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Grading Contours not true to slope values

52 REPLIES 52
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Message 1 of 53
SDcivil
2858 Views, 52 Replies

Grading Contours not true to slope values

I am creating some simple grading objects from feature lines.  I am targeting my existing ground surface with a 2:1 fill slope.  My resulting surface shows me 5' contours, whcih I think should all be spaced out at constant 10' offsets, but they are not.  When I measure the offsets I get different vaues ranging on 10.2' to 9.7' (depeding on the section of the feature line I am looking at).  Anyone know a setting to make the slope build properly with a true 2:1 offset between contours?  What good does this grading tool do if you can't get a properlty constructed slope?

52 REPLIES 52
Message 21 of 53

Most typical sections would be constructed perpendicular to the base line. If 2:1 is exceeded in that direction, the onus is on the designer to corect the typical section to provided for the inevitable skewed slope exceeding maximum.

 

 

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People)

HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
12 GB Ram


Note: Its all Resistentialism, so keep calm and carry on

64 Bit Win10 OS
Message 22 of 53
wfberry
in reply to: SDcivil

I am on the CADvisers side in this.  A 2:1 slope on a typical highway project is measured perpendicular to the direction of the roadway.  Send a knowledgeable surveyor to set slope stakes and he sets them based on perpendicular distance from his hinge point on the edge of the slope.

 

Bill

 

Message 23 of 53

The OP won't post his drawing with the example, but I suspect CADverse is correct, he isn't building his 2:1 slope correclty, but adamantly believes that he is. I'm done looking at this until he responds with his file.  Still, there probably is room for improvement with contour construction, but I can't see how such a miniscule deviation would matter in the case of land development. Most state and local regulations I believe allow some deviation in construction.  If something needs to be that precise I recommend the OP to create the points himself and send those to survey; good luck constructing it that accurately.  I've never had an asbuilt completed that didn't show at least a tenth or two in deviation.


"Very funny, Scotty. Now beam down my clothes.
Message 24 of 53
fcernst
in reply to: SDcivil

Right it's simply a Convention matter... Like Ms. Mercier said long ago, Gradings (and Corridors) construct their Daylight projections perpendicular from the Feature and the contour output is a function of that Convention.

 

So, it's not "wrong" per se. It's correct for its Convention.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 25 of 53
BrianHailey
in reply to: SDcivil

Another thing to think about, why is the requirement 2:1? Well, it's because 2 is a nice easy number to remember. Why not 1.94:1? Why not 2.37:1? Did someone actually go out and calculate that in this particular situation, the optimal slope for this design, based on good engineering judgement, life and safety considerations, cost of the project, ease of constructability, is exactly 2:1?

 

Absolutely not!

 

Instead it probably went something like this, "I've used 2:1 in the past in areas like this and it seems to work pretty well, we should probably use 2:1 again in this area."

 

So, ask yourself this, why is slightly steeper then 2:1 in this area a bad thing and the answer is most likely, "because the city/state/county/<insert other reviewing agency> said we can't go steeper than 2:1".

 

I had the same type of thing come up when I was doing design for some utilties. The design criteria said that the sanitary sewer and the waterline had to be a minimum 10' apart. In my design, for about 2', they were only 9.5' apart.

 

Who came up with this magic number of 10'? Again, it's a nice easy number to remember and is close to (error on the side of caution) what the actual minimum distance between the two utilities actually should be.

 

And, yes, I ended up redesigning the utilities.

Brian J. Hailey, P.E.



GEI Consultants
My Civil 3D Blog

Message 26 of 53
Joe-Bouza
in reply to: BrianHailey

Better yet add some coordinates to those contours to get extra close Smiley Wink

 

 

"All Contours are lies"; all it take is one look to see - flip a face for example

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People)

HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
12 GB Ram


Note: Its all Resistentialism, so keep calm and carry on

64 Bit Win10 OS
Message 27 of 53
fcernst
in reply to: BrianHailey

2:1 max slope is definitely per Code (IBC, IRC, HUD) that US jurisdictions follow. 

 

  • Must check steepest slope for max criterion compliance: The steepest path on a slope is perpendicular to contours.

In that regard, I acknowledge the OP's original concern of the use of C3D projection grading tools by his staff, if they do not understand the projection grading convention. 

 

 

 

 



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 28 of 53
BrianHailey
in reply to: fcernst


@fcernst wrote:

2:1 max slope is definitely per Code (IBC, IRC, HUD) that US jurisdictions follow. 


Ok, so, what's my slope? 2:1. I label it 2:1, not 2.0:1 or 2.00:1 so, if I have a slope of 1.6:1, that rounds to 2:1, right? Smiley Wink

Brian J. Hailey, P.E.



GEI Consultants
My Civil 3D Blog

Message 29 of 53
fcernst
in reply to: BrianHailey

Regarding that,  I would not want to give the other side's attorney any opportunity to demonstrate the design is below 

code in deposition.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 30 of 53
Joe-Bouza
in reply to: fcernst

Wow ! So now we need a projection grading that finds the first contour then turns to be perpendicular to it?

 

Add more coordinates!

