Gable End and Interior Shearwalls 4

[medeek]

I've been mucking around with the Woodworks software and reading through some of its documentation. I noticed that the uplift forces being calculated for the holdowns was different than I was calculating manually for gable end shearwalls. Looking through the help files I noticed that the height being used to calculate the holdown force was not the wall height but actually the average height to the roof diaphragm for that segment (see diagram below):

When a roof like the one shown above is composed of closely spaced trusses (max. 24" o/c) my thinking was they would act like mini shearwalls of their own and bring the diaphragm load down to the ceiling level where it would be transferred to the walls. I suppose the same argument can be made for interior shearwalls as shown above as well. However, I am now having to rethink this assumption.

The exterior shearwalls parallel to the ridge obviously are same height as the wall height but how is everyone else handling the gable end and interior shearwall heights?

A confused student is a good student.

 

[woodman88]

What your sketch shows IMHO is a non drag truss (gable truss without diagonal webs, which will require shear sheathing applied) being placed over the shear walls. The interior condition assumes a truss (drag?) to the wall and a non drag truss over the wall. This is a worst case assumption, so it will always work.
If you are requiring drag trusses to be connected to the shear walls, than where the force is applied depends on your details.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[medeek]

Let's assume the gable truss is a drag truss and has diagonal webs as well as the vertical webs for sheathing, I've seen lots of these on various construction sites. Now back to my original question, when I go to calculate the uplift (tension) force on the holdown at these shear panels on the gable end what is the appropriate height to use?

A confused student is a good student.

 

[jayrod12]

well if your gable truss is a drag truss with a bottom chord elevation at the same height as the side walls, then I would use the same wall height.

 

[DaveAtkins]

When a sloped diaphragm transfers its reaction to a shear wall, the force is along the top of the drag strut truss. In other words, the force is not at the top plate of the wall, as you assumed, nor at midheight of the wall, as Woodworks assumed, but rather on the diagonal, up and down along the top of the truss.

So, the truss will experience overturning, which must be resisted by the holddown at each end of the truss.

The truss, in turn, transfers the diaphragm force into the shear wall along the top plate of the shear wall.

DaveAtkins

 

[woodman88]

DaveAtkinS
Woodworks appears to assume a balloon wall (not a drag truss). As such the shear force will be applied at the mean height of the sheathing as shown in the above sketch. It will also require straps from the gable to the wall.
When a drag truss is used, where the uplift is taken out depends on the connections details.


Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[medeek]

Explain more about the straps and where they would go assuming a balloon framed wall.

A confused student is a good student.

 

[woodman88]

The straps would be needed at the vertical red lines in the sketch. So the force flow per the assume shear walls can be transferred per the sketch.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RWW0002]

For a balloon framed walls - If studs are continuous from the foundation to the outriggers then there would be no need for straps at the gable wall.

On a separate note, for walls with full height gable studs, the shear force for OTM should be applied at the average height of the roof for "gable" sections (similar to the area between windows in your sketch) and at the highest point along the wall for "sloped" sections (similar to the end gable wall segments in your sketch) in order to maintain equilibrium. This is assuming the shear force is delivered uniformly to the shear wall along the (sloping) sheathing.

See attached link.

http://www.shearwalls.com/resources/appendix-b.pdf

 

[medeek]

Thank-you for posting the paper. I will read through this and then come back to this discussion.

A confused student is a good student.

 

[DaveAtkins]

I never see balloon framing. Always platform framing and drag strut and gable end trusses.

DaveAtkins

 

[medeek]

The holdown forces for a sloped shearwall (symbolic) are then:

http://design.medeek.com/images/misc/SLOPED_SHEARWALL.jpg

A confused student is a good student.

 

[medeek]

A confused student is a good student.

 

[medeek]

Note that up until now I have been treating the gable truss as a drag truss in my analysis which means the holdown force I've been using is given in the first box above:

T = F(cosθ)w_h/b

This holdown force is less than the holdown force given by Matteson's paper (second box) and also less than holdown force given by the average wall height method (third box).

The other thing thats stands out to me from Matteson's paper is the suggestion to apply a strap at the low end of the sloping shear wall that passes over the top plate of the shear wall and connects to the chord. I've never seen this done, which doesn't mean it hasn't been done or shouldn't be, but I'm oblivious to it. Does anyone have any examples of this being done in practice, picture or diagrams would be educational. The downward force that must be provided by the strap is: F_y = F(sinθ)

A confused student is a good student.

 

[medeek]

My findings based on this discussion are presented below:

Which brings me to one additional question which was already partially addressed by Dave Atkins above. Every truss in the roof that is subject to a lateral load along its top chords will experience an overturning moment with an uplift force given as:

V/2(tanθ)

The lateral force on each truss cannot be resisted by the walls beneath it since they are perpendicular to the load so the lateral load will be collected up by the diaphragm and resisted at the shearwalls. However, the vertical uplift force I would argue can be resisted by each truss connection to the wall beneath it hence the only loads that need to be dealt with at the shearwalls are the total horizontal component tributary to each shearwall. Once you have the total shear for the shearwall then if the truss above cannot act as drag truss you then have to analyze it according to the method proposed by Matteson or the avg. height method. In this reqard the Matteson method appears to be a more accurate and rational method of the two.

A confused student is a good student.

 

[woodman88]

All the design values for lumber are considered as converative. Until the lumber values are increased to more accurate values. A simple analysis, as in average height, is quite good enough to design by. IMHO

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[RWW0002]

I have not combed through your calculations with any detail but things look OK to me; although I do not know that is necessary to divide the sloped wall into sections as the paper shows. The force can be applied along the roof slope with a moment arm (perpendicular to that force) to the point of overturning. It has been a while since I have worked through the geometry, but I think it comes out to Ta = Vc*Hmax/B, and Tb = Vc*He/B for the sloping wall.

As long as there is no break in the sheathing at the eave height, I do not see any reason for any intermediate ties within, or at the top of the wall.

Also, you might want to consider distributing forces to walls in a line based on panel relative stiffness rather than simply wall length. There has been some discussion concerning this on here recently. (Also see 2008 AF&PA SDPWS 4.3.3.4) This will have the added benefit of drawing load away from the tall slender wall where uplift forces are highest.

 

[RWW0002]

Woodman,
All this being said, I agree that many engineers simply use the average wall height for the segment in question for balloon framed walls. Similarly, I think most use the eave height for interior shear walls with a drag truss and count on additional uplift being transferred through the truss and resisted by the truss ties at the bearing ends.

 

[medeek]

Rww0002, that is my next area of further study, how to deal with shearwall load assignment based on a more rational approach. Take for instance the interior shearwall in the image at the top of the discussion. Based on standard approach this shearwall will see double the load that either exterior gable shearwall will see. Being that this wall is only a small segment of the entire wall length this is not realistic, the ext. shearwalls will take a larger fraction of the load. The question is how to determine what is appropriate.

A confused student is a good student.

 

[woodman88]

medeek.
I would recomend you read up on some basic laterial design of buildings. The Reinforced Masonry Engineering Handbook: Clay and Concrete Masonry, Fifth Edition Hardcover – March 5, 1998 by James E. Amrhein (Author) should be a good one.
Please note that such designing of shear walls is not typically done for wood buildings.


Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.