Effect of Moisture Content on Dowel-Bearing Strength

Dowel bearing strength (embedment strength) is a critical component of wood connection design. Previous tests have concentrated on defining the relationship between dowel-bearing strength, specific gravity, and fastener characteristics such as diameter. However, because adoption of yield theory indefining connection strength is relatively new in the United States, few studies have been conducted that completely define the factors influencing dowel-bearing strength. One such factor is moisture content. In this study, the dowel-bearing strength of two groups of specimens was determined. One group was made up of approximately 200 clear Southern Pine pieces distributed evenly among five different moisture content environments (4%, 6%, 12%, 19%, and green) and loaded with 12.7-mm- (0.5-in.-) diameter bolts. The second group included Southern Pine, Douglas Fir-Larch, and Spruce-Pine-Fir specimens in two moisture content environments (6% and 20+%) that were loaded with 3.33-mm (0.131-in.) (8d) smooth shank nails. An empirical linear relationship was developed between dowel-bearing strength and moisture content using the first group of specimens, which compared favorably with results from the second group. These results show that the dowel-bearing strength-moisture content relationship was not dependent on species or fastener type, and therefore, those parameters were not included in the model. Auxiliary tests verified previous research that has shown that dowel-bearing strength (parallel-to-grain) is positively correlated with ultimate parallel-to-grain compression strength

Damage Assessment of a Full-Scale Six-Story Wood-Frame Building Following Triaxial Shake Table Tests

In the summer of 2009, a full-scale midrise wood-frame building was tested under a series of simulated earthquakes on the world’s largest shake table in Miki City, Japan. The objective of this series of tests was to validate a performance-based seismic design approach by qualitatively and quantitatively examining the building’s seismic performance in terms of response kinematics and observed damage. This paper presents the results of detailed damage inspections following each test in a series of five shake table tests, and explains their qualitative synthesis to provide design method validation. The seismic test program had two phases. Phase I was the testing of a seven-story mixed-use building with the first story consisting of a steel special moment frame (SMF) and stories 2–7 made of light-frame wood. In Phase II, the SMF was heavily braced such that it effectively became an extension of the shake table and testing was conducted on only stories 2–7, making the building a six-story light-frame multifamily residential building instead of a mixed-use building. All earthquake motions were scalings of the 1994 Northridge earthquake at the Canoga Park recording station with seismic intensities ranging from peak ground accelerations of 0.22 to 0.88 g. The building performed quite well during all earthquakes with damage only to the gypsum wall board (drywall), no sill plate splitting, no nails withdrawing or pulling through the sheathing, no edge tearing of the sheathing, no visible stud splitting around tie-down rods, and reasonable floor accelerations. On the basis of damage inspection, it was concluded that it is possible to design this type of building and keep the damage to a manageable level during major earthquakes by utilizing the new design approach

Damage Assessment of a Full-Scale Six-Story Wood-Frame Building Following Triaxial Shake Table Tests

In the summer of 2009, a full-scale midrise wood-frame building was tested under a series of simulated earthquakes on the world's largest shake table in Miki City, Japan. The objective of this series of tests was to validate a performance-based seismic design approach by qualitatively and quantitatively examining the building's seismic performance in terms of response kinematics and observed damage. This paper presents the results of detailed damage inspections following each test in a series of five shake table tests, and explains their qualitative synthesis to provide design method validation. The seismic test program had two phases. Phase I was the testing of a seven-story mixed-use building with the first story consisting of a steel special moment frame (SMF) and stories 2-7 made of light-frame wood. In Phase II, the SMF was heavily braced such that it effectively became an extension of the shake table and testing was conducted on only stories 2-7, making the building a six-story light-frame multifamily residential building instead of a mixed-use building. All earthquake motions were scalings of the 1994 Northridge earthquake at the Canoga Park recording station with seismic intensities ranging from peak ground accelerations of 0.22 to 0.88 g. The building performed quite well during all earthquakes with damage only to the gypsum wall board (drywall), no sill plate splitting, no nails withdrawing or pulling through the sheathing, no edge tearing of the sheathing, no visible stud splitting around tie-down rods, and reasonable floor accelerations. On the basis of damage inspection, it was concluded that it is possible to design this type of building and keep the damage to a manageable level during major earthquakes by utilizing the new design approach. DOI: 10.1061/(ASCE)CF.1943-5509.0000202. (C) 2012 American Society of Civil EngineersKeywords: Experimentation, Shake table tests, Damage assessment, Wood structures, Performance, Full-scale tests, Damage, Full-scale experiment, Earthquakes, Multi-story buildings, Shake tabl

