highway design ce 424...r = vehicle's front outer wheel drawn the radius (the radius of the...
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Highway DesignCE 424
Cenk Ozan, Ph.D.
Adnan Menderes University
Engineering Faculty
Civil Engineering Department
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Turning on Curves
-It is desired to travel at a speed on curves as much as on
alignments.
-But, small radius curves must be designed especially in
mountainous regions .
-Horizontal curve radius depends on vehicle types, vehicle
dimensions and speed.
-While radius of curvature is endless at beginning of curve
(on alignment), radius of curvature constantly decreases
to curve radius on the curve.
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Vehicle Stability on Curves
A vehicle which moves from alignment to curve is
exposed to centrifugal force. Centrifugal Force has
skidding effect and turning over effect.
Rv
gWF
2.
W= weight of vehicle (kg)
V= speed of vehicle (m/sec)
R= radius of curve (m)
g= acceleration of gravity
F= centrifugal force (kg)
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Forces Affecting a Vehicle on Curve
h= height of center of gravity of
vehicle from the ground (m)
P= side friction force which balances
centrifugal force (kg)
me= coefficient of side friction
P= me.N = me.WRg
vW
Rv
gW
WP
e .
22
.m
(it is assumed that P= F )
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Vehicle speed km/h, g= 9.81 m/sec2
Rv
e .4,127
2m
Skidding speed on the curve without superelevation
ReskidV .3,11 m
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Moment balances turning over effect of centrifugal
force on curves.
Moment = W.(e/2)
2..e
WhF 2
...2
eWh
R
v
g
W
heR
turnoverV
mgvv
.0,8
2sec/81.96,3
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ehe
turnoverV
skidV .
0,83,11 m
ehe
turnoverV
skidV .
4,1m
Example: me= 0,40; h= 0,70m and e= 1,80 m
56,070,0.40,04,180,1
turnover
Vskid
V
Accidents on the curves occurs to skid rather than turning over.
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In practice, superelevation is designed to balance skidding
and turning over effects of centrifugal force which affects
negatively vehicle’s stability.
Critical speeds cause skidding and turning over on
superelevation with curves:
mm
tge
tgeR
skidV
.1
)(3,11
*11.3* e RVskid
m
İf Tg= 0
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2.
)2
.(3,11 etgh
etghR
turnoverV
Minimum radius of curves for design speed:
).(4,127
).1.(2
)min( m
m
tge
tgepV
skidR
)2
..(4,127
).2
.(2
)min( etgh
tge
hpV
turnoverR
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Lateral Accelerationand
Rate of Change of Lateral Acceleration
F’ = F- W.tg
tggmR
vmpm ..
2..
p= lateral acceleration
(m/sec2)
V= vehicle speed (m/sec)
m= mass of vehicle (kg)
d= superelevation (%)
R= radius of curve (m)
d= tg=d/100
dR
vp
dg
R
vp
.0981,0.96,12
2
100.
2
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Lateral acceleration occuring on curve the
change in unit time is called to Rate of Change
of Lateral Acceleration (Sademe).
t= L/v or 3,6L/v
L
dv
LR
vp
L
vd
L
v
R
vp
t
p
dt
dpp
.7,36
.
..7,46
3'
.6,3..0981,0
.6,3.
.96,12
2'
'
Rate of Change of
Lateral Acceleration is
felt from P’= 0,3
m/sec3.
Max. Rate of Change
of Lateral Acceleration
is 0,6 m/sec3.
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Minimum Radius of Curve
)(127
2
min
.127
2
.
2
..
2.
.
2
ed
pv
R
ed
R
v
etg
Rg
v
tgRg
vee
tgRg
v
m
m
m
mm
As can be seen in next
formula, Centrifugal force
is balanced superelevation
and side friction
Side friction coefficent on
dry ways: 0,40-0,50.
If speed increases, it
decreases.
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Exchange rate of side friction coefficient
Design Speed
(km/h)
50 70 90 100 110 120
Side Friction
Coefficient
0,16 0,15 0,13 0,13 0,12 0,12
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Superelevation
If lateral acceleration is balanced with superelevation,
lateral acceleration means zero. It is called theoretical
superelevation.
