Pipes cast in concrete beams

[pwht1]

Hey all,

Does anyone have any good references on how to analyze a concrete beam with a pipe cast into it? The pipes run the full width of the beam, perpendicular to the span.

The Hydraulics guy wants to run 5 100mm diameter sleeves through a concrete beam at 150mm centres, 200mm from the bottom of a 500 deep beam. My ultimate design actions at that section is a 1000kNm moment and a 250kN shear force.

Thanks,
Paul

 

[ishvaaag]

I think that the inset openings are far too small to be properly dealt with "standard" procedures for concrete beams with openings; hence I assume one will find better appraisal of what happens using a FEM approach, in which your main concerns would be the maximum compressive stress to avoid crushing at the reduced section and of course maximum tensile stresses to ascertain the adequacy of the reinforcement.

 

[csd72]

I have some goo references at home, I will see if I can find them.

Basically you analayse it as 2 columns one in compression and one in tension with both undergoing bening due to the shear. Treat these columns as fixed either end of the penetration as far as shear bending is concerned.

 

[rscassar]

I think csd72 is on the right path with this one. I would be looking to create two coupled beams 200mm deep. This would involved four layers of reinforcement total with confining reinforcement to the top and bottom flange and possibly to the vertical link element as well. Congestion will be a concern.

I would have concerns with the 250kN shear force.

Best of luck.

 

[csd72]

sounds like your holes may be a bit close together and a bit to close to the bottom.

when you say 150mm centres, is that 50mm of concrete between the holes?

also 200mm from the bottom, is that 100mm of concreteunder the holes?

I would tend to either bunch them close together in a single rectangular hole or space them further apart (say 3 to 5 diameters) so they act as individual holes.

 

[pwht1]

Thanks all and sorry for not getting back to you yesterday.

Ishvaag, afraid I don't have any decent FEM software, at least none that could handle this sort of thing.

CSD72, that sounds like it could work I'll have a play around and see what I can come up with. Yes there is 50mm of concrete between the pipes and there is 200mm of concrete below the pipe.

Also the hydraulics guy specified the spacing, not to sure on the reasoning. I'd like them at least 400-500mm apart but don't have a decent explanation other then a gut feeling.

Have any of you seen/detailed a similar situation? If I was to have 2 separate cages above and below the sleeves how would the top cage be supported?

Thanks again.

 

[csd72]

pwht1,

The mechanical guys will always give you their ideal situation but it is up to you to try and get these put in a way that works with the structure. With experience you will be able to predict these things and allow for them beforehand, at least to a certain extent.

I would think that spreading these over 2.5m is probably too much to ask.

You will have a void former or formwork for the penetration and they can chair the upper cage off this.

I have not got the articles yet as I was home late last night.

 

[pwht1]

This project has been running for a year and half now and they only just realised (last week) that there was insufficient headroom to run the pipes beneath the soffit. And our beam depths haven't changed during that period.

I was just assuming they'd use plastic sleeves, but a formed void is a better solution.

No sweat, thanks for all your input.

 

[csd72]

Yes, God bless them, the emchanical engineers are always the ones that are last to sort things out.

The good thing is that there is always someone else holding up the program.

The downside is that you always get hit with these lastminute things.

 

[csd72]

one of the articles:

http://eprints.utm.my/671/1/MAMansur2006_Designofreinforcedconcretebeams.pdf

 

[pwht1]

Wow, that's an awesome paper. Thanks so much.

 

[Brad805]

csd, you are a pretty helpful guy That was a good paper.

Brad

 

[pwht1]

Sorry guys, one more question. This beam will have bonded PT above the pipes. As the PT is only running across the top half of the beam, I'm assuming:

1) Secondary moments from PT are calculated based on the top chords properties and are resisted only by the top section.

2) The bottom chord will be stressed but not have any secondary moments.

Is that the correct path?

 

[csd72]

pwht1,

I would treat the whole section (both top and bottom chords) as the section under PT as it will be the whole section either side of this that is stressed. I must caveat this with the fact that I am not a PT expert, but this makes logical sense to me.

Both top and bottom chords will have moments from their proportion of the shears also.

 

[rapt]

pwht1,

With your penetrations that close together, I would be treating them as a single rectangular penetration. Then use the virendeel method discussed in csd's paper or or any good steel design text book should be used.

Regarding the PT, more details of the arrangement would be needed before comment can be made!