The application of traffic load is straightforward, but there will be a variety of loaded lengths and positions of tandem axles and special vehicles must be chosen to suit the influence lines. This particularly applies to bending in the arch for the tie beam, the locked in bending moment can be controlled by adjusting the hanger lengths. The influence line for a vertical hanger usually comprises a single positive peak as shown below right.ĭead load effects will normally comprise a large proportion of the design stresses for main elements, and it becomes very important to allow fully for the erection method. Note that there are both positive and negative parts. A typical influence line for bending in the arch is given below left. However, maximum bending will occur with just part of the span loaded. It is obvious that maximum axial forces are generated when the whole span is loaded. A circular arch will always have greater bending moments in the arch members.Īlthough the tied arch is essentially just a simple span, the influence lines of some of the elements are worth examining. However, the presence of the tie beam contributes stiffness to the system and this means means that there are some moments, especially around the arch springings. Hence there will be an outward movement at the free end.Ĭoncrete deck (precast units on ladder deck, before in-situ concreting)Ī parabolic arch is the best shape for structural efficiency because, under uniform load there should just be axial forces in the arch members. The arch will deform downwards, and it will try to spread its feet, but this is limited by having to stretch the tie beam. This creates thrust in the arch, which is balanced by tension in the tie beam. If a load is placed on the deck, it is transferred to the arch via the hangers, as the global stiffness of the arch is greater than the bending stiffness of the deck. The arch is held longitudinally at one end, with the other end free to expand or contract under varying temperatures. Looking at the diagram above, it can be seen that a tied-arch is really just a simply supported beam. It can be seen that one end will still require a longitudinal restraint to carry wind, braking, acceleration and skidding forces, and that the other end is permitted to move longitudinally. The tied-arch is sometimes referred to as a bowstring arch.īy taking the arch thrust through the tie member, the primary requirement for the substructure reduces to only carrying vertical loads. The tied-arch offers a solution when it can be arranged that the deck is at such a level that it can carry the horizontal force as a tie member, as shown below.
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