Chun Ni

Self-tapping screws are becoming increasingly popular for use in modern timber structures. The axial stress distribution of self-tapping screws due to a mechanical load has been previously studied. However, the stress distribution of self-tapping screws due to moisture swelling-induced load from wood has not been explored so far. This research presents an analytical model to predict the axial stress distribution in self-tapping screws embedded in mass timber products under the combined effects of axial mechanical loading and wood moisture-induced swelling. The analytical model has been validated with numerical simulation. The input properties of the analytical model can be determined from withdrawal tests of self-tapping screws and the manufacturer’s guide of screw and mass timber products. A simple program has been developed to predict the stress distribution and maximum axial stress in self-tapping screws for a range of effective penetration lengths under a pre-load and moisture content change. Correctly predicting the maximum axial stress in self-tapping screws under the simultaneous action of a pre-load and wood moisture swelling-induced load can help design safer timber structures. This research provides a practical method for practicing engineers to predict the maximum axial stress in self-tapping screws due to pre-load and wood moisture swelling.

Self-tapping screws (STS) are used in wood-to-wood and wood-to-steel connections in timber structures. Premature failure of STS during the construction phase has been reported by structural engineers and contractors in relation to a few North American mass-timber projects. This STS failure type is suspected of having been precipitated by additional axial stress exerted on STS from swelling of the wood resulting from prolonged wetting. This study investigates the axial stress distribution of STS installed in two mass timber products, cross-laminated timber (CLT) and glulam, under axial loading and changing moisture conditions in the linear elastic regime. The focus is on modelling the stress distribution of STS during wood-wetting. Properties of self-tapping screws under axial loads, such as tensile and withdrawal properties, along with swelling properties of CLT and glulam, have been investigated. A numerical method to predict the axial stress distribution of the screw from these material property tests has been developed. The numerical model has been calibrated with test results. The method developed in this research helps in understanding the premature failure of self-tapping screw connections under the moisture content variation of wood. The next step will be developing an analytical model to predict the axial stress distribution of STS.

Light-frame wood structures are the most common type of construction for residential and low-rise buildings in North America. The 2015 edition of the National Building Code of Canada has increased the height limit for light-frame wood construction from 4 to 6 stories. With the increase in building height, it was noticed that light-frame wood structures may be governed by inter-story drift under wind and seismic loads. To reduce the inter-story drift, a hybrid system, consisting of CLT cores and light-frame structures, is proposed. The efficiency of this hybrid system is dependent on the performance of the connections between the two sub-systems. In this project, self-tapping screws (STSs) were used to connect the CLT core and light-frame wood structures on the floor level. Monotonic and reversed-cyclic tests were carried out on CLT-wood frame connections connected with STSs inserted at 45°, 90°, and mixed angles (45° and 90°). The connection performance was evaluated in terms of strength, stiffness, ultimate displacement, ductility, and energy dissipation capacity. Results show that a joint with STSs inserted at 45° had high stiffness and ductility but low energy dissipation, while connections with STSs installed at 90° had high ductility and energy dissipation but low stiffness. Connections with STSs inserted at mixed angles (45° and 90°) achieved the advantages of both configurations when the STSs were inserted at 45° or 90° individually, i.e., high stiffness, ductility, and energy dissipation. The ductility and energy dissipation were significantly improved compared with connections with STSs only inserted at 45° or 90°. This mixed angle connection can be an ideal design for connecting light-frame wood structures to a CLT core to resist wind and seismic load.

For three-member nail joints under double shear loading, the Canadian timber design standard specifies a minimum thickness of the middle member of eight times the diameter of the nail (8d). This requirement has created difficulties in the specification of midply shear wall because of the use of the relatively thin wood-based sheathing as middle member in the sheathing-to-lumber joints. The objective of this project was to evaluate if the European Yield Model adopted in the Canadian timber design standard can still provide acceptable prediction of strength and failure mode of double-shear nail joint that has middle member thickness less than 8d. Six groups of double-shear nail joints, covering two nail sizes and three sheathing thicknesses, were tested under both monotonic and reversed cyclic loads. Six parallel groups of single-shear nail joints were also tested. The effect of nail penetration into the point-side member was investigated through testing of two groups with different nail lengths with all other parameters being equal. The test results show that the European Yield Model can still be used to estimate the strength and failure mode of double-shear nail joints when the center member has a thickness much less than 8d. Therefore, a value of 2.5d is recommended on the basis of this test project. The nail penetration into the point-side member does not affect the strength of joints when the penetration length is more than 5d.

The structural response of mid-rise hybrid light wood frame buildings, incorporating portal frame and shear wall, under either cyclic load or earthquake action were investigated via finite element analysis. Based on the structural behaviour of the one- and five-storey hybrid buildings under cyclic loading, a method to estimate the ductility ratio, μ, and ductility-related force modification factor, R_d, for the light wood frame building containing two types of lateral load resisting systems exhibit different ductility characteristics was proposed. Four- and six-storey buildings with layouts that rely on shear wall only or a combination of shear wall and portal frame were designed using the R_d factors estimated from the proposed method. Natural frequency extraction and non-linear time history dynamic analyses were performed on these multi-storey hybrid buildings using a macro element model to represent the lateral resistant element in ABAQUS. The influence of horizontal diaphragm flexibility was considered in these analyses. The results show that the use of a R_d value higher than the lowest R_d factor of the two lateral load resisting systems suggested by the National Building Code of Canada may be justified.

A high-performance wood-frame shear wall system, named the MIDPLY truss wall (MTW), was developed for application to midrise wood-frame buildings. The basic MTW system consists of metal-plate–connected wood trusses, sheathing panels, and steel tie-downs. The sheathing panels of the MTW system are placed between the wood trusses and connected with mechanical fasteners. The main concept of the MTW system is to increase the lateral load resistance of a shear wall by reconfiguring the wall frame members into lateral load–resisting wood trusses. In this study, the feasibility of the MTW system was investigated. Eight 2,440×2,440 mm MTW walls were constructed and tested under monotonic and cyclic loading in accordance with ASTM test standards. Shear stiffness, strength properties, and ductility of the MTW walls were evaluated and compared with those of comparable MIDPLY walls. Overall, the MTW system shows significant improvements in shear stiffness, yield load, peak load, and ductility. This paper cont...

The performance of portal frame walls was studied using a finite-element model verified with results from tests of full-size portal frame walls. Parameters such as wall height, metal strap type and location, doubling of the bottom plate, sheathing placement, and nailing pattern were investigated. Results indicate that in all cases, lateral load capacity and stiffness are greatly reduced with the increase of wall height. Considering the effect of different metal strap types and locations, the tensile strength of metal straps has the greatest impact on the lateral load capacity and stiffness. Walls with metal straps placed directly on the framing members outperform walls with metal straps placed over the sheathing. For walls with a double bottom plate and two rows of nails fastening the bottom plate, the stiffness and lateral load capacity are slightly increased compared with walls with a single bottom plate. For walls with unblocked sheathing at midheight, the lateral load capacity is the same as in walls with continuous sheathing running from the bottom of the wall to the top, but the stiffness is slightly less.