Wednesday, March 5, 2014

Design of Dowel bars - Rigid Pavements.

Hi,
How you doing? Let's discuss the design of the dowel bars. So, what are dowel bars first?

  • Dowel bars

 Dowel bars are steel bars, generally round in shape,  and are used in the expansion joints which, are provided in the concrete pavements(rigid pavements) to allow the expansion of the concrete slab.
The main function of the dowel bars is to transfer the wheel load from one slab to the adjacent slab through the shear and bending moment.

Dowel bars are embedded on both the slabs, and on one slab there is a dowel cap which provides space for the bar to occupy it when there is expansion in both slabs.
This empty space is filled with some easy to contract material so, that it may adjust its volume according the need of the dowel bar.

Design of Dowel bars:
In order to design the length and diameter of the dowel bar we have to check its failure under the action of wheel load on the pavement slab.
Failure may occur in three ways:

  1. Failure of the dowel in shear
  2. Failure of dowel bar in flexure
  3. Failure of concrete in bearing.
Steps:
  1. According to Indian roads congress(IRC) the following formulas(given in image) are used to find the load capacity of the dowel under shear, flexure and that of concrete in bearing. Three formula are given in the image below.
    Dowel bars- design formula (Courtesy: Highway Engg. By S.K. Khanna and C.E.G.Justo)
  2. Generally the thickness of the expansion joint and diameter of the dowel bars are fixed.  We first find out the length of the bar by equating the second formula with the third formula assuming that the load capacity of the concrete in bearing is equal to the flexure capacity of the dowel bar. The formula is given below in image, length has to be found by trial and error method.
    Length of Dowel bar ( Source: Highway Engg. by S.K.Khanna & C.E.G. Justo)
  3. Then use that length to find out the three load capacities corresponding the these three formulas. The lesser of the three will be the capacity of the dowel bar because, failure at any case is the failure of the pavement system.
It is assumed that the wheel load is transferred to a distance of 1.8.l from the application of the load. The capacity factor of the dowel system is defined as the ratio of the load capacity of the dowel system to the load capacity of the single dowel bar.
We must check the dowel system to be sufficient to provide the corresponding load factor and that will decide the spacing of the dowel bars and therefor the numbers of dowels required.

Thanks for your visit!



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