Cantilevered Beam

[XR250]

I know this has been discussed previously, but wondering if there is any new insight.
I have a wood floor system that needs the live load capacity upgraded.
Easiest thing to do is to add an I-beam under it. Due to footing constraints, the beam will span 22 ft. with a 7 ft. cantilever at one end. The i beam will be fastened to the joists with a nailer.
Two Questions:

1) If I add web stiffeners in the beam at the columm, is the bottom flange considered braced at that point or do I need to add knee braces up to the floor system?
(Due to Arch. considerations, the column is going to be much larger than needed.

2) If the end of the cantilever is not braced against twist (only the tension flange is braced), should I use 14' for the unbraced length? It is possible for me to add knee braces up to the floor system to brace the end, but the arch does not want it.

Thanks

 

[structSU10]

A brace for a beam must be able to prevent twist - lateral bracing at a flange can possibly do this - and AISC says for cantilever members an end brace shall be attached to the tension flange. The stiffener at the column can be analyzed as a brace following appendix 6 of AISC, but it also has to be checked for the brace force from the column as well, and may be hard to develope the stiffness required.

I suggest the paper by Yura titled 'Fundamentals of beam bracing' in the AISC journal 2001 for some good information on bracing of beams.

 

[jayrod12]

Can you not just provide some bottom chord bracing back up into the last supported floor joist for the cantilever end?

 

[XR250]

I can but, as previously stated, the Arch would prefer it not to be there.

 

[Triangled]

Off the top of my head.....
1. I think you have a kind of hinge at the bottom of beam and need to brace or design the beam web accordingly. Will think more about this......
2. I think you're looking for the inflection point in the backspan, which may be around 14', but can vary based upon loads and loading conditions. I think I would consider a condition with live load on cantilever only when looking for this inflection point. Will think more about this one too.

 

[theonlynamenottaken]

This may be helpful. Normal loading is one in which the load acts through the shear center of the beam, and can't translate laterally as the cantilever deflects in a lateral-torsional manner. Destabilizing loading is one in which the loading does not act through the shear center, or can translate laterally to act eccentrically with respect to the shear center as the cantilever deflects in a lateral torsional manner. From the description of your problem it sounds like you are considering a "Normal Loading condition" in either "Support case b)" for web stiffeners, or "Support case c)" for knee braces.

 

[JAE]

structSU10 is correct that the cantilever simply needs its tension flange braced at the end to create an unbraced length = cantilever length.

On the back span, the column/stiffener arrangement does brace the beam against twist assuming the connection between column cap plate and beam bottom is adequate per Appendix 6 of AISC.

Additional lateral braces along the back span could be accomplished by using a steel cross-member. This cross-member could be rigidly fastened to the top flange of the beam and extended either way parallel to the supported joists (and located up between the joists). This could simply be a channel or even a vertical plate fastened to the wood joists. (the cross member would interrupt the wood top plate to allow rigid fastening to the top flange of the beam). With some additional stiffeners in the web of the beam at the cross-member brace you would create a rigid "elbow" that would prevent twist and yet be above the ceiling line.

 

[Triangled]

Assuming sufficient compression face lateral bracing at the column per the posts above, and if there was no further compression flange bracing along the cantilevered section, I would be looking at something like 2.0 x cantilever length for unbraced length of cantilever.

 

[JAE]

...unless the cantilevered end tension flange was braced...then you'd use the cantilever length.

 

[JAE]

This previous thread from 2012 is similar - we discussed use of a Kc value in a paper that was linked.

thread507-320359

 

[XR250]

Thanks for all the replies. The tension flange of the cant. is braced. The beam passes at 2x cant. anyway so I should be fine either way.

 

[RFreund]

Because this is wood you would need to use table 3.3.3 (2005 NDS) for 'Le' right?
Which is a different procedure for checking LTB than is typically used for steel.
Good reference for that

http://www.awc.org/pdf/tr14.pdf

EIT

http://www.HowToEngineer.com