Dead hang chain constantly failing

[mdarcy]

Hello all

 

When I try to suspend a subhung truss by 3 or more connections, the central chains give the error "chain is under pressure and has failed", regardless of the subhung load or capacity of the deadhang. The only thing that corrects this is adding a third hoist to the truss system above. 

 

In real world practise, I can not see how the chain would fail, especially if rated to 1000kg with a subhung load of 36kg across 3 points.

 

Any help would be appreciated, file is attached.

Failed Chain.vwx

[jcogdell]

You are getting this error message because the dead-hangs are reacting to the deflection of the upper truss.

 

There are 3 ways that you can fix the error message

 

The first method is to attach a middle point to your pre-rig truss to reduce the vertical deflection in the middle of the truss

 

The next is to turn on the 'Compensate Drops' option in the Braceworks calculation preferences, this tells Braceworks to automatically compensate for the deflection of the upper truss to achieve the correct trim height. This simulates how your rigging crew would work on site leveling the truss

 

The last method is to manually adjust the dead-hangs to compensate for the deflection using the 'Chain Shorten' field in the OIP,

taking the data from your file the left hand dead-hang would need to be shortened by 19mm, the middle by 20mm and right hand one by 19mm.

(Turn off all the influence line display classes except 'du' to get the deflection values from the upper truss)

 

The advantage of using the last method is that the dead-hang and the calculation report will display the chain shorten data making it easier for your site crew to correctly adjust the dead-hangs.

 

On the subject of the error message itself I will contact the dev team to get a better explanation of why this particular message is used for this scenario

 

 

 

[mdarcy]

Hi Jesse.

 

Thanks for that, I thought it was an issue with deflection, but was confused by the wording of the error.

 

I hadn't considered the "Compensate Drops" option which would solve my problem. Adding a central hoist isn't an option for this application but I did notice that it fixed the issue when experimenting. 

 

Thanks again,

Mark