Structural Engineering Riddle of the Day
Structural Engineering Riddle of the Day
(OP)
Dear All,
Perhaps this question warrants a major refresh of classic structural engineering mechanics (speaking for myself).
Why does the stiffness of the horizontal beam in system 1 have absolutely no bearing on the moment distribution in the vertical elements? i.e. the vertical elements in the frame of system 1 are exactly the same as modelling an infinitely stiff joint (encastre). Changing the stiffness of the beam has no effect on moments.
Whereas, the stiffeness of the horizontal beams in system 2 have a marked effect on the moment distribution in the vertical elements, i.e. modelling infinitely stiff ends obviously results in higher vertical end moments.
See picture below, the wording isn't 100% accurate it should read "vertical bending moment distribution"

Perhaps this question warrants a major refresh of classic structural engineering mechanics (speaking for myself).
Why does the stiffness of the horizontal beam in system 1 have absolutely no bearing on the moment distribution in the vertical elements? i.e. the vertical elements in the frame of system 1 are exactly the same as modelling an infinitely stiff joint (encastre). Changing the stiffness of the beam has no effect on moments.
Whereas, the stiffeness of the horizontal beams in system 2 have a marked effect on the moment distribution in the vertical elements, i.e. modelling infinitely stiff ends obviously results in higher vertical end moments.
See picture below, the wording isn't 100% accurate it should read "vertical bending moment distribution"







RE: Structural Engineering Riddle of the Day
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RE: Structural Engineering Riddle of the Day
_________________
Jones & Wagener
www.jaws.co.za
RE: Structural Engineering Riddle of the Day
System two is statically indeterminate. The vertical element internal forces are dependent on the stiffness and forces in the other members.
RE: Structural Engineering Riddle of the Day
RE: Structural Engineering Riddle of the Day
RE: Structural Engineering Riddle of the Day
DaveAtkins
RE: Structural Engineering Riddle of the Day
RE: Structural Engineering Riddle of the Day
Now as to what the riddle is, I have not idea.
RE: Structural Engineering Riddle of the Day
are the blue boxes UDLs ?
why would a UDL on a standing cantilever have anything to do with the RHS sketch, whatever that means ??
ok, the LHS of the 2nd sketch is a double cantilever, and so redundant; big deal, easy to solve.
RHS of 2nd sketch means what ?
another day in paradise, or is paradise one day closer ?
RE: Structural Engineering Riddle of the Day
Some would say that democracy is three wolves and sheep voting on what to eat for lunch... Ok, but admittedly in this case it sincerely wasn't the best way to convey the question.
Ignoring the sketch, the question really was why is the BM distribution in a certain frame independent of the stiffness of the connecting members?
Bootlegend seemed to understand the somewhat (ok, very) backwards way of asking the question so Kudos to him.
The answer is that in a statically determinant system the bending moment distribution is independent of the relative stiffness of the connecting members.
_________________
Jones & Wagener
www.jaws.co.za
RE: Structural Engineering Riddle of the Day
The sketches seemed clear enough to me, and whilst the answer to the question should be pretty obvious to any experienced structural engineer, it doesn't hurt to take a fresh look at things we take for granted, and for a student/inexperienced engineer I think this sort of question helps to generate understanding of structural behaviour.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Structural Engineering Riddle of the Day
In System 2, the columns on the right would have the same moment diagram as the single column on the left if both beams were infinitely rigid and the upper beam was prevented from translating horizontally or, to put it a little differently, if the columns on the right had the same boundary conditions as the one on the left.
BA
RE: Structural Engineering Riddle of the Day
Loads in the first cantilever can only go in one direction. Easy to distribute. Loads have a choice in the second figure, so stiffness comes into play.
Rotations and deflections are another matter -- stiffnesses (is that a word?) of all the elements are important in figuring out where these elements move to under loads.
"We shape our buildings, thereafter they shape us." -WSC
RE: Structural Engineering Riddle of the Day
Had to laugh at BAretired's take on it, as correct as it is.