Simplified Strength Model
The racking performance of light gauge steel frame / wood panel shear walls is similar to that of wood framed shear walls. It is assumed that when a shear wall is subjected to lateral loading, the steel frame distorts as a parallelogram in which the top and bottom tracks maintain a horizontal position. The screws along the perimeter of a panel rotate about the flange of the studs; however, no obvious rotation of the screws connected to the interior studs occurs. The steel frame member connections act as hinges, which means that no lateral resistance develops in the frame itself. Rather, the lateral load is resisted by the composite action of the wood panels and steel framing through their relative rotation. The external work applied to the shear wall was assumed to be absorbed by the rotation of the screws.
In order to develop a model which can be used to predict the shear capacity of a light gauge steel frame / wood panel shear wall, some secondary behavioural characteristics need to be neglected or simplified. The following assumptions, which are similar to what was proposed by Kallsner & Lam (1995) are applied in the model:
i) Deformation of the studs and tracks does not occur. These steel members are hinged to each other.
ii) The panels are rigid in their own plane and adjacent panels have no contact or overlap with each other.
iii) The relative displacements between the sheathing and framing are small compared with the panel size. The wood and steel also do not separate from each other during loading.
iv) No relative displacement exists between the centre of the sheathing panel and the corresponding centroid of the steel frame.
v) No horizontal panel joints exist in the same storey. Although in engineering practice, such joints are allowed, no tests with such configuration were included in this research.
vi) The shear wall is fully anchored onto the support or lower storey.
vii) The external work done by the racking loads is completely absorbed by the distortion of the sheathing-to-frame connections.
viii) The sheathing-to-frame connections have the same capacity in all directions.
The displacement of the sheathing relative to the steel frame can be viewed in Figure 4. All of the studs have rotated about their bottom ends through the angle y, while the sheathing panel has rotated as a rigid body to an angle ф. In the simplified model these two rotations, which are taken as independent variables, result in the force distribution of the sheathing-to-frame connections as shown in Figure 5. Based on the assumed force distribution the shear capacity of the wall segment, Sywan, can be expressed as shown in Equation 1. The shear capacity is dependent on two factors, the first being the wall configuration including the connection pattern and the second the shear capacity per connection. This equation was originally presented by Kallsner & Lam (1995) in their elastic model for wood framed shear walls.
Where Syconn is the shear strength of the individual sheathing connector (Tables 2&4), H is the height of the wall, x and y are the position of the fasteners (Fig. 5) and N is the number of sheathing connections.