Crane Deflection Requirements 1

[Skipdeiter]

I am going to be putting a 10 ton and a 5 ton crane on a 52' span. As I am a MechE, the structural code is new to me (CSA S16.1), and I am curious as to the reasoning behind a 1/800 deflection limit. This seems excessive as stresses become non-existent with this requirement.

 

[johnp]

That's exactly the point, Skipdeiter,
Structural engineers think in terms of centimeters of deflection - mechanical engineers think in micorns.

A 25 mm deflection in a roof memeber may be negligible. For a crane that may be significant. Crane girders are usually designed on deflection limits, and not stress. 1:500 is not unuaual for a light crane. 1:800 is healthy for a heavy installation. The vaues are established as rule-of-thumb guidance from practical experience.

My feeling is that the deflection limit has more to do with vibration, than in trouble powering the crane up a 1:400 slope. But if you want some more solid guidance contact the Crane Manufacturers Association.

 

[RWF7437]

See thread

thread507-164583

 

[johnp]

Thanks RWF for the link to previous discussion.

Fatigue is another factor to take into consideration. Working cranes do amazing things to welds and web plates. These effects take place well below normal static bending stresses.

A 3 inch deflection under load on a girder carrying twin cranse can be unnerving. I've seen people have heart failure observing deflections smaller than that.

A crane can be looked at as a system of springs and levers, each contributing to the overall natural vibration frequency. This results in bounce at the moving suspended load. The only way to reduce this bounce under dynamic starting and stopping loads is to limit the deflection of the components - the winch cable, the mobile bridge beams, and the static rail girders. The recommended deflection limits are based on practical observation of successful installations - and shonky ones. The recommendations may not be the complete answer for all crane design applications, but they provide a fairly reliable starting point.