Drainage manhole design

[tsli001]

Hi there:

In a project, I am ask to design a drainage manhole structure. I haven 't done it before. Is there anyone got any tips on it? I will also need to provide the structural aspect of the design, such as the re-bar, concrete etc. If there is a example will be great.

tsli

 

[JedClampett]

Best way to do it is to ask an more experienced engineer to help you out. That's how we all learn. But another way, if you're in the USA, is to go to your local government website (county, city, etc.) and see what their typcial details look like. I can't speak for foreign typical details.

 

[bimr]

One would think that you will be using a precast product:

http://www.concrete-pipe.org/pdfs1/DD_20-06.pdf

 

[tsli001]

Hi guys:

Thank you for your input. Actually I had practice in U.S. before, I even got a PE in Mass. Anyway, I'm in Hong Kong right now due to lack of job back home. I need to design a 10mX11m special manhole. Well, I think is more like a shaft or tank. So, any idea I can got some tipes. By the way, got anything in metric?

tsli

 

[Lan123]

Unless you provide more details it is very difficult to provide information

What is the expected flow through the manhole?

Is this a sewer or a rainwater MH?

Is this a junction MH

How (or through what) does the water enter and leave?

Rebar will have to be designed according to the HK structural design code

 

[tsli001]

Hi Lan123:

I got some of the information for the above mentions questions.

1. I've been asked to design two manhole for 3x2400mm concrete pipes. The first manhole flow is 10.585 cms. The second manhole flow is 9.981 cms.

2. It is for stormwater drain.

3. Both of the manhole are junction manholes.

What do you think 'JedClampett' suggestion?

Thanks for looking into it.

tsli

 

[Lan123]

First you need to understand and tackle the hydraulics and size you MH. Then as bimr suggested you need go for a precast MH rather than trying to do the structural design from the scratch. I am sure you should be able find lot of precast manufacturers in HK. I guess you guys use British Standards

Can you share with a us a plan and a profile of the proposed arrangment.

What are your min and maximum flows through each branch? You said junction MH. So are you turning you flow directions?

What are your downstream boundary conditions? Rather into what body are you discharging?

 

[tsli001]

Hi Lan123:

Sure, I can share something with you. In the attached word document is the hydraulics modelling Plan&long profile. The Red line on plan is the route that for our drainage alignment. As you can see the water from North to south on plan. On the profile you will find maximum flow within the profile. That is all the information that I can provide for our discussion. The proposed manhole is place on each connection with a 35 degree of blend. Looking forward to your feed back.

tsli

 

[cvg]

are you kidding? I highly doubt that anybody produces a 10 meter by 11 meter pre-cast manhole designed for three barrel 2400 pipes - this is a custom structure. yes, you could pre-cast it - but it sounds like this should be cast in place. I also doubt you will find any standard details.

Design for

- smooth transitions
- allow passage of large and small debris
- provide safe access
- check for surge pressures
- provide ventilation, perhaps grated cover
- provide divider walls which provides structural support to the ceiling and reduces turbulence
- works better with rectangular shapes than it does with circular pipes

 

[tsli001]

Hi cvg:

Thank you for a engineer understand my situation. Thats why I have been ask to 'Design' Thats why is a insituate pour . That why why in the beginning I said is more like a shaft or tank. So, I will like to know did anyone got any idea or any helpful link to design it? Again, most of my experience in US is drainage. I have not done structural before.

Come on guys, help me out. Dont let the others think the PE from US can't do anything.

tsli

 

[JedClampett]

If you have the size and the elevation of the structure, you need to figure out the shears and moments in the walls. If the structure is buried, soil pressures are likely to control (as opposed to internal water pressure). If you have a soil report, use the at-rest pressure. If you don't and you're above the water table, use about 60 pcf. Below the water table, use 100 pcf. Add the overburden pressure plus a two foot surcharge to simulate vehicles.
You can model the walls as plates using computer software or use The US Bureau of Reclamation "Shear and Moments on Flat Plates" or something like that. Start with 12 inch walls as a minimum with two layers of bars each way. Make sure all the corners and the base slab can develop the moments from the walls. Design the one largest wall and use the same reinforcing for all the walls. The top plate needs to be designed for the soil over it plus a vehicle surcharge.
Don't let anyone tell you these are easy structures. There's about a million things to think of (maybe ten) and go wrong. You need to get some "go-bys" to use as a starting point to make sure you're using good standard practice. There's a lot I didn't discuss due to lack of time and space.

 

[tsli001]

Thanks. JedClampett

 

[Lan123]

Can't open the file

 

[tsli001]

Sorry about that folks let me re-upload a PDF file for it.

tsli

 

[cvg]

there are several good manuals on the USACE site:

http://140.194.76.129/publications/eng-manuals/

specifically, try EM 1110-2-2007, -2104, -2502 and -2902

 

[gbam]

tsli001 -
I could not view your PDF; but, I have designed a number of custom vaults like you mention, none this large however. I typically use standard details for retaining walls to start out. I then have a structural engineer finalize the design for the top slab and pipe connections. An open junction structure is easier to maintain but not as aesthetically pleasing. I also will typically use non-shrink grout the shape the benching within the vault. The length and width of the vault is based on the minimizing hydraulic loss.

Do you have standard details available in your area? If not do a google search for DOT Standard Details in the USA. There should be quite a few.

 

[Trent2003]

Why don't you just go with a precast manhole? All you have to do is get the pipe inverts that connect to it and have the shop drawings reflect those inverts. I guess you could design your own but why. Look up the design standards for your state and go from there. It's quite simple. the most common are either a J-8 or a P-8 manhole. Start with precast companies. They have tons of approved precast manholes with ton's of shop drawings. Don't design it, put the responsibilty on the precast producer!