Metal Plate Connected Trusses - Advanced Attic Truss Theory 4

[KootK]

Not really a question but, rather, a public service announcement of sorts. Pre-fab attic trusses are a nifty innovation that sometimes cause engineers a bit of discomfort, particularly during renovation assignments. This one was designed recently by a friend and one of the best truss engineers in the business. Some interesting features of this, particularly elegant, attic truss:

1) Pass through vertical webs from the lower to the upper truss segments which simplify the tricky plated connections and are good for stability during fabrication and in service.

2) Perfect triangulation of the panel points between the upper and lower truss portions.

3) Creation of a sort of moment frame up and over the attic space.

The only drawback of the configuration that I can see is that the clerestory probably complicates the diaphragm design a bit.

Too purdy not to share...

 

[r13]

pham,

No. We are taking about the same thing, only taking out of the consideration of walls as diaphragm sometimes. I consider a diaphragm is a horizontal element - a concrete slab, rigid metal deck, or made out of panels with structural elements, such as beams, purlins, joists, rafters. For the latter without the panel, there is no diaphragm, but still there is a "diaphragm action" to complete the load path - the loads are carried and passed over by beams, purlins to rafters, braces and columns, then to the foundation. Our difference is I don't see the diaphragm and diaphragm action are equal, although they are directly related. I would agree that a building can't stand without the ability to pass the loads through diaphragm action.

 

[KootK]

- I notice that there are no eave overhangs. Seems weird if this is being built in the lower mainland with their humid climate and building envelope challenges. Seems like there is more to the story...

Not my design so I couldn't say with any certainty. Some possibilities:

1) This roof might be sandwiched between some other roofs.

2) The entire first panel on each end might be the "overhang".

3) Might be an agricultural or cheopo commercial building with strapping and a short "sheathing" overhang.

4) Occasionally, builders will request that they overhangs be scabbed on to the top chord in the field.

- I wonder why the clerestory is there, since it seems like there wouldn't be much ability to draw light or otherwise add windows there. What am I missing?

1) It would almost certainly be for light, ventilation, or aesthetics.

2) For some occupancies, the owner won't much care if there's sticks interrupting the light.

3) For wall we know, the trusses may be doubled up and spaced greater than 2' to reduce the interruption of light.

4) This could be a localized condition in an otherwise non-clearstory roof. Kinda like a dormer.

- What limits the span of these? Is it a typical span/depth ratio thing, the span of the bottom chord as a floor joist, both?

Both of those but, really, most issues can be overcome other than, perhaps, reasonable shipping and erection limitations. I've seen nifty version of attic trusses like the one shown below where the bottom chord is actually a sub-truss in it's own right. Bad ass. That said, most attics I see are in the 30' range limited by a desire for 2x8 top chords and 2x10 bottom chords. Like anything, the sky's the limit if your pockets is deep enough.

 

[r13]

...but without any sheathing or temporary bracing. It's truly just a heap of sticks.

If not for the cost, can't you make the temporary bracing into permanent braced condition. It is uninhabitable without roof/floor panels (service function), but unable to resist the loads because unstable (structural function)? It is up to the designer then.

 

[KootK]

If not for the cost, can't you make the temporary bracing into permanent braced condition. It is uninhabitable without roof/floor panels (service function), but unable to resist the loads because unstable (structural function)?

Sure, one could needlessly spend gobs of money to buy themselves a wood framed building braced only by discrete bracing elements (1x let in bracing, metal strapping, etc). In the photo that I posted above, you may have noticed that much of the temporary bracing is outside of the plane of what will eventually be the building envelope.

 

[r13]

Thanks for spending time with me. I cease my case

 

[KootK]

No sweat. We're all entitled to a few volleys of considered dissent. I live in an all glass house in that respect.

 

[Craig_H]

Ooh nifty use of a floor truss indeed. Thanks!

 

[milkshakelake]

The only problem is that these systems are proprietary with moment capacities. Mere mortals like me can't calculate it. But they're still great and innovative.

In my area, it's used more in rural/suburban areas. Urban areas don't like crane permits so they do stick builds.

 

[phamENG]

milkshakelake - welcome to immortality: TPI 1-2014

 

[milkshakelake]

@phamENG I didn't know that existed; I thought Mitek software was the only way. Thanks!

 

[vitalstatistix]

KootK,

I am in the Vancouver area and looking to get new attic trusses for a reno my own place. Would love to work with a truss engineer/supplier that displays the level of thought shown in your original post. Any chance you can share contact of that company or your friend specifically?

Cheers

 

[KootK]

Reach out to Govinder Dhesi at West Coast Home & Truss LTD: Link. I don't imagine that you'll have much trouble getting in touch with him but, if you do, report back and I'll find a kosher way to put you two in touch.