Research on the Pull-out Behavior of Light Gauge Steel-Fast Growing Timber with Self-Tapping Screw Connection

The application of timber to buildings can bring out the environmental characteristics of timber and make the building aesthetically. Fast-growing timber is abundant in China and can be used in the construction to reduce dependence on imported timber and reduce construction costs. However, due to the weak strength and uneven structure of fast-growing timber, there are some limitations in the application of construction. The introduction of light gauge steel can weaken the natural defects of timber, while timber can also solve the problem of instability of light gauge steel. Light gauge steel-fast growing timber structure is a new type of building structure that introduces steel into timber structure. It has the advantages of both steel and timber, and has certain prospects for application in building structures such as village houses. Since the mechanical behavior of self-tapping screw-connected light gauge steel-fast growing timber structures is influenced by the pullout resistance of self-tapping screws, and the existing studies on the pull-out resistance of self-tapping screws for such structures are not well studied. Therefore, a nail joint pullout test was conducted on steel-timber composite members made of domestic fast-growing fir timber and thin steel plates graded Q235. The thin steel plates and timber blocks were connected with self-tapping screws and placed in a homemade fixture. Using a testing machine to pull out the screws from the specimens to study their mechanical properties. Two important experimental parameters, including the thickness of the steel plate and the grain direction, were selected during the test to obtain the failure mode, ultimate load, and the ductility of the self-tapping screws. The results show that the failure mode of the specimens is all the damage of self-tapping screws pull-out. The threads are obviously damaged after the nails are pulled out. There is an accumulation of iron and timber chips in the threads, and the steel plate reamed and bulged. The dispersion of the specimens is low when the screws are pulled out perpendicularly in the direction of the grain, and the dispersion can be reduced by increasing the thickness of the steel plate. Simultaneity, the addition of light steel can significantly increase the ultimate load of specimens. The ultimate load at the end is lower than that in the other two directions, and the difference decreases gradually with the increase of steel plate thickness under the same conditions. When the self-tapping screws are pulled out from the radial direction and the chordal direction, the difference of load carrying capacity is not much, and the difference of ultimate load in the two directions was within 10%. When the steel plate thickness is 1.5 mm, the specimen shows better ductility than other specimens, while the overall ductility coefficient of the specimens is lower than 3. Finally, based on the European Code for the design of timber structures and Chinese Code for the design of steel structures, a formula to calculate the pull-out load capacity of steel-timber composite members composed of fir timber and thin steel plates connected by self-tapping screws was proposed, and the error of the formula was within 15%, which verified the applicability and accuracy of the formula.