Rebars Compression Splash Point

[khinz]

Please give references of how a rebar behave when there is say a 2mm gap in the column. Would the rebars just bend outwards for a given load? Let's say the rebars is a 20mm grade 60. So do you compute for the axial load needed to bend the bar in the gap or just splashing outwards in compression in the 2mm gap portion until it just breaks? For calculation purposes. Let's assume there is only 4 corner bars in a 0.3x03.m column.

I can't find this unique theoretical topic in the internet. Many thanks.

 

[Ron]

khinz...do you enjoy being in the weeds? Forget the obtuse theoretical and get the practical solved.

 

[khinz]

Just want to take the opportunity to understand this. A long column would buckle. A short rebar won't buckle in the gap but I wonder how to compute for the load where the rebar can just flatten out in the middle of the 2mm gap in the column. Nothing wrong with being interested in this for stock knowledge for future. Thanks.

 

[TomDOT]

Ron,

I'm having doubts that it is actually a theoretical question. Given the prior discussion, I am envisioning a field "repair" pour ending up with a gap.

I thoroughly hope I am wrong.

 

[briancpotter]

In short: the column would have essentially zero capacity.

A 2mm gap is indeed too short for the rebar to buckle according to the Euler bucking equation. So what you have amounts to a section of the column with greatly reduced area and moment of inertia. For the information you've given, the area at that section is about 1.4% of the area of the full column - with 60ksi rebar, it's theoretically possible to actually have sufficient compression capacity if the column was lightly loaded in the first place.

But that's before taking into account second order effects.

The moment of inertia of the 4 bars in the example you've given is around 3% of the total column. Taking into account that steel has an E value of around 7.25 that of regular concrete, your moment magnification factor reaches 1.4 (the limit prescribed by ACI) at just over 4% of the full column capacity. And that's before you get into all the other reasons the code would forbid this.

Don't ever do this.

Brian C Potter

http://simplesupports.wordpress.com

 

[khinz]

It's theoertical question for practical use.

I personally inspect all columns at the ground floor of the 3 storey building. I discovered there is a bad column at the ground floor (instead of the second floor). In the lower 1 foot of a ground floor column, there is just stone and almost no mortar. If we demolish the 3-storey building, it would cost about $150,000. So we would remove the bad 1 foot and put 4000 psi concrete inside (from a concrete truck). I'd try to find pressurized formwork. But in case there is left a gap 2mm. What would happen. Has no one repair such before? You mean if you have a 10 storey building and you discovered a bad column below. You demolish the entire 10-storey building, honestly? Thanks.

 

[khinz]

briancpotter, just saw your message after I sent the above.

I realized what you are saying.

How about a beam, can it have gap?

Iaybe i'll demolish the column from the ground up to the third floor but temporarily support all the beams in the column with steel supports, and then build new column and reconnect the beams (but what if there is gap in beams) to it. Has anyone tried this?

 

[briancpotter]

A beam could, theoretically, have enough capacity, since removing the concrete might not change it's (theoretical!) moment capacity much.

However, once again it's the change in section properties which screws you.

This time, you've effectively put a hinge in the beam at this point. For a cast-in-place beam, this changes the max moment from wl^2/12 to wl^2/8 - a 50% increase. What's more, it puts it at precisely the location where the steel in the beam would be the least. This is just the first reason off the top of my head why this won't work.

Brian C Potter

http://simplesupports.wordpress.com

 

[khinz]

Oh my. Say you have a 5 storey building and you discovered one of the ground floor columns have bad concrete and a portion needs to be replaced (1 foot). Do you have to demolish the entire 5 storey building and is there any repair that can be done to replace that bad portion in the column without any gap?

If a pressurized concrete is used to repair the 1 foot bad concrete in the column, it can still gap when the concrete settles.

 

[JacksPanic]

Ever heard of props and freeflowing non shrink grout?

 

[khinz]

I think hokie mentioned about leaving 1 inch or so of space after putting repair concrete into the removed 1 foot portion of the column and putting non-shrink grout. Has anyone actually done this? non-shrink grout can be 4000 psi too? this can help save $150,000.

 

[khinz]

http://www.sakrete.com/uploads/downloads/Sakrete NS Grout Tech Data 6-11.pdf

the above says non-shrink grout can even reach 8000 psi. But it has no aggregates or gravel.. can it be as strong as the ready mix concrete with aggregates and gravel? if so.. then why do we have to use aggregates and can't just use non-shrink grout in construction? Thanks.

 

[khinz]

braincpotter wrote:

In short: the column would have essentially zero capacity.

A 2mm gap is indeed too short for the rebar to buckle according to the Euler bucking equation. So what you have amounts to a section of the column with greatly reduced area and moment of inertia. For the information you've given, the area at that section is about 1.4% of the area of the full column - with 60ksi rebar, it's theoretically possible to actually have sufficient compression capacity if the column was lightly loaded in the first place.

But that's before taking into account second order effects.

The moment of inertia of the 4 bars in the example you've given is around 3% of the total column. Taking into account that steel has an E value of around 7.25 that of regular concrete, your moment magnification factor reaches 1.4 (the limit prescribed by ACI) at just over 4% of the full column capacity. And that's before you get into all the other reasons the code would forbid this.

Don't ever do this.

Briancpotter, you meant above that steel has E value or Modulus of Elasticity value 7.25 that of regular concrete. I understood it. But how or what formula did you arrive at the moment magnification factor of 1.4 at just over 4% of full column capacity in connection to the moment of inertia? Thanks.

 

[Ron]

The 2mm gap is not the only consideration. You have to consider the development length above and below the 2mm gap....now do you have a buckling issue?

 

[khinz]

Ron, the gap is just concrete gap in case it accidentally got undetected if the repair has to go thru that risky path, the bars are continuous from foundation to the second floor roof top so development length not a problem. I'd just like to understand how braincpotter derived the magnification factor from the moment of inertia.