huangyhg

cantilevered steel i-beam capacity
should be a simple question for a structural engineer. i need to lift a fountain from a lake and plan to rig a cantilevered steel 6"w x 4"h i-beam of .25" thickness. a 12" wooden piling will support the middle of the 11 foot beam and the rear will be well anchored to two pilings. what weight could i expect the cantilevered five-foot section to support? the fountain weighs about 1000 lbs and with chain fall and lifting sling, the total supported weight would be about 1100 lbs. would a 6"w x 6"h i-beam be a better choice? any help would be appreciated. thanks
as per my advice to all looking for a definitive answer, see a local engineer, do not seek an exact solution here.
sc
hi rodkent
depending on how the pilings are attached to the beam it may not be a cantilevered beam it maybe a simply supported beam in which the supports are not symmetricaly placed about the beam centre. if your case is a cantilevered beam then you have a beam which is statically indeterminate, to expand a little a cantilever beam is a beam that is built in at one end and supported such that the built in end is not free to rotate in the plane of the load.please confirm
how the wooden pilings are attached and i maybe able to help you further.
regards desertfox
desertfox,
thanks for your reply. maybe what i plan is not actually a cantilevered beam, but is depicted below. total length of the main i-beam is 11 feet with the center resting (unsecured) on a 12" diameter wooden piling. the rear is bolted to the bottom of a purpendicular 48" i-beam that is secured to two 12" diameter wooden pilings with large lag bolts through welded end flanges. the rear support pilings are spaced 48" apart.
the desired result is to hoist an 1100 pound load with the unsupported end of the beam using a chain fall. the middle support piling is at the end of a deck and this arrangement would give me a five foot reach out over the water to hoist the fountain. would the 6"w x 4"h, .25" mild steel beam be expected to bear this load without buckling or should i use a 6x6 beam, or support the outboard end of the beam with a brace?
plan view of pilings:
o
o
o

elevation view of beam:

============================x 48" i-beam (end view)
o chain fall || main beam bolted under
| || cross beam at rear
^ sling ||
/ \ || beam resting on 12" dia.
load piling at middle

any help will be appreciated, but the worst case is that the fountain falls back into the lake. no liability problem here
thanks,
rod kent
hi rodkent
nearest info i could find here was a 6"x6" x 16lb/ft web thickness 0.24".
now i am only considering the beam and not the strength of the pilings.
using simply supported beam theory i estimate using a 5'.6"
overhang the following figures:-
reaction at centre support piling= 2200lb(acting
skyward)
reaction at the main beam end = 1100lb (acting down to
floor)
max bending moment on i beam is at centre support = 6050lbft
max bending stress in i beam = 7177,65 lb/in^2
for a beam of this size made from 43a grade material (steel)
the above stress would only be about 1/10 of its minimum tensile strength therefore:- the beam itself would do the job, however this assumes that the beam is loaded in bending only and that the supports can safely react the forces given above, this is about the best i can do,
regards desertfox
desertfox.
thank you for this analysis. prompt, thorough, and useful.
rod kent

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