Double Stud or Single Stud at the Butt Join between Sheets of Bracing?

I used to work as a timber frame and truss detailer before becoming an engineer.  During that period of my life, I prepared shop drawings for the prenailed timber frames of many homes around South East Queensland.  As a detailer, I had to know exactly where to place studs in order to make it easier for the various trades on site to do their work.  For example, plumbers wanted to have blocking directly behind the toilet cistern, so I would detail a decent sized piece of timber in the right spot.

When it came to plybrace, I always provided an extra stud at the join between two sheets of bracing (as in the photo above).  As I recall, chippies wanted the extra stud so that they could nail off the sheet without worrying that the nails might miss the stud.  As one chippie told me recently, "If your stud is out by a poofteenth, all your nails are going to miss."  This double stud detail was, and still is, perceived as best practice.

Unfortunately, the double stud detail is less than ideal from an engineering perspective.  When we design a shear wall, we are designing a structure that transfers the lateral racking forces from wind or earthquake, which are coming from the ceiling diaphragm at the top of the wall, down into the foundations.  The image below illustrates two ways in which the racking force is transferred from the top of the wall to the base of the wall.  The rigid body component is restrained by tiedowns and the shear component is restrained by nails between the bracing sheet and the timber frame.  There is also a sliding component (not shown), which is restrained by bolting the bottom plate down to the substrate.

When we fix two sheets of plybrace side by side, engineers are expecting the shear component to be transferred from one sheet of bracing into the next.  If there is only one stud at the join between the two sheets, then the shear transfer is direct and assured.  On the other hand, if there are two studs at the join between the two sheets, then there is a problem.  To illustrate the problem, I have drawn a close-up of the join between the two sheets and exaggerated the deformations for clarity.

When you look at this image it becomes clear that the heads of the nails in the lefthand bracing sheet (red) are being pulled down and to the left and the heads of the nails in the righthand bracing sheet (blue) are being pulled up and to the left.  If we focus on the studs, we can see that the lefthand stud is being pulled down by the racking and the righthand stud is being pulled up.  If the studs aren't properly nailed together, they will slide past each other, which will relocate all of that shear force into the top and bottom plates.  This effect can be clearly seen in the next photo of an actual racking test done in the laboratory.  In this photo, you can see that the stud on the right has moved up relative to the stud on the left (about 10mm).  This pushed the top plate up and pullout nail failures can be clearly seen on the lefthand bracing sheet.

The results of the lab test closely match predictions of what would happen if the double stud detail is used without providing sufficient connection between the two studs.  That is, the studs slide past one another, which concentrates the vertical shear forces on the top and bottom plates which, in turn, concentrates the forces on nails through the lefthand sheet into the top plate (and, it turns out, also those nails through the righthand sheet into the bottom plate).

Does the residential timber-framed construction standard AS1684 provide any guidance on whether to use a single stud or a double stud at the butt join between sheets of bracing?  The answer is NO.  Most builders use Method B of Detail (h) in Table 8.18 of AS1684 Parts 2 and 3 (see below), which provides a rated capacity of 6 kN/m.  As you can see, the standard makes no mention whether a single stud or a double stud is used at the join between the sheets.  Frame and truss manufacturers and chippies can hardly be blamed if they do use a double stud detail.

To find out what common practice is, I spent a few days driving to building sites around Brisbane, talking to carpenters and taking photos (see photo at the top of this blog for a typical example).  The double stud detail was used at every single building site I visited, regardless of whether prenailed frames were being used or the chippie was stick framing.  I could not find a single instance where a single stud detail was used at the butt join between two sheets of plybrace.

I also had a look at construction practice in other parts of the world to find out what details they use.  The American Wood Council stipulate that all timber framing in shear walls must be 2" thick (i.e., 45mm) and, where a double stud detail is used, they must be nail laminated together with enough nails to transfer the shear force (see clause 4.3.6.1.1 of SDPWS 2015).  

The double stud detail is assumed as standard in the EU.  Eurocode 5 (EC5) makes it clear that shear transfer must be verified between adjacent panels (see clause 9.2.4.2(13) and 9.2.4.3.1(4)) using basic statics calculations.  At a minimum, EC5 requires the connectors between the two studs to be able to transfer a shear force of 2.5 kN/m.

Given that the double stud detail is standard construction practice in Australia, I would recommend that we also make it a requirement that the studs be nail laminated with enough connectors to transfer the shear forces between adjacent panels.  

What follows is a worked example using the 6 kN/m bracing detail in AS1684.  I've sketched a free body diagram below and included the shear between adjacent panels.  For sake of simplicity, let's assume there are no vertical loads acting on the shear wall, so we are only concerned with racking loads.  If we let h = l = 2.4m, then the shear force between the two panels is 6 kN/m.  If we are using 3.05mm diameter framing nails and MGP10 timber (JD5 joint group), then the characteristic capacity of one nail is 640 N (interpolated from Table 4.1(B) of AS1720.1).  Adopting a capacity reduction factor phi = 0.95, the design capacity for one framing nail between two studs is 610 N.  As such, ten nails are needed per metre of stud to ensure that the shear force is transferred between adjacent panels.

For this example then, double studs at the butt join between bracing sheets should be nail laminated with 3.05mm diameter nails at 100mm spacings to ensure that the design shear force is transferred between the two sheets.

References:

American Wood Council. (2015). SDPWS - Special Design Provisions for Wind & Seismic with Commentary (2015 Edition). American Wood Council: Leesburg, VA.

European Committee for Standardization. (2004). Eurocode 5: Design of Timber Structures - Part 1-1: General - Common Rules and Rules for Building (EN 1995-1-1:2004). European Committee for Standardization: Brussels, Belgium.