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...

 

[skeletron]

Hey @KootK: are any of your contacts in the truss industry willing to talk about that stream of construction? I was thinking about writing an article for an industry newsletter in BC. I see, and spec, a lot of these trusses for residential repairs. Let me know if I can contact you for further info.

 

[KootK]

I would love to help. Or, at the least, help to scare up the right person to help. Check this out and let me know if you have any difficulty sorting out my contact address: Link

 

[Craig_H]

Nice share, KootK! Interesting approach with the clestory for sure. I'd hate to hijak your thread, but this made me curious regarding how best to handle the floor diaphragm when using attic trusses. I assume that it's necessary to apply the diaphragm on the ceiling below rather than the floor surface, as it wouuld be impossible to get a continuous diaphragm in the void spaces on either side of the attic. How does that normally get handled? I can't say that attic trusses seem very popular in my market, but it would be good to know the most economical way to handle this for when it eventually does come up...

 

[KootK]

I can't say that attic trusses seem very popular in my market...

Don't be so sure, my market is your market.

I'd hate to hijak your thread, but this made me curious regarding how best to handle the floor diaphragm when using attic trusses.

1) To my knowledge, nobody in our market pays the diaphragm any serious attention for this kind of construction. So there's that.

2) Usually there not a clearstory on these things which makes the roof diaphragm basically no different from any other pitched roof.

3) The attic floor does indeed need some kind of lateral stabilizing which might be:

a) Bottom chord gypsum sheathed diaphragm although we don't normally attempt to quantify that (and it's discontinuous).

b) Discrete 2x bracing although you'll pretty much never see this done, nor done anywhere close to properly.

c) The setup below were I try to get it done by the book and without relying on the ceiling.

 

[Craig_H]

Thanks! Interesting how much variation there potentially is across the province in terms of construction preferences.

I like the interior shear wall approach - hadn't thought of that approach, and just assumed I'd need a plywood layer between the bottom chord and the ceiling below. I suppose gypsum could be used, but like most, I try to avoid it.

 

[r13]

To my knowledge, nobody in our market pays the diaphragm any serious attention for this kind of construction. So there's that.

The word "diaphragm" seems over emphasized in many occasions. It is only one of the lateral load carrying/distribution element in the lateral load resisting system.

 

[jayrod12]

Be that as it may, Koot's comment about lack of attention to diaphragms is true for my area as well. I feel except for the people in high seismic regions in Canada tend to just ignore it. I'd be lying if I didn't say I'm much less attentive to diaphragm details then many of you on this board, but I'm in a low (read:non) seismic area and generally the diaphragms are of the size that they're generally ok with the bare minimum connections.

 

[KootK]

The word "diaphragm" seems over emphasized in many occasions. It is only one of the lateral load carrying/distribution element in the lateral load resisting system.

In the space that is mass produced, residential construction, it has been my experience that diaphragms really are the only horizontal, lateral load resisting and distributing elements. Consider that, in this space in North America:

1) Discretely braced floor decks, other than for temporary erection stability, are all but unheard of and;

2) When discrete bracing is used for temporary erection stability:

a) it's usually employed at a fraction of what would be required for a complete lateral load resisting diaphragm and;

b) even this is difficult to get installed well enough that buildings don't flop over and kill construction workers.

I feel except for the people in high seismic regions in Canada tend to just ignore it.

My impression to date is that it is ignored to nearly the same extent even in those regions. I think that, once things scale up to large scale multi-family dwellings, you start to see some attention getting paid. The truss that is the subject of this thread was designed near Vancouver and will be installed near Vancouver. As you can imagine, with that clerestory setup, it's highly likely that there will be some holes in the diaphragm load path.

 

[r13]

I think the illustration below provides a very good description of the function of the typical diaphragm, and its essence.

 

[phamENG]

r13 - I certainly don't disagree with what you posted, but it feels a bit out of context. Showing a generic LFRS description with a diagram of a multistory moment frame building doesn't seem to fit the discussion of diaphragm considerations in light frame single family dwellings with pre-manufactured roof trusses.

I think in the generic sense, we can "qualitatively" call on the mysterious redundancy of wood construction to say that the load path can easily circumvent the roof diaphragm in many occasions. That's probably how most get away with it. There's also the fact that very few buildings have ever seen or will ever see their design lateral load, making it hard to "catch" the bad designs. But...the only really reliable load path is through the diaphragm (and specifically designed and detailed connections between them and the structure).

