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Concrete Junction Box Design Criteria for Inside Height

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oengineer

oengineer

Structural
Apr 25, 2011
708
I am working on the design of a junction box connecting two RCBs. The RCBs are both 4x3. The flow is the same for both RCBs.

Due to the height of the entering & exiting RCBs, the I am calculating that the overall inside height (excluding top & bottom wall thickness) needs to be 4'-6".

Is there a criteria for Junction Boxes that states the minimum required inside height? Do workers typically enter inside junction boxes?

Any suggestions/ comments are appreciated.
 
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Questions - what is RCB and 4x3 (4"x3", 4'x3')?
 
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I still don't know what is RCB, but seems it is a device/structure for water flow. Anyway, a junction box/chamber services as point for cleanout, therefore, yes, workers would require to enter it during maintenance tasks. If this is for a utility company, or government agency, they usually have their standards regarding the size.
 
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RCB = Reinforced Concrete Box (culvert)

Rod Smith, P.E., The artist formerly known as HotRod10
 
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See Image of RCB's below:

rcb_ngresd.png
 
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Reinforced Concrete Box Culverts (RCB's)

Attached are 2 sheets showing a TxDOT junction box example. It might help you organize your sheets and design.

I'm not sure that there is a minimum height, but if there is a manhole going down to the junction box, then I would think you would want the box to be at least 4ft tall so that someone could get down there if needed.

Also, 2 things I like about this TxDOT detail....

1) The culverts coming into the junction box are a minimum of 6" above the box bottom. I think this allows for construction error; and also for you to get at least 1 continuous piece of rebar under your pipes coming in.

2) The detail shows at least 1ft of concrete above the crown of your pipes coming in. This gives you room for some continuous rebar above the pipe coming in.

 
 https://files.engineering.com/getfile.aspx?folder=448f1254-370e-4aba-9edf-71cb209fced9&file=TxDOT_Junction_Box_Houston_District.pdf
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See plan view configuration of Junction Box with RCBs below:

jb1_htr0pj.png
 
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@JoelTXCive

Thank you for the information.

I was thinking that 4.5 ft would be too short for someone to enter inside the junction box. It seems to shallow. I was thinking about adding an additional 2 ft to get the inside height to 6.5 Ft. So that someone can actually stand inside the box.
 
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Linked is a junction box detail from Georgia DOT.

Link
 
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@retired13

Thank you. Based on the detail, it appears a clearance of 4'-6" is sufficient for the interior of the junction box.
 
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I agree that 4.5 ft is pretty short. If you have room for a larger box, then definitely bump it up.

Often times the box height is strongly influenced by flowline elevations set by the civil/hydraulic engineers. They will have their flowlines set at a certain elevation and if you suggest lowering them to get more headroom in a box; they will freak out.
 
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@JoelTXCive

Does someone actually enter inside these junction boxes or is a machine used to service it?

I wonder because there are no ladders shown on the drawing in the junction box near the manhole for the TXDOT detail.
 
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I have seen many times of sanitary/storm sower works, the worker enter the manhole/junction box using a ladder. I suggest to ask your client to see if access for workers is required.
 
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@JoelTXCive

Are you aware of any clearance requirements that TXDOT may have for junction boxes?
 
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Here is detail from TXDot, [link ftp://ftp.dot.state.tx.us/pub/txdot-info/ftw/specinfo/new/jb-ftw.pdf]Link[/url]

Note that most of the DOT document are accessible by general public, search using local DOT name and subject will bring you the result.
 
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@retired13

Thank you for the link.

I noticed that the detail referenced in the link is for Junction Boxes without Risers. In these cases, these JBs are probably not expected to be accessed so the inside clearance height would not matter, correct?
 
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I believe this likely be the case. The needs are to be assessible for inserting cleanout rod, and occasionally clean up the accumulated garbage. As long as the dimension is not less than the RCB, it should work out (as suggested by all those detail sheets).
 
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retired13 said:
I believe this likely be the case. The needs are to be assessible for inserting cleanout rod, and occasionally clean up the accumulated garbage. As long as the dimension is not less than the RCB, it should work out (as suggested by all those detail sheets).
Click to expand...

In my situation, the junction box does have a riser. So it appears that the inside of the junction box will need to be accessed by either machine or service crew.
 
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I think verification with the client is the best approach. Why guess?
 
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