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calypso699

Drawing Strom Drains in Profile

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When drawing a storm drain system in profile view, why is the pipe diameter shown in the vertical direction as opposed to perpendicular to the slope of the pipe? Drawing the pipes in the vertical direction seems counterintuitive since in the field when a pipe is placed along a slope the diameter is measured from perpendicular. When checking for cover/clearance, wouldn’t drawing the profile exactly how it will be placed in the field provide more accurate results...especially when the pipe slope gets steeper? I realize that profiles are generally drawn with a vertical exaggeration, but for the purpose of this discussion let’s assume the profile is drawn 1:1.



I've been racking my brain as to why this is done this way and I can't come up with a reason. Can someone please explain it to me so I can finally get some sleep :)


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For some, it is just easier to offset a line instead of drawing a line perpendicular and copy and pasting along the reference line. At a 1:1 scale, there is no real trouble, but when you start distorting the profile (lets say 40:4), then it is not as simple as drawing a perpendicular line the length of the diameter - you have to draw your reference line not just the pipe diameter, but based upon the slope of the pipe and a combination of the horizontal and vertical scales.



To compansate for this, I will just draw the pipe from invert to invert (either calculated or designed) and offset vertically at the invert the pipe diameter based on the vertical scale. Regarding the more accurate design results - I have never once met a contractor who looks at the profiles. They all just take the inverts shown on the plan and run with it. If you are < inches away from a conduit on the design, chances are the actual design diameter shown on the profile will be the least of your concerns.


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Most CAD software programs automatically draw the pipe to scale when drawing profiles. The part that gets complicated is the pipe thickness, especially when dealing with gravity fed systems. You can account for the standard pipe size & thickness, but CAD doesn't account for the bells/connections of the pipe. This is why most agencies require a certain amount of minimum clearance slightly more than what is really necessary (like 18" instead of 12").


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Did somebody say storm drain? You know who should probably weigh in on this....


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Thanks for the feedback everyone. So from what I gather the "correct" way to properly draw in the pipe diameter would be to offset parallel/perpendicular along the slope versus going in the vertical direction, but in practice drawing the profiles this way is difficult if the scale isn’t 1:1. In addition, there are safety factors for minimum clearance requirements so precision isn’t really a factor. Does that about sum it up?


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When drawing a storm drain system in profile view, why is the pipe diameter shown in the vertical direction as opposed to perpendicular to the slope of the pipe? Drawing the pipes in the vertical direction seems counterintuitive since in the field when a pipe is placed along a slope the diameter is measured from perpendicular. When checking for cover/clearance, wouldn’t drawing the profile exactly how it will be placed in the field provide more accurate results...especially when the pipe slope gets steeper? I realize that profiles are generally drawn with a vertical exaggeration, but for the purpose of this discussion let’s assume the profile is drawn 1:1.

I've been racking my brain as to why this is done this way and I can't come up with a reason. Can someone please explain it to me so I can finally get some sleep :)

Good question.

I think the main (if not the only) reason is for ease of drafting. If profiles where always 1:1 then, sure, drawing the SD pipe at true diameter might make sense. But if you are drawing with, say, a vertical exaggeration of 5, then how much would you offset the pipe invert/flow line of your 18” pipe so that it is still true diameter? See what I mean? If we wanted to always draft pipe IDs at true diameter, then everything would always have to be drawn 1:1 first, and then vertically exaggerated (unless you felt like doing tedious math). If you have an 18” pipe on a 9.37% slope at a vertical exaggeration of 5 already drawn but then decide to use a 24” pipe, all you have to do is grab your top of pipe, turn your ortho on and move it straight up .5x5=2.5 feet. Done. What would you do if you were trying to keep the pipe at true ID?

Also, moving straight up is “good enough”. For instance, a 36” pipe on a 10% slope copied up 3’ instead of offset 3’ ends up with a 2.9851’ diameter instead of 3.0’ (before vertical exaggeration). You couldn’t pick that up using a scale (even after vertical exaggeration, the difference is negligible). You’ll also notice that pipe walls are often not shown on plans, so that shows you that exact drafting precision isn’t typically a requirement for SD profiles. It’s really the flow line IEs (along with the pipe size) that matters for hydraulic calcs and construction.

I hope that helps.

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