Why has my workflow produced results that differ significantly from truss manufacturers specifications?

[Mike Wright]

I've just worked on a project which involved a moderately complicated indoor truss structure supporting an LED screen.  I worked with the project manager to choose the correct truss products, spans and hoist types.  My resulting braceworks report indicated that none of the trusses were above 50% of their load limit even after applying chain shortening to account for the rigid nature of the LED screen.

 

The venue asked that a ticketed rigger look over the loads involved, a perfectly reasonable request.  Their default was to look at the weights and check them against the manufacturers load table.  The configuration was as near as damn it 5th point loads and the table indicated that two of the trusses would be 110% of the safe load.  My rig was therefore rejected.

 

I then had a very experienced rigging engineer and another very experienced rigger look over the drawings and they were all in agreement that even before the dynamic loading factors were applied, the system was beyond or close to the edge of the capabilities of the truss.

 

This has raised a lot of questions for me. 

 

Have I been over reliant on braceworks as a proof of concept, should the manufacturers tables have been my first point of call?

 

Is the truss system here too complicated for braceworks to reliably calculate?

 

Have I over stepped the abilities my moderate level of rigging experience gives me?

 

There's a screen grab of the system below.  I can't share the file at the moment but will attempt to re-produce it in a simpler form.

 

The problematic trusses were the two larger spreaders and the end solution was to add two more of them so the load was (roughly) quartered between them.

 

I'd be interested in peoples opinions/advice

 

Thanks

 

[jcogdell]

Hi Mike

 

Could you DM me the file, along with the exact settings you used in the Braceworks calculation? (the file will be kept fully confidential)

Once I've looked it over we can share a screen and go through it to see where the issue is.

[Mike Wright]

Hi.

 

I'll do my best to get something over soon (I'm on site at COP26 in Glasgow which is a.....challenging.....site).  It might take me a few days to find time.

 

For the sake of not getting told off I should probably say this is not related to COP26

 

Thanks

Edited November 2, 2021 by Mike Wright

[jcogdell]

All good, hope everything is goes well.

 

[Art V]

Not a braceworks user but I did have a similar issue of differences between calculated and real world. One thing to check with the manufacturer's tables is whether these are calculated or measured data.

 

In my case the supplier was using calculated data and there was a rather significant difference with measured data in this particular case even though their and my calculated data (based on theoretical values) were basically in agreement. With things like trusses I would assume that the manufacturer has done load tests etc. and use that for their tables but you never know for sure until you ask.