Jobghost.blogspot.com
explains most important topics in Civil engineering for the GATE-2016
examination. Here the “TRANSPORTATION ENGINEERING-Horizontal curves ” topic is explained.
Each and every word given below is important for all your competitive
examinations. And also the GATE-2016 practice sets, Example problems, previous
year Question papers are available in jobghost.blogspot.com.
Horizontal
Curves:
Whenever there is
a turning in the road, the vehicle has to be turned without skidding or
overturning with the design speed. So the curve should be designed for the
gradual turning. Hence these are the “curves used to transfer the direction of
roads by providing gradual transformation of centrifugal force without offering
skidding or overturning to the vehicle”.
Equation
for centrifugal force:
Centrifugal force of the vehicle
R ----->
Radius of Curve
w ----> Weight
Super
elevation:
Whenever
the loaded vehicle moves with the design speed in the horizontal curve the
centrifugal force due to the vehicle load will push the vehicle off the road.
It may cause over turning of vehicle. So these forces are counteracted by
providing super elevation at outside of the curve.
Super elevation is nothing but gradual
increase of road height with respect to center or inner curve of the road to
resist the centrifugal force of the vehicle moving with design speed.
Following relation is used to design the super elevation and
co-efficient of friction of the road.
e ---->
Rate of change of super elevation
f ----->
Co-efficient of Friction
v ---> Speed
of vehicle in m/sec
(If given
in KMPH convert to m/sec by multiplying
(5/18)
Grade
compensation:
The lesser value of the above values will be taken as grade
compensation. (whichever is lesser)
Widening Of
Curve: (We)
The rear wheel
of vehicles passing the horizontal curve
will not immediately follow the front wheel. The rear wheel of the vehicle will
go off the track due to its rigidity of the axil. So the curve should be
widened to avoid the off tracking of the wheal.
There are two
factors to be considered while designing the extra widening.
i. Mechanical widening Or Off tracking
distance
Increasing the
width of the road to account the off tracking due to rigidity of wheel base is
called mechanical widening.
i.Psychological widening
Increasing the
road width for the driver’s psychological factors is actually called
psychological widening.
Widening of curve (We)
= mechanical widening (Wm)+ Psychological widening (Wps)
n-----> No. of lanes in road
l----> Length of Wheel Base of
vehicle
R -----> Radius of curve
V------> Design speed in Km/hr
If R< 50 widening provided inner side
of curve
If R> 50 widening provided Outer side
of curve
Example Problem:
1.A carriage way of 7.5 m has a co-efficient of friction 0.15
and length of vehicle wheel base is 6.2 m. The design speed of 80 Km/hr. is
allowed for the provided super elevation 1/20 then
i. What is the radius of curve?
ii. What is the total width of pavement at curve?
Ans:
Width of road =7.5 m=7.5/3.5=2
lanes (3.5 m per lane)
Friction coefficient (f) = 0.15
Super elevation (e) =1/20=0.05
= 5%
Speed (v) =
80 Km/hr=22.22 m/sec
Then the
total width of road is the sum of extra widening and the actual with of road.
Extra
widening required is
We =0.685m
Hence the total
width is = 7.5+0.685=>8.185m
Note : “Speed of
the vehicle should be in Km/hr for this equation”
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