Analysis of Wood Cantilever Loaded at Free End

A wood cantilever loaded at the free end was analyzed using the anisotropic elasticity theory. This report presents a two-dimensional numerical example of a Sitka spruce cantilever in the longitudinal- radial plane. When the grain slope is zero, i.e., the beam axis coincides with the longitudinal axis of wood, the stresses in the beam and the deflection of the beam are the same as those for an isotropic beam; when the grain slope is different from zero, the stresses and the deflection can increase significantly

Equivalent Circuit Modeling of Wood at 12% Moisture Content

Electrical impedance spectra (EIS) were collected from southern pine (Pinus spp.) wood equilibrated to 12% moisture content. Cylindrical graphite electrodes were embedded in the wood so that they met nearly end-to-end along a line parallel to the grain, and the EIS properties were characterized as functions of electrode spacing and electrode contact pressure at frequencies between 1 x 10-1 and 3 x 105 Hz. The data show a narrow distribution of relaxation times, which can be fit using a Debye model or, even better, a model with a constant phase element (CPE) in parallel with a resistor. The relaxation time is sensitive to contact pressure applied to the electrodes but not electrode spacing. The width of the CPE distribution is sensitive to electrode spacing but not contact pressure. On first inspection, the data suggest a finite contact resistance between wood and electrode, but further examination reveals that the "contact resistance" is an artifact caused by electrode fringe effects

DESIGN APPROACHES FOR CLT CONNECTIONS

Various design approaches for establishing the resistance of connections in cross-laminated timber (CLT) structures have been developed and adopted in timber design standards worldwide. Although the fundamental principles are similar, the new design provisions for CLT connections have been aligned in some standards with the existing design philosophy and format adopted for sawn timber and glulam using traditional fasteners such as dowels, nails, and wood screws for consistency and simplicity, in the other standards, alternate approaches have been developed. This article presents a snap shot of the various design approaches for connections in CLT adopted in Europe, Canada, the United States, and New Zealand. The intent is for the reader to have a better knowledge of the underpinning assumptions, principles, and the adopted design rules in each of these standards

A Small-Scale Test to Examine Heat Delamination in Cross Laminated Timber (CLT)

This paper examines the strength of wood adhesive bonds at high temperatures. The goal of this research is to better understand the conditions of heat delamination in cross laminated timber (CLT) that is exposed to fire. Heat delamination in CLT occurs when one lamination detaches from the composite panel before the char front reaches the bondline. Timber that falls from the panel, as a result of delamination, contributes additional fuel to the fire, which can cause fire regrowth, while the loss of a lamination causes a sudden loss in strength. Currently, to demonstrate that an adhesive does not delaminate, it must pass a full scale (6 m by 3 m) compartment fire test as prescribed in the PRG-320 product standard. In this work, we scaled down the mechanical loads and temperatures to 300 mm lap shear specimens. Seven different adhesives were tested and compared against solid wood controls with the same geometry as the lap shear specimens. Quasi-static tests were run where the specimens were loaded to failure at 25 °C and 260 °C, when the samples were at thermal equilibrium. Additionally, creep tests were performed where the load and temperature ramp was matched to the adhesive bondline temperatures measured in the large scale PRG-320 tests. With the exception of some of the polyurethane formulations, all adhesives passed the scaled-down creep test that resembles the PRG-320 standard. Of the polyurethane adhesives tested, only one formulation remained intact for the duration of the test. These results can be used to help better predict which adhesives may pass the PRG-320 test prior to full scale testing

Ability of finger-jointed lumber to maintain load at elevated temperatures

This article presents a test method that was developed to screen adhesive formulations for finger-jointed lumber. The goal was to develop a small-scale test that could be used to predict whether an adhesive would pass a full-scale ASTM E119 wall assembly test. The method involved loading a 38-mm square finger-jointed sample in a four-point bending test inside of an oven with a target sample temperature of 204°C. The deformation (creep) was examined as a function of time. It was found that samples fingerjointed with melamine formaldehyde and phenol resorcinol formaldehyde adhesives had the same creep behavior as solid wood. One-component polyurethane and polyvinyl acetate adhesives could not maintain the load at the target temperature measured middepth of the sample, and several different types of creep behavior were observed before failure. This method showed that the creep performance of the one-component adhesives may be quite different than the performance from short-term load deformation curves collected at high temperatures. The importance of creep performance of adhesives in the fire resistance of engineered wood is discussed.