Vertical Curve Design

I'm not sure about what's in the book, but are you saying it's not curved now as it exists?

It is typical to approximate the existing profile with 50' segments.

Driveway design is typically controlled by local ordinances. Any driveway within Sate or County right of ways are controlled by their design standards and sometimes for a specific distance beyond..

Are you having problems with the math?

Paul in PA, PE, PLS

Dan Patterson, post: 381817, member: 1179 wrote: I'm not sure about what's in the book, but are you saying it's not curved now as it exists?

It is curved, but not to any shape that any uniform vertical curve will fit well enough to call it "conforming". The road was built 100 years ago and has had a lot of repairs since.

I'll try VPI's every 50' to see if it meets the state design manual guidelines (which are usually overly conservative).

I was just trying to keep the design conforming with AASHTO if possible.

Pardon me if I'm not seeing this through your eyes. Your design effort ends at the point of conform, i.e. you do not and should not try and make the exist meet geometric standards, Exist is what it is, exist. You can depict it however the geometry is best depicted such as points of grade changes or by best fit curves. As a designer your goal is to have the new alignment profile meet the exist profile at the same grade. Determine the grade of the exist at the points of conform and use those as the grades of the design ends. Attempting to make the exist profile adhere to geometric standards is not part of the project scope. Exist is what it is, if it is way out of geometric standard then you might discuss expanding project scope, but generally the project approval process took into account the limits to be designed and brought into 3R standard..

Paul in PA, post: 381821, member: 236 wrote:
Are you having problems with the math?
Paul in PA, PE, PLS

Funny you should say that...I spent the morning solving for Lmin. based on SSD and couldn't figure out why my answers were not matching the AASHTO charts. The answers appeared to be way too large. Took me awhile to wake up enough to see that when you are dealing with small grade differences and short vertical curves, there becomes a point where the formula just isn't accurate because the line of sight is over the apex and your answers are negative. Then you fall back to L=3V.

Kind of more of a learning exercise than anything. I tend to forget that many of these formulas are approximations that are only useful under specific conditions.

clearcut, post: 381832, member: 297 wrote: Pardon me if I'm not seeing this through your eyes. Your design effort ends at the point of conform, i.e. you do not and should not try and make the exist meet geometric standards, Exist is what it is, exist. You can depict it however the geometry is best depicted such as points of grade changes or by best fit curves. As a designer your goal is to have the new alignment profile meet the exist profile at the same grade. Determine the grade of the exist at the points of conform and use those as the grades of the design ends. Attempting to make the exist profile adhere to geometric standards is not part of the project scope. Exist is what it is, if it is way out of geometric standard then you might discuss expanding project scope, but generally the project approval process took into account the limits to be designed and brought into 3R standard..

Sound advice - thanks.

I really kind of got off task but learned a few things along the way.

If there there were design speeds 100 years ago, they'd probably be about 20 mph, right? Trying to force an existing road to current standards could create other problems with adjacent driveways and grades. Improvements could actually have adverse consequences. Show what exists, but be weary of proposed improvements.
On a side note, I once designed improvements to an old cart path at the recommendation of the town at their local road standards (adopted from the state design manual for highways). The road less traveled became a shortcut for highschool kids. I'd be willing to wager the long time residents of that road didn't care much for my work.

Andy,
What is the design speed for that alignment?

Carefully, very slow and meticulous.

Where I used to work you would have to bring an extra lunch, because that would be a scheduled double shift.

Paul in PA

In response to your question:
....does the AASHTO Green book (A Policy on Geometric Design of Highways and Streets) give any guidance as to when the need for vertical curves are triggered?

With reference to the 1984 edition to which I presently have access, the question in general is addressed in Chapter II, Design Controls and Criteria. Discussion about vertical curves is provided in Chapter III, Elements of Design in the sections beginning with Vertical Curves, General Considerations. A later section titled "General Controls for Vertical Alignment" comes closest to directly answering your question. However, reading should continue on into the section "Combination of Horizontal and Vertical Alignment."

In the AASHTO manual, I see no simple answers to "when the need for vertical curves are triggered." Thorough study and experience are a requisite. Hopefully, a competent person will be assisting you with design?