The action of long-span bridges, notably suspension bridges, under wind has long been of concern. This first of two papers summarizes the pertinent experimental and other prerequisite data and proceeds to the linear dynamic analysis of bridge flutter for bridges having vibration modes that are not necessarily simple, i.e., that may involve motions beyond pure flexure and torsion. A review is made of the energy considerations involved in the assessment of aerodynamic stability.The paper lays the groundwork for the one that follows.
1. INTRODUCTIONt
['he aerodynamics of most important suspended-span bridges are studied today in the wind unnel. The most common activity to this end has been the section model study wherein t relatively short, typical section of the bridge span, built rigidly and to model scale (say ./100 to 1/25), is spring mounted and tested under wind flow for its oscillatory tendencies. Vlost such tests have been performed under laminar flow conditions, though a recent study i1] suggests how tests under turbulent conditions might be performed and interpreted.
Although the earliest section model tests of bridge decks were considered as directly 9 epresentative of prototype bridge action (the first "bending" and "torsion" frequencies )f the prototype being duplicated to scale by the spring mounting) it is now generally recogaized that the section model best serves only as an analog source of aerodynamic data 'ather than as a completely similar dynamic model. Its usual springing is such that only .~vo bridge deck motions (bending and torsion), assumed to be uncoupled, are represented, md it is implied further, for similarity to the prototype, that the modes of these motions 9 etain identical form throughout the prototype span. Many actual bridges are in fact arched 9 n the vertical plane of the roadway centerline, and one recent design [2] is strongly curved n plan. With such bridges natural structural vibration modes tend no longer to be simply ancoupled into "bending" and "torsion" but to be fully three-dimensional in character, uith components of vertical, torsional, and lateral sway motion all occurring at the natural ."requency of the particular mode in question. In such cases the section model falls considerably short as a direct dynamic analog of the full prototype bridge; still, one very proper ,*xploitation of this very useful device simply reverts back to its service essentially as an analog source of basic aerodynamic data that stem from the faithful geometric shape of the model.
The alternative of building and testing a full aeroelastic model of a given prototype has been resorted to on a number of occasions. This is a productive direction in which to proceed but it involves considerable cost--amounting to several times section model costs, a large t The original material of these two companion papers was somewhat lengthy. Therefore the decision was made to break the presentation into two parts. This first paper, Part I, containing less new material, ~erves as a kind of state-of-the-art summary--in rather specific form---of the background needed for Part II.