Vertical Curves and ASSHTO

[CPENG78]

I have a private access road (driveway) serving 8 units that are to be a considerably lower in elevation than the frontage street. The city engineer has notified us to follow ASSHTO guidelines. Does ASSHTO require a vertical curve after a specific grade break? All of my quick research points to the ASSHTO greenbook but I dont have one in the office. I would put a vertical curve regardless but I would like to find out what is the minimum and what length. The slope from lip of gutter to back of walk is approximately 2% and then slope must drop as quickly as possible. I would like to maximinzse this at 15%, however it all depends on the required length of the curve and placement of the PVI. Vehicle speed to be between 5 to 10 mph. Any input greatly appreciated.

 

[civilman72]

Vertical curve design in AASHTO Green Book will not apply for such a low speed driveway.

Your main concern should be to design a vertical curve sufficient enough to safely pass larger vehicles (i.e. moving vans, pickups with trailers, etc.) through the vertical deflection without dragging or getting stuck.

I would start with a K-value of '5'. L=K*A, where A is grade break (15%-2%), so the vertical curve is approximately 65' long. You can then overlay some standard vehicles with your vertical curve/profile and check your clearances.

 

[WL123]

You also might want to consider designing for larger vehicles, oil delivery, furniture moving, etc...

 

[CPENG78]

Civilman72 / WL123,
Thank you for your input. The maximum 15% slope is based on a fire truck entering this road. However, my concern is not just the fire truck but also a low profile vehicle. Unfortunately I am very restricted on what I can do. Like I mentioned before it is a private road and I can not take in any sotrm water from the frontage road. The storm system within the property is private since the city did not want to deal with it for maintenance.

With regards to vertical curves, any publications that may have K values based on such low speeds? Most literature that I have available at the moment is for higher speeds; streets and highway design.

 

[Maury]

AASHTO has a publication "Geometric Design of Very Low-Volume Local Roads (ADT<400)". I costs about $25, but the criteria for vertical curves is based on sight distance, not vehicle clearance, so the curve length may be longer than than what you would want.

"Highway Engineering Handbook" by Brockenbrough and Boedecker quotes from Ohio DOT Location and Design Manual by showing 5% grade breaks spaced at 20'. You might try to access this source directly.

Civilman's suggestion to plot a trial curve and use vehicle templates as a check seems a good idea. You can create your own templates using wheel base dimension and minimum clearance. Ask the fire department or a moving company for help.

 

[CarlB]

Was discussed here before with pretty much same conclusions

http://www.eng-tips.com/viewthread.cfm?qid=185538&page=13