Continuous Steel Beam - Stability Bracing

[strucguy]

Is there a way out of not having to provide stability bracing at inflection points when designing continuous beam? The AISC specification is not clear in this regard. Your input is greatly appreciated. Thanks in advance.

 

[ash060]

It is clear, Appendix 6.3.1 of AISC 360-05 requires lateral bracing near the inflection point at both flanges for beams subject to double curvature

 

[cap4000]

I have the 2008 Galambos book and in it he requires bracing at both the T&B flanges at the inflection points.

 

[frv]

ash060 got it right.. Crystal clear.

 

[BAretired]

The beam does not necessarily require bracing at inflection points, but bracing of top and bottom flange is required at each support (brace point nearest inflection point).

BA

 

[slickdeals]

I am not sure I agree with that interpretation.

I think the code mandates that both the top and bottom flange of the beam be braced at the location nearest to the inflection point and not just the compression flange.

I don't disagree that it is a good detail to brace the top and bottom "at" the inflection point, but again the inflection point is a moving target depending on the loads.

 

[slickdeals]

@BA:
You beat me to it. I was commenting on ash060's interpretation.

 

[strucguy]

Thanks for the responses guys. But, what is one's option if the bracing can't be provided. Can't a stiffer beam solve the problem. If yes, who do I approach the problem? Do, I need to overestimate the actual unbraced length provided. I am sure there are tons of case where bracing isn't practical. Any thoughts on how this needs to be dealt with. Thanks.

 

[BAretired]

strucguy,

This is where some of the forum members part company with me. I know because we have discussed it before.

Load two adjacent interior spans to cause maximum distance to each inflection point (live load on each adjacent span and alternate spans beyond). Say the inflection point is 'a' from the support on one side and 'b' from the support on the other side. Design the beam using a moment equal to the peak moment at the support and an unbraced length of (a + b). Repeat for each of the other interior supports.

BA

 

[ash060]

You have to have a bracing point at the support, so if that is the nearest brace point that is the one that has to have top and bottom flange bracing

 

[paddingtongreen]

strucguy, you need a laterally or vertically stiffer, or combination thereof, beam. You may find yourself using a column shape. Do you not have charts that show moment capacity versus unbraced length. The AISC manual used to have them as a guide.

Michael.
Timing has a lot to do with the outcome of a rain dance.