Building/runway impacts for overhead cranes.

[DKDMA]

Reviewing section 4.9.3 in ASCE 7-16 states the following:
The maximum wheel loads shall be increased by the following percentages.

Vertical Impact Force:
Monorail Cranes (powered) - 25%
Cab-operated cranes - 25%
Pendant operated cranes - 10%
Hand-geared - 0%

I have been using 25% impact for all underhung cranes in my design work.
But now have been confronted that this is incorrect.
What constitutes Monorail Cranes? Or is the logic that it has a pendant 10%?
Can someone help and clear this up?
Thanks for your time.

 

[dauwerda]

I'm not sure that I understand your question. Using 25% for all cranes is not wrong though, it at least meets the minimum requirements for powered cranes and goes above and beyond the requirements for pendant-operated bridge cranes and hand-geared cranes. The actual excerpt gives a little more information than what you listed above (mainly that the 10% is only for pendant-operated bridge cranes).

The way I interpret this is that a powered monorail crane (however operated) needs to be designed for a 25% increase while pendant operated bridge cranes need only be increased by 10%.

The commentary also points to a few other references that may go into more detail on this, see excerpt below:

 

[DKDMA]

dauwerda,
Your are right, I should have been more clear.

I have been designing (powered) underhung cranes using 25% impact. Hand geared cranes require no impact adjustment.
Be that they may be a double girder underhung crane or single girder underhung crane, but controlled with pendants still requires 25% impact on design.

It was explained to me that a "Monorail crane" is a monorail beam (singular), thus the name "mono"-rail. So 25% should only be used for monorails.
Double girder cranes that are operated by pendant only require 10% impact design. Whether it is a top running crane or an underhung crane.

I assumed that a crane, double girder or single girder, hanging from the lower flange of the beam should be 25%, thus the "monorail crane" statement.

So I am questioning which approach is correct? Am I right in my assumption?

 

[dauwerda]

I believe what was explained to you is correct.
A monorail crane consists of a single stationary beam that a crane trolley can travel lengthwise on (so it only has one axis of travel). A bridge crane, whether single girder or double girder, allows for the lifting device to travel up and down the girder (or "bridge") part as well as forward and back on the runway beams on either end of the girder (so it has 2 axes of travel). See this link for a more thorough explanation:

https://www.mazzellacompanies.com/Resources/Blog/what-are-the-different-types-of-overhead-cranes

It sounds to me like you have an underhung bridge crane. If it is pendant operated you can use the lower 10% impact force increase.

That said, I have seen more and more bridge cranes getting installed with both pendants and remotes, which would require the 25% increase for structure design. An owner may want to change a pendant operated crane to remote operated (or at least add the remote option) in the future as well, so it may be best practice to always use the 25% as you have been doing to allow for future changes without requiring structural modifications.

 

[DKDMA]

Thanks so much for your response dauwerda, I do know some states do require 25% for underhung cranes.

 

[ajh1]

A lot of this is based on the travel speed of a crane. If a crane is pendant operated or hand-geared it can only travel as fast as the operator can walk under it. Thus the impact forces are lower as there is less dynamic action. With a cab controlled or remote controlled crane, the crane speed is independent of the speed that an operator can walk under it.