Reliability-Based System Factor for Serviceability Design of Wood Floors

A structural analysis model for parallel-member wood joist floors is developed that includes the effect of component creep. Viscoelastic material models are calibrated using the data from a recently completed experimental program conducted as part of this overall study. Using this system model, deflection serviceability reliability analyses of parallel-member wood systems, including the effects of creep deformation, are conducted. Stochastic load models are used to simulate the time-varying nature of applied loads. Multiple limit state definitions for deflection serviceability of parallel-member wood floors are considered. Monte Carlo simulation is used to evaluate limit state probabilities. Reliability indices for current serviceability design provisions are also evaluated, and a serviceability system factor for Load and Resistance Factor Design (LRFD) is recommended

Creep and Creep-Recovery Models for Wood Under High Stress Levels

Forty small clear southern pine specimens were loaded under third-point bending to examine creep and creep-recovery behavior for wood under high stress levels. Stress levels of between 69% and 91% of the predicted static strength were applied for 23 h with 1 h allowed for recovery, and the resulting deflection vs. time behavior was studied. The experimental creep and creep-recovery behavior was modeled using modified power law functions. The results indicate that these functions provide the best fit to both primary and secondary experimental data. The empirical models can be used to simulate the viscoelastic behavior of wood under high stress levels. The simulation will provide a useful tool in future studies to examine duration-of-load (DOL) effect, which is one of the more important factors in wood structural design

Finite Element Analysis of Moso Bamboo-Reinforced Southern Pine Osb Composite Beams

A finite element (FE) analysis was performed to investigate the flexural properties of a structural composite lumber—Moso bamboo (Phyllostachys pubescens) reinforced southern pine oriented strand-board (OSB). Parametric analyses were conducted to investigate the stress and displacement distributions. Various beam configurations as affected by glue, web structure, flange composition, and bamboo-OSB combination were considered. The comparison of the numerical results from the selected models with those from bending tests was also performed. Finally, a rational design criterion for this type of composite beam was proposed based on the analytical and experimental studies. Bamboo is capable of improving the flexural properties of the OSB for use as a structural beam or joist. At a given cross section of about 30 X 140 mm, for instance, two-layer (6.4-mm thickness each) laminated bamboo flange can increase the OSB beam's maximum bending stress by 60 to 70% and double its stiffness. The total flange thickness, rather than the thickness of each layer, controls the beam deflection while the flange with a thinner layer (3.2 mm) resulted in higher bending, vertical, and transverse stresses but lower in-plane shear stress. More reinforcing material in the composite beam could reduce the maximum bending stress but would likely increase beam weight and processing cost. From this study, it is suggested that a two-layer flanged composite beam would be favorable from a material processing standpoint as well as superior in engineering performance over other configurations of bamboo-OSB composite beam product

Effects of Wood Decay By Postia Placenta on the Lateral Capacity of Nailed Oriented Strandboard Sheathing and Douglas-Fir Framing Members

The effect of wood decay on the single shear strength of nailed oriented strandboard (OSB) sheathing to Douglas-fir framing member connections was investigated. The connections evaluated in this study were representative of those present in lateral force resisting system components of light-framed wood structures, including shear walls and horizontal diaphragms. Strength and stiffness of the nailed connections were characterized using monotonic testing of samples exposed for increasing intervals to the brown rot fungus, Postia placenta. After the destructive tests, portions of the sheathing and framing member from the samples were further evaluated for dowel bearing strength and weight loss. The results indicated that existing yield models used for design of nailed connections can predict nominal design values for nailed connections of OSB sheathing and Douglas-fir framing members with various levels of decay damage, provided that the dowel bearing capacity of the wood materials can be assessed

Sheathing Nail Bending-Yield Stress: Effect on Cyclic Performance of Wood Shear Walls