It is obtained that lateral acceleration formula is
equalized to zero.
Rpv
pvvanddR
vp
2.00786,0
teotionsupereleva
0.81,9.96,12
2
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Disadvantages of Excessive Superelevation
-If vehicles stop on the curve or travel slowly,
vehicles can skid or turn over to center of curves.
-Thus, Max. Superelevation is %8-10.
-It is kept lower in snowy, frozen regions and
urban highways
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superelevation
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In fact, lateral acceleration is balanced with
superelevation and side friction. In this case,
superelevation;
Rpv 2
.00393,0tionsupereleva
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Superelevation Formula in Turkey
2superelevation 0,00443.
vpR
Max. superelevation: %10
Vp= design speed (km/h)
R= radius of curve (m)
LR= length of Raccordement
LR(min)= 45 m
30,0354*
vpLR R
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Superelevation Applications
There are three basic methods for developing superelevation on a crowned pavement leading into and coming out of a horizontal curve.
In the most commonly used method, case I, the pavement edges are revolved about the centerline. Thus, the inner edge of the pavement is depressed by half of the superelevation and the outer edge raised by the same amount.
Case II shows the pavement revolved about the inner or lower edge of pavement, and case III shows thepavement revolved about the outer or higher edge of pavement.
Case II can be used where off-road drainage is a problem and lowering the inner pavement edge cannot beaccommodated. The superelevation on divided roadways is achieved by revolving the pavements about the median pavement edge. In this way, the outside (high side) roadway uses case II, while the inside (low side) roadway uses case III. This helps control the amount of “distortion” in grading the median area.
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Case I
Case II
Case III
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Superelevation Applications
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Superelevation Applications
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Superelevation Applications
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Superelevation Applications
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(1/3)Ld
(1/3)Ld
Superelevation Application Without Transition Curve
TO
TF
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Transition Curves
Vehicles which move from alignment to curve are
affected by centrifugal force. That centrifugal
force is balanced with superelevation and
transition curves which are placed between
alignment and curve.
Due to transition curve, centrifugal force effect is
distributed equally along transition curve and removed at
beginning of curve.
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Conditions about Transition Curves
-Vehicle can be travelled on transition curve with a speed
as much as on alignment.
- When vehicle steering is turned constant angular speed,
vehicle should reach the largest rotation angle at the
entrance of curve.
- Transition curve should be tangent to alignment at the
beginning (radius of curvature is endless) and should be
tangent to circular curve at the end ( radius of curvature
is equal to radius of curve).
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K= curvature
Case of without transition curve
K
Transition Curve
Curvature change on horizontal curves
Curve
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L= length of transition curve
ÜA= start of transition curve
ÜE= end of transition curve
k= curvature
-Curvature at Lx distance
k=(K/L)*Lx
-Curvature at the point of ÜE K=1/r
k= Lx/(L*R)
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Transition Curve Types
• Clothoid (the most widely used in the world)
• Lemniscate
• Cubic Parabola
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Transition curves
Clothoid
Cubic parabola
Lemniscate
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Length of Transition Curve
It is desired that vehicles should not exceed certain a
value of Sademe on the account of length of transition
curve.
Vehicle has zero lateral acceleration at the beginning of
transition curve. When entering curve, vehicle reachs
v2/R value of lateral acceleration. This changing occurs
during t= L/v.
Lateral acceleration occuring on curve the change
in unit time is called to Rate of Change of Lateral
Acceleration (sademe).
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LR
v
v
LR
v
p.
32
'
Speed km/h
'..7,46
3
pR
pV
L
Rate of Change of Lateral Acceleration (sademe)=p’= 0,3-0,6 m/sec3
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Clothoid
It is used in high-standard roads
R.L= A2 (A= clothoid parameter)
Clothoid is actually a spiral. The spiral is the one most commonly used in
highway design. The degree of curve varies gradually from zero at the
tangent end to the degree of the circular arc at the curve end.
By definition, the degree of curve at any point along the spiral varies
directly with the length measured along the spiral. In the case where a
spiral transition connects two simple curves, the degree of curve varies
directly from that of the first circular arc to that of the second circular
arc.
