Timber building systems, based on platform-frame technology, were born and have been developed in the USA, but are recently spreading across the world. In high seismicity regions, i.e., North America, Italy, Japan and New Zealand, the application of the platform-frame technology as a seismic force resisting system (SFRS) has already proved to be very effective, thanks to its lightness and intrinsic dissipative capacity, when properly designed [1
]. Platform-frame systems generally benefit from high shear deformability and their dissipative capacity is mainly diffused in connections (mainly nails, screws or staples) between frame and bracing panels.
Also, hybrid systems are of great interest in the current construction practice. Coupling different materials allows the exploitation of their intrinsic properties and the reduction of their limits, improving the overall behaviour of the building. Steel and timber can be integrated into components and/or in a layered construction system (e.g., steel connections with timber frames or timber walls, hybrid frames, steel frames with timber panels) [3
]. Examples of hybrid construction systems have already been built and tested: steel beams or frames combined with cross-laminated timber (CLT) panels [4
] or with timber-frame shear walls ([8
]) were studied through experimental tests and numerical modelling. Dubina [11
] analysed the results of monotonic and cyclic-loading tests of full-scale shear walls realized with cold-formed steel frames braced with oriented strand board (OSB) panel or corrugated sheet. These systems showed a high ductility and proved to be reliable as SFRSs.
As a response to users’ needs, new hybrid light frames have been proposed for optimizing their performance [12
]. The innovation is primarily in the materials used for the frame, for the bracing system and for the ductile connections to fasten the panel to the frame. The bracing panels are usually realized with timber-based materials (usually OSB sheets), whereas gypsum or plaster of cement are used as a finishing layer. The influence of these brittle materials on the performance of the timber-frame shear walls is reported in [13
]. Another variation of hybrid systems is the use of cold-formed steel components for the internal frame.
For a safe use of these innovative systems in seismic areas, it is necessary to evaluate their mechanical properties by means of experimental tests. In [14
] a summary of experimental and modelling studies of light-frame timber shear walls performed in the last two decades of the 20th century is presented. More recently in the USA, the seismic behaviour of typical platform-frame systems has been studied [15
] for improving their design procedure and for proposing solutions for retrofitting existing buildings [16
The hybrid system analysed in this work is composed of an internal frame realized with tubular steel columns and timber beams, which is externally braced with OSB panels on both sides, fastened to the frame with proper dowel-type fasteners. The new building system is an evolution of the one tested and described in [10
]. The previous system was composed by a wood frame coupled to tubular steel columns, which supported the vertical loads. In the new system, the steel columns are part of the seismic-resisting frame, in addition to supporting, as previously, the vertical loads. This simplification optimizes the system, making the assembly easier and cheaper with respect to the previous one [10
], due to the lack of the timber frame. The removal of the vertical timber posts means that the strength, stiffness and ductility of the new hybrid system rely on the behaviour of the steel-to-panel connections realized with self-drilling screws.
For this purpose, a shear wall was tested according to the quasi-static cyclic loading protocol of EN 12512 [17
] with the aim of characterizing this structural system in terms of strength, stiffness, ductility and hysteretic behaviour.
2. Structural Components of the Shear-Wall System
The investigated shear-wall system is a lightweight platform frame realized with steel columns having a square hollow section braced at both sides with OSB panels. The steel columns have the aim of supporting dead and live vertical loads, whereas the OSB panels allow the wall to withstand lateral loads thanks to self-drilling screws, nails and staples, which fasten the panels to the columns and to timber crossbeams within the wall. This precast system is conceived to be modular, with constant spacing of the columns equal to 125 cm. However, the width and height of the wall can be properly adapted to the geometry of the building or to special needs, changing the dimensions of the OSB sheets. Dimensions of the main components and spacing of the fasteners have to be specifically designed for each building. Here, a description of the main components is given. In the following sections, dimensions, thicknesses and spacing chosen for the tested specimen are listed.
The shear wall is composed of:
shows the details of the main components of the shear wall. All these structural elements confer to the wall the lateral stability and the necessary in-plane strength, stiffness and dissipative capacity to resist to earthquake action. The in-plane shear strength of the wall and the dissipation capacity are given by screws and nails, which fasten the OSB panels to the columns and to the crosspiece beams. The steel brackets are placed for supporting floors and to anchor columns at the foundation, in order to avoid the uplift of the shear wall due to rocking behaviour. These brackets are made of the same steel element of the column, diagonally cut, with a steel plate welded below. Finally, the U-shaped bottom rail is placed to transfer shear forces between OSB panels and foundation, avoiding sliding deformations of the wall. To comply with the capacity design approach, the steel brackets, the connections with the bottom rail and the anchoring to foundation should be sufficiently over-resistant with respect to the connections between the OSB panels and the hybrid frame, in order to confer to the wall a diffuse energy dissipation due to shear deformation and to avoid anticipated brittle failures.
