Module 3: Geometric design of highways

Lecture 15 Horizontal alignment II

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Guidelines on superelevation

While designing the various elements of the road like superelevation, we design it for a particular vehicle called design vehicle which has some standard weight and dimensions. But in the actual case, the road has to cater for mixed traffic. Different vehicles with different dimensions and varying speeds ply on the road. For example, in the case of a heavily loaded truck with high centre of gravity and low speed, superelevation should be less, otherwise chances of toppling are more. Taking into practical considerations of all such situations, IRC has given some guidelines about the maximum and minimum superelevation etc. These are all discussed in detail in the following sections.

Design of super-elevation

For fast moving vehicles, providing higher superelevation without considering coefficient of friction is safe, i.e. centrifugal force is fully counteracted by the weight of the vehicle or superelevation. For slow moving vehicles, providing lower superelevation considering coefficient of friction is safe, i.e.centrifugal force is counteracted by superelevation and coefficient of friction . IRC suggests following design procedure:

Step 1

Find $e$ for 75 percent of design speed, neglecting $f$, i.e $e_1=\frac{(0.75v)^2}{gR}$.

Step 2

If $e_1\le0.07$, then $e=e_1=\frac{(0.75v)^2}{gR}$, else if $e_1>0.07$ go to step 3.

Step 3

Find $f_1$ for the design speed and max $e$, i.e $f_1=\frac{v^2}{gR}-e=\frac{v^2}{gR}-0.07$. If $f_1<0.15$, then the maximum $e=0.07$ is safe for the design speed, else go to step 4.

Step 4

Find the allowable speed $v_a$ for the maximum $e=0.07$ and $f=0.15$, $v_a=\sqrt{0.22gR}$ If $v_a\ge v$ then the design is adequate, otherwise use speed adopt control measures or look for speed control measures.