Continuous Beam

[m1ke99]

I have a two consecutive simple span steel beams, designed accordingly for uniformly distributed load and only positive moment (wl^2/8). If the beam at the central column makes the beam continuous over the column, I will have a negative moment at the column. 2 questions:

1) If I do not have adequate negative moment resistance in the beam, will the system fail? Or since the problem is valid for 2 simple spans, what will happen?

2) What do I use for the unbraced length on the negative moment portion (bottom flange) of the beam for calculating lateral torsional buckling? Load is applied through with joist framing connected to the top flange.

 

[Lion06]

1. For a two span continuous beam the negative moment over the interior support is (wl^2)/8. If you can't develop that moment, then the maximum moment that you can develop is what will remain in the beam once it reaches that moment - assuming it's ductile and can continue to rotate. Once that moment capacity develops, you technically have a "failiure" at that section, but it's still stable and additional load will simply result in two simple span conditions.

2. The unbraced length is the distance between points of lateral support for the compression side of the beam. It sounds like your unbraced length for negative moment will be L (between supports).

 

[GalileoG]

Lion06, can the unbraced length for negative bending be taken as the distance between inflection points?

 

[JAE]

Not according to Yura. Use the full "L" and the appropriate Cb factor.

 

[Lion06]

JAE is correct. AISC also explicitly states this in the 13th edition manual.