Hairpin Bend

Hairpin Bend In alignment of a hill road, it is frequently necessary to insert sharp turning angles with in which it is very difficult and sometime even impossible to lay out curve  by normal geometrical design. In such case a case it is preferable to round off the route by circumscribing the curve around the turning point. Such compound curves are called hairpin bend. A hair pin curve bend is located on the section having minimum cross-slope and maximum stability. It must be safe against land-slide and ground water seepage. Design parameters of hairpin bends Design speed %inimum radius of main &urve %aximum length of transition curve %aximum gradient with in bend Super elevation in curve portion

!"#m$hr   'm ()m !.)* ("*

 +ow from eometrical design, angent angent length of reverse curve / 0 r tan β/2 1here, r 0 radius of reverse curve β0 Deflection angle

 Again, A2 0 34 0 5%  portion in between reverse 7 main curve

%0 6ength of straight

hen, from ∆A82, tan β 0 82 0

9 . 0 9

::::::.. (/

A2

5%/

r tan β/2 + M

9 0 9adius of main curve 4rom trigonometric law we have, tan ; 0

! tan  $; !

:::::: !/

( - tan! ;$ ! tan ;$!

Solving (/ 7 !/

0

-m5< m! 5 9!r59/ r5!r

=ence the distance of the apex of reverse curve to the center of main curve is A8 0 830 5%/ 0 9 . cos;

sin;

6ength of main curve &r/ 0 >9? ? 0 ('"5!;-@/  

('"

otal o tal length of =airpin 3end is S 0 !&r5%/ 5&

&r 0 length of reverser curve 0 >9; ('"