I'm on the east coast of the US, just barely within the Hurricane Prone Region of ASCE 7-10. Diaphragms in wood structures are often ignored here, too. I've seen quite a few shear wall designs that paid zero attention to floor diaphragms. And if they did, they didn't address how the roof diaphragm would behave or connect. I try to correct that where I can, but then you have the inertia of the contractors. The "you want me to do what?" argument sure gets old at times.

 

[KootK]

The "you want me to do what?" argument sure gets old at times.

Right. If I pitched the plywood option below, I'd get stripped naked and beaten to death in the parking lot.

...and just assumed I'd need a plywood layer between the bottom chord and the ceiling below.

 

[phamENG]

Well if they aren't tarring and feathering you first, you have a better work environment than I do...

 

[r13]

but it feels a bit out of context. Showing a generic LFRS description with a diagram of a multistory moment frame building doesn't seem to fit the discussion of diaphragm considerations in light frame single family dwellings with pre-manufactured roof trusses.

I am curios how you distinguish a diaphragm in a multistory moment frame building and a wood frame building (assume properly braced), aren't they function the same manners - deliver/receive the loads, passing the loads to the jointed framing members, and stiffen the jointed framing members in resisting the lateral load? Neither me, nor the article/illustration dismiss the role of diaphragm in any building that incorporates it in the design, rather, I like to call for attention to the stiffness of the diaphragm (for stiffening purpose), if you are employing one in your building system. Can building survive without diaphragm? Surely can, but not without a strong framing system.

 

[KootK]

I am curios how you distinguish a diaphragm in a multistory moment frame building and a wood frame building (assume properly braced), aren't they function the same manners - deliver/receive the loads, passing the loads to the jointed framing members, and stiffen the jointed framing members in resisting the lateral load?

They do indeed serve the same, fundamental purpose. That said, there are important differences:

1) Relative to a commercial structure, a residential wood diaphragm will be much, much less rigid. So much so that the the code asks you to look into that and tailor your design approach to suit.

2) A discretely braced (trussed) horizontal diaphragm is all but unheard of in wood frame, residential construction.

3) Chords, collectors, and their attachment to things are much less "automatic" than they tend to be in steel and concrete structures where, to a large extent, you "get that for free".

Can building survive without diaphragm? Surely can, but not without a strong framing system.

Your average, wood framed residential building almost certainly wood fall down if its diaphragms were removed. This is because wood frame construction is very much old school, trabeate construction. Truly post & beam-ish as compared to steel and concrete construction where almost all joints can be expected to transmit meaningful amounts of moment.

 

[phamENG]

I can't think of anything else to add, so I'll simply go with "what he said."

 

[r13]

I think the main difference between the average residential building and other buildings are the cost. For all buildings, the basic load resisting systems are categorized as braced frame, moment frame, shear wall, and mixed/duel system. I can only think of the diaphragm is indispensable in the match box structure that consisted of walls and slabs only. I could be wrong though.

 

[phamENG]

r13 - maybe we have a terminology difference?

For all buildings, the basic load resisting systems are categorized as braced frame, moment frame, shear wall, and mixed/duel system

.

I generally agree there. But these are all VERTICAL elements of the LFRS.

The diaphragm is a horizontal element (or at least has an element of horizontal resistance when looking at a sloped roof diaphragm). Without some horizontal element to gather the lateral loads and take them back to vertical elements, your load path will be incomplete. In most structures, it's a diaphragm. In a lot of industrial structures (mill buildings at the large end, maintenance platforms at the small end), discrete bracing is used in lieu of a diaphragm. That's one reason some basic diaphragm analysis techniques break it down as a simplified diagonal brace.

 

[Craig_H]

KootK, back to your purrdy truss photo, I've got a few questions:
- 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...
- 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?
- 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?

Definitely a cool way to build attic space. Shame that the geometry of this one will all be covered up.

 

[KootK]

I can only think of the diaphragm is indispensable in the match box structure that consisted of walls and slabs only. I could be wrong though.

An unsheathed wood framed building is a good deal less stable than a wall and slab system which will have some, inherent rigidity at the joints. Imagine the situation shown below but without any sheathing or temporary bracing. It's truly just a heap of sticks.