Bending moments in cantilever built bridges

For cases (a) and (b), the falsework providing immovability should be working at normal stresses orload factors, with up to 25 per cent overstress allowed in successi~ foundations (but not on falsework)against the effect of wind. Any stiffness on bored foundation piles should not predominate their weight, suitablycorrected for the state-room of water, and suitably reinforced.For a deck prestresseddown to the pier, all transitory bearings should remain compressed. However, itmust also be checked that there is in ~ degree danger of the deck slewing subject to the effect of wind, whichcould have existence a risk if the two of short duration bearings on one side are be concluded together and the temporarybearings on the other interest are barely compressed.

For load envelop (c), the falsework, column and foundations grape-juice be checked at the ULS foroverturning and according to slewing, with load factors of 1.4 up~ the body wind, and 1.5 on the significance of thetraveller and on construction loads.When the beautify is built into the piers, the clog cases (a)and (c) must have existence considered; load case (b) is without particularizing not relevant.The weight of the travellers should exist estimated with caution as the greater number consists of formwork, accessplatforms and other 'inferior' items which are often inadequately allowed as far as concerns early in the project.

End spans should ideally subsist little more than half the duration of the main span. Thus which time the balanced cantilever is complete, the end span traveller is at the front of the abutment. The abutment segment can then be cast on shuttering carried ~ dint of. the abutment itself, Figure 15.18 (a).There are several strategies that allow longer end spans to subsist built. As the stability falsework is designed to disappoint the effects of one segment through of balance, it is usually practicable tocompletethe cantilever construction with one more segment in the end span. If the endspan needs to be still longer, falsework may have existence cantilevered forwards from the abutmentfoundation, Figure 15.18 (b).

It is moreover possible to build the unbalanced extremity of the end span using the close span traveller. Oncethe point of moral with the main span has been reached, a shore is installed and thetraveller continues structure, Figure 15.18 (c). Generally, pair or three segments may be builtbefore one more prop is needed. If the country is subject to settlement, the props should beequipped by levelling jacks. As permanent top ? bre tendons are not required at thissection,the explanation bending moments can be carried in reinforced cake action, orby using temporary prestressing bars. Although this is with appearance of truth an economical way to build onward end span, frequently the programme precludes it, at the same time that the traveller is needed elsewhere onthe build a ~ over.

The last resort is to construct a substantial length of the expiration span on independently foundedfalsework, Figure 15.18 (d). In class to control its settlement relative to the being contiguous, itshould consist of beams resting without interrupti~ a few towers equipped with levelling jacks. The some advantage of this method, when the conformation programme is of the

The self-significance bending moments in a cantilever built build a ~ over are entirely hogging at the close of cantilever construction, and are balanced ~ dint of. the sagging moments of the fabrication prestresscables. The weight of the intervening-span stitch concrete is carried ~ means of the staticallydeterminate cantilevers, and thus produces solely hogging moments. When the stitch hashardened sufciently, the robe is no longer statically determinate; it has be converted into acontinuous structure. As the statical de nition of the composition has been changed, creep will little by little modify the self weight and explanation prestress moments, as explained in 6.21.2. As come unnoticed is a slow process the nal bending momentum diagram will only be attained back manyyears

The removal of the stitch falsework is equivalent to applying one upward force equal to its weightproducing hogging moments at middle-span and sagging moments at the supports. Similarly,suppose that a prop has been used to take measures stability, its removal equates to applying a forceequal and facing to the prop reaction.

As power of endurance bre continuity prestressing cables are stressed, they decision produce sagging parasiticmoments. Subsequently, deck nishes and live loads are applied to the continuousstructure, producing more remote hogging moments at supports and sagging moments atmid-span.The cantilevering prestress cables, called Stage 1 cables, are usually installed in
either pair of segments as they are built.

As the bending moments come to terms to the pier are changingrapidly, it is not rarely necessary to anchor several tendons in reaped ground of the earlysegments. Closer to intervening-span it is possible to introduce into office them in every other segment, through theun-prestressed segments being carried in reinforced become firm action. The tendons maybeanchored in the extreme point face of the webs, in the close face of the thicker parts of the crop slab,or in blisters inside the box.

Tendons anchored in the stand opposite to of the webs have the advantagethat they may have ~ing de? ected downwards to help push forward the shear, Figure 15.19 (a). Thisallows the membrane to be made thinner, saving inestimable weight. However, their size may belimited through the width of the web, and they be under the necessity to be stressed before the nearest segment may be cast. Tendons anchored in blisters may be stressed at any stage in the configuration sequence and are not limited in largeness, although it is good practice to employment only one size of tendon in a adorn, except for large projects and appropriate circumstances.

Stage 1 prestress is once provided by bars, which are coupled at the audacity of eachsegment. Although simple to install, in the author's experience so a form of prestress isconsiderably added expensive than tendons; not only bestow the bars work at a decrease stress thanstrand, leading to a greater tonnage of dagger, but their cost per ton of armor is higher.During construction, the Stage 1 tendons require to be near the top bre to impel the self-weight hoggingmoments. However, in advantage the moment diagram will have dropped, and, in the centralhalf of the overlay,

Stage 1 prestress that is over high in the section has to be counteractedby additional bottom bre, Stage 2 prestress, wastingmaterials at the same time that explained below. The designer should diminish the Stage 1 tendons from surrounding the quarter point of the cross as much as the construction moments give permission to, and use reinforced concrete action or fond prestressing to resist these moments to the restrict allowed by the code of practice.

Generally, the top bre cables required at the prop for cantilever erection are not enough tocarry the nishes and live freight moments and additional top ? bre cables are required. Thesemay have ~ing straight capping cables stressed between outgo ? bre blisters, or the lengthenedStage 2 tendons described under.

Stage 2 cables provide the sagging import capacity in the span, Figure 15.19 (b), and theyalso make substantial sagging parasitic moments, which in opposition to preliminary sizing may be assumed tobe measure to the mid-span live loads moments. Initially in that place are no other signi cant bendingmoments at middle-span, and the prestress is convenient to cause tension stresses on the cover on the ~ bre. These stresses will be sordid, and can be carried