Message 510 of 521 (841689)
10-19-2018 9:47 AM
Reply to: Message 509 by Percy
10-18-2018 5:51 PM
Re: A Sort Of Climate Change Related Question
Disclaimer again - not my area of expertise - but here goes with another possible-explanation.
But how is "no blocking" stronger than blocking? It seems that blocking done correctly should even stronger since two parallel studs with no horizontal block between them have to be weaker than when there are one or more horizontal blocks.
Yes, you are correct if taken literally.
That is... blocking is better than no blocking if you're not going to add any plywood to the outside of the wall.
However... a wall is not as literally described here. The sheathing (the plywood on the outside) acts just as well as the blocking as far as bracing for lateral movement. In fact, it acts even better because it's larger and connects multiple studs instead of just two specific studs. It also has multiple nails in it per-stud or per-connection... where each blocking only has one point of connection to each stud.
To prove this point... think of an Ikea-ish bookshelf. Have you ever put one of these together?
You lay the long sides down, you put the top and bottom on and you add in the shelves.
Here, the shelves act like the blocking would in a framed wall as far as lateral-movement is concerned.
However, if you've ever stood up a shelving unit at this point, you know it's very weak against lateral-movement still. If you push it from one side, the two side-ends (the "studs") push over together and the "blocking" shelves barely do anything to stop this motion. Of course... the shelves aren't nailed into the studs where the blocking would be... but this isn't adding a lot of structure... only the connection point of the single-nails-per-blocking-per-stud. It's still relative-ly weak against lateral movement.
However, there's always that super-flimsy back piece you put on those shelves. Put that on... and the entire shelf suddenly becomes super-rigid. This back piece is the sheathing... the plywood we put on an exterior wall. It's very good at preventing lateral movement because it connects the top and bottom and all the studs together into their "full square shape" with multiple connection points (multiple nails per stud or top/bottom piece.)
A non-windstorm plywood is good... but if it is jointed in the middle it means it's not connecting the top and bottom and studs all together. It's only connecting the top and studs... or maybe just the bottom and studs. This is "good" but connecting all 4 sides... top, bottom and studs... all together with one large-enough windstorm-plywood (or whatever it's made of) is much better for bracing against lateral movement.
Also, there's a step-back view of the whole structure.
Windstorm plywood is larger... so you have less pieces in total. Regular plywood is smaller and would require more smaller pieces to fit together in order to cover all external faces.
When dealing with structural support... larger one-piece things are always stronger than smaller-multiple-pieces-connected-together things. Therefore, even without blocking, the less-pieces-because-they're-bigger idea of the windstorm plywood covering is stronger than the smaller-multiple-pieces-connected-together idea of the non-windstorm plywood.
|What is the "gable end hinge point"? Is that the top of the gable? And what is the "bottom chord"?|
What is a top chord? Is this the underside of the roof?
Let's go through some pictures:
Just need to look at the top-most picture on the right side here.
A Truss is one-triangle-piece of a roof support system.
Bottom chord is the base of the triangle.
Top chord is either one of the two top-sides of the triangle.
Examine the blow-up of the overhang. Understand that the "butt cut" is the part of the roof-truss that is exposed to the exterior wall-side, but not covered by the truss's top chord.
Gable End Frame
I believe a "gable" is a bunch of trusses all together to form a roof. As shown in this picture.
Notice how the wind is hitting the flat-face of the end-truss of the gable. This is the gable end frame.
The same wind direction is used in the following pictures:
Gable End Hinge Point
We're now looking at the side of the gable... that's why we no longer see the triangle shape.
Notice here that the wind is bending the structure where the gable meets the base structure (roof meets top of wall.)
Notice how this gable has it's own sheathing and the base structure has it's own. This separation is causing most of the gable end hinge point problem.
Now with a single piece of sheathing going from bottom of base structure to top of gable:
No More Gable End Hinge Point
Here, the wind's force is distributed across the entire face of the windstorm sheathing. (The picture doesn't clarify this very well at all - in fact, it makes it seem like it isn't... but it really is - that's just physics.)
The roof and wall system are now "connected" and the hinge point disappears.
|How do plywood sheets that extend all the way up to the roof eliminate the need for hurricane clips or at least something to fasten the roof to the top plate? Do these longer plywood sheets somehow get attached to the roof?|
However it is done, how does it prevent "lift and roll over"? How would it "prevent lift" since this whole junction between plywood sheath and roof is covered by the soffit?
I think these questions go back to the first picture again. Remember the "butt cut?"
If the butt cut is not covered by plywood, or covered with it's own plywood and not connected to the wall - because the wall-plywood was not large enough - this would make "lift" a difficult problem. It would only be single-point hurricane clips holding the roof to the wall structure.
Now, add in windstorm sheathing that goes from the bottom of the wall structure up to the top chord of the roof truss - covering the entire butt cut as well.
This, again like the flimsy-back-piece of the Ikea shelves, adds much more structure and many more connection points between the wall structure and the roof structure.
"Lift force" from the wind still exists - this will always exist as long as the overhang exists.
However, going from single-point-connection hurricane clips to mutliple-point-and-single-large-piece windstorm sheathing connections is such a huge gain is structural stability that it reduces it so much some claim it to be "eliminated."
The soffit isn't structural... it doesn't add in to this in any significant way. Any lift force generated because of wind and the overhang (regardless of it going on the soffit or not) will be transferred to the structural connection of the hurricane clips or the windstorm sheathing. The roof will want to come off all together ("rolling") this puts strain on the connection between roof-and-wall/base.
|If there are no or fewer stud to plate connectors and floor to floor connector straps, doesn't that just mean using more nails? I'm still not getting how it's possible to ever eliminate hardware. Are nails not considered hardware?|
I think this is more of a laymans vs. expert terminology thing.
I really don't know, because I'm very much a layman here as well.
My guess is that nails are technically "hardware" in the sense that you will find them in the hardware section of any hardware store.
However... they are considered so much fundamental hardware that they are sometimes not included when discussing "using hardware" especially when the context is around using other-connection-things that are not nails/screws/bolts... like hurricane clips or plate-connectors or straps or anything else like that.
So I think this is a definitional/contextual confusion issue more than anything else.
Maybe if you're talking about "getting nails" you can say you're getting hardware for the job.
However, if you're talking about all-the-other-connection-types... and you mention dealing with "the hardware" you would be referring to everything-that-connects-stuff but not really including the nails (because nails are so fundamental and obviously required).
|Is wall air leakage a significant contributor to exerting destructive force on a structure during high winds?|
This one... I'm really not sure.
Maybe? Or maybe something else entirely like R-Value of the wall in general (nothing to do with hurricanes or winds?).
As far as high winds are concerned... my initial thought is that gaps/seams would help (assuming all other structure aspects are equalized somehow)... because wind blowing through something causes a lot less force than wind getting trapped by something. That why I think this might have something to do with non-hurricane-winds benefits.
But maybe it's touching on the idea of more-seams = more-little-pieces-connected-together-instead-of-solid-one-piece-things... which would also imply that the structure is weaker against wind forces (more joints = more possible bending/weakening locations).
|This message is a reply to:|
| ||Message 509 by Percy, posted 10-18-2018 5:51 PM|| ||Percy has responded|
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| ||Message 511 by Percy, posted 10-21-2018 8:51 AM|| ||Stile has responded|