Jib Crane Design

[TND333]

The situation is a little harder to explain and not as quite as critical as the example I'm providing.

For Example: if I'm interested in designing a free standing Jib Crane.

How do I go about sizing the column? Resolving the moment from the load and Beam is easy enough, but I'm having some trouble incorporating the length of the vertical column into the calculation.

Thank You

 

[rb1957]

so the cable goes from the end of the boom to the top of the mast; the boom reacts the load at the two ends, at the outbd end this is the vertical component of the cable tension, the lateral componet is applied to the top fo the mast, bending it.

start with the load, make a freebody of the boom, determine the cable tension, make a freebody of the mast ...

another day in paradise, or is paradise one day closer ?

 

[drawoh]

TND333,

I assume your post is solidly mounted at the bottom and unsupported to the full height. Under these conditions, your bending moment will bend your post. Your jib crane will be loaded from the side. This will turn your post into a solidly supported cantilever beam with its maximum stress at the base.

You have told us nothing about the load, your requirements for rigidity, who is operating the crane, and who or what everything will land on if it fails.

--
JHG

 

[TND333]

Thank you for the responses,

I guess to further simplify it, if i have a solidly supported cantilevered beam with only a bending moment acting on it what determines the maximum stress?

The beam has a length (l) but should theoretically have a constant moment across the length. How do you incorporate beam length in the stress calculation?

Tom

 

[fegenbush]

This is a fairly common beam and beam-column problem for a structural engineer, but cranes can be dangerous so I would advise caution. The beam portion has flexure and point loading (including deflection) considerations. The beam-column (as you state it, column) will have compression and flexure. Both will combine to determine the capacity of the crane.

For the beam, please reference Chapter F of the AISC Steel Construction Manual (or AISC 360-10).
For the beam-column, please reference Chapter H of the same.
You should also consider second order effects (P-Delta). See paragraph C2.1b of the same reference.

Note: On small crane systems, deflection often controls. You should keep your deflections small (L/1000 where L is the length of the beam) in order to avoid having your trolley roll to the far end under design load.

I would be happy to help further, should you require specific direction.

 

[rb1957]

"if i have a solidly supported cantilevered beam with only a bending moment acting on it what determines the maximum stress?" ... is this a jib crane to anyone ? isn't this a cantilever ??

for a cantilever (assuming uniform section), maximum stress is at the base (where the bending is maximum).

when you say "with only a bending moment acting on it" ... do you mean that the beam is loaded with a moment ?

another day in paradise, or is paradise one day closer ?

 

[rb1957]

i thought you had ...

but it sounds like you have ...

pix from google search ... no advertising here ...

another day in paradise, or is paradise one day closer ?

 

[drawoh]

TND333,

In a perfect world, your jib crane post will never see a side load. In the real world, it will. Someone will pull or push your load from the side, possibly with a fork lift. Fork lifts make entertaining reading. I suggest you do a search and find out what you are up against.

--
JHG

 

[TND333]

To Assuage everyone's fears, this is for a personal installation to be used by me on my property. Most of the dimensions I've taken directly off of a commercially available jib.

But it left me thinking about a moment load on the end of a sported cantilevered beam (Say the Post of a jib crane) and how I would incorporate the length of the post in the stress calculation.

I know there is combined loading, additional compressive force of the weight of the load etc. but, for a pure moment loading MC/I seems lacking.

Tom

 

[rb1957]

from the 2nd pic, you can draw a free body of the crane, so the base is reacting direct load (the load in compression) and bending ('cause the load is off-set) ... P/A+(Pd)c/I

if it's a bought crane, it'll come with a load rating.

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

For a vertical load, the bending moment is constant along the length of the column. The length does not change the stresses.

 

[fegenbush]

If you check out the reference I listed, you will find that the length of the beam-column is included in the value Pc (design axial compressive strength). The components of this relating to length are the slenderness ratio, KL/r (to determine if the beam-column has slender elements) and Fe (the elastic critical buckling stress, or Euler buckling stress).

Another issue that has not been covered is how you plan to anchor the crane to the floor.