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

[hippo11]

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.

 

[hippo11]

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

 

[COEngineeer]

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

 

[hippo11]

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

 

[CJJS]

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

 

[Lion06]

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.

 

[hippo11]

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...

 

[JLNJ]

Are you asking about a bent-plate diaphragm?

 

[hippo11]

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

 

[JAE]

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.

 

[CJJS]

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?

 

[hippo11]

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?

 

[JAE]

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

 

[JLNJ]

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?

 

[JAE]

For smaller projects, we usually don't see the ridge as a chord, but I would think that in "real life" there is a need.

This has been discussed in the past here:

thread726-131641

thread507-178730

And a whole bunch of links to threads here that might help: thread1066-95784

 

[hippo11]

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?

 

[CJJS]

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.

 

[Lion06]

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.

 

[Lion06]

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.

 

[hippo11]

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)