Holes in Concrete Grade Beams

[tdlnyc]

I have a 18" wide x 24" deep grade beam spanning about 20 feet.
The contractor wants to place a 6" diameter pipe through the beam (before the pour)
I have done this before in the range of L/3.
Is there any design aspects I need to consider?
Thank you

 

[youngstructural]

make sure the entirety of the hole is in the tension zone of your beam; Not just outside of your Whitney stress block depth (since this is a simplification), but below the transform section's neutral axis.

Alternatively you could consider the actual loss of strength involved and see if this is acceptable. You may also consider adding compression block wings (tabs to each side to create a T-Beam) or you may introduce a steel tube to permit the penetration and tranfer the load around the penetration if it must be in the compression zone.

Also you will want to look at the effective cover for your reinforcement... You need to ensure that the cover is provided both around the beam and inside the beam (to any free edge, such as your penetration).

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[lkjh345]

IMHO, as long as the sleeve is approximately near the centerline (vertically) of the beam, and somewhere between .2L and .35L away from a support horizontally, you shouldn't have any trouble.

You may wish to consider adding a couple of #4's or #5's x say 5'-0" long top and bottom and on each face of the beam, but these probably are not neccesary.

 

[tdlnyc]

Wow, I just signed onto this web, what a quick reply.

I have looked at many of the posts before but never signed up till today.

Thanks so much for the replies.

Tom
NYC

 

[hokie66]

For your dimensions, locating the sleeve at mid-depth and anywhere within the middle half of the span should not affect the beam strength. This is because you will be in a low shear area where the area reduction is allowable, and in a high moment area, but the top and bottom "flange" are intact.

 

[msquared48]

No Aluminum Pipe!!!!!!!!!!!!!!!

PVC sleeve and Cast Iron pipe OK.

Mike McCann
MMC Engineering

 

[youngstructural]

Good point Mike! 35 year Petrosky effect is definately NOT something to embed in your foundations...

On a wee side-bar, I keep catching contractors with Aluminum tools here in New Zealand. Even though they insist on calling them Aluminimum (add, and pronouce, the extra 'i'). Sometimes very surprising that experienced trades people won't know.

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[csd72]

YS,

Aluminium is the correct english term which is used everywhere except in the USA (and maybe canada by osmosis).

 

[LOKSTR]

msquared48,

Could you please explain why we should avoid "Aluminum pipe"?

I am completely ignorant about this fact and want to know the reasons behind this.


Thanks

 

[msquared48]

Chewmical interaction of the Alumi'nium (Alu'minum properly spelled - accents added) and concrete to the extent of locally destroying the integrity of the concrete matrix.

Mike McCann
MMC Engineering

 

[youngstructural]

Here is my (humble) understanding of why all Al to PCC contact must be avoided:

- The aluminum ions react with the cement paste to cause the creation of hydrogen gas. The rise of the gas actually increases the volume of the plastic concrete/grout. Done correctly, this can be made to give rise to a volume gain in direct proportion to the shrinkage... This was used to create many of the early "non-shrink" grouts.

Unfortunately, when not rigidly controlled, this will cause the concrete to expand in an unpredictable manner.

- Where things start to go wrong is a bit more hazy, but I understand there are a few potential dangers:
1) All that hydrogen in micro-bubbles is next to all sorts of different minerals and chemicals in your aggregates. I have been told annacdotally that it can give rise to Hydrogen Sulphide, which is a well known concrete deteriorator.
2) The hydrogen itself weakens the bonds in the concrete matrix.
3) The continuing evolution of hydrogen in the concrete will not abate simply because the concrete is set, but rather continues until it exhausts the supply of free components or reaches a steady state under pressure. Apparently the steady state under pressure is quite common, but given the porosity of concrete the gaseous hydrogen excapes, reducing the pressure, and repeatedly (or perhaps even continually, but probably not continuously) reactivating the hydrogen gas producing reaction. These pressures can be substantial, and when not stealing components of the concrete matrix and thereby directly weakening the concrete, the hydrogen spends its time stressing your concrete from the inside out.

So your pipe, and anything embedded into concrete, should definately not be Aluminum.

Note that the actual interaction is quite complexe, and is similar (but not identical to) the sodium-concrete reaction which also gives rise to hydrogen.

I would really love to hear anyone's opinion on what I've written; Further clarification, and corrects if needed, would be greatly appreciated!

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...

 

[hokie66]

YS,

I think that's all good, but I don't think there is anything wrong with using aluminum tools. They are not in contact with the concrete long enough to make a difference.

For those who spell it aluminium, including myself, because I now live in Oz, I still maintain it is wrong. We Americans claim we invented it, so why can't we spell it any way we like?

I am prepared for abuse.

 

[msquared48]

Abuse...Abuse...Abuse.

Mike McCann
MMC Engineering

 

[youngstructural]

Lol... No abuse here Hokie... And I'm never, NEVER going to spell a car's wheel as anything other than a "Tire".

Cheers,

YS

B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...