Calculating Angled Truss

[Mike Rock]

Can braceworks handle load calculations on trusses that are not horizontal in front view? 

In the example file I have a 40' truss supported by a 1 ton hoist on each end. When I raise one end of the truss I would expect the higher hoist to see more of the load. Looking in VW help and the form I can't seem to find anyone else with this issue. Am I missing something? 

Tilted Truss Example.vwx

[Scott C. Parker]

This is an often misunderstood concept. Several factors support the weight loads of each hoist being the same regardless of the system tile.

• The truss you have is set to use a rigid cross section.
• The two hoists are placed acurately to maintain a vertical chain, which removes any sideways pull on either.
  • If the high hooks were attached to a stationary pic point and then you change the tilt of the truss, the chains would start to pull in/outward which would cause dinamic shifts in load and lead to pulling the system in one direction. If this were the case, the balance could then differ.
• The truss sections are solid and transfer the weight evenly between the two pic points.
  • In the real world, and when using spansets, they would shift slightly and the difference would be present. 

Think about is this way.
You have a vertical truss that weighs 100lbs. One person is holding up the top and another person is holding up the bottom. If both share the load equally, each exerts 50lbs of effort. Now, change the same truss to horizontal. Each person is still holding up 100lbs between them, 50lbs each.

 

PS: so others know, the file had only two hoists. The image above shows three and the weight would be distributed differently, but the calculations still hold true.

[Mike Rock]

@Scott C. Parker

I have not observed this behavior you are describing in the real world on automated truss systems with load cells. Both in situations where the upper hooks are fixed and situations where the upper hooks track to keep the chains plumb. 

 

Attached is an excerpt from Entertainment Rigging by Harry Donovan. Maybe a better way to phrase the question would be " does vector works take into account the center of gravity when determining loads" which based on the response is no.

 

Edited October 22, 2024 by Mike Rock

[Scott C. Parker]

15 hours ago, Mike Rock said:

Maybe a better way to phrase the question would be " does vector works take into account the center of gravity when determining loads" which based on the response is no.

Agreed. The various calculations offered do not fully deal with the inherent changes that occur when dealing with factors that shift the weight. An easily observed example is the shift in the perceived attachment points when using slings. VW and Braceworks use an attachment point based on the centerline of the truss and misses the shift caused by the rotation or tilting of the truss. 

In the photo above, the shift from the tilt causes extra mass to shift to the left, beyond the pickup point, and the amount of mass appears substantial. 

In the posted file, the amount of shifted mass is much less.

In retesting the posted file, I found that the tilted systems offer about eight pounds of different results compared to a flat version of the system. I shifted the hoists to attach over the third light from the end for each. The issue, though, is that the pickup points are not accurate as they are from the centerline of the truss. At the end of the day, it'll be up to the engineers to whom the Braceworks reports are sent to make the final reports.

 

In my first reply, I used an example of people holding the truss from the ends of the truss. In that case, there would not be a shift in mass beyond the implied pickup points.

 

I could see a future Braceworks improvement that adjusts the calculation pickup point to be offset from the centerline of the truss. This would require users to always choose a desired pickup symbol that tells BW what the offset is. It would also require info from the pickup point object manufacturers as to the side load bending resistance strength of their solid pick point objects. A wide and deep rabbit hole to get stuck in.

 

Great conversation. Thanks, Scott

[Mike Rock]

I figured it was a big ask but wanted to see if it was a limitation of the software or a user error. At least I know it isn't a me thing.