STR-931: TIMBER I-JOISTS WITH WEB OPENINGS: REINFORCEMENT, CAPACITY PREDICTION AND SENSITIVITY ANALYSIS

Timber I-joists are a popular product in light-frame wood construction in North America. The design with timber I-joists, however, has not yet achieved the same level of refinement compared to reinforced concrete or steel structures. One of the reasons is that timber I-joists have higher variability in their material properties than more homogeneous building materials. Additionally, although very commonly applied in practice, engineers and practitioners have limited knowledge and guidance for I-joists with web opening. As a result, in many cases the design of timber I-joists based on manufacturer’s specifications lead to very conservative solutions. The present research predicts the capacity of unreinforced and reinforced timber I-joists with openings from experimental results. A total of 100 unreinforced and 100 reinforced I-joists with opening were tested under four point loading. The capacity of the I-joists with opening was predicted from regression analysis. A sensitivity analysis was performed for the predicted equations using Meta-model of Optimal Prognosis (MOP) to evaluate the contribution of each parameter on the model responses. The research demonstrates that the reinforcement technique was efficient for I-joists with openings and the proposed equations were very accurate to predict the I-joists capacity

Development and evaluation of composite insulated beams.

Decline in supplies of old growth wood coupled with increased demand for structural timber by the construction industry led to creation of engineered timber products (EWPs) comprising wood waste. The author designed, fabricated and tested composite insulated beams (CIBs) which are foam filled sandwich panels constructed from EWPs. CIBs in many cross-sections and materials were limit-tested for structural performance, long-term durability, thermal and dynamic behaviours. Varaiation in material properties was overcome by statistical sorting of beams with different stiffness. Some types of CIB were found to provide better structural performance than equivalent timber and glulam I-beam sections and the CIBs maintained a high strength to weight ratio. A parametric study based on Eurocode 5 determined the governing design criteria for CIBs. The study showed that in identical loading conditions CIBs offer longer spans than conventional EWP I-beams, together with lower beam depths for similar spans.Injected polyurethane foam improved long-term durability, bearing capacity and damping ratio of beams, but reducedthermal loss and reduced weakening effect of a web opening on shear strength of beams

Timber and Timber-Timber Composite (TTC) Beams with Openings: Laboratory Experimentation and Nonlocal Finite Element Simulation

