Edinburgh Napier University

  Throughout the U.K, Timber Platform Frame is increasingly being utilized in the high volume, low cost housing market. Here, the client’s desire for fully detached housing often leads to narrow properties with openings concentrated in the shorter wall. This can cause design considerations since the number of full height wall panels deemed to give resistance to racking forces is reduced. In order to achieve the required racking resistance, remedial measures such as the application of additional sheathing, improvement of the sheathing fixity and even the use of steel portal frames, are often employed. Whilst these measures can be considered structurally effective they can also lead to the over specification of materials resulting in a decrease in overall project efficiency. In order to prevent this, research is currently being carried out at Edinburgh Napier University into the development of a series of racking solutions which are optimized in terms of performance, installation and cost.
This paper considers factors influential to racking strength and stiffness such as the degree of fixity between the sheathing and timber frame, nailing intensity and layout and the degree of overturning restraint. A series of racking tests were carried out and the results are used to assess the accuracy of a simplified plastic model for the prediction of racking strength. The paper also highlights the need for a ―stiffness check‖ in order to ensure that structures do not suffer from excessive deflections. A review of an existing analytical model for predicting panel deflection is undertaken and it is found that the method gives acceptable results when predicting the racking strength of imperforate wall panels with varying degrees of overturning restraint. It is also found that estimating the deflection at the wall head based only on the nail slip of the sheathing fastener and the shear deformation of each sheet leads to an overestimation of wall stiffness performance.