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NDS wood diaphragm Table 4.2--what if inclined roof?
3

NDS wood diaphragm Table 4.2--what if inclined roof?

NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
In the NDS 2005 wood diaphragm Tables 4.2A, B, and C, values are given for different nailing patterns, sheathing thickness, grade, etc...but what if my roof is a 30 degree gable?  Does anyone know where I can read about how inclines affect unit shear resistance values, etc., for a sloped roof diaphragm?

Thanks.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Also is there a max slope to where I can no longer assume diaphragm action?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

I went to 2005 NDS and chapter 4 is for sawn lumber.  I am not sure what tables you are talking about.

http://www.swijetty.com
Sea Water Intake and Jetty Construction

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Sorry, I meant Table 4.2 A,B and C of the Wind and Seismic Special Design Provisions, 2005.  The thin green/gray book.

It just seems that a wood paneled diaphragm wouldn't work that well in diaphragm action if it's too steep.

thanks

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

I think you would have to resolve the force into normal and parallel to the plane of the diaphragm.  

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

It shouldn't matter about the slope of the diaphragm.  The values are based on unit shears.  
I would agree with CJSchwartz about resolving it into components when calcing the shearwall loads.  I would add this, however:  Add in the downward vertical force (from the diaphragm) for designing the compression chord, Add in the upward vertical force (from the diaphragm) for designing the tension chord and the hold downs.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
thanks...so I would take wind (or equake) * cos 30 for my in-plane sheathing shear check/bdry element check, and wind (or equake) * sin 30 to check flexure of the sheathing out-of-plane...

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Are you asking about a bent-plate diaphragm?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
bent plate?  i am talking about 7/16" OSB wood panels nailed to the top chord of gabled pre-engineered trusses...sloped wood diaphragm...thanks

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Keep in mind that when you resolve your horizontal shear to an inclined in-plane shear, the shear value will go up (the sloped diaphragm is less efficient resisting the horizontal shear at its inclined angle)

....but the diaphragm also should be checked using its sloped length (which is also longer) to get the shear per foot of diaphragm.  

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE,
It seems to me that if the horizontal shear is the resultant shear, resolving into normal and parallel to the plane shears would result in respectively smaller values of shear.  What am I missing here?  

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Yeah I agree with CJ.  You would take wind * cos 30 for the in-plane diaphragm forces, and then wind * sin 30 for the normal-to-plane diaphragm forces.  

If you look at the forces as a right triangle (angle between a and b is 90 degress), with the wind blowing left-to-right, the wind is the hypotenuse (c), while the components are the in-plane (a) and normal-to-plane forces (b).
         

         /`
     a  /   `  b
       / _____`

           c

No?
 

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Hmmm...don't know about that.  I see it this way:

HORIZ component of the diaphragm shear = HORIZ lateral wind force.

The DIAGONAL shear along the plane of the diaphragm is the RESULTANT force  (R) made up of a horizontal component (A) and a vertical component (B) such that R2 = A2 + B2.

A = horizontal component of the resisting diaphragm
B = vertical component of the resisting diaphragm
R = Resulting in-plane diaphragm Shear
H = lateral wind load

Where H = A


 

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE

Do you then design a chord member at the ridge or is the chord force negated by combining chord forces from each side of the sloped roof? Do the length-to-width calcs use the full roof width or just the (projected) width from eave to ridge?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Are we talking about the same thing? With a 30 degree (angle from horizontal) roof:

We have 6 forces/force components here, say H = 20 kips

1.  The applied wind, which is horizontal, H = 20 kips
2.  The parallel-to-diaphragm component of H...equal to H*sin 60=17.32
3.  The perpendicular-to-diaphragm component of H...equal to H*cos 60=10

4.  The parallel-to-diaphragm resisting force R
5.  The horiz. resisting component of R = R cos 30
6.  The vert. resisting component of R = R sin 30

CJ says I need to make sure 4 is greater than 2, and I need to make sure that the sheathing in bending and shear works for 3

JAE says I need to make sure 5 is greater than 1

I think it depends how we assume we resist "H".  JAE is right if I depend on membrane action (in-plane) only from the sheathing, and depend on no bending resistance from the sheathing.  CJ is right if we assume the sheathing acts both in bending and membrane action.

Sort of like JAE is making a truss analogy and CJ is making a frame analogy.

Technically either way is right.

No?