Introducing openings/holes in joists/beams of flooring systems is usually necessary to pass through building services without increasing the floor-to-floor height. In timber beams, the openings reduce the stiffness and limit the ultimate load-carrying capacity by promoting tensile stresses perpendicular to the grain and longitudinal splitting shear stresses. Furthermore, quasi-brittle failure of the timber in shear and tension (promoted by openings) leads to localisation of strain, spurious sensitivity of finite element (FE) simulations with respect to mesh size and orientation and ultimately poor or erroneous damage initiation and failure load predictions. Moreover, the interaction between the openings in the web and slabs of timber-timber composite (TTC) floors which have gained popularity in recent decade, remain largely unexplored in terms of experimental results, and numerical simulations, and design provisions are only limited to bare timber beams without slabs. Accordingly, this research project aims at producing benchmark experimental data and developing reliable FE models for assessing the effect of geometrical irregularities such as notches and openings on the failure mode and load carrying capacity of the timber and TTC beams. The research project consists of an experimental program and a numerical (FE) simulation that involves derivation, computer implementation and application of a nonlocal continuum-damage model for timber. In the first stage of laboratory testing, pushout tests were performed on symmetric LVLCLT and GLT-CLT joints with coach screw shear connectors to establish the load-slip behavior, stiffness, and peak load of the coach screw shear connectors. In the second stage, laminated veneer lumber (LVL) and glued laminated timber (GLT) beams with symmetric circular and square openings were tested under three-point bending tests to establish the governing failure mechanism, produce the load-mid span deflection curve and determine the reduction in stiffness and peak load of the LVL and GLT beams due to openings. In the final stage of laboratory experimentations, TTC beams were fabricated by connecting CLT slabs to LVL and GLT beams using coach screw shear connectors and then three-point bending tests were performed on the fabricated LVL-CLT and GLTCLT composite beams with web opening size, shape, and location identical to the LVL and GLT beams tested in the second stage of the testing program. The digital image correlation (DIC) results acquired during stages two and three of the experimental programs shed light on the mechanism of strain localisation and failure promoted by the openings. Moreover, the test results elucidated the major contribution of the coach screw shear connectors in conjunction with CLT slabs to the failure mode and load-carrying capacity of the TTC beams with web openings. After the experimental program, the existing design criterion for evaluating the ultimate load-carrying capacity of the bare timber beams with holes was modified and applied to TTC beams with web openings. In this regard, an analytical Timoshenko composite beam model was utilized to estimate the shear stress and normal stress profiles in the joist (web) cross-sections and accordingly, the relevant terms in existing design criterion were modified to take into account the composite action between the slab and joist and the reinforcing effects of the screws around the opening areas. The proposed modified design equation had a great agreement with the experimental results. The numerical part of this research project focused on development, implementation, and validation of a constitutive law for nonlinear FE analysis of timber beams with stress concentrators such as notches and openings. The complex orientation-dependent behaviour of timber accompanied with nonlinear ductile hardening and brittle softening of the timber in compression, tension and shear were captured by an enriched 3D multi-surface continuum damage model. To alleviate the localisation of strain and spurious FE mesh sensitivity associated with brittle/quasi-brittle behaviour of the timber in tension and shear, a nonlocal integral model was developed and incorporated into the 3D continuum damage material model of timber. Apart from a standard attenuation function that suffers from boundary effects at the edges of notches and openings, a symmetric attenuation function was adopted in the nonlocal integral model to minimise the boundary effects in the nonlocal FE simulations. The developed symmetric nonlocal material model was implemented as a user-defined material subroutine (UMAT) in ABAQUS software, and the nonlocal FE simulations of the tested timber and TTC beams with openings were carried out to demonstrate the adequacy and accuracy of the nonlocal FE model for predicting the failure mode, load-displacement, and peak load of the timber beams in the face of strain localisation. The results of experimental program and numerical simulations revealed that the CLT slab thickness and penetration length of screw shear connectors around the opening areas have major impact on the structural behaviour of the perforated timber beams. It was demonstrated experimentally that different opening shapes of equal area could result in similar reduction of the loading capacity in the perforated timber beams. In addition, the numerical models revealed that local constitutive models cannot simulate the failure of timber materials. Indeed, the local material models must be enriched with a strong localization limiter to prevent strain localization and mesh dependency associated with quasi-brittle failures (softening behaviour) of timber. In the numerical simulations, it was shown that adopting nonlocal integral technique in the material model of timber effectively resolves the strain localization and mesh sensitivity issues

Structural behaviour of structural insulated panels (SIPS)

The Structural Insulated Panel System (SIP system) has recently attracted continuingly growing interest since it is strong, energy efficient, easy to use in construction and hence has a potential to become a new alternative building material. It is anticipated that Structural Insulated Panels (SIPs) are required to withstand loads in various directions either individually or in combinations, e.g., the axial, racking and transverse loadings. Very few publications report the performance of SIPs when subjected to loads in multiple directions. Moreover, when applying SIPs as a load bearing material, there is another major concern related to their long-term performance, mainly caused by creep. This research presents studies on structural behaviours of the SIPs under both short-term and long-term loadings under single and multi-axial loadings together with two typical joint designs i.e. mini-SIP and dimensional timber spline joints with and without openings by experimental, analytical and numerical investigations. It has been demonstrated that the developed numerical models can well predict the initiation of failure load and the failure mode of SIPs. Interactive failure load curves between axial and transverse loadings have been developed by carrying out a parametric analysis for SIPs with/without openings by using two types of joint construction

Evaluation of remediation techniques for circular holes in the webs of wood I-joists

