Beam on ground 1

[mar2805]

Hi guys!
I have this question thats been bothering me for some time now.
I have a ground beam suporting columns, lets say 3 columns each distance 5m making a beam 15 meters long + 0,5 at each end.
Beam is desigined in longitudinal direction for maximum bending moments.
When designing a ground beam is it safe to assume that, even thou the beam is cast in 2 phases, it will act as one whole area?
Meaning the full cross sectional area of 30x100cm in my case (hatced area), with top and bottom reinforcement designed for maximum postive and negative bending moments.
Another problem that I have understanding is that my design code says that section subjected to bending must always have a minimum reinforcement. In my case 0,15% of the cross section area.
If you look at the picture attached that would be cross section 1-1 wich should be designed for generated ground pressure in transfersal direction.
This criterea always governs the design and gives large amounts of reinforcement area!
Should I provide this amount no matter how crazy the results are?

Please help.

 

[EngineeringAdam]

As far as I understand it the ACI allows horizontal shear to be mitigated by using web reinforcement ties applied per 17.6.

17.6 references 12.13 which covers the development of web reinforcement.

I cannot speak to the minimum required by your code, since I am not sure what code you are using. However, you might find an "out" since the beam you are designing is basically and foundation grade beam and might not have the same requirements as a standard flexural concrete beam.

 

[mar2805]

I agree with you on that one.
Maybe section 1-1 should be analysed as it was a strip foundation and there is a wall siting on it?
This would then be the only section that should be reinforced, only in the lower part.

Regarding phase casting and treating cross section as one, whats your opinion on that?
Thanx!

 

[EngineeringAdam]

You could do that - have the lower section act as your beam, and the upper part act as a wall sitting on top.

As far as the phase casting, I believe it shouldn't be much of an issue structurally if you are using the lower first phase reinforcement as your beam. However, there are other issues to be aware of, such as water proofing at subterranean cold joints. I am no waterproofing expert but, I have been warned about it in these situations.

Like I mentioned before, as long as you follow the ACI horizontal shear section you can use the full section structurally.

 

[mar2805]

So your opinion that the shaded area represent no problem even if its cast in 2 phases. I ean the coross section 30x100 can be used in design.
As for the cold eniviroment and water influence at this phase joint - you have a point. You asssumed that the section is weaker here, water could penetrate and if this is inside the frost zone, concrete cracks.
But doesnt this confirm my doubts as this shouldnt be watched &analysied as a whole section?
Then again we are what aboth all those inverted T section that are used and designed like this?

 

[EngineeringAdam]

I still think that horizontal shear at cold joints can be mitigated by the proper application of ACI 318-08 17.6.

I have seen some that argue a cold joint might involve shear friction, which is also addressed in the ACI. Either way, both have different ways of addressing transverse reinforcement across the critical section. But I think 17.6 is most applicable in this case.

Waterproofing should be mandatory if you are counting on a earth exposed cold joint to perform well. After reading your post is seems that your cold joint isn't below grade, therefore, I think water proofing probably isn't needed. Decisions like this need to be addressed on a case by case basis, so use your judgment.

 

[mar2805]

This beam would be 0,70-1,00m under ground!
Section would be designed for shear as usual. Links would be provided minimum 25cm distance.
This "cold joint" you are saying is phase stop profile, boundary from lower to upper part?

 

[mar2805]

Any chance I could read that part of ACI tat explains this kind of a situation?
I asking couse my code doesnt say anything aboth this matter (phases casting)

 

[mar2805]

To simplfy the problem.
Two beams of equal width "B" and height "H" cast seperatly on top of each other.
Opossite, one beam of height "2H" and width "B" cast at once will be more stiffer have greater shear strenght.
Basicly this could be explaind due to 2 beams bending independently, adjcent phase planes will want to slide off. There should be some force ressiting this.
In 2 beams egsample it will be "friction A" and steel bars area (links).
In one beam of "2H" height it will be "friction B" (shear strenght of concrete) wich is greater then "friction A" and steel base area (links).
I think this might stand but how to determine this "friction A" parametar?

p.s. but how to explain this phenomenom? This is opossite of what I just said :-(

http://www.youtube.com/watch?v=tTbkNWwaxDM&feature=related

 

[EngineeringAdam]

I am not sure if it's okay to post the ACI here for you to read, might violate copyright or forum rules.

Anyway, off the top of my head I am not sure the easiest way to compute the friction you are trying to find with two beams acting independently. However, the ACI details a prescriptive method to insure the horizontal shear is transfer across cold joints. This is accomplished buy using traverse reinforcement across (perpendicularly) the cold joint. The ACI doesn't have you calculate the friction directly.

 

[mar2805]

Yeah I think your right...
Have you seen the video, how do you explain this?

 

[RFreund]

If you are trying to find the 'friction force' between the two beams wouldn't this be a shear flow calculation q=VQ/(I*b)?

EIT

 

[EngineeringAdam]

I did see the video, and it isn't related to shear friction.

What happens is by cutting a member down the center you are left with two more flexible sections. Because of this flexibility the sections can deflect more easily without rupture. Of course they need to be strong enough to handle the deflection without failure.

It should be understood that cutting the members only allow for more deflection, not strength.

With the wood post experiment the post can deflect more without rupture, but it isn't as strong in moment or axial capacity.

 

[mar2805]

Bending= aplied moment= amount of rotation.
The second beam could bend more.
Can you explain what you meant by saying "isnt stronger in moment as the first one".
I think this is due the shear forces. Splited part of the beam cannot attract more shear forces on the section since it is splited.
Moment is basicly a sum of an area under a shear force diagram.
Because of this smaller amount of moment is created at the fixed base of the beam.

 

[mar2805]

To simplfy the problem.
Please look at the picture.
Reinforced concrete beam of lenght 5m is subjected to bending.
If beam is cast in 2 seperate phases, can it be analyzed and design as it was cast all at once?
Can anyone post an calculation egsample?

 

[RFreund]

Well I still think this is a shear flow problem like adding a cover plate to a steel beam. However my above post is incorrect because it assumes a linear elastic section. Concrete design utilize a plastic section. Unfortunately I am not sure what the shear flow equation looks like for a plastic section.

EIT

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