Jib Crane Deflection Does it Matter if Mf & Pf at Jib Arm and Mast column is below Mr & Cr

[Deension]

Hello All,

I am evaluating a free standing over braced jib crane that is constructed of 170" high mast column. A 52" high pivot column installed at the top front of the mast column. A 200" long jib arm pin connected to the middle of the pivot column and a 1" tie back rod is connected to the top of the jib arm at about 165" from face of mast column given a cantilever length of about 35" more or less to the free end. The tie rod is connected to the top of mast column at the opposite end.
Mast is HSS 9" x 0.25"; Pivot column is HSS 6" x 0.25"; Jib Arm is W8 x 10 or 13 or 15.

I have checked for strength limit state design and the ratio of moment and axial force in all components of the crane meets and exceeding the requirements. However; there appears to be rather large deflection about 4 times as much as the limited value ... I know that the industry standard for jib crane deflection limit is L(in)/150 but does it really matter how much it deflects provided that there is no concern for hoist sliding on jib arm and that strength limit is not reached (Mf/Mr + Cf/Cr <=1). Does the deflection matter from failure of material point of view? I am not quiet sure on this so thought I could ask the group for in put. I thank you all in advance for you inputs!

 

[TehMightyEngineer]

I'd say it does matter. That much deflection could make it hard for the operator to push a trolley toward the support. You suggest that there's not concern for the hoist trolley sliding down the job arm, make sure this is the case.

Also, even though you didn't ask for it; make sure you check the load case of a fully loaded jib turned 90 degrees to the support. The eccentricity where the supporting hinge pins are offset from the centerline of the support can cause a pretty hefty torsion on the support.

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[Deension]

TehMightyEngineer; thank you for your comment.

There is no torsion on the jib as it will simply rotate when jib is rotated at 90 degrees. Torsion can only be induced if load is restrained which in this case is not the case. My question is leaving the serviceability issue a side, does it matter how much deflection do we get in a cantilever beam with column (jib crane) from material failure point of view? We have tested the jib crane to 125% of the rated capacity, the deflection is elastic. What we have is the deflection of column and beam combined that induces a very large deflection. It is worth mentioning that the maximum lift occurs very rarely (once in 3 years).

I am not concerned with strength limit state. But what I want to make sure is that the large deflection does not causes material failure. Which I think it does not not; however, I want to make sure that I am not missing anything.

Regards

 

[TehMightyEngineer]

Ah, deflection should not be a strength limit from my understanding as long as all your other strength checks for flexure and cracking were achieved. About the only issue I could see with large deflections is P-delta effects and/or invalidating the small-displacement assumptions in your elastic analysis. However, for a jib crane I don't see these being too much of an issue.

As for the torsion, I attached a PDF showing what I mean.

Professional and Structural Engineer (ME, NH)
American Concrete Industries
www.americanconcrete.com

 

[dhengr]

Deension:
The total deflection is primarily a serviceability and operational issue, as long as all of the stress/strength issues check out satisfactorily. The main deflection issue out on the boom will be that the trolley works o.k. and some shock effect as regards the springiness of the boom tip under load. Can they move the fully loaded trolley over the length of the boom without needing a whole team of guys? And, if the load is left hanging some distance in from the boom tip, is there a chance that it will just roll back out to the boom tip, moving out on the sloped boom? You can do a little adjusting to partly compensate for this condition, by setting the boom tip a bit high (above level) when under no load. Just tighten the turnbuckle on the 1" dia. tension tie rod, lifting the boom a bit. I would normally prefer to use slightly larger members to low the total tip deflection. The workers will appreciate this when moving a load on the boom.

 

[Deension]

dhengr and TehMightyEngineer; thank you both for your inputs.

 

[RWW0002]

Attached is a document outlining serviceability and strength requirements of jib cranes that might come in handy. There is a good discussion of the torsion issue and a simplified method of analysis.