Concrete Basics 2

[struct4me]

Why the allowable bending compression in concrete is always greater than axial compression?? Thanks.

 

[r13]

I don't recall the allowable compressive stress in ASD, but for strength design, the ultimate stress for both case is 0.85fc'. The difference lies in strength reduction factors, 0.9 for beam, and 0.65 or 0.7 for column, as the confidence on the strength of flexural member is higher than the compression member.

 

[MegaStructures]

*Disclaimer* I’m not opening my code and I don’t work with concrete often, but I don’t think it’s accurate the the allowable strength for concrete in compression in a column is always lower than a beam, but it is more often, depending on the type of loading the member is experiencing. If the member is compression controlled I.e. the concrete fails before the steel yields the safety factor is 0.65 and if it is tension controlled I.e. steel yields before concrete fails the safety factor is 0.9. So the difference is basically from the danger of the failure modes. Concrete fails sudden and catastrophically, so the code protects against this with a higher safety factor. Steel yielding is a “slower” ductile failure that presents some warnings of impending failure, such as excessive deflection.

 

[dik]

I'm not sure... statistically flexural bending stress could be slightly higher since you are only stressing a small part of the cross-section to maximum compression.

Dik

 

[r13]

dik,

I think interaction diagram provides the answer - for a given cross section, the concrete stress is constant (0.85fc'), but P & M varies with given eccentricity.

 

[MegaStructures]

retired13 is right, but it's important to understand WHY the safety factors are different for an axial member and a bending member.

 

[dik]

Yup... thought he was talking plain concrete... difference between beam flexural strength and splitting and pure... same thing with compression.

Dik

 

[r13]

Dik,

You could be right, OP didn't mention reinforced or not.

 

[MegaStructures]

Good point Dik. Maybe OP can clarify

 

[HTURKAK]

Which WAS the CODE ??
The allowable bending compression stress in concrete is always greater than axial compression stress..This is true... I am familiar with old DIN 1045 ..Moreover, the allowable compression stress changes for T beam, one way slabs, two way slabs...

I found a table .. For example ; for concrete C 30 ( the cube strength is 30 MPa ) , The allowable compression stress;

for rectangular beams , one way or flat slabs and T beams d ≤ 80 mm σ b= 9 MPa
for rectangular beams , one way or flat slabs d > 80 mm σ b= 10 MPa
for columns ( axial load ) σ b= 8 MPa

In case of axial load + bending (beams, columns, slbas )

for rectangular sections , uniaxial bending σ b= 11 MPa
for rectangular sections , biaxial bending σ b= 12 MPa
for T beams , compression stress at table σ b= 10 MPa

Nowadays we are lucky that , the safety is defined with probabilistic approach...

 

[r13]

Why the allowable bending compression in concrete is always greater than axial compression??

Not true, especially per present practice of reinforced concrete design in the US. But in another sense, we can treat the strength reduction factor as an adjustment of the permissible concrete compressive stress for bending and compression. The reason is the confidence level on behavior of concrete under each type of loading.

 

[struct4me]

"Which WAS the CODE ??"
This is as per Indian concrete Code IS 456, which under its working stress (WSM) Annexure (now WSM is part of Annexure, where LSM are in the main code) says the following under a table (Pic)
I always have taken it for granted without asking why??

 

[Blackstar123]

An axially loaded cylinder cracks considerably if strained beyond the maximum stress f'c. Whereas, flexural tests on beams have shown that concrete does not normally show any visible cracking or spalling even though the strain at extreme fiber is greater than that corresponding to the maximum stress. This is probably due to the fact that concrete close to neutral axis is strained less than the surrounding. This could be one of the reason for higher permissible stress for concrete in a flexural members than in an axial members.

 

[HTURKAK]

I looked to the attached table.

Although I am not familiar with IS 456, IMO, the reasons for the allowable bending compression in concrete is greater than axial compression are;

i ) The flexural bending , produces compression stress at only a small part of cross-section ( as Mr dik (Structural) pointed out also ) and code writers aware of the parabolic stress distribution , that is, when the compression stress reached to the max. allowable, there is still margin,

ii ) The bending moment is in general max. at a local point along the length while pure compression member suffering through out the full length..

Old DIN 1045 defines the FS as a composite number depends on (material precast or cast in place,loading, calculation base, unforeseeable  factors..)

 

[struct4me]

Thanks everyone for sharing your viewpoints, well appreciated. Many Thanks!!