how do we use the beam selection tables for cantilevers
 

how do we use the beam selection tables for cantilevers
i'm having trouble interpreting what the method of analysis
is to determine which steel beam section size to use for a
cantilever problem. if i have a beam that is fixed at one
end, and has a point load at the other (let's say span of
the beam is 10 ft, and the point load is at the end of the
beam and is say 500 lbs.) the moment is then 5000 lb.ft and
the shear is 500 lbs. my question is this: can we just
go to the beam selection tables and select a beam with this
info, and if so, what do you use for an unsupported length?
i was under the impression that the tables were just for
simply supported beam conditions with different unsupported
lengths. what would be an acceptable section to choose here
and why?
find a job or post a job opening
you need to worry about stability and untraced length. why don't you do a simple maximum bending moment and select a beam with an acceptable section modulus?
good luck.
i guess that's what i'm asking. i have my maximum bending
moment (5000 lb.ft) from the applied load. can i then go
to the beam selection tables and select a section, and if so
what do you use for an unbraced length?
tobman,
a cantilevered beam as in your case(if i understand correctly), actually has multiple loading, at point of connection (a, fixed or pinned), cantilever support (b, usually pinned), and cantilever point load (c), in addition to uniform loads applied to the spans (if any). due to multiple loading, standard tables would not apply. the unbraced length would be the center to center distance of joists attached to the beam (e.g. 5 ft o.c., etc), if decking is applied directly to the beam unbraced length would = 0.
erv:
my beam is fixed at one end and is free at the other with
a point load on it. there is no lateral support except at
the fixed end. again, my question is:(and to recap: the
length of the beam is 10 ft) can i use the beam selection
tables, and what would the section be? and if i can't use
the tables, what formula do i work with to figure out the
resisting moment of the beam in this case? mr= mc/ix perhaps?
tobman - as lutfi has stated, the unbraced length (of the compression flange) is the primary concern. using load case 22, part 2, aisc manual of steel construction, asd, 9th edition as a guide (page 2-303 in my copy) the compression flange is the bottom flange for the entire 10 foot length. assuming that the cantilever connection provides lateral support (to the compression flange) the unbraced length is 10 feet. please note that it does not matter if lateral support is provided at the 500 lb. loading point or not - the unbraced lenght is 10 feet in either case.
from this information the allowable moment in beams charts (page 2-175 in my copy of the aisc manual) show that a w6x9 is the lightest section that meets the 5 kip-foot moment and 10 ft. unbraced length critera. this assumes a-36 steel and a conservative cb=1. any beam directly above this point on the chart will work also - such as a w8x10.
best wishes
slideruleera:
you have answered my question completely. i was under the
impression that the selection tables were only for simply
supported conditions, and that the cantilever condition did
not apply. this will make this problem a whole bunch easier, as i'm often asked to put up beams here with a bit of an over hang past the supports for
rigging purposes, and i've always been unsure of what to use for the unsupported length.
thankyou very much.
your are very welcome.
tobman - in rereading your latest post i picked up on something you may need to consider: a beam with an overhang (perhaps like load case 26 on page 2-305) is not the same thing as a cantilever beam (load case 22 discussed above). if your situation is like load case 26 come up with a hypothetical setup for discussion - the beam selected may be different.
slideruleera:
my beam in this case is like load case 22. however, i am
asked to put up monorails quite a bit of the time (load case 26) for rigging motors and such. when i do these, i
generally use tables put up by the monorail manufacturer's
association, using the max load on the monorail multiplied by an impact factor (usually 1.5). i then check for 20% of the load acting perpendicular to the monorail and 10% longitudinally down the monorail for both the monorail and the connections to the supporting beams. i then check the lower flange buckling from the charts and select my monorail. however, the charts here again do not state what to do if you have a monorail that overhangs a support with the load at the end as shown in case 26, a very common condition in the field. i have been checking the overhang by mr=mc/i and ensuring that i don't exceed the capacity of the monorail that way. i'm curious if the same solution as
you mentioned before using the overhang as the unsupported length and checking with the beam selection tables in the steel code to ensure that the moment and shear capacity are not exceeded would be prudent here as well.
hi tobman,
your problem is also very common to me; and i would suggest that you use 2x cantilever length as the unsupported length. my understanding is that the compression flange is very similar to a compression