Structural Concrete Sewer Chamber 3

[EntryLevelEIT]

Hey fellas,

I'm reviewing a concrete sewer chamber in which I have no prior experience. I'm looking for some experienced input. Please view both the plan view and secion. My thoughts are as follows:

1. The design seems to combine a chamber and a riser cone manhole, not sure how typical this is but it doesn't look right to me.

2. The W8 x 31s interrupt the rebar shown in the secion, creating a cantilever effect. The W8 x 31s also leave an area just adjacent to the manhole with no support, other than the small section of concrete the would pour around the W-shape.

3. I could remove the W-shapes and heavily reinforce the area around the manhole opening.

4. What prevents the conical section from failing due to shear under a truck load. I understand they are reinforced and manufactured in those shapes. Is the best to contact the intended manfacturer for this situation.

Thanks for any help!

 

[EntryLevelEIT]

plan view

 

[cvg]

nothing wrong with the riser pipe, it should be manufactured to AASHTO standards and able to handle traffic loading. However, I would slide it over so it bears on the two walls. I prefer the manhole shaft to be in vertical alignment with a wall, it makes it easier and safer to climb in and out using the stairs unless you are an orangutan.

I would try this without the wide flanges, I have never seen a manhole yet that required this sort of reinforcement.

 

[beej67]

Is the VCP sewer over or under the storm? That box is awfully close. If it were my stamp, I'd have the sewer removed and replaced with DIP for that stretch. Especially since it looks from the angle of incidence like the VCP gets closer to the proposed 42" RCP as you head up (north?) on the plan view. If it's well deeper of the storm then maybe it's okay, but it'd still give me grief. VCP is crap, and as long as it's been since VCP was allowed as a construction material at all in my region of the country, I have to expect it's old as hell. You'd hate to have to replace it in five years with a lot of new storm drain monkeying with your ability to get at it.

Just something else to think about.

Structural reinforcement of junction boxes isn't my bag, but I do see your concerns about those steel beams cutting off the rebar. Never seen beams like that in a detail before. I would think the thing to do would be to leave the rebar continuous across the whole section, and if necessary just beef it up. Fair warning, I haven't touched reinforced concrete structural design since undergrad.



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

http://www.campbellcivil.com

 

[cvg]

vcp is crap? must be a local thing, we don't all feel that way... I've seen miles of it constructed in the last 20 or so years. I don't think I'm old as hell yet... I've seen plenty of non-vcp sewers that needed premature sliplining and well installed vcp sewers lasting 60 - 70 years with no problem

 

[bimr]

I agree with CVG's comments.

It looks like a structural engineer designed the chamber and then someone stuck a riser on top of it. There are no details on the riser but many on the chamber.

1. There are no details for the connection of the pipe to the chamber.

2. There is no flow channel shown.

3. Riser connections and gaskets are not detailed.

4. The VCP will have to be replaced if you are installing a new sewer under it.

5. The steps should be above the flow line of the sewer.

6. See the attached example.

 

[beej67]

Interesting, CVG. VCP is a big no-no down here. Been drawing up plans since the mid to late 90s and I've never seen it called out once. My experience working for three different firms has been "don't call it out, and replace it if you see it." Last time I saw VCP anywhere it was on a survey for a pipe bursting job we were doing, and it was what we were bursting.

Perhaps our municipalities down here are simply the subject of malicious marketing? I've personally got no bad experience with the stuff, because I've never been on a project where we called it out.



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

http://www.campbellcivil.com

 

[EntryLevelEIT]

I'm really only reviewing this from a structural standpoint.

cvg, I agree this should be done without the wide flanges and that the walls should be used as a bearings where possible.

bimr, If I use that detail the manhole base will need to engulf all 3 pipes which would greatly lengthen the concrete reducer cap. I'm concerned with how much load that cap would be albe to take if extended 'x' feet further from the edge of the wall.

Disregarding my particular design, does anyone have a procedure for analyzing openings in reinforced slabs and reinforced vertical walls. ACI 318-05 13.4 doesn't seem to go into much detail. If I punch a circular opening through a two-way slab reinforced T & B how many kips/ft can it take. Likewise, if I punch an opening at the corner of a manhole and the uniform load rests on the slab/wall how much liner load can it take?

 

[bimr]

If you are only reviewing this structure, just mark it up, send it back and ask for calculations and details. For example, the designer is showing not any details on the riser. One would think that not enough information has been submitted to review it.

The manhole detail was uploaded to provide you with general arrangment details, not a a substitute design.

 

[cvg]

typically heavier reinforcement and thicker slabs are used. diagonal bars are often used to reinforce around manhole penetrations. bearing the sides of the manhole shaft on two walls at the corner will also take the majority of the vertical axial load from the shaft. Otherwise BIMR hit it, ask the other engineer to prove it works. A sketch just won't cut it.

 

[EntryLevelEIT]

I intend to have the engineer prove it works, but I still need to understand the calculations when it's resubmitted. Otherwise, if it's inacurate I'll never know. That's why I posed the question above as an example.

In reality it's also more complicated than the sample because most of the walls intended to bear the load also have pipe penetrations.

 

[msquared48]

In retrospect, and from a structural aspect, the WF over the concrete wall is superfluous. In all likelihood, the wall is stiffer than the WF. Delete it.

The other WF breaks up the integrity of the top slab. Designer should have just used beam steel with a thicker slab if warranted. Delete that too.

As for the riser, the walls of the riser should align with the exterior walls of the vault to minimize impact on the top slab. The small concrete lateral restraint ring at the bottom of the riser is not necessary either. This cam be done with a small inset in the top slab to receive the riser rather than another small pour.

Mike McCann
MMC Engineering
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