3. Preliminary Laboratory Tests
Preliminary tests were performed at the testing laboratory for construction materials of the University of Padova to determine the mechanical behaviour of the OSB-to-column self-drilling screws subjected to a displacement-driven monotonic load applied at a rate of 2.0 mm/min. The tests were performed in a universal testing machine with a load capacity of up to 250 kN. A conventional push-out test configuration was adopted, which induces pure shear loading conditions in the connection by compression (Figure 2
The specimen was composed by a cold formed steel hollow section of 100 mm × 100 mm having thickness equal to 2 mm or 3 mm and two 18 mm thick OSB/3 panels. Each panel was connected to the tubular element with four 5.5 mm × 50 mm self-drilling screws. The use of two different thicknesses of the columns was chosen to analyse the possible different response of the connection. Figure 2
b shows the geometrical details of the specimens. A symmetric disposition of the panels was chosen in accordance to the intended use of this system and to avoid out-of-plane deformations. A total of six specimens were tested, three for each analysed thickness of the column. This is the minimum number of specimens needed to compute the 5% characteristic value of a property according to EN1990 [24
], if the coefficient of variation is unknown from prior knowledge. The test results are here reported in terms of failure load and force-displacement curves recorded for each specimen. Table 1
lists the main mechanical parameters and the mean and characteristic values among the three tests: yielding point (Vy
), ultimate displacement Vu
, maximum shear strength Fmax
, stiffness for the elastic and post-elastic branches (ke
), ductility μ. The 5% characteristic values were obtained using a kn
factor equal to 3.37, according to annex D on EN1990 [24
] for a number of specimen equal to three. Figure 3
shows the force-displacement curves up to failure of the tested specimens.
The results clearly show a good force-displacement response of the OSB-steel connection with a characteristic failure load per screw equal to 2.84 kN for the 2 mm thick column and 2.63 kN for the 3 mm thick column. It is worth noting that the thickness of the column does not influence the strength of the connection. Conversely, the displacement capacity recorded for the 2 mm column was higher than for the 3 mm column, and the failure mode was different. For the 2 mm column, a preliminary elastic phase was followed by wood embedment and formation of one plastic hinge near the screw head. Then, a second higher post-elastic stiffness was recorded due to rope effect, followed by a final softening branch after the maximum strength, and failure due to penetration of the screw head in the OSB panel. For the 3 mm column, a higher elastic stiffness was evidenced, together with a less ductile failure due to premature shear cutting of the connectors after slight wood embedment and formation of a plastic hinge. The failure modes are clearly shown in Figure 4
. It can be seen in Figure 4
a that the 2 mm specimens showed a marked rotation of the screws and penetration of the head. Conversely, the screws in the 3 mm column showed reduced rotation and failed for shear (Figure 4
These preliminary results led us to choose the 2 mm steel column for the following full-scale test, to obtain a better seismic response of the shear wall. It is clear that the thickness of the column has to be verified also for withstanding vertical loads. Another outcome of these tests was that, using a 2 mm thick steel column, the response of the OSB-steel connection is independent from the length of the screw, as can be noticed in Figure 4
a. This led us to choose shorter 5.5 × 38 mm screws for the full-scale test.
A novel timber lightweight platform-frame system composed of steel columns braced with OSB panels has been presented. The use of steel columns, which can be continuous from the foundation to the roof, permits the avoidance of the compression perpendicular to the grain of the crossbeams, which is a typical issue of traditional multi-storey platform-frame buildings.
Preliminary monotonic tests were performed to analyse the behaviour of the OSB-to-column connections with self-drilling screws. Then, results from a cyclic-loading test of a full-scale shear wall showed that this system is characterized by high in-plane shear strength and good ductility and dissipative capacity, and is classifiable into the Medium Ductility Class according to the European Seismic Code. The good balance of these mechanical parameters with the simplicity, speed of construction and cost effectiveness makes this system an interesting alternative to traditional timber platform frames for low- and medium-rise buildings in seismic-prone areas. It has to be highlighted that only one full-scale specimen was tested. Therefore, to generalize the results and to compute characteristic mechanical parameters proper of the shear-wall system, further tests are needed.