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The design speed for two-lane road is 80 kmph. When a design vehicle with a wheel base of 6.6 m is negotiating a horizontal curve on that road, the off - tracking is measured as 0.096 m. The required widening of carriageway of the two lane road on the curve is approximately _______
1. 0.55 m
2. 0.65 m
3. 0.75 m
4. 0.85 m

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Correct Answer - Option 3 : 0.75 m

Concept:

Off-tracking: 

The phenomenon of the rear wheel not following the front wheels while negotiating the curve is called off-tracking. It depends on

  1. Length of wheelbase
  2. Turning angle or radius of the curve.

\({ off -tracking}\; = \;\frac{{{l^2}}}{{2R}}\)

The extra widening (We)of pavement is given by

We = Wm + Wp

Wm = Mechanical widening, Wp = physiological widening

Mechanical widening: The widening required to account for the off-tracking due to rigidity of wheelbase is known as Mechanical widening. It can be calculated as:

\({W_m} = \frac{{n{l^2}}}{{2R}}\)

n = Number of lanes

l = length of wheelbase = 6.1 m

R = Radius of circular curve

Psychological Widening: Extra width of pavement provided for psychological reasons such as overhangs of vehicles, greater clearance for crossing, etc is known as Psychological Widening.

\({W_{p}} = \frac{V}{{9.5\sqrt R }}\)

V = Design speed (kmph) 

Calculation:

Given - l = 6.6 m, n = 2, V = 80 km/hr

off-tracking = 0.096 m

\({0.096}\; = \;\frac{{{6.6^2}}}{{2\times R}}\)

⇒ R = 226.875 m

\({{\rm{W}}_{\rm{e}}} = 2 \times 0.096 + \frac{{80}}{{9.5 \times \sqrt {226.875} }}\)

We = 0.752 m

 

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