huangyhg

cranked beam with rotational freedom
i have a problem involving a cranked beam. i have a retrofit structure which is basically a large central column with two bracing members at high level. the bracing members must brace back to set point on the existing structure.
in order to reach this point both of the bracing members must be cranked.
the problem occurs because we do not know the exact angle we need for the crank so need to detail an arrangement which will allow for angular rotation at the joint, while maintaining fixity.
we are looking at an arrangement with a threaded rod spanning across the angle which can be tightened or loosed in order to push apart or pull together the members. think of a pair of compasses and the central wheel for adjusting them.
i am concerned about a couple of things:
1) the force required to push the members apart is too great for a person to exert. it must be installed manually.
2) the axial forces within the bracing are trasferred to the bar and strip the threads returning the joint to a pure pin. which would lead to failure.
if any members have any thoughts i would be glad to hear them.
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i am not sure what you mean by a crank beam, but if i follow your description you need two brace elements at an unknown angle with a fixed joint between the two members. if you are able to attach a gusset plate to your column you might be able to attach the two braces to the gusset plate with a single bolt. this would allow the angle between the two
by a cranked beam i mean a beam that has a bend on plan somewhere along its length (as opposed to a straight beam), but you have followed what i was saying correctly (its always harder to explain than draw).
with hindsight i also forgot to mention this structure is on the bottom of the north sea, and will be installed by saturation divers. generally welding and drilling subsea is best avoided if possible so we were trying to come up with an arrangement that can adjust the angle mechanically, but still maintain stiffness.
this sounds a lot like the mast on a boat (or at least very similar to).
could you use a cable system ? you could use a "spreader", a relatively short compression member, and possibly a fixed length of cable connecting the end of the spreader to the "mast" (making a triangle) possibly with a turn-buckle to adjust the length on installation, and a long cable with a turn-buckle to reach the ground. the turn-buckle allows you to apply whatever tension load you require, and it can fine tune the shape of the "mast".
good luck

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