Designing, Manufacturing and mechanical investigating of joint in laminated stringer of spiral stairs from poplar wood

This research was carried out with the aim of making a stringer of wooden spiral stairs using laminated wood product from poplar wood and achieving an optimal connection to convert the stringer to smaller dimensions in order to achieve a suitable design for the construction of wooden spiral stairs. For this purpose, 3-layer plywood of poplar was made using two-part polyurethane glue with a surface layer thickness of 0.5 mm and a middle layer of 2 mm in dimensions of 120 x 120 cm, and then the produced plywood panels were used to make the desired laminated product. Mechanical properties and failure modes of the product were investigated. Then, 5 types of end to end joints including finger joint, tail joint, half and half, diagonal and modified diagonal joints were designed for the stringer of the wooden spiral stairs and made in the dimensions of length 65, width 20 and thickness 2.1 cm and subjected to out of plane bending test. Modulus of elasticity of the stringer in the in plane bending test was obtained as 1598 MPa, 2441 MPa, 1669 MPa, 1047 MPa, 957 MPa and 652 MPa, respectively for control sample, modified diagonal joint, diagonal joint, half-and-half joint, finger joint and tail joint. In addition, modulus of rupture of the stringer in the in plane bending test was obtained as 32 MPa, 27 MPa, 23 MPa, 19 MPa, 12 MPa and 13 MPa, respectively for control sample, modified diagonal joint, diagonal joint, half-and-half joint, finger joint and tail joint. The result of one- way ANOVA showed that there is a significant difference among bending strength and modulus of elasticity of the stringers at 99% confidence interval (α≤0.01). According to the results, the stringer with modified diagonal joint would be the most suitable alternative in wooden spiral stair.