Jib Crane Support 4

[dik]

I often encounter jib cranes with a circular post welded to a larger circular base plate. This connection is reinforced with numerous 'gussets' in a radial pattern. I would think that using a thicker base plate and eliminating the gussets would be less costly. This would include grinding the edge to reduce any tripping hazard. Is there a reason for using the gussets? Just to increase the stiffness of the connection?

Just curious...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[GC_Hopi]

My understanding is yes to both. It increases the stiffness and reduces fatigue cracking at the post to baseplate weld.

Also depends on the capacity of the crane. The stiffeners seem to be added at capacities over 1,000 lbs based on my observations.

 

[SWComposites]

Probably to increase stiffness and to avoid stress concentrations in the circular weld joint leading to fatigue failure.

 

[jhnblgr]

https://www.towernx.com/downloads/Technical_Manual_MP_BasePL.pdf

Provides insight into circular base plates and fatigue.

 

[human909]

While fatigue may be an issue even without fatigue there are other factors....

I would think that using a thicker base plate and eliminating the gussets would be less costly.

When you are used to dealing with I or H sections that has been my experience and my approach right up to extremely large members.

But with I and H sections you have twice as much perimeter area for welds. Furthermore your weld are loaded non eccentrically. Whereas with HSS your have less welds and eccentrically loaded welds.

I've dealt with HSS a bit recently including deep dives into guidelines and complex FEA (IDEASTATICA). Your welds really struggle without gussets or other solutions.

 

[dik]

Yes... fatigue... hadn't considered that. Thanks, gentlemen.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[bugbus]

If the structure is subject to cyclic loading, I would avoid stiffeners altogether. The AASHTO code on sign structures recommends a thicker baseplate with no stiffeners vs a thinner baseplate with stiffeners. Cracks will tend to form at the upper ends of the stiffeners before anywhere else.

I understand that the additional fabrication cost of adding stiffeners would normally exceed the added material cost of a thicker baseplate.

 

[dik]

Confused... that was my original thought. I have to do a little more digging...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Holbewoner]

Perhaps de-lamination of the base plate becomes an issue when increasing the thickness of the base plate.

Without gussets should be more fatigue resistant because continuous geometry. However, the load is not continuous: the base plate (assumed bolted) is bolted on the "ground". These bolts are not continuous, but discrete. And each bolt pulls stress to the nearest position in the circular weld. This creates a "wavey" stress pattern in that weld, with peaks equal to the amount of bolts. The gussets flatten this out -- just like a thicker base plate flattens this out.

Additionally, the welds of the gussets can be considered sacrificial against fatigue, as they can be inspected and repaired more easily than the main circular weld.