Arijit Sinha

Structural Performance of Glued Laminated Bamboo Beams

This paper presents a study aimed to characterize the structural performance of laminated bamboo lumber (LBL) and bamboo glulam beams (BGBs) as a first step to evaluate their potential application as a structural material. LBL was tested to determine their flexural, tensile, and shear properties, whereas BGBs were tested for their flexural, shear, and compressive properties. The BGBs were fabricated using two different adhesives: isocyanate resin (ISO) and phenol-resorcinol formaldehyde (PRF). BGBs with ISO performed better in bending strength, whereas the stiffness of glulams with both glue types was equivalent. Irrespective of the glue type, failure modes and shear test data showed BGB bending strength was limited by interlaminar shear in the LBL used in BGB fabrication. From the experimental study, it is concluded that the LBL does possess higher allowable and average strength values in tension and bending and comparable stiffness values, with much less variability to a commonly used structural species of wood, Douglas fir. The potential of using LBL in framing applications exists. However, certain impediments need to be addressed and researched before acceptance of LBL and BGB in the construction marketplace.

Viability of Hybrid Poplar in ANSI Approved Cross-Laminated Timber Applications

AbstractThe development of cross-laminated timber (CLT) technology has opened up new opportunities for low-density hardwood species, which have traditionally not been rated as construction-grade materials for structural engineering applications. Several characteristics of CLT, namely thermal performance, seismic behavior, and speed of construction, have raised interest among designers. The CLT technology has recently been used for residential and nonresidential multistory buildings and it has been identified as one of the ways of achieving tall timber building construction. As CLT gains acceptance in the industry, low-density wood species, not specified in current ANSI standards, need to be investigated for potentially successful use in CLT panels. This paper presents a study that demonstrates the viability of a Forest Stewardship Council (FSC) certified sustainable plantation grown low-density species, hybrid poplar (marketed as Pacific Albus), for use in performance-rated CLT panels by following the ANS...

Thermal degradation of bending properties of structural wood and wood-based composites

Abstract Wood and wood-based composites are being used extensi-vely in single-family residential dwellings. Therefore, it is important to categorize their response when exposed to elevated temperatures for a sustained period of time. In fire-resistant design for wood structures, the main goal is to ensure that enough structural integrity is maintained, during and after a fire, to prevent collapse and to maintain means of egress. Another goal is an ability to assess post-fire structural integrity and residual strength of an existing structure. The objectives of this study are: (a) to study the effect of temperature and exposure time on bending strength (MOR) and stiffness (MOE) of solid sawn lumber (SSL), laminated veneer lumber (LVL), oriented strand board (OSB) and plywood; and (b) to develop predictive relations between different temperatures and times of exposure and the thermal degradation of strength. A total of 1080 samples were tested in static bending under various heat treatments. The results indicated that exposure to elevated temperature caused significant degradation of bending strength and stiffness. A statistical regression-based model and a kinetics-based model were developed and evaluated for predicting the strength loss of wood and wood-based composites as a function of thermal exposure temperature and exposure time. The kinetics-based model fit the data better and predictions consistently matched the observed values, making the model preferred over the regression approach.

The effect of elevated temperature exposure on the fracture toughness of solid wood and structural wood composites

Fracture toughness of wood and wood composites has traditionally been characterized by a stress intensity factor, an initiation strain energy release rate (Ginit) or a total energy to fracture (Gf). These parameters provide incomplete fracture characterization for these materials because the toughness changes as the crack propagates. Thus, for materials such as wood, oriented strand board (OSB), plywood and laminated veneer lumber (LVL), it is essential to characterize the fracture properties during crack propagation by measuring a full crack resistant or R curve. This study used energy methods during crack propagation to measure full R curves and then compared the fracture properties of wood and various wood-based composites such as, OSB, LVL and plywood. The effect of exposure to elevated temperature on fracture properties of these materials was also studied. The steady-state energy release rate (GSS) of wood was lower than that of wood composites such as LVL, plywood and OSB. The resin in wood composites provides them with a higher fracture toughness compared to solid lumber. Depending upon the internal structure of the material, the mode of failure also varied. With exposure to elevated temperatures, GSS for all materials decreased while the failure mode remained the same. The scatter associated with conventional bond strength tests, such as internal bond and bond classification tests, renders any statistical comparison using those tests difficult. In contrast, fracture tests with R curve analysis may provide an improved tool for characterization of bond quality in wood composites.

Effect of grain angle on shear strength of Douglas-fir wood

Abstract The effect of grain angle (GA) on shear strength of Douglas-fir has been evaluated. Shear block specimens with a GA varying from 0 to 90° was loaded in the shear plane, resulting in failure mode transitioning from parallel to grain shear to rolling shear. As expected, shear strength decreased as the GA increased from 0° to 90°. A root-mean-square equation was found to be suitable to predict the relationship between GA and shear strength. Traditional Hankinson formula and the Tsai-Wu criteria were less effective with this regard.

