How to interpret a sewer profile drawing properly

[vt001]

This may seem like a bit of an embarrassing question, but I am a bit confused.

So I understand that
A) on a sewer profile drawing the length between manholes is measured from manhole centre to manhole centre right.
B) And the inverts of the pipes coming into and out of the manhole are also shown at the centre line.

My confusion is arising from
1) why isn't the actual length of the pipe ie from inside of the manhole wall to inside of the next manhole wall shown?
2) doesn't this length from centre to centre mess up your grade calculation? You now have a longer length. Say on a 2m diameter manhole your actual pipe length is 2 m shorter. When I look at profile drawings the grades don't seem to reflect this ie they take a centre to centre length in the calc.

Am I missing something obvious here and confusing myself for no reason? I tried searching the web, but haven't really been able clear up my confusion and asking a colleague what seems like such a basic fundamental question is kind of embarrassing.

 

[LittleInch]

I don't think you're missing anything, but the key to this is how much difference does it make and is it good our bad? I don't know what your average distance between manholes is, but 2m would not seem to make a huge difference if it's say 200m between them. Having a slightly longer pipe for your grade calc means that in reality the grade would be very slightly steeper, so a good thing.



My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[vt001]

I agree that on long lengths at flat grades it probably doesn't make much of a difference especially given construction tolerances, but on shorter lengths it might. In my mind it's confusing that what is being shown on the profiles isn't what is actually being built when it not that difficult to calculate "properly". Am I just being a bit pedantic about this?

 

[LittleInch]

Over a set of chambers it could add up, but still only be a few cms. Do two versions next time, one the standard way and one your way and see if it makes an appreciable difference.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[dik]

vt001... pretty much the same here, except I usually stipulate the inlet and outlet inverts at catchbasins and manholes... as everyone noted the difference in length is likely only a 'smidge'...

Dik[ul]
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[cvg]

bid items for pipe generally are from center of manhole to manhole or from center of valve or other fittings, so the length shown on the plans is for bidding purposes.

actual invert of the pipes at the manhole should be shown since there is generally a requirement to "drop" through the manhole. this drop compensates for head loss through the structure.

 

[chicopee]

Bidding is the key word to your OP. Bidding contractors will not spend the time to precisely figure out all the details that they would face on job sites if they are successful in their bids. Anyway, it is always better to have a little more than a little less when you are on job sites constructing things. If you are short on your inventory while you are in construction, that will mean delays and obviously greater expenses than if you have a little more inventory. Contractors are always mindful of penalties if you don't finish on time and always mindful of the possible bonuses if you finish ahead of schedules.

 

[Twinkie]

I was always taught, you can always cut the pipe, you can't make more ...

I always label and do my calcs on center to center

 

[gbam]

Our local wastewater agency has a requirements to show both lengths on the profile; ie. 100LF (95LF), with the "parrans" being the length of pipe for reviewer to verify slope. I dont see why you couldn't do something similar as long as you clarify oit in the general notes or notes on the profile sheets.

 

[LincolnPE]

Here's an explanation from one of the state DOT manuals (regarding storm sewer lengths) that may be helpful:

Pipe slope is calculated using the difference between the inlet and outlet flowline elevations
divided by the horizontal distance measured from inside wall of the upstream structure to inside
wall of the downstream structure. The actual required length of a pipe for construction is
calculated along the slope of the pipe. To make measurement easier for payment purposes, the
Department measures along the ground from center of structure to center of structure. Therefore,
the measured payment length is often different from the required installation length. The designer
should ensure that pipe lengths used in the design provide the appropriate pipe design slope
value, which in turn is used to determine the pipe design velocity and capacity.[/i]

 

[beej67]

The short answer is this:

If you go back and recalculate your slopes based on actual lengths instead of center-to-center lengths, all your slopes will go up a little bit, which increases their capacity a little bit, so the error is a conservative one. Using center-to-center lengths also means you don't have to recalculate all your hydraulics whenever you change manhole sizes.

That said, I've seen software do this both ways. Eagle Point back in the 90s used to actually calculate your pipe lengths for its pipe charts instead of going center-to-center. Worse, it didn't have an option (back then anyway) to switch back to center-to-center calculations. It was a huge headache.

Rule one as always is to read your software documentation.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Althalus]

Ditto to what beej67 said.

Here's what happens in the field:

The surveyor calculates the required pipe elevations from the drawings. He marks the inverts at the manholes. But he only places the first stake at a reasonable interval away from the manhole (a minimum of 5ft for example). Most of the pipe is laid out according to the slope and elevations indicated on the drawings. But the two end segments by the manholes are tilted slightly steeper to meet the invert elevations shown on the drawings.

So, short answer, yes, you are being pedantic. That is a good thing to be when you're an engineer. But you have to know when to accept the "not quite right" method is actually a better one.

 

[LincolnPE]

In sewer design, there is not only a capacity criterion to consider, there is a velocity criterion that needs to be met. The design flow is required to be above a certain minimum velocity, and below a certain maximum velocity. Typically min 2 ft/s and max 10 ft/s for sanitary for example.

The engineer needs to use the correct length and design slope to ensure these design parameters are met. Large sanitary trunk lines can go down to some very, very flat slopes. We need to get the surveyors the correct slope to use.

Our Civil software now can easily label on our plan and profile sheets the 2D pipe length from inside edge of structure to inside edge of structure that is used in hydraulic design.

It can also label the 3D length for payment. For example, WSDOT states that it shall pay on the pipe length measured along the pipe slope invert.

 

[concua]

http://www.wsscwater.com/home/jsp/content/pdmpart2.faces

Part Two - Sewer Design Guidelines (WSSC)

 

[eea]

I was taught that sanitary sewer was center to center because structures were often originally brick built up off of / around the pipe with the crown exposed to make a structure. There should be a smooth gradient across the main sanitary sewer at a MH structure, hence paved inverts. Now that precast MH's are all the norm, the pipe runs are starting and stopping at the structure, but many details still allow for block or brick structures.

Storm sewer on the other hand is usually measured inside face to inside face of the structure. A slight sump is often tolerated, thickness of the pipe sitting on the base which helps with sediment similar to a catch basin. I have not seen many agency details requiring a paved invert in a storm structure.

I would say that this is negligible, expect when it comes time to pay the contractor. Check the agency's standard specifications and show the lengths according to the measurement for payment.