Summary

This study discusses the various approaches of which the superstructure of the proposed Cebu-Negros suspension bridge can be designed.  The two-lane per direction suspension bridge, approximately 6.8 kilometers in length, will specifically connect Looc of Negros Oriental and Liloan Point of Cebu passing through the Ta�on Strait in the Philippines. 

The preliminary design discussion of the bridge superstructure is grouped into four (4): (a) design of the deck of the bridge; (b) design of its stiffening truss girder; (c) design of the suspension main cables and hangers; and (d) the various design approaches of the pylons.  All of which present basic configurations, methods and procedures.  None of which are detailed. 

The study was pursued based on the recommendation of the pre-feasibility study conducted for this proposal. 

The orthotropic bridge theory is first considered to design the deck system of the Cebu-Negros suspension bridge.  The main objective in using this theory is primarily to �obtain optimum structural performance from materials�.  This concept resulted from the engineers� efforts in Europe, particularly Germany, to develop lightweight-steel bridge-decks that are not only very economical but also possess excellent structural characteristics (this was in the period of Post World War II).  It is shown in this study that the �Pelikan-Eslinger Method� is the most practicable method of deck analysis.  This method is based on the application of the equation known as the method of elastic equivalence.  A substantial discussion is presented in the paper.  In addition to that, a numerical calculation process is also presented leading to the preliminary configurations and dimensions of the bridge deck system. 

The stiffening truss girder acts multitude of functions.  One of its basic functions is to limit the deformation of the cable and to distribute any concentrated, unsymmetrical, or non-uniform loads.  If the cable design is parabolic (which the presumption in this case it is), the truss-type girder keeps the hanger tensions in constant proportion or equal.  Another important function it does is to hold the cable in place or in its initial curvature of equilibrium.  This in turn limits the deflection of the structure and resists the setting up of vertical oscillation.  Since all primary member forces are all axial loads and the open web system accommodates a greater depth than an equivalent solid web girder, this gives a stiffened truss girder a structural advantage by imparting more rigidity to the bridge and reduces deflection.  A preliminary configuration is developed for such truss-type girder.  It was subjected to a test using the available software (Staad-III Ver.22.0-W).  The objective of this stage is to come up with the basic configuration and dimensions of members.  However, the calculations need counterchecking and presentation requires detailing because such items are already beyond the scope of work of the study.

The preliminary design of the cable system is chiefly divided into two main parts: (a) the design of the geometric configuration of the cable system and the  (b) structural design of the cables based on stress analysis.  The geometric design of the cables is derived from the simple consideration of the parabolic cable.  This approach gives more emphasis on the form of the cable member while at the same time the stiffening girder is just assumed to be beam-like structure hung therefore.  It is strongly recommended, however, to perform the catenary cable analysis for the geometric design.

The second part of the cable system design takes into consideration the effect of the stiffening girder on the cable stresses.  Three developed theories used in designing and analyzing cable stresses are discussed: (a) the Rankine Theory, (b) the Elastic Theory, and  (c) the Deflection Theory.  The discussions over the concepts of these methods cover its apparent advantages and disadvantage.  It is recommended to use the third concept due to the strength it conveys: it tackles and analyzes the actual and realistic behavior of the structure.

The discussion for the pylon design covers only the different possible approaches a pylon can be designed and analyzed.  The contribution from the pylon to the steel quantity is of considerable complexity as the required dimensions of the pylons depend not only on the in-place forces from the cable system but also on wind and on other lateral forces acting on the pylon itself as well as on most part of the superstructure.  Steel is the only material that is being used and considered in this study.  It offers several advantages, though other hybrid material may be more superior.  In any rate, the use of steel offers a possibility of greater pylon height.  This also ensures more favorable sag ratio.  Thermal expansion of the pylon balances the difference in inclination of the main and side span cable and this minimizes temperature effects present with indeterminate types.

The study is very much a first-step move on the technical side of the proposed suspension bridge.  There is, of course, so much room to fill for further preliminary studies.  One suggestion is to conduct a preliminary study for the substructure component of the bridge.  Furthermore, much detailed study is very much encouraged.