Cantilever beam LTB restraint 2

[Enhineyero]

Hi all, I have cantilever beam which is laterally restrained on top by purlins. I would like to know what bracing system you use for decreasing the unrestrained length on the compression flange, to prevent lateral torsional buckling. in my case I attach tension rods at the bottom flange (like those of a cross-bracing).

 

[connect2]

Seems to me there is info available from AISC or CISC if you're freezing in the snow, also Guide to Stability Design of Metal Structures or, Nethercott paper, yadda. The restraint at the root of the cantiliver is as if not more importatnt than the tip. You will find it odd but if you find the original Nethercott papers, or Galambos Stability Design, you need to be very interested in the restraint condition of the tension flange.....as well as the compression flange.

 

[homostructural]

I am using angles like the picture displayed here.

All about Eurocodes:

http://www.DesignToEurocodes.com

 

[msquared48]

Kinda depends on the connection over the support and what you want to see. Just use your imagination...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[Enhineyero]

Thanks for all your replies.

Homostructural - The bracing you are using is for beams supporting a roof, right? how about if your cantilever beam is supporting RC slab?

This is kind of out of topic (but still on the same subject), what do you consider as the unrestrained length of a fixed end beam (supporting a slab) with compression on the bottom flange near the support and compression on the top flange at midspan, considering the moment at supports. I heard mixed opinion regarding this subject. I myslef would consider the unrestrained length as the distance from the face of support till the inflection point, others would say it would be the total length of the beam. Any opinion regarding this?

 

[homostructural]

Yes, you are right! The bottom flange of the beam is stabilised with stiff diagonal bracings (angles) that connect the beam with the roof purlins. As for your question, I am not sure... You could probably embed properly anchored steel sections in concrete slab prior curing.

All about Eurocodes:

http://www.DesignToEurocodes.com

 

[RFreund]

Here is a brief article on the subject if you are using AISC design manual:

http://files.engineering.com/getfil..._Interchange_»_Design_of_Cantilever_Beams.pdf

Also in regards to your fixed - fixed beam with bracing only on the top flange. Once again if using AISC - I believe you would take the entire length as unbraced as AISC defines a bracing point for lateral torsional buckling as able to restrain lateral displacement of the compression flange or able to brace the section against twist of the cross-section. However you are able to use the "Cb" factor (see section F1 in the 13th edition) which increases the LTB capacity. Also - you may argue that if you can develop some sort of moment connection to the slab w/possibly some sort of stiffener that you have now created an "LTB brace point" as the section cannot twist (See Appendix 6 for bracing requirements) but I'm not sure I would go that route.

EIT
www.HowToEngineer.com

 

[Enhineyero]

Thanks for this Rfreund.

side question: If i have an I beam supporting a slab, with a hogging moment near the support, then i added stiffener plates (to tie the top & bottom flange) say at every 20"(500mm) can i consider my unbraced length for the hogging moment to be 20"? My justification for this would be that there would be no lateral translation since the beam is restrained by the RC slab, and the buckling restraint for the compression flange is at every 20". Hence 20" for unrestrained length.

May i have your insight on this?

 

[structSU10]

I am attaching a paper on beam bracing which discusses some of the stiffness parameters required. Use this in conjunction with AISC to determine the appropriate system. I would tend to think you would need a pretty substantial stiffener to act as a brace on its own, I would tend to think you also need kickers of some sort.