Limiting stress value for steel beam flanges in composite sections

[graduatequestions]

When looking at a steel concrete composite section you need to calculate the build up of stresses in the extreme fibre of the steel beam's flange. This is relevant under hogging moments but im not 100% sure why.

What sort of failure do you get when the yield stress is reached in the flange? Is it to do with torsion or is literally just a bog standard thing - checking the limiting stress value isnt reached in the beam and I suppose its more relevant as the concrete is cracked?

 

[Lion06]

The concrete is only in compression, it is not taking any tension, why do you care if it cracks?
What is a hogging moment.
For a composite section, you will typically assume the yield stress has been reached in the entire steel section - possibly all tension or some compression and some tension.

What in the world is a "hogging" moment?

 

[graduatequestions]

Apologies - 'hogging' is commonly (here anyway) to describe the moment over a pier/support, i.e. where the concrete is in tension and the steel in compression rather than a 'sagging' moment at the midspan of a structure where the steel is in tension and the concrete in compression.

In a 'hogging' section you tend to assume any concrete in tension is cracked, which is fine. I was just curious as to the relevance of checking the stress capacity in the flange in particular in these sections.

 

[Lion06]

You would typically assume the bare steel section only is taking the moment in the case of a "hogging" moment.

 

[kslee1000]

THe reason is your bottom flange is not continuously braced while under compression, buckling could be a problem.

 

[kslee1000]

Correction: shall say "if the bottom flange is not properly braced, buckling could.."

 

[civilperson]

Does no one count the reinforcement in the concrete for tension strength? AISC 16.1-84, I3.2b. 1),2) &3).

 

[Lion06]

civilperson-
We don't typically count on reinforcement, but have when necessary.

kslee-
I sort of see your point with buckling, but........
1.) that has nothing to do with built up stresses if the bare steel alone is taking the negative moment. It is whta it is. M/S. That simple there is no need for superposition of stresses since there is no composite section for negative bending.

2.) unless you have stacked construction (e.g. the typical case of beams framing into girders with the same top of steel elevation), the beam framing into the member in question will serve as a brace point for both flanges (assuming the shear connection extends more than halfway down the web of the member in question).

 

[miecz]

graduatequestions-

Are you considering the rolled section to be composite with the reinforcing in the negative moment region?

civilperson-

I have used negative reinforcing, for bridge superstructures. For this condition, AISC doesn't seem clear that the plastic neutral axis will move toward the reinforcing and away from the compression flange, making the composite section more susceptible to buckling. That is, a compact rolled shape is not always compact when composite with reinforcing. I think I3.2(b)(1) should read "The composite section is compact, according to Cases 2 and 11 of Table B4.1..."

 

[kslee1000]

AtructuralEIT's 2 point is correct. Usually the transverse beam is adequate to act as full functional brace, however, there are more than a few cases that the transverse beam is much smaller than the main girders, in such case, high compression stress in the bottom flange tends to cause instability (buckling) to occur.

 

[civilperson]

meicz,
The compactness of the compression flange is unaffected by the movement of the PNA upward toward the reinforcement. The compactness of the web in compression may be affected, but only in a lesser degree since the height of the compression part of the web divided by the height to the PNA is limiting the ratio of hc/tw.