Cantilevered Beam
Cantilevered Beam
(OP)
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
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






RE: Cantilevered Beam
I suggest the paper by Yura titled 'Fundamentals of beam bracing' in the AISC journal 2001 for some good information on bracing of beams.
RE: Cantilevered Beam
RE: Cantilevered Beam
RE: Cantilevered Beam
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.
RE: Cantilevered Beam
RE: Cantilevered Beam
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.
RE: Cantilevered Beam
RE: Cantilevered Beam
RE: Cantilevered Beam
thread507-320359: Unbraced Length of a Cantilever
RE: Cantilevered Beam
RE: Cantilevered Beam
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
www.HowToEngineer.com