Structural Steel

Design of Other Elements

A few special conditions relating to elements of arch bridges other than the ribs and ties should be considered in design of arch bridges.
Floor System. Tied arches, particularly those with high-strength steels, undergo relatively large changes in length of deck due to variation in length of tie under various load conditions.
It therefore is normally necessary to provide deck joints at intermediate points to provide for erection conditions and to avoid high participation stresses.
Bracing. During design of the Bayonne Bridge arch (Art. 14.8), a study in depth explored the possibility of eliminating most of the sway bracing (bracing in a vertical plane between ribs). In addition to detailed analysis, studies were made on a scaled model to check the effect of various arrangements of this bracing. The investigators concluded that, except for a few end panels, the sway bracing could be eliminated. Though many engineers still adhere to an arbitrary specification requirement calling for sway bracing at every panel point of any truss, more consideration should be given to the real necessity for this. Furthermore, elimination of sway frames not only reduces costs but it also greatly improves the appearance of the structure. For several structures from which sway bracing has been omitted, there has been no adverse effect.
Various arrangements may be used for lateral bracing systems in arch bridges. For example, a diamond pattern, omitting cross struts at panel points, is often effective. Also, favorable results have been obtained with a Vierendeel truss.
In the design of arch bracing, consideration must be given to the necessity for the lateral system to prevent lateral buckling of the two ribs functioning as a single compression member.
The lateral bracing thus is the lacing for the two chords of this member.
Hangers. These must be designed with sufficient rigidity to prevent adverse vibration under aerodynamic forces or as very slender members (wire rope or bridge strand). A number of long-span structures incorporate the latter device. Vibration problems have developed with some bridges for which rigid members with high slenderness ratios have been used. Corrosion resistance and provision for future replacement are other concerns which must be addressed in design of wire hangers. While not previously discussed in this section, the use of inclined hangers has been employed for some tied arch bridges. This hanger arrangement can add considerable stiffness to the arch-tie structure and cause it to function similar to a truss system with crossing diagonals. For such an arrangement, stress reversal, fatigue, and more complex details must be investigated and addressed.


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