The objective of this project was to evaluate methods to remediate a wood I-joist with a single, circular hole in the web while leaving utilities in place. The methods were experimentally evaluated with a full-scale bending test using four equally spaced point loads. There were three depths of joists with varying flange widths and two span lengths (4.88 m & 2.44 m). Failure modes for the long span joists without holes were in the flanges in either tension, compression or lateral buckling; but once a hole was introduced the majority were classified as shear failures. Effectiveness of the remediation was evaluated based on three criteria: strength, stiffness, and ease of installation/cost. The OSB collar patch was effective for 8 out of the 12 series tested. A maximum reduction of load from a joist with no hole to one with a hole was 49% for the long span and 58% for the short span. The OSB collar was not as effective in returning stiffness to the joists, but was easier to install and less expensive than the LSL patch

Lateral capacity and seismic characteristic of hybrid cold formed and hot rolled steel systems

This thesis addresses the application of hybrid cold-formed steel (CFS) - Hot-Rolled Steel (HRS) structures, as a new lateral force resisting system for light weight steel framed buildings in seismic regions. The study considers hysteretic behaviour, as well as maximum lateral load resisting capacity through comprehensive testing and advanced numerical analyses. The study identifies the advantages and disadvantages of the proposed hybrid system and provides in depth knowledge about performance characteristics of this innovative structural system, in order to facilitate the use of this system in earthquake-prone regions. The project is divided into three main parts: experimental, numerical and analytical studies. A comprehensive literature review is performed as a part of this study, in order to discover the existing gaps in the current knowledge regarding the structural performance of CFS structures and the methods for lateral performance enhancement. The literature review suggests that although CFS walls are not new, and have been used as non-structural components for many years, their application as main load-bearing structural frames is relatively new. That is, appropriate guidelines that address the seismic design of CFS structures have not yet been fully developed in the literature. In addition, the lateral design of these systems is not adequately detailed in the available standards of practice. There have been several attempts to improve the seismic performance of such structural system by different bracing or sheathing configurations. However, there is minimal background information available on hybrid systems such as hot rolled-cold formed structures. In this study, a series of CFS-HRS hybrid shear walls are constructed in order to investigate the lateral behaviour of the walls with different configurations to obtain the optimum combination of HRS and CFS. Different configurations are considered to provide the most efficient load transfer pattern from cold formed steel part of the wall to the Hot Rolled section, which is responsible for withstanding the lateral loads. The CFS part is aimed to transfer lateral loads to HRS part without any internal local failure. The ideal failure condition is the HRS yielding. Therefore, the optimum rigidity of the HRS part is of great importance to prevent any local failure happening prior to reaching the maximum lateral capacity of the HRS. For each experimental specimen, the hysteretic envelope curve is plotted, and different characteristics are evaluated. Since the failure mode of such systems is very complicated, the test results will provide the possible failure modes to be utilised for any further investigation or any optimisation analysis in numerical and analytical studies. In addition, Non-linear finite element (FE) analysis is employed using the ABAQUS software [1], in order to investigate the seismic performance of the proposed hybrid shear walls in multi-storey light steel frames. The nonlinear analysis accounts for different structural characteristics, including material non-linearity, geometric imperfection and residual stresses. The numerical models are verified based on experimental test results. The principal objective of this part of the study is aseismic optimisation of the proposed hybrid system and finding the corresponding dimensions and configurations to improve the strength and stiffness to achieve the objective. Using the hybrid wall panel system, a 4-storey building in an earthquake prone region is designed as per the relevant codes of practice. For the designed 4-storey building, the CFS part of the panel only bears the gravity loads, while a hot rolled steel collector transfers the lateral load to the HRS part acting as the main lateral load resisting system. Finally, the building is designed using different lateral load resisting systems and the results are compared with those from the proposed hybrid system in terms of cost. Furthermore, based on the real failure mode shapes obtained from test specimens, a Finite Strip Method program is developed to evaluate the elastic buckling mode shapes of a single stud with an arbitrary section detail. The code is helpful for design of CFS studs as explained in Chapters 3 and 5