Concrete Moment Frame Beam

[slickdeals]

What's the typical detail in your firms on how far additional top bars at columns are extended? I have seen L/4 and L/3. Would appreciate if anyone had a typical detail that you could share.

We are putting together a spreadsheet for moment frame beams based on ETABS outputs and wanted to add checks to ensure that the reinforcement is developed past the point it's not needed.

 

[KootK]

Those are typical extensions for non moment frame beams but I doubt they'd be sufficient for moment frame beams. In a symmetrical setup with lateral loads dominating, your inflection points are near midspan, right?

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.

 

[slickdeals]

Yes it is. You would have continuous top reinforcement supplemented with additional top reinforcement at the supports. The key is to find out the right balance between the continuous bars and added bars that results in maximum efficiency. I am trying to see if there's a good starting point (L/3) or (L/4).

 

[KootK]

Ahh... That bi-triangular moment diagram would suggest that your steel demand would drop by half at the 1/4 points. Following that logic and developing those bars past inflection, L/3 seems as though it would be the way to go.

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.

 

[JAE]

I've always used L/3 for the cutoff length but as you mentioned, include continuous top bars as needed for special conditions.
Also watch out for short spans next to longer spans where the short span see little or no positive moment.



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