Please classify this Steel Web Truss 1

Home-made steel Warren trusses?

Are they cantilevering from the back wall? Big galvanized plate at the back of the truss!

Maybe left over components from a scaffolding/shoring system and "bastardized" to suit.

They sort of look like small transmission tower legs....especially with the galvanizing.

I was told by the site assessor that the homeowner said he did salvage them, but he did not understand where.

Here is a photo of how they are attached at the front of the garage.

You have been engaged to determine the structural adequacy, or otherwise, of the roof framing system?

Some 'funky' connection (or lack of) at this detail:

I am a structural engineer, but I am used to deciding if wood structures are adequate. This garage customer wants to put a solar panel system on his roof. So I need to decide if the structure is adequate to bear the code bearing loads and the PV system.

You don't know the steel material (yield strength), or the type of weld material, or perhaps the weld sizes, or the pipe thicknesses.

So you'd have to either sample and test pieces of these members or it seems like you might need to load test this to know for sure.

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Almost zero chance you get any info about the origin of the trusses here. As JAE said, you're going to have get wall thicknesses, weld details, etc...and just go through the analysis. I would start with a very conservative analysis assuming a low steel strength and maybe assume schedule 40 pipe - maybe with all worst case assumptions you're only at half capacity. That will give you an idea how deep you have to dig to verify the roof can take the additional load (or withstand code minimums in the first place). If testing is required, maybe the homeowner has some left over components you can have.

Side note - your diagram of the truss doesn't match the picture. The diagram shows the last web member connecting at the end of the top chord...picture shows the last web member framing in short of the end of the truss. Big difference for the top chord.

CANPRO said:

I would start with a very conservative analysis assuming a low steel strength and maybe assume schedule 40 pipe

Click to expand...

I would think assuming sch. 40 is un-conservative. Start with 18ga fence tubing and go from there

Yeah that doesn't look like pipe to me.

Other Fun Concerns:
I don't think you have any real top chord bracing. by the photos it looks like your bottom chord is not braced which means your truss model doesn't work.... Bar joists need bottom chord bracing for the compression-web model to work properly.

Diaphragm?

Wall Capacity?

To be honest, you are going to have to say that this is not adequate for any additional loads and you cannot verify if it can support existing loads at this time.

They look very much like the proprietary trusses used in scaffolding (as Ingenuity posted). Have you been able to spot a manufactures stamp? Most of these systems are an alloy so be careful if your assuming steel.

Picture for comparison:



As pointed out earlier, there are insufficient/missing chord ties for this "system" to have any real design strength.

Looks like rigging or light/speaker trusses from the entertainment industry......perhaps part of a stage or theater setup that is quickly put up and taken down as the production moves from one venue to the next.

XR250, I agree that sch 40 wouldn't be conservative in checking the lower end capacity...poor choice of words on my part. I think it would be a reasonable assumption to make in estimating the capacity of the truss - but I would most definitely confirm.

Might not be so bad.

- you have top chord bracing at 2' o/c.

- with CHS top chord and end rotation restraints you can probably get by without bottom chord bracing for gravity load at least.

- webs shouldn't give you any grief as they're pretty stocky relative to the flanges.

- At 38, span to depth ratio is high. But, then, it's 26" on centre steel in a light weight system.

Maybe start with some simple deflection estimates then gauge whether it's worth going any further.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

Daniel Kaufman PE:
I think it is important, and it’s certainly time, to sit down with the client, and explain the facts of life. After all, you aren’t a miracle worker, even if he does think you should be. He built it on the cheap and now wants it to meet current code and support added loads, and that’s pretty unreasonable, given that it probably wouldn’t meet min. stds. as is. You’ve got a fair amount of ammunition from the posts above. Ask the client for the original plans specs., design info. and calcs. Ask him for the manuf’ers. name and spec. sheets/shop drawings for the roof trusses. Explain why you need these if he wants his questions answered. We know what the answers will likely be, but he is now asking you to make something out of nothing, and is he willing to give you the free rein to do what needs to be done and to pay you for your time and effort? If he can’t provide the answers and info. you need for a reasonable analysis and investigation, you will need to do material testing, etc. to develop this design info. Make sure that he understands that you are not willing to just act as his reinsurer, or insurer of last resort, just for a small engineering fee.

As dhengr says, talk to the owner but suggest adding to the existing "trusses' sufficient added members to at least carry some degree of COMMUTABLE loading, plus assume the lowest working stress for the steel once you have determined tube wall thickness. However, it might be a "good" job to walk away from.