Rising global emission have led to a renewed popularity of timber in building design. However, there remains a gap in understanding the mechanical behavior of wood, particularly at the scale currently seen in mass timber structures (e.g. greater than six stories). Whilst wood presents a sustainable alternative to concrete and steel, further research is necessary to successfully utilize this material. Our work therefore focuses on novel simulation methods for wood mechanics. We are developing a lattice model to simulate the microstructure of timber using curved beam elements, which allows for representation of the cellular nature of this material without undue computational costs. We are also working on a robust prediction model for wood creep and its resulting effects in full-scale structures.
Wood Creep
This study lays the groundwork for wood compliance prediction models for use in timber design.A thorough review of
wood creep studies was conducted and viable experimental results were compiled into a database. Studies were chosen
based on correlation of experimental conditions with a realistic building environment. An unbiased parameter identification method, originally applied to concrete prediction models, was used to fit multiple compliance functions to each data curve. Based on individual curve fittings, statistical analysis was performed to determine the best fit function and average parameter values for the collective database. A power law trend in wood creep, with lognormal parameter distribution, was confirmed by the results.
Current work includes application of the aforementioned wood creep model to structural modeling. This study focuses on the interaction between concrete and timber in hybrid buildings over long time spans. Additionally, our group is modeling the behavior of creep in glue-laminated beams/columns (glulam) and cross-laminated timber panels (CLT) under varying moisture conditions.
Wood Lattice Model
A lattice model to simulate the microstructure of timber is under development, the main component of such lattice model is a 3D beam formulation characterized by (a) a general geometrical curvature and torsion of the axis as well as (b) an irregular cruciform cross-section. The various branches for the cross-section represent the walls of the wood cellular structure and the beam axis is the line at which various the cell walls meet. The beam formulation has been implemented in a finite element code by using the isogeometric analysis (IGA) technology. The implementation of connections between branches from different beams is undergoing, most of the fracture behaviors will be enforced at these connections, certain fracture constitutive laws used for quasi-brittle materials such as wood will be included.
Collaboration with the University of Maine has led to a series of small-scale experiments for use in calibration and validation of the aforementioned lattice beam model. These experiments include notched fracture tests with respect to multiple grain orientations (pictured) as well as matchstick bending tests.
Computational modeling of micromechanics of wood, cross-laminated timber (CLT), and other fibrous materials
A 3D discrete connector-beam lattice (CBL) model to simulate the mechanical behaviors of wood and engineered wood products (e.g., wood panel, CLT) at the mesoscale has been developed. The basic element of this lattice model is the 3D curved beam characterized by (a) arbitrary curvature and torsion of the beam axis and (b) an irregular, cruciform cross-section. The various branches for the cross-section represent the cell walls of the cellular wood microstructure and the beam axis is the line at which various cell walls meet. The Isogeometric Analysis (IGA) technique has been employed to describe the beam geometries accurately. The joints (both transverse and longitudinal) between neighboring beams are modeled with the “connector” elements, characterized by the smeared crack model for the cell wall failure and by the cohesive fracture laws for the softening strain-stress relationship of quasi-brittle materials (including wood, fiber-composites).
The beam lattice and connector elements have been implemented with Abaqus user subroutines “UEL” and “VUEL” for both implicit and explicit analyses. A preprocessing-analysis-postprocessing pipeline has been formed. The codes has been optimized to allow the simulation of large models (number of elements > 10 millions), the length scale of specimens can be up to tens centimeters as a mesoscale model with element sizes of 10~100 microns. Collaborating with experimentalists from the University of Maine, this model has proved its capability in accurately simulating the orthotropic nature, as well as the annual ring-oriented fracture behaviors of wood materials. This model can elucidate mesoscale-level understanding on how the cellular morphology influences the macroscopic properties of the material. This insight can be harnessed for the design of natural and engineered “wood-like” materials with special functionalities, such as anisotropic programmable material properties, stress redirection, and oriented impact energy dissipation.