Assessing the role of adhesives in durability of laminated veneer lumber (LVL) by fracture mechanics

Abstract This study explored the suitability of fracture toughness properties for durability assessment of wood composite panels by observing changes in fracture toughness during crack propagation following cyclic exposure to moisture conditions. The main objective was to develop a new method for ranking the role of adhesives in the durability of wood-based composites. This new approach was compared to conventional mechanical performance tests, such as observing strength and stiffness loss after exposure. Comparing changes in fracture toughness as a function of crack length after moisture cycling shows that this approach can distinguish different adhesive systems on the basis of their durability, while conventional tests fail in this regard. The most and least durable adhesives (polyvinyl acetate and phenol formaldehyde) could be distinguished based on steady-state toughness alone, but this was not the case for the performance of two other adhesives (emulsion polymer isocyanate and phenol resorcinol formaldehyde). Further analysis of experimental R curves (toughness as a function of crack length) based on kinetics of degradation was able to rank all adhesives confidently. Probably, the failure of conventional tests in this context is that they are based on initiation of failure, while the fracture tests require consideration of fracture properties after a significant amount of crack propagation has occurred.

Thermal Degradation of Lateral Yield Strength of Nailed Wood Connections

This study investigated the effect of exposure to elevated temperature on the yield strength of single-shear nail connections when subjected to lateral loading. Solid sawn lumber and laminated veneer lumber were used as framing members and two different thicknesses of both oriented strand board and plywood were used as the sheathing members. The connection geometries evaluated were typical of those encountered in lateral force resisting systems such as shear walls or roof diaphragms. The connection geometries were (1) edge connection— nail positioned 19 mm from the panel edge, loaded parallel to the grain of the main member, and (2) plate connection—nail positioned 19 mm from the panel end, loaded perpendicular to grain of the main member. Data collected from monotonic tests on 480 nail connections, after exposure to elevated temperatures, were analyzed for yield strength. In addition, 210 dowel bearing strengths were evaluated for the same treatments. The results indicated that exposure to elevated temperature caused significant degradation in lateral yield strength after exposure. The highest degradation occurred when exposed to 200°C for 2 h. For example, for plywood (11.2 mm) and solid lumber connections, the decrease in yield strength after exposure to 200°C for 2 h was 26% for edge connections and 56% for plate connections. The results further indicated that, given thermal degradation of the dowel bearing capacity of a material, the existing yield models stipulated in National Design Specifications (NDS) can predict yield load values for nailed connections for a given sheathing and framing member combination under those thermal degradation conditions. Additionally, the yield models predicted a predominant yield mode (IIIs), consisting of a single plastic hinge being formed just beneath the surface of the thicker member. These predictions were consistent with observed yield modes. DOI: 10.1061/(ASCE)MT.1943-5533.0000233.

Performance of Steel Energy Dissipators Connected to Cross-Laminated Timber Wall Panels Subjected to Tension and Cyclic Loading

AbstractThis paper presents a new alternative energy dissipation solution to be used with cross-laminated timber (CLT) self-centering walls. CLT is a relatively new building product in North America and could potentially be used for high-rise construction. The development of high-performance seismic design solutions is necessary to encourage innovative structures and the design of these structures to new heights. The objective of this paper is to propose a wall-to-floor connection system that is easy to install and replace (structural fuse) after the occurrence of a large damaging event. The proposed energy dissipators are fabricated following concepts used in developing steel buckling restrained steel braces (BRB), having a milled portion, which is designed to yield and is enclosed within a grouted steel pipe. The connection system is investigated experimentally through a test sequence of displacement-controlled cycles based on a modified version of the test method developed by the American Concrete Inst...

Thermal degradation modeling of flexural strength of wood after exposure to elevated temperatures

Abstract Wood is being used heavily in single-family residential dwellings. Therefore, it is important to categorize their response when exposed to elevated temperatures for a sustained period of time. An important aspect of structural fire design is to assess postfire residual strength of existing structures. This study addresses this issue by developing models to predict strength degradation of wood after exposure to elevated temperature. The objectives were to (a) study the effect of exposure time on bending strength [Modulus of Rupture (MOR)] of wood at elevated temperatures, (b) interpret any relationships between different temperature and time of exposure using kinetics and a statistical approach, and (c) compare the two approaches. Two hundred thirty-two samples in total were tested in flexure as a function of exposure time and several temperatures. MOR of wood decreased as a function of temperature and exposure time. Rate of degradation was higher at higher temperatures. These results were fit to a simple kinetics model, based on the assumption of degradation kinetics following an Arrhenius activation energy model with apparent activation energy of 37.4 kJ/mol. A regression-based statistical model was also developed. The kinetics-based model fit the data better with one less parameter and predictions consistently matched the observed values, making the model preferred over the regression approach.

Evaluation of a Wood-Strand Molded Core Sandwich Panel

AbstractGreen building practices have created an opportunity for the forest products industry to become a leader in the creation of innovative building products derived from renewable materials. In...