# Rigid Cross section should calculate as a stiff rigid unbending object, no?

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If I'm using truss that is set to "Rigid", I shouldn't be getting flexible calculations, no?

Picture a 60' truss, shouldn't matter the brand if it's set to Rigid (I'd think). Placing a hoist at either end gives green.

Placing a second hoist at one end, about 6' in from the end, results in the outer hoist chain being compresses and failing. Yes, in the real world the middle span of the truss deflects down and the inner hoist acts as a fulcrum. But, if all is rigid, in theory, the end hoist shouldn't know about deflection.

What am I not understanding?

Thanks, Scott

Edited by SCParker
• 2 months later...

Using rigid cross section data confuses the calculation algorithms.  You do not seem to have weight on this unless its hidden, and its hard to read if it is.  But basically with more than one hoist and rigid cross section means that the weight is all calculated on the closest hoist point.  At least this has been my experience in chatting with the vectorworks engineers in my service cases.  You must also take into account the chain lengthen feature to make sure all the chains are level.  if you think in the real world, the middle motor would have deflection in it, thus the middle motor would be an inch or two shorter than its outer partners.  So with all these calculations fighting against each other, with a rigid cross section makes the numbers not really reliable.  Certainly use paper and pencil to make the numbers verified.  You also have to take into account that VWX uses a 2:1 safety factor by default I was told, so you may have to check your settings.  The engineers are working so hard to try and get data uploaded for each truss manufacturers cross section data, so in my opinion, just like mr brooks... only use the truss that is in vectorworks and stop using tomcat, thomas, etc... maybe they will fess up and give them the numbers... who knows, maybe Thomas truss really is and they know it, thats why they wont release the numbers!

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