In this video we will try to understand the camber values given in RDSO standard drawings of Open web girders.
In all RDSO standard open web girders drawings a table is given showing camber values under different conditions. On left hand side A, B, C values are given which represent different loading conditions. On the right hand side panel points of girder bottom chord are mentioned starting from first panel point that is L Zero and ending at mid length panel. Camber values up to the mid panel point are given which covers half of the girder. Camber values on next remaining panel points may be obtained from these values because the arrangement of girder is symmetrical in nature.
In the example taken here there are six panel points shown, covering five panel lengths. The camber value written against A represents camber on jack. It refers to the camber required for mitigating the deflection due to dead load, super imposed dead load called side, live load and impact load. This is also called camber on jack because at the time of trial assembly, the panel points will be raised by this much value when girder is supported on jack at every panel point. This represents values of camber when no load has been transferred to the girder and girder is resting on the jacks.
Next values are given against B and it is called camber after removal of jack.
So, when jacks are removed, the dead load gets transferred to the girder and it consumes some amount of camber mentioned under A in the form of deflection. So now this camber written against B represents camber values due to SIDL, Live load and Impact load as dead load stands transferred.
Now the girder is dismantled and it is sent to site. It is brought at site and track, guard rail, chequered plates etc. are laid over it which are basically SIDL i.e. Super imposed dead load. Once this SIDL is also imposed the remaining camber is called C, that is residual camber. So residual camber C is basically actual camber after final erection of girder at site, which is ready to receive the live loads. Now only live load and impact load remain to come on the girder.
Residual camber is very critical parameter to indicate the health of the girder in general. Ideally when the live load for which the girder has been designed, comes on the girder at design speed, the girder should assume the nominal shape. Or in simple words bottom chord of girder should become horizontal. That’s why in the table the value of C at mid span L5 which is noted as 65.44mm is also the value of D, which is deflection under load testing under design EUDL and design speed.
So, This was all about this topic of residual camber. Watch another video to understand what to do if this residual camber falls short or in simple words when there is loss of residual camber.
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