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...why should I...
7

...why should I...

...why should I...

(OP)
...design the roof trusses if the responsibility for the metal plate-connected wood trusses is really that of the truss manufacturer?

I am the engineer of record for a small timber building - for which I've decided to use metal plate-connected wood roof trusses.

RE: ...why should I...

Has the client asked you to design these?

If this is the case then explain to them that each truss manufacturer has a slightly different way of making these depending on their abilities and economies. If you were to design the truss for them then this would force them to build it in a less efficient way and could end up actually costing more.

RE: ...why should I...

you shouldn't.

RE: ...why should I...

(OP)
Fair enough. Here's the deal: I am being challenged for running "design calculations" for several roof trusses. From the very start, I stated I would specify metal plate connected wood trusses - knowing full well that the truss manufacturer - of the construction contractor's choice would be responsible for the design.

I came up with what I called a "representative roof truss" regarding span, webs and chords assuming a certain wood species. I determined that the top chord, for example, wouldn't handle unbalanced snow loads unless it was a minimum 2X6- that is, I established a minimum condition that I was satisfied with. I did not and never had the intention of doing any truss plate design.

My ultimate goal was to draw a representative truss (I'm doing my own drafting) that I could show on a drawing and, in the event that someone asked me if it was "strong enough" or something like that, I'd be able to state, with confidence, "yes."

The span tables at my access (Alpine's, et al) did not address the heavy snow load of the project area, etc.

RE: ...why should I...

It's still debatable if the manufacturer will follow your diagrams. Generally here in Texas they are viewed as "recommendations" more than anything. Usually because a number of engineers don't address the application beyond a structural analysis (for somewhat obvious reasons) leaving the truss designer to apply the diagrams to a realistic framing usage.

RE: ...why should I...

dave-
Who has asked you to design the trusses or challenged you about not designing them.  We use performance specs when we use wood trusses.  We never show a complete section with web members (only partial for sections showing blocking, etc..), and we never specify any chord member sizes.  If the truss manufacturer can get the top and/or bottom chords to work with 2x4's (higher grade and/or better species obviously), then we just check their calcs when they come in.  A small part of our business is a local convenience store and we perform CA on about 40/year and have never had a problem.

RE: ...why should I...

(OP)
Good points.

1. I'm not expecting the manufacturer to follow my diagrams - but I do expect him to conform to a certain roof slope, minimum overhang, etc. Also, I needed to give something to the architects for their drawings that would work in reality - if, say my drawings didn't make it to the manufacturer or whatever.

2. I'm being challenged by "management" for "spending too much time" when I could have simply drawn a triangle on top of a square and labled them "truss" and "wall" respectively, and been done with my structural engineering work. Management in this instance is light on design experience and heavy on insanity experience. StructuralEIT, that's a good point that the manufacturer could make things work with a different species, etc. However, IMO I am giving up too much control over design. Wait... wait. The project will be value-engineered (part of schedule, no surprises) and if it's detemrined that calling-out a minimum 2x6 is too cost-restrictive, then maybe I'll bend - if they don't have to use Zebra wood to get the strength... There is also the very, very real possibility that *if* I get the manufacturer's calcs (out of my control) they will not be on time, the pressure will be on to rubber-stamp them, etc. I owe to to my profession (yeah, yeah, yeah) to be a little bit diligent; the truss manufacturer (of high integrity, I'm sure) isn't the user and isn't goign to look at the finished project year after year.

thanks for your posts!

RE: ...why should I...

Your should also be aware that you will have to complete the design of the anchorage for the permanent continuous lateral bracing as per the TPI code.  For this you will have to review the stresses in the web members or you can just specify some other method to provide the necessary stiffness to the web members to prevent them from buckling.  This is particurlarly crucial with heavy roof loads.

RE: ...why should I...

Dave,

Read through and show your client chapter 2 of the ANSI TPI1-02.  This chapter clearly delineates everyones responsibilities as it pertains to metal-placte connected wood trusses.

woodengineer

RE: ...why should I...

(OP)
oldrunner - ABSOLUTLEY. The fact that "management" didn't understand that intuitively really "upset" me. It upset me to the point that I called them "morons."

RE: ...why should I...

DaveVikingPE;
If you have determined from tables/design/experience the need for 2x6 top chord trusses due to heavy loading, then that should become part of your specification. A truss fabricator increased or reduced chord size can affect the architectural appearance of your project which you might not have designed for or the owner may not approve. A stick diagram depicting truss profiles or building sections will clarify the truss requirement, but you should not show the webing of how the trusses are to be manufactured. A truss fabricator will want the freedom to optimize configuration based on their capabilities, and you want the manufacturers product liability to cover their design work, not yours. On your engineering truss specification sheet you might consider these:
  • This project has been engineered for truss profiles shown and manufactured using 2x6 top chord with both top & bottom chords of SYP 19%MC lumber
  • Substitution of different size materials, lumber species, or profile changes will require additional engineering expense and design time
  • Truss calculations shall be submitted to the project engineer for approval with sufficient time to design permanent bracing of truss components into an engineered roof system

RE: ...why should I...

(OP)
Excellent! Thanks!

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