No?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE,
I am curious now as to which way is the correct way.  This, I think is similar to resolving a vertical force acting on a sloped rafter into parallel and normal to the slope components.  The vertical force is the resultant, just as the wind load (horizontal) is the resultant in this case.  
If someone has any more difinitive reasoning on this, please advise.  Thanks.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE-
Doesn't your method require a vertical wind force?  If the resulting in-plane diaphragm shear is the resultant of the vectors then there is a net vertical reaction from the wind.  That is not the case.  The Horizontal wind force is the resultant, with no vertical reaction.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Here is my thinking:
Draw a free body diagram of the node at the wall - sloped diaphragm interface.  You have a horizontal wind force (say 10k) coming from left to right into the node.  That must be balanced by a net force from the diaphragm coming from right to left into the node.  
If we say the sloped diaphragm force is the resultant (and not a component), then there is clearly a net downward force from the diaphragm that is not balanced by the horizontal-only wind load.  
Statics says this can't work.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Hmmm...EIT has a point...but I think that the vertical component of the diaphragm force gets balanced not by the applied wind, but somewhere else, like through the nails in shear/tension.  Think of a FBD of the entire panel.

I think we are all kind of talking about the same thing in different ways.

I think as long as you either do either of the following you're okay:

1.  Assume your wind is the resultant, and properly design for the resultant only (don't need to resist the wind components)

or

2.  Resolve your wind into components, and properly design for ALL wind compenents (don't need to resist the resultant)

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

StructuralEIT,

Using your example, I have a 10 kip horizontal force coming in at the node.

The node has only two elements attached to it:  the sloping diaphragm and the vertical wall.

Free body diagram

The 10 kip force can only be resisted by the horizontal COMPONENT of the diaphragm.  There is nothing else to resist it.  

Thus, there is a sloping RESULTANT which is comprised of a horizontal COMPONENT (equal to the 10 kips and is LESS than the sloping Resultant) and a vertical COMPONENT.

So (Horiz 10kips)2 + Vertical2 = Sloping Resultant2

The vertical component is a downward force resisted by axial force in the vertical wall.  Both the vertical wall and the sloping diaphragm have axial compression.

ΣF = 0 in both directions.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

I may be coming over to the dark side, I just want to think about it a little more.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE,
OK.  I see what you are saying now.  I think both ways are correct as hippo11 notes.    

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
Not to get too philosophical about this...but the whole idea of "horizontal" and "vertical" components is really just man's attempt to simplify and categorize a truly 3-D world of infinite axes into 2 axes, artificially called by someone "X" and "Y" or "horizontal" and "vertical".

You can legitimately resolve any resultant into components on of any set of axes you want, called "P" and "Q", oriented at 22.32345 degrees and 112.32345 degrees (as long as they're perpendicular to each other).

I think as long as you fully design for either

1. your resultant OR

2. ALL your components

that you are being geometrically correct.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

JAE,
How does the shear from the windload on the wall get into the roof diaphragm if the wall can only take axial load?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Well, the wall can take only axial load via the top node, but yes, the wind is actually delivered to the diaphram via bending and shear.

Here's a layout of the FBD:





RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Thank you.  That helps clear it up.  So, the two ways to check the diaphragm are:
1.  Calculate the horizontal unit shear (H divided by the projected horizontal length) and compare it to the tabulated allowable.
2.  Calculated the unit shear along the lenth of the diaphragm (R divided by actual length of panel) and compare it to the tabulated allowable.  

Correct?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

You take the H from the wall analysis, and it is applied horizontally along the eave.

(my sketch above shows it in the form of a concentrated load in kips, but in reality, the lateral wind comes into the diaphragm along the wall in lbs./ft.)

  This is like a horizontal uniform load on a "horizontal" beam...the beam being the diaphragm spanning from end-wall to end-wall.

Then you calculate the "R" value in lbs/ft (= H/cosΘ).  This is your uniform load on the diaphragm.  

Calculate your diaphragm end shear = 0.5 x (R x Length of wall)/(depth of diaphragm along the slope).  This is your diaphragm shear to be compared with the IBC tables or equivalent.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

(OP)
how'd you upload an image?  useful

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

hippo11 -

key on the "Process TGML" button below the message window and it will show you how to insert an image (Under the "Image" section).

You have to have a website where you can upload pictures and link to them.  I use VillagePhotos.com but there are a number of sites out there.  Then you just insert the URL of the photo on the site into a message box on Eng-Tips.

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

I think I agree with JAE now.  That means that the studs need to be designed for a significant axial load in addition to the out-of-plane wind load.  Weel, it can be significant if the roof has a decent slope to it.
Does this normally happen - adding in that compression?

RE: NDS wood diaphragm Table 4.2--what if inclined roof?

Well I haven't fully given in to the 'Dark Side' yet

Wouldn't the FBD shown assume the diaphragm is a support as drawn? I thought the idea of a diaphragm is the support is the shearwall which typically is on a wall perpendicular to the wall drawn.  I would think the diaphragm would need to be thought of more as a 3d plate element with horizontal supports along the shearwalls.

Of course this discussion is neglecting any actual uplift or downward forces from the wind directly applied to the diaphragm

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