Superelevation of highway | Maximum Superelevation | Design of Superelevation (as per IRC guidelines)
Superelevation of highway | Maximum Superelevation | Design of Superelevation (as per IRC guidelines)
INTRODUCTION :
Superelevation (e)
When a vehicle travels in a circular path or curved path, it is subjected to an outward force which makes a vehicle to overturn and skid due to Centrifugal force. To overcome this force and for safe travel of a vehicle, the outer edge of the road is raised above the inner edge. This is known as superelevation or banking of road.
v2
e + ft = gR
e = Rate of superelevation
ft = Design value of transverse or lateral friction
coefficient (0.15 as per IRC guidelines)
v = Design
speed vehicle (m/s)
R = Radius
of the horizontal curve (m)
g = Acceleration
due to gravity = 9.8 m/s2
Maximum
Superelevation
In order to account for mixed traffic conditions in India, IRC has defined the maximum limit of
superelevation (enas ) as given in Table 1.11
Recommended maximum limit of superelevation
7 % |
- Plain and
rolling terrains and in snow
bound areas |
10 % |
- Hill
roads not bound by snow |
4 % |
- Urban road
stretches with frequent intersections |
Minimum
Superelevation
From
drainage considerations it is necessary
to have a minimum cross slope to drain off the surface water. If the design superelevation works out to be
less than the camber of the road surface, then the minimum
superelevation to be provided on horizontal curve may be limited to the camber of the surface.
Thus, after elimination of the crown a uniform cross slope equal to the camber
is maintained from outer to inner edge of pavement at the circular curve.
In very flat curves with large radius,
the normal cambered section may be retained on the curves. However, in such
cases, a check is performed for negative superelevation against allowable
lateral friction coefficient.
The IRC recommendation giving the radii
of horizontal curves beyond which normal cambered section may be maintained and
no superelevation is required at horizontal curves, are presented in Table
1.12, for various design speeds and rates of cross slope.
Recommended radii beyond which
superelevation is not required
Design
Speed (km/h) |
Radius (m) of
horizontal curve for camber of |
||||
4% |
3% |
2.5% |
2% |
1.7% |
|
20 |
50 |
60 |
70 |
90 |
100 |
25 |
70 |
90 |
110 |
140 |
150 |
30 |
100 |
130 |
160 |
200 |
240 |
35 |
140 |
180 |
220 |
270 |
320 |
40 |
180 |
240 |
280 |
350 |
420 |
50 |
280 |
370 |
450 |
550 |
650 |
60 |
470 |
620 |
750 |
950 |
1100 |
80 |
700 |
950 |
1100 |
1400 |
1700 |
100 |
1100 |
1500 |
1800 |
2200 |
1600 |
Design of
Superelevation (as per IRC guidelines)
·
The superelevation is calculated for 75% of design speed neglecting the friction
(0.75v)2
e = gR
·
If the calculated value of ‘e’ is less than the specified maximum limit of superelevation
(enas ) the value
so obtained is considered as design value of superelevation.
·
If the
calculated value of ‘e’ exceeds enas then enas is considered as design value of superelevation and
developed lateral friction coefficient is verified at the full value of design speed.
v2
ft = gR − enas
·
If ft calculated is less
than 0.15, then enas is accepted as the design
superelevation.
·
If not, either
the radius of the horizontal curve has to be increased or the speed has to be restricted to the safe value va given in equation 1.6 which will be less than the design
speed.
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va = Æ’(enas + ft)gR
·
Appropriate
warning sign and speed limit regulation sign are installed to restrict and
regulate the speed to va at such curves.
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