analytical procedure is described in detail. A comparison is also made between this method and the traditional transformed area method.