pipe trouble on a friday afternoon

[Hidden_Brain]

can someone please set me straight on this one? please see attached screenshots. i have the upstream structure rim elevation at 33.4. i want the outgoing pipe to have a 1m cover, so i have the pipe start crown elevation at 32.4 (33.4-1). i have a 300 mm pipe with a 7 mm wall thickness. i also have a 0.3m sump for the structure.

so, the invert out of my pipe should be

= rim-cover-top pipe thickness-inner diamater

=33.4-1-0.07-0.3=32.03

Civil 3D reports it as 32.1 (33.4-1-0.3)

the sump elevation of my structure should be

=rim-cover-top pipe thickness-inner diameter-bottom pipe thickness-sump depth

=33.4-1-0.07-0.3-0.07-0.3 = 31.66

Civil 3D calculates it as 31.8 (33.4-1-0.3-0.3)  

it seems like it is neglecting the pipe wall thickness (7mm) in both cases. why would it do that? am i missing the obvious? running Civil 3D 2011 SP1 on a Win 7 64 bit desktop with 8 GB RAM. any insight is very appreciated.                                                        

[Neilw_05]

I can't say whether C3D is designed to consider pipe wall thickness, but I have to ask, what is the scenario that requires the depth to be within a tolerance of the pipe wall thickness (i.e. 7 mm in your example)? Can anyone build it that close?

[Hidden_Brain]

good point Neil. i would still like to know if Civil 3D considers the pipe wall thickness or not, or if i am actually doing something wrong in my workflow.

[g_k50]

Consider that the pipe crown is the inside top of the pipe and not the outside top of the pipe.

[Hidden_Brain]

g_k50,

i am not sure if i am following you. are you saying that Civil 3D treats the inside top as the pipe crown, or are you suggesting that i use the pipe inside top as the crown for my calculations? is that the right approach?

<<editing the original message to make it easier to read>>

in my example:

rim=33.4

desired cover = 1

desired sump depth=0.3

in the pipe properties, i type in my desired start crown as 32.4 (=33.4-1)

based on this, civil 3d should calculate the start invert as 32.4-0.07-0.3= 32.03. it actually calculates as 32.4-0.3= 32.1 (ignores wall thickness). 

the correct elevation for my structure sump should be= 32.03-0.3= 31.73

civil 3d calculates the sump elev as=32.1-0.3= 31.8. again, not quite correct because it ignored the wall thickness previously. 

am i getting the concept right? can others please weigh in?

[andrewpuller3811 · Accepted solution]

Not to be rude, but your numbers aren't right.

7mm = 0.007 not 0.07

So pipe invert = 33.4-1-0.007-0.3 = 32.093 (if rounding is 1 decimal place then 32.1)

and sump elevation of structure = 33.4-1-0.007-0.3-0.007-0.3 = 31.786 (if rounding is 1 decimal place then 31.8)

[Hidden_Brain]

oh well, this is embarrassing....lol!

i cannot believe i had that math wrong (0.007)!!! you were not rude at all Andrew, instead very helpful. thanks so much for steering me in the right direction. now i can get back to enjoying my weekend 

[Neilw_05]

For those of us who work in imperial units, 7mm/25.4 mm/in=.2755in/12 = .023 ft. A very small tolerance indeed.