Pre-camber vs Carriageway Vertical Alignment

[HighPanda]

For prestressed bridges, pre-cambering is needed to offset deflection due to creep and self weight. And, for long span bridges, the pre-camber may be, say 150mm. My question is how the pre-camber marries with ideal highway vertical alignment which may affect drainage design on the bridge deck?

For example, if there is a bridge, sloping in elevation, and pre-cambered with the crest at the midspan, does it mean there is a section of the bridge deck concave upward near the midspan?

 

[Drumchaser]

Depending on the method of forming or slab falsework required (plan details) the bottom of slab location in relation to the top flange of the I-Beam or girder will be established after the beams are profiled and "graded". Box beams, slab beams etc. are obviously a different slab system.

HP

The "completed" top of slab or riding surface should follow the vertical curve stated in the bridge layout. Before the slab is poured the top of slab grades are to parallel the vertical curve -plus deflection-, so that when the bridge is poured the settlement of the beams will be accounted for.

At times vertical curves give many folks fits. It is best to plot the full vertical curve so as to see exactly what is happening with the grade line approaching the bridge, over the bridge, and beyond the bridge.

I hope this helps.

 

[DRC1]

For Box beams where no slab is istalled, Typically at mid span there is a crown. It is generally not as pronouced as steel camber. Typically a membran is applied to the suface and rainage of water above the membrane is provided at the low points. Typically I have seen asphalt pavementu used to provide the finihed profile.

 

[Zambo]

In my opinion there are 2 precambers. There is the precamber required to take account of the longterm effects of shrinkage and creep. This precamber should be calculated by the designer to ensure that at a time in the future, say 10,000 days, the bridge vertical alignment is acceptable. The designer should then take this into account in all aspects of his design including drainage.

There is then the precamber for during construction. This should be calculated by the contractor and should include for method of construction including the application of deadloads such as asphalt. The contractor should ensure that at the time of completion of the project the geometry of the bridge consists of the vertical alignment at 10,000 days plus the precamber required by the designer.

 

[MichSt]

Unlike steel girders, the camber of prestressed concrete girders is not designed based on the roadway profile and anticipated deflections. The difference between the camber in the concrete girder and roadway profile is made up by haunching the deck at the beams, which can result in some large haunch depths. The haunch weight can be significant and must be accounted for in the girder design and screed elevation calculations.

 

[HighPanda]

Thanks for all advice.

Here is an attachment where my question came from.

If you look at Fig 7.49, the max sag of the camber curve is not at midspan span because it is for balanced cantilever construction.

Does anybody has any clue how it fit with highway vertical alignment.

 

[Zambo]

You are using balanced cantilever construction, are you using precast segments or cast-in place?

Assuming you are precasting the segments then you need to calculate the geometry of the segments so that after construction you reach the required design precamber. I would call this "construction engineering" and you need advice on the adjustment of the forms and the correction of casting errors.

If the balanced cantilever is cast in place, with travelling forms for example, you still need to calculate the geometry at the various construction stages, but correction of (minor) casting errors can more easily be carried out.

At any point during construction you need to know the expected geometry at that stage. If the geometry is following your expectation you will know that the final geometry is going to be correct. If you start to find a deviation from the expected geometry at a particular stage you may have to make adjustments to ensure the final geometry is correct.

 

[HighPanda]

Zambo,

Thanks for your advice.

I understand that precamber probile is for creep and/or shrinkage, deflection under S/W + ......other effects. The brdige deck profile will turn to "ideal or desired" profile after say 50 years when all these effects (e.g. creep) has already occured.

However, the camber curve (e.g. see my attachment) doen't fit with the highway alignment (at least on day one when the bridge is completed) as there is bump away from the midspan.

 

[Zambo]

HighPanda,

there is no bump. When you hand over the bridge on day 1 the vertical alignment will be similar to the final alignment at 10,000 days or whatever but just a bit higher. The horizontal alignment will also be similar but just needs allowance at the bearings for shortening.

I think you are confusing the precamber during the stage by stage construction with the final alignment. So for example as you construct the cantilever with temporary prestressing you will need to set the elevations higher so that deflection of the cantilever due to self weight is compensated for. The same for the length, you will need to cast the cantilevers longer so that when the final permanent prestressing is applied the span becomes the design length (to do this there needs to be a movement joint somewhere in the system)

I doubt you will be working this out by hand calculation. You need a strutural engineering programme that can give you the stage by stage analysis of the structure so that you know the geometry of the bridge at all construction stages. This will then give you the segment geometry.