Finite Element Modeling of underground junction box, for storm drainage, with holes for pipes

[Spellbinder]

Hi,
I am modeling a junction box for storm drainage in RISA3d. Its an underground concrete box with some hols for pipes.
When i model the box with circular or rectangular cutout for pipes the shear concentrates a lot, which seems unrealistic. I need ideas or reference, that will help me reduce this shear.
Please let me know if you need more information

 

[WARose]

For stress concentrations, a good reference 'Roark's Formulas for Stress and Strain'.

In situations where a homogeneous, ductile material is subjected to loads that will not create fatigue.....stress concentrations are generally ignored. (But that probably wouldn't work for RC.)

 

[JGard1985]

Whats the shear coming from? In the model are your pipes being supported by the concrete box. Usually pipe bedding ensures all piping is self-supported I'd think that box and piping would settle at nearly the same rate

Jeff
Pipe Stress Analysis
Finite Element Analysis

http://www.xceed-eng.com

 

[AskTooMuch]

Warose,

Where in roaks book can I find stress concentration can be ignores?

 

[WARose]

Warose,

Where in roaks book can I find stress concentration can be ignores?

I don't think it comes out and says it anywhere. I was referring to a industry practice with a ductile material (like A36 steel) not subjected to cyclic loads.

The hard part is (of course) knowing if the stress is indeed a concentration and how widespread it is. That's where a reference like Roark's comes into play.

With something like reinforced concrete.....that's a different ballgame. It's hard to give a one-size-fits-all solution there......but you can (for example) average the effects for highly concentrated forces/stresses.

 

[Spellbinder]

I have uploaded 2 files. the loaing is the soil pressure from outside.
one showing the shear for my model. this boxes without any hole has a highest shear of 14.5 kips per ft. But with the holes the shear goes as high as 31 kip per ft.

https://files.engineering.com/getfi...-16b4-42c1-a8f2-70fe119aea6f&file=shear_x.png

I have another file showing where the shear is higher than 14.5 kips per feet. Hopefully that will give a better picture of what i am talking about.

https://files.engineering.com/getfi...9aeb6ecbeb2&file=shear_over_15_kip_per_ft.png

Thanks for the replies. Hope to get a better understanding of whats going on from the discussion.

 

[GC_Hopi]

This is a modeling issue. I am not familiar with RISA3d but you should try refining your mesh and check how you are applying the loading especially around the openings. The most common/simplest method of resolving this is to average the shear stress of several elements in an area. This will give a more realistic idea of the shear stress.

From a design perspective, I try to put develop a bar at areas where you see stress concentrations. This reduces the chance of cracking at that area.

 

[TehMightyEngineer]

Select only the section you're checking for shear and use the section cut feature of RISA 3D. This will give you much more realistic forces in a unit width of plates.

Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL, HI)