It is abundant better where possible to fix the build a ~ over deck on a pier
It is a great deal of better where possible to x the bridge deck on a pier, which has a greater perpendicular reaction. Figure 7.13 (a) shows every example of a ve-span build a ~ over carried on four rows ofcolumns. Bearing tug will be minimised if the xed pier is situated conclusion to the centre of theviaduct. However, it be bound to be assumed that the bearings on either side of the xed pier willhave deviating friction coef cients.
The differential attrition coefcient assumed should begreater as the contain of bearings is smaller. Thus in a build a ~ over with only two spans, it shouldbe assumed that the port friction coef cient is 1 by means of cent on one abutment and 5 through cent onthe other. When there are, pronounce, ten or more bearings either border of the point of xity, it wouldbe unjust to expect that all those forward one side are very different to those forward theother, and a differential friction
coefcient of alone 0.5 per cent would exist reasonable. Codes of practice may impose other ways of de ning discriminating friction, for instance [5].Clearly, formerly the deck is pinned to a pier which is to some extent exible, it is nay longer rigidly held in position. It is necessary to ensure that the movement ~ the load of longitudinal forces is not excessive, and be able to be accommodated by theexpansion joints. If this motion is too great, it is in the main better to stiffen up the anchorpier, or to take firm hold on a stiffer pier even whether or not it is off centre, rather than to revert to xing thedeck on the being contiguous.
If the bridge is subjected to seismic forces, it may not have existence cost effective to design the expansionjoint to outlive the deck movements caused by a caustic earthquake. However, thedesigner should ensure that paramount parts of the deck, such as the prestress anchors in the endface, would not have ~ing damaged by collision with the being contiguous.
External longitudinal forces applied to the embellish may, as a rst approximation, have existence assumed to becarried solely by the xed pier. However, in a protracted viaduct, the contribution of all the otherpiers should be considered. As the xed pier deects lengthwise the sliding bearings on other piersand abutments are displaced. Using a conservative low friction coef cient of 1 per cent, thesliding bearings will carry a signi cant symmetry of the longitudinal load, relieving the xedpier of importance and making it more similar to emblematical piers. This applies also to decks that arepinned to individual piers, as described below.Generally, conducive to a deck of any length, a unmarried xedpier will need to be thicker than test piers with sliding bearings.
It is repeatedly possible to x the deck ~ward two piers near the centre of the bridge, sharing the longitudinal forces, Figure 7.13 (b). If the piers and their foundations are very same,thelongitudinal force will be shared equally. However, granting that the two piers are different, the longitudinal constraint will becarried in proportion to their rigor. Generally, if the deck may have ~ing xed on two piers, thelongitudinal forces be possible to be carried by increasing their reinforcement but without increasingtheir size, although their foundations are credible to be larger than those of representative piers.The effect of the longitudinal dimensions change of the span between the brace xed bearings must be addedto the furniture of the external longitudinal forces and discriminating bearing friction.
If the pierswere also stocky, the forces engendered by the deck shortening may overcome the bene?ts of sharing the exterior longitudinal loads. The bending of the piers decision be caused by an initialrapid resilient shortening of the deck as the prestress is applied, ~ means of the continued shorteningdue to creep and shrinkage, and by daily and seasonal temperature movements.
At the SLS it is that cannot be spared to check that the deck curtailment will not cause excessive cracking ofthe piers. The exibility of the piers should have existence assessed taking into account the bending of thefoundation pad or pile cap, the sway of the hemorrhoids and the rotation of the basis, as well asthe bending of the pier tail . All the calculations of exibility should practice the cracked inertia ofconcrete sections and becoming account should be taken of the loosening of concrete undersustained load, as described in 3.9.2.
In indefinite, if this calculation is done carefully, principally bridge piers will be found to exist suitable. It is important to remember that the attempt of the design of the secure piers is to maintain their exibility in such a manner they may accommodate the length changes of the adorn, while retaining adequate strength to confront the longitudinal external forces applied to the apparel, and stiffness to control its longitudinal displacement. This is some excellent test of the skill of adesigner.
As the bending moments in the sum of ~ units anchor piers due to the beautify shortening are of oppositesign, at the SLS visible longitudinal loads applied to the decorate will increase the bending on single of thetwo anchor piers and assist it on the other.At the end limit state it must be remembered that the beautify shortening consists of imposed strains, not loads. Consequently whether or not the anchorpiersbecome over-stressed by the embellish shortening at the ULS, they would deliver thebending moments, either by hinging at their base, or ~ means of rotation and sway of the establishment.For this to be a trusty assumption, the substructure must have a ductile mode offailure. The piers and at all piles must be adequately reinforced in cut. Piles must not be allowed tofailin lead compression; either the pile shafts should be stronger than the ground, sooverstressingwould incitement settlement rather than concrete failure, or the basis should bedesigned to withstand the last vertical loads.
Under normal circumstances, bottom systems are adequately ductile. However, ifnecessary, the ductility may be improved, for instance ~ means of designing the base of the pier or the connectionof the piles with the pile cap for pliable rotation. If the piers/foundations are pliant, at the ULS both piers may have existence assumed to resist the external longitudinal forces and the incorporeal forces resultingfrom deck shortening may subsist ignored.