 

The 2010 maintenance code fro ADA forces a maximum 2% cross slope ... Actually 1:48 which theoretically is 2.08%, but reviewers are stuck on 2%. So now when I grade an ADA accessible route that will obviously have a longitudinal slope less than 5% - 2% of that will invariable lead that some point being a tad over 2%. What's the answer? 1.9%

 

sometimes you need to adjust for code values. If ambient conditions are going to create a daylight slope to exceed 2:1 then my suggestion is to use a value slightly flatter to insure maximum is not exceeded.

Thank you

Joseph D. Bouza, P.E. (one of 'THOSE' People)

HP Z210 Workstation
Intel Xeon CPU E31240 @ 3.30 Hz
12 GB Ram


Note: Its all Resistentialism, so keep calm and carry on

64 Bit Win10 OS
Message 31 of 53
jmayo-EE
in reply to: Joe-Bouza

"Wow ! So now we need a projection grading that finds the first contour then turns to be perpendicular to it?"

 

Apparently those contours are also going to need some legal advice aas well. 😮

 

If anyine in this office or a reveiewing agency complained about contours being spaced 9.7' to 10.7' when it should be ten feet I would first need to contain the laughter, next advise them that their scale may need to be calibrated (holding in the laughter once again) and then remind them that the contours are only accurate to 1/2 the interval so with +/- 1 foot, 9.7' and 10.7' are really 10 to 11 feet and I meet the grading criteria.

 

🙂

John Mayo

EESignature

Message 32 of 53
fcernst
in reply to: jmayo-EE

John you have to keep up with us now...  It's not a question about contour construction. The C3D contours are correct as constructed. That's been established. Grab some coffee.

 

It's a question of whether the designers are aware the projection slope used in the C3D routines does not necessarily determine the surface slope. The OP mentioned in Section we typically  show a 2:1 max and the intent is to not exceed that for surface slope per code, but in many cases the resultant surface slope is steeper than that.

 

Hailey had a question about slope labeling precision not contour construction. 

 

 

 

 

 

 

 

 

 

 



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 33 of 53
jmayo-EE
in reply to: fcernst

Oh Fred my laughter from this thread woke me. I understand exactly what is being discussed. I am just not sure if the issue is more funny than the indignation. 🙂

John Mayo

EESignature

Message 34 of 53
fcernst
in reply to: jmayo-EE

??? Your post talks about contours not being accurate. 

 

The legal I'm talking about, is the expert demonstrating for the other side, that the design surface slope exceeds code.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 35 of 53
troma
in reply to: Joe-Bouza

Joe mentioned a grading 'finding the first contour and then turning perpendicular'.  Sounds awkward of course.

But in message 14 on this thread, Brian reminded us how to add vectors.  Kudos to Brian.

 

So, what is really needed (or at least wanted by some) is the ability of the grading slope to subtract vectors.

 

Let me put it to you this way:

Say, within the grading settings there was an option you could turn on and off, lets call it "Remove Parent Object Slope from Grading"

When turned on, this setting would subtract the vector of the slope of the parent feature line from the required slope of the grading; thus resulting in a true slope of what the user inputs.

 

Would you object to such an option?

Would you use it?  Why, or why not?


Mark Green

Working on Civil 3D in Canada

Message 36 of 53
Jeff_M
in reply to: troma

Just for the sake of stirring the pot, this topic was covered quite well.....7 years ago.

http://forums.autodesk.com/t5/AutoCAD-Civil-3D/Create-slope-perp-to-contours/m-p/1849529#M34354

 

I had thought there was another, similar, thread that James Maeding had started, but I could not locate it.

Jeff_M, also a frequent Swamper
EESignature
Message 37 of 53
fcernst
in reply to: troma

No, because it would be utterly meaningless. This thread should have been over after Message #3:

 

Is your feature line sloped? Grading occurs perpendicular from the feature line and not from the contours.



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 38 of 53
fcernst
in reply to: Jeff_M

This doesn't really stir anything up. Engineers need to evaluate the surface gradient (normal to contours) for geotech slope stability and code items. Code items and design criteria based on free body diagrams of people and cars sliding around.

 

Code items such as ADA, handicap parking areas can not exceed 2% in any direction (resultant), in Colorado we can't have greater than 4% in any direction (resultant) in parking lots due to ice. 

 

Surveyors and contractors want offset staked. Engineers have to make sure everything is safe when things are offset staked.

 

What's to stir up?



Fred Ernst, PE
C3D 2024
Ernst Engineering
www.ernstengineering.com
Message 39 of 53
jmayo-EE
in reply to: fcernst

I agree Fred and forgive me for being rude before. If the model exceeds the max slope I just back down the grading criteria from 1:2 to 1:1.9 and move on. If contours seem spaced to tight I'll do the same. I might even just through a txt string into a profile, section or plan calling out a max. slope ignoring the model or without modeling anything. We are constrution the information on the plans not the model.

 

 

 

John Mayo

EESignature

Message 40 of 53
Neilw_05
in reply to: SDcivil

I say, slap a slope label on the plan view and add a disclaimer not to scale the drawing. Let the contractor take the liability. Smiley Wink

Neil Wilson (a.k.a. neilw)
AEC Collection/C3D 2024, LDT 2004, Power Civil v8i SS1
WIN 10 64 PRO

http://www.sec-landmgt.com

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