Abstract
In recent years, timber structures are popularly applied for lattice shell roofs to create a long span space due to lightweight and aesthetics. This research proposes several types of steel connections for timber lattice shells improving structural performance and architectural appearance. The out-of-plane flexural strength and rotational stiffness of the proposed connections are examined by full-scale experiments. Based on the experimental results, the seismic performance of timber lattice shell roofs using proposed connections is investigated.
Dimensions of a single-layer timber lattice shell are assumed to be 24 m × 24 m × 3.2 m. The five types of connections are proposed for this single layer lattice shell as shown in Fig. 2. Rectangular laminated timber members are fixed to tee or wide flange section with lag screws. These connections basically resist to out-of-plane bending moment by the pull-out force of lag screws and bearing force between the timber members and steel connections as shown in Fig. 3. Tests on three specimens for each type of connection were carried out to confirm the rotational stiffness, flexural strength and fracture mode with respect to out-of-plane bending.
Numerical simulations of a single-layered timber lattice shells illustrated in Figs. 7 and 8 using proposed five types of connections were conducted to investigate their buckling loads. The strength reduction factor for elastic buckling load against linear buckling load α0 was estimated approximately as 0.9. Fig. 10 shows that the strength reduction factor due to connection stiffness β(κ), stipulated in AIJ Recommendation for Design of Latticed Shell Roof Structures (AIJ Rec for Shell), and found to be less than the calculated values in numerical simulations with evaluated connection stiffness in the tests. The effect of timber shear stiffness on the strength reduction factor was also investigated. At last, elastic buckling load of timber lattice shells could be estimated multiplying reduction factors α0 and β(κ) to linear buckling load. The findings are summarized as follows.
(1) The maximum flexural strength of the proposed connections ranges 0.34 to 0.87 times the laminated material bending strength Mb, and the normalized rotational stiffness ratio κ (Eq. (56)) ranges from 7.5 to 22. This rotational stiffness ratio values indicate that the proposed connections are categorized as medium stiffness for single layer timber lattice shells.
(2) In tee flange connections, the rotational stiffness and flexural strength increase as the wrap length elongates. The wide flange connections exhibited maximum flexural strength, though the rotational stiffness is smaller than other connections with equivalent wrap length,
(3) Evaluation formulas to calculate the rotational stiffness and the flexural strength for the proposed connections are provided. The values calculated by the formulas generally correspond to the test results with a safety margin.
(4) The knockdown factor β(κ) for the buckling load of timber lattice shells in of each connections range from 0.74 to 0.87, which is applicable for practical design of single-layered timber lattice shell roofs.
(5) The elastic buckling load of timber lattice shell roofs with the proposed connections can be evaluated multiplying the reduction factor γt , the knockdown factor β(κ) f and the reduction coefficient α0 which can be assumed 0.9 to the linear buckling load.