This study investigated the effects of sheathing nail bending-yield stress (fyb) on connection properties and shear wall performance under cyclic loading. Four sets of nails were specially manufactured with average fyb of 87, 115, 145, and 241 ksi. Nail bending-yield stress and the hysteretic behavior of single-nail lateral connections were determined. The parameters of the lateral nail tests were used in a numerical model to predict shear wall performance and hysteretic parameters. The competency of the numerical model was assessed by full-scale cyclic tests of shear walls framed with Douglas-fir lumber and sheathed with oriented strandboard (OSB). The parameters of the shear wall model were used in another program to predict shear wall performance for a suite of seismic ground motions. The single-nail connection tests and wall model computations suggested that increased fyb of the sheathing nails should lead to improved wall stiffness and capacity. In both single-nail lateral connection and shear wall tests, the probability of nonductile failure modes increased as fyb increased. The peak capacity of the walls increased as fyb of the sheathing nails increased up to 145 ksi, but wall initial stiffness, displacement at peak capacity, and energy dissipation were not significantly affected by fyb. Sheathing nail fyb greater than 145 ksi did not enhance the overall cyclic behavior of wood shear walls

Predicting Strength of Matched Sets of Test Specimens

Five different methods for a priori estimating bending strength of wood and wood composite specimens are compared in this paper. They are: (1) edge-matching, (2) matching specimens by normal distribution, (3) matching specimens by log-normal distribution, (4) simple linear regression, and (5) multiple linear regression. It was found that the square root of mean square error (RMSE) of percent difference (PD) of predicted modulus of rupture (MOR) is the key measure in comparing the five methods. Multiple linear regression was found to be the best method to predict MOR of a specimen in an edge-matched set. Finally, how to create the prediction limits for mean MOR of a subgroup of specimens is discussed. The prediction limits for predicting MOR make it possible to quantitatively determine the effect of various treatments of wood materials

Radiation hardness qualification of PbWO4 scintillation crystals for the CMS Electromagnetic Calorimeter

This is the Pre-print version of the Article. The official published version can be accessed from the link below - Copyright @ 2010 IOPEnsuring the radiation hardness of PbWO4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered

Measurement of the t t-bar production cross section in the dilepton channel in pp collisions at sqrt(s) = 7 TeV

The t t-bar production cross section (sigma[t t-bar]) is measured in proton-proton collisions at sqrt(s) = 7 TeV in data collected by the CMS experiment, corresponding to an integrated luminosity of 2.3 inverse femtobarns. The measurement is performed in events with two leptons (electrons or muons) in the final state, at least two jets identified as jets originating from b quarks, and the presence of an imbalance in transverse momentum. The measured value of sigma[t t-bar] for a top-quark mass of 172.5 GeV is 161.9 +/- 2.5 (stat.) +5.1/-5.0 (syst.) +/- 3.6(lumi.) pb, consistent with the prediction of the standard model.Comment: Replaced with published version. Included journal reference and DO

Combined search for the quarks of a sequential fourth generation

Results are presented from a search for a fourth generation of quarks produced singly or in pairs in a data set corresponding to an integrated luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in 2011. A novel strategy has been developed for a combined search for quarks of the up and down type in decay channels with at least one isolated muon or electron. Limits on the mass of the fourth-generation quarks and the relevant Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a simple extension of the standard model with a sequential fourth generation of fermions. The existence of mass-degenerate fourth-generation quarks with masses below 685 GeV is excluded at 95% confidence level for minimal off-diagonal mixing between the third- and the fourth-generation quarks. With a mass difference of 25 GeV between the quark masses, the obtained limit on the masses of the fourth-generation quarks shifts by about +/- 20 GeV. These results significantly reduce the allowed parameter space for a fourth generation of fermions.Comment: Replaced with published version. Added journal reference and DO

Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV

A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7 TeV is presented. The data were collected at the LHC, with the CMS detector, and correspond to an integrated luminosity of 4.6 inverse femtobarns. No significant excess is observed above the background expectation, and upper limits are set on the Higgs boson production cross section. The presence of the standard model Higgs boson with a mass in the 270-440 GeV range is excluded at 95% confidence level.Comment: Submitted to JHE