Nominal Bracing Capacity of Plasterboard Walls is Conservative (maybe)



In my last blog post I noted that full-scale testing by the Cyclone Testing Station (CTS) in Townsville had revealed that about 40% of the lateral load on a building is resisted by the plasterboard sheathing and cornices (Satheeskumar et al., 2016).  As surprising as this might seem, it really shouldn't be at all surprising to structural engineers.  We know that architectural finishes add to the stiffness of a building.  This fact is clear from the dynamic behaviour of a finished building where the measured natural frequency of the building is almost always higher than the predicted natural frequency from software models (i.e., SpaceGASS, Etabs, Strand7, etc.), which typically neglect the non-structural elements.

 As part of my research, I am developing a suite of finite element (FE) models of mid-rise timber buildings (4 - 10 storeys) in order to predict the loading profile on the shear walls under earthquake and cyclonic loading conditions.  This will help me to plan the experimental phase of my work.

 What guidance do the standards give regarding the stiffness from plasterboard sheathed timber framed walls?  The Australian Standard for residential timber-framed construction AS1684 permits, under certain conditions, a nominal bracing capacity of 0.45 kN/m and 0.75 kN/m for walls with sheathing on one side and both sides, respectively (image below).  If we assume that this load produces deformation equal to the serviceability criterion (height / 300) then the initial estimate for wall stiffness will be 50 - 56 N/mm per lineal metre for single-sided walls and 83 - 94 N/mm per lineal metre for double-sided walls (range is for different height walls, 2.7m and 2.4m).

To accurately predict the behaviour of an actual building under these loading conditions, I need to model with greater accuracy the stiffness of non-structural walls in the building.  Luckily for me, someone has done all the hard work on this topic about a decade ago at Melbourne University (Liew, Gad & Duffield, 2006).  They conducted monotonic shear wall tests using four different types of plasterboard as the sheathing material.  The image below shows the load deflection curve for just one of these double-sided 2.4m x 2.4m walls (solid line).  The ultimate load is between 1.2 kN/m and 4.5 kN/m.  If we focus on the linear-elastic portion only:

  • yield load is between 1 kN/m and 3.4 kN/m (still higher than nominal bracing capacity in AS1684); and
  • stiffness is between 275 N/mm and 425 N/mm per lineal metre.



Based on this data from Liew et al. (2006), it might be tempting to form the view that the nominal bracing capacity in the Australian Standard is overly conservative.  But, we need to be careful in how we interpret these results for a few reasons:

  1. The lateral load was applied directly to the top plate:
    • For non-loadbearing walls, the load would actually be transferred from the ceiling diaphragm into the wall via cornices and partition brackets.
  2. The authors do not describe the hold-down details:
    • The nominal fixing requirement is 2/2.8 dia nails at 600mm centres.  Actual hold-downs in this test may have been more robust than nominal fixing, but we don't know because the authors don't report this information.
  3. The walls were restrained from both horizontal and vertical movement at the top plate:
    • For non-loadbearing walls, there is typically a gap between the top plate and the structure above.  Restraining vertical movement is an unrealistic thing to do and the author's justification for doing it is not convincing.  If the walls had been allowed to deform vertically, it is likely that they would have been less stiff and weaker.
  4. The tests did not consider the effect of wetting on the performance of these plasterboard clad walls:
    • Wetting is likely to occur during a cyclone as a result of window breakage and subsequent water ingress.  Studies have shown that wet plasterboard loses strength as a result of wetting (e.g., Escarameia, Karanxha & Tagg, 2007).

My 'take home' message here is the importance of careful experimental design to ensure that the data I collect is directly relevant to real world building practices.


Escarameia, M, Karanxha, A, & Tagg, A. (2007). Quantifying the Flood Resilience Properties of Walls in Typical UK Dwellings. Building Services Engineering Research and Technology, 28(3), 249-263.

 Liew, YL, Gad, EF, & Duffield, CF. (2006). Experimental and Analytical Validation of a Fastener Bearing Test as a Means of Evaluating the Bracing Characteristics of Plasterboard. Advances in Structural Engineering, 9(3), 421-432.

 Satheeskumar, N, Henderson, DJ, Ginger, JD, Humphreys, MT, & Wang, CH. (2016). Load Sharing and Structural Response of Roof-Wall System in a Timber Framed House. Engineering Structures, 122, 310-322.

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