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Author Topic: Shearload  (Read 345 times)

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Offline bannerd

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Shearload
« on: October 12, 2021, 08:13:58 AM »
(Long one, skip to the question on the bottom if you want.. :D)

Hey all, I want to learn more about shearloads, lateral loads... etc.  Our 30ft wide by 50ft long house has a minor "Racking".  I really have to throw my body around to get it to move.. I would have never noticed this until I put a jug of water on the deck and notice how much it moves.  Or if I pile a stack of wood I can topple the wood right over at two feet high.  Which brings me into an issue.. I have no sheathing.  I don't know if I CAN do sheathing because our wall insulation will be light straw clay.  I had an engineer look at the house and it's beyond him.  I found another guy in vermont and he wants me to sheath the entire outside in 2x6's at 16 OC and then build the larsens from there to a 2x2.  This will give me enough gap to allow the walls to breath and allow the clay slip on the inside to move humidity from the air and bring it back into the living area.  It's a modified truss which will hold the slip/straw mix but be structural as well.  Once that is done we would run horizontial strapping across the larsen truss and then run either vertical/diagonal or horizontal sheathing on the outside of that.  I started to work on this last night but it just doesn't make sense to me. 

The entire weight of this structure is going against the BTX screws I put into the 2x6's.  As the wind blows it will pull on the screws or apply some pressure depending on the direction.  I did some quick math and the 12ft high wall by 50ft long @ 20lbs of wind load (55mph wind) is almost 480lbs on each screw.  The screw shears @ 595lbs.  This doubles or tripples on the amount of screws I would think.  Of course, down pressure is more than adequate because of the numerous 8x8's in the home which I think also helps with racking depending on the direction of the wind and roof load and "live loads".  After a few more pictures we decided to put knees right into the main sill at each corner of the house to see if this takes up the compression loads.  We could sheath the outside of the home and yes that would fix the issue but we would lose the exposed beam look.  We also lose air quality and R value as these walls need to run at least 10-12" for insulation.

I still have yet to do the entire house in the 2x6's, I'm running 3" BTX big timber screws and putting about four into them top and bottom.  I also need to make some knees and drive them into the sill and see if that helps.

So my question is.. do structures really depend on the screws or nails?  Sheathing is very important in the term of racking.  I've never thought of this before... typically I just do construction work and move on but with this being my personal house... I do a lot of thinking now and in my head I'm getting the "What ifs".


Offline snobdds

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Re: Shearload
« Reply #1 on: October 12, 2021, 12:05:28 PM »
Typically, the bones of the house, (the framing) should all be in compression loading.  Then you put sheathing up to lock in as many joist (6 usually) to create racking strength, which is the inability to fall over sideways.  Sheet goods are a new thing and prior to them, builders cut in diagonal's across a wall to inset a board to create racking strength.  

Traditional screws only hold in compression not sheer, and should only be used in compression loading.  Newer structural screws can be used in shear loading. Nails, the common type, will never break, just bend.  That is why they are good to fasten compression loads to shear loads.

If you don't use sheeting, you will need to create racking strength by using knee bracing. A new was to create racking strength is by using closed cell spray foam. It's amazing how much strength is gained by "Gluing" the entire structure together.


Offline bannerd

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Re: Shearload
« Reply #2 on: October 12, 2021, 04:01:16 PM »
Thanks, I put some temp knees on the bottom and it took up some racking, I think once the sheathing is up all around it will be very minimal.

Offline bannerd

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Re: Shearload
« Reply #3 on: October 12, 2021, 04:03:51 PM »
 


Offline Don P

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Re: Shearload
« Reply #4 on: October 12, 2021, 10:20:03 PM »
If you go to awc.org and click on the publications tab there is a link to the WFCM. When that page loads there is a link to a free web version. In section 2, the engineered provisions scroll to pg 61 and the tables begin, lateral loads among them. Table 2.5a and b are a good place to get the lateral force per foot hitting the building, don't forget the roof is part of the sail. Then for instance calculate the force on the 50' wall, it must be resisted by the 30' walls and vice versa. This was written for stick frame and you are using it well outside of its limits stated around pg 1 ( an inspector will tell you to get an engineer) but the loads and forces in those tables are the same, how you resist them is the problem of the day.

Can you run X's of flat metal strapping over your 2x6's, attach to each and really attach through to the frame when crossing it.
The future is a foreign country, they will do things differently there - Simon Winchester

Offline bannerd

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Re: Shearload
« Reply #5 on: October 13, 2021, 07:44:49 AM »
Thanks Don,

This is the system we're going with;

Light Clay ? TRC Timberworks

I'll take a look at those documents. 


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