When columns are in compression why do we need then reinforcement? 1

[lolobau]

Hi

a strange question I know but still I wish I could see the forces of each reinforcement bar in a columns. Does someone has such a graphis?

When our concrete columns are in compression (pure central load, no moments, etc)why should we then still need reinforcement?


Why can't we have the column then without reinforcement?
Reinforcement is meant for TENSION forces only, or?
How comes that you still fine in a column the rebars staying under compression.
Otherwise why would we need the stirrups around them if they would be in tension only?

Somehow it appears to be the we need the stirrups (links) becasue the bars are getting compressed?
Strange....

Often you see horizontal cracks in columns at the spots where the crank is (overlapping) because hte bar got compressed there, why?

 

[lolobau]

http://debug.pi.gr/BookimagesEn/01120.jpg

 

[oldestguy]

Maybe research the testing done by various studies and you will find the answer.

 

[Stenbrook]

Ties are used for confinement of the concrete and shear capacity as well as holding the vertical steel bars in place. Think of having a bunch of sand compacted in a cylinder. You take the cylinder off and then step on it. The sand will most likely completely collapse. Now if you leave said cylinder on it, you could stand on it without it collapsing.

Vertical steel is used for many reasons. How likely is it that in real life you actually have 0 moment on your column? If you have the minimum vertical reinf you don't need to worry about any eccentricity that may be on your column. It does also add some compressive capacity to the concrete as well considering the fy=60000ksi as opposed to your concrete which would range between 3-10 ksi.

At the end of the day, I would suggest that if you are interested in why concrete is reinforced in the way it is, you should take a class about concrete design.

 

[dik]

They did that for millennia before the ACI came along <G>. Reinforcing provides a moment resistance greater than the kern e value.

Dik

 

[KootK]

A few things:

1) While it's rarely done in practice, most codes allow for plain concrete in select circumstances and, therefore, unreinforced concrete compression members.

2) As Stenbrook alluded, for heavily loaded columns, the rebar is intended to take compression. Most modern highrise columns wouldn't work without that even though they are generally quite insensitive to the presence of floor level moments.

3) Regardless of why a designer chooses to include reinforcement, once they have, that reinforcement will participate in resisting compression loads. That's simply a function of the strain compatibility that exists between materials. As such, even reinforcement intended for tension capacity will participate in compression resistance and thus require ties to prevent local buckling.

4) Due to creep and shrinkage in the concrete, and the absence of it in the rebar, reinforcement tends to attract more compression load than conventional reinforced concrete design procedures would suggest. This effect is more pronounced as low reinforcing ratios which is one of the reasons minimum levels of column reinforcement are specified.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[BAretired]

Plain concrete, i.e. unreinforced concrete has been used for pedestals with small eccentricity for many years. Pedestals are similar to columns in that they are compression members. If it is known with certainty that a column sustains pure compression, they would not require reinforcement, but this is rarely the case.

BA

 

[cliff234]

In addition to the reasons already mentioned here are two more (these may have been mentioned - I did not read all of the responses),

Ductility: Unreinforced concrete is just like unreinforced masonry. It is brittle. Every ten years or so there is a big earthquake somewhere in the world and tens of thousands of people are killed. Did you ever notice that most of these disasters (with large numbers of fatalities) are in places like China, Iran, Turkey, Haiti, etc.? The loss of life is due primarily to people living in brittle old masonry structures. No reinforcing steel. Ductility is important.

Nothing is perfect: There is no such thing as a perfectly straight column with absolutely zero moment. Minor imperfections will cause moments. Small load eccentricities will cause moments. The steel is needed to insure that there is some moment resistance.

 

[ARKeng]

Much of this was tackled by other posters, but let me put things in different terms of how I think of them and maybe it will help you understand in a different way.

When our concrete columns are in compression (pure central load, no moments, etc)why should we then still need reinforcement?

Though theoretically this can happen, it is very rare that a column has zero moment. Especially for all design loading conditions expected during the life of a structure.

Why can't we have the column then without reinforcement?

You can if your load is low enough and your column short and stocky enough.

Reinforcement is meant for TENSION forces only, or?
How comes that you still fine in a column the rebars staying under compression.
Otherwise why would we need the stirrups around them if they would be in tension only?

Reinforcement isn't just for tension. You get higher strength in compression when a column is reinforced. Remember that the Young's modulus, E, of the steel is significantly higher than the concrete (i.e., it's more rigid) so it absorbs more of the load on a pounds/square inch basis than the concrete. The ties help confine the rebar has someone else mentioned. Also, keep in mind that compression in a column doesn't only generate axial (downward) stress. The Poisson's ratio effect means that when the column shortens, it also tries to flatten out (Think of what would happen if you loaded a column made of artist's clay). This creates "bursting" pressures that the ties help resist too.

 

[Agent666]

I'd disagree with those saying it is theoretically possible. Think of a concrete test cylinder, its a small column, when loaded too far what occurs, bursting forces as ARKeng noted. I'll take the confinement of reinforcing any day even if there is no moment.... Your mileage may vary though, certainly in some countries they seem to view the reinforcement as optional as evidenced by many a story on the 6 o'clock news....

 

[BAretired]

Plain concrete has been commonly used for pedestals where l/h ≤ 3 and for walls not exceeding 3m in total height with continuous vertical support such as a strip footing. When using plain concrete in compression, the permissible stress and eccentricity must be kept extremely low but I agree, plain concrete is not suitable for use in a column for reasons already stated by others.

BA

 

[apsix]

A666
Think of a test cylinder only loaded to 50% of its failure load; it's like a small column with a Factor of Safety of 2.

 

[BAretired]

Think of a test cylinder only loaded to 50% of its failure load; it's like a small column with a Factor of Safety of 2.

Correct, but according to A23.3, unless it has changed in the last few years, the factored load cannot exceed 0.80φ[sub]c[/sub]f'c for a plain concrete pedestal. Since φ[sub]c[/sub] is 0.60, the compressive stress cannot exceed 0.48f'c for the factored load, so the factor of safety is substantially more than 2.

Needless to say, plain cannot be considered where uplift is a possibility.

BA

 

[lolobau]

Hi guys

thanks you so so much for the very informative feedback. It makes totally sensse what you all write

but since the vertical reabrs receive so much compression then the links (stirrups) become then also very important to keep the vertiacl steel in place.
Some engineers say that this steel is basicaly only for the casting to keep the vertical bars in position. This can't be true. It takes also some tension forces when the main vertiacl bars want to bulge out.
but now the code is not really giving a option to calcualte it.

All the programs just give you the vertical bars but don't really pay attention to the links

For example a round column. Can you use "open" rings or spiral "endless" cage?

A circular ring would open up since it is not anchored or hooked somewhere, or?
But you can still see that such round links are beeing used

lolobau

 

[bridgebuster]

The ends of a circular "ring" or tie have to overlap otherwise they won't work as intended.

 

[Tomfh]

Same reason table legs aren't connected with hinges.

 

[MacGruber22]

...this steel is basically only for the casting to keep the vertical bars in position

If that were the case, there would be no codified prescriptive requirements for ties in reinforced columns. I would interpret anything these engineers say from here on out as suspect. This is part of structural engineering 101.

All the programs just give you the vertical bars but don't really pay attention to the links

If the program doesn't directly recommend tie sizes/spaces/anchoring, then it is your job to design/provide as the code requires. Simple evidence that only experienced engineers (or young engineers with constant, quality guidance) should be using the more complicate design software.

"It is imperative Cunth doesn't get his hands on those codes."