vertical curves and horizontal spirals, friends or ennemies?

[TheParp]

Hello,
we are currently designing a mainline for an american company.

On some horizontal curves we have to include spirals.
In the AREMA Practical guide, there is a mention saying that it is strongly recommended not to put a vertical curve in the spiral element of an alignment if we are using the new method to calculate the vertical curve lenght.

-First, my curiosity would like to know why (why in the curve its ok and not in the spiral)

-Second, is "strongly recommended not to" can be interpreted as "do it if there is no other alternatives"

 

[chicopee]

I am confused with your statement. Are your referring to a spiral curve on a horizontal plane, or a spiral curve on a steep grade, or just a circular curve on a steep grade that would resemble a portion of a corck screw? To me, a spiral could be a curve similar to the shape of the cochlea within the inner ear or the shape of a spiral stair case or a cork screw.

 

[TheParp]

When you have an horizontal curve composed of this : tangent-chlotoid-curve-chlotoid-tangent on an horizontal plane, why does the AREMA "strongly suggest" not to put a vertical curve in the chlotoid portion of the horizontal curve?

 

[dik]

Might be the geometry gets a tad confusing as well as any distortion to the rail.

Dik