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O= the beginning point of the
clothoid
P= the end point of the clothoid
X,Y= coordinates of point of P
Xm, ym= coordinates of center
point of curvature
NP= Tk= length of short tangent
NO= Tu= length of long tangent
DR= transition proportion
A point to be taken into account in clothoid tangent with the
horizontal axis is τ that point, the value of angle radians
R
L
2
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x
yarctg
X
Ytg
yxS
RYRm
yR
gYXu
Ty
kT
RYm
y
RXm
x
A
L
A
L
A
LY
A
L
A
LLX
D
22
)cos1(
cot.sin
cos.
sin.
...10.42240
11
6.336
7
2.6
3
...8.3456
9
4.40
5
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Superelevation
Applications with
transition curve
without runout
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Superelevation Applications with
transition curve (revolved about the
centerline with runout
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Travelled Way Widening on Horizontal Curves
Widening is needed on certain curves for one of the
following reasons:
•The design vehicle occupies a greater width because the
rear wheels generally track inside front wheels
(offtracking) in negotiating curves
•Drivers experience difficulty in steering their vehicles in
the center of the lane.
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R
lb
2
2
l = The distance between the front and rear axles of
vehicle (or vehicle length)(m)
R = vehicle's front outer wheel drawn the radius (the
radius of the axis of the curve) (m)
B = the amount of widening for a single lane (m)
Above equation can be valid on low speeds. But, the higher
the speed, the opposite lane violations may increase.
Therefore, widening on high speed section is more
calculated.
R
pV
R
lnb
R
pV
R
lb
.05,0
2
2.05,0
2
2
Vp= design speed (km/h); b= widening on n lanes roads
Widening
on a single
lane road
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Application of Widening
Widening increases linearly along transition curve.Widening reachs max. value at the end oftransition curve.
Widening is made two patterns;
• Center line is constant, equal widening on insideand outside edges
• After inside edge is widened, center line is shifted.
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Application of widening in transition curve case
Widening on horizontal curves
Transition curve
Transition curve
Alignment
Alignment
curve
curve
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TO
2/3Ld1/3Ld
b/2
Application of widening without transition curve case
widening
(center line is constant, equal widening on inside and outside edges )
Widening begins at the start point of length of raccordement.
Widening reachs max. value from point of TO to distance of 1/3Ld .
Center Line
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TO
1/3Ld
b
widening
Center line
2/3Ld
Widening without transition curve
(widening on inside edge)
New center line
b/2
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Application of superelevation on horizontal curve which has transition curve and widening
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Application of Superelevation with Transition Curve
• When end of length of run out ( the start point ofrun off ÜA), inside and outside edge’s crown shouldbe equal.
• From point of ÜA (the start point of run off) topoint of ÜE (the end point of run off)superelevation increases and reaches its max.value.
• Superelevation goes on its max. Value in the curve.
• At the exit curve, above operations continueadversely and reaches alignment
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Lenght of Run out (inside edge profile is determined)
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Lenght of Run out (center line profile is determined)
12
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Lhk
Profile
Ru
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Sight Distance on Horizontal Curves
obstruction
line of sight
Sight distance = S
MM
RR RR
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Sight Distance on Horizontal Curves
Sight distance is decreased by obstructions,
cut slope in horizontal curves etc.
Solution is;
• if possible, removing that obstruction,
• if not, radius should be increased or location
shoul be changed. If there is cut slope, cut slope
should be trimmed.
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object
Obstruction
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AC2= AD2 + x2
AD2= R2 – (R – x)2
AC2= 2Rx
Accepting AC= ½ S
S2/4= 2Rx
X= S2/8R
S<d
d= total length of circular curve from
TO toTF measured along its arc.
Center line
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EF= d= total length of circular
curve from TO toTF measured
along its arc
S= d+ 2l
l= (S – d)/2
On the other hand, from ACD,
ADO and EAO triangles
AC2= AD2 + x2 AD2= AO2 – (R – x)2 AO2= l2 + R2
accepting AC= 1/2S
x= d.(2S – d)/8R
(shift distance)
S<d
Center line