Daniel P. Hindman

Applying digital image correlation to wood strands: influence of loading rate and specimen thickness.

Abstract Previous studies were devoted to various applications of digital image correlation (DIC) to wood and wood-based composites. However, the focus of these studies was qualitative strain distribution. Overall, there is a lack of testing protocols of DIC for quantifying the elastic properties of woody materials. The objective of this study was to investigate the effects of different specimen thicknesses and loading rates on measurement of Youngs modulus and Poissons ratio by DIC. Youngs modulus from DIC decreased as thickness increased at a loading rate of 0.254 mm min-1. Comparing the different loading rates at a thickness of 0.794 mm, Youngs modulus from DIC was not in agreement with the value obtained by means of the extensometer regardless of loading rate. However, Youngs modulus from DIC at a thickness of 0.381 mm and a loading rate of 0.254 mm min-1 was in good agreement with the corresponding Youngs modulus obtained by an extensometer. Poissons ratio measured from different loading rates and specimen thicknesses was not significantly different between the two measurement systems. From the testing arrangement applied for this study, it is recommended that DIC should be applied at a loading rate of 0.254 mm min-1 or slower for strands with a thickness of 0.381 mm or less.

Mechanical Properties of Southern Pine Cross-Laminated Timber

AbstractCross-laminated timber (CLT) is a novel wood building material suitable for floor, roof, and wall assemblies in low- and mid-rise construction as an alternative to concrete and steel. CLTs are considered to provide good seismic resistance, fast erection times, carbon sequestration, and a significant reduction in greenhouse gas emissions over other construction materials. Southern pine is an abundant forest resource; yet it has not been commercially used for CLT manufacturing. The purpose of this paper was to measure the bending and shear properties of CLT material fabricated using southern pine lumber compared to product standards. Bending strength, bending stiffness, shear strength, resistance to shear by compression loading strength, and resistance to delamination were measured according to product standards, and test results were compared with values given in the standards. Bending tests were conducted on jointed beams due to test length requirements. Bending strength, bending stiffness, and wo...

Strength of Sawn Lumber and Wood Composite Dowel Connections Loaded Perpendicular to Grain. I: NDS Design Equations

Connections are important design elements in timber engineering. Connections loaded perpendicular to grain for structural composite lumber (SCL) materials have received little attention. Connection strength can be determined as the minimum of the Technical Report 12 (TR-12) equations, a generalized form of the National Design Specification (NDS) connection equations, or a shear stress check for reduced beam section. This paper presents test results and compares the 5% offset yield and capacity of double-shear single bolt connections with steel side plates loaded perpendicular to grain to TR-12 equations for solid-sawn lumber and two SCL products. Testing considered three different span/depth ratios and three different loaded edge distances. Splitting failures were observed at the 4D loaded edge distance and mixed mode failures contained splitting at the 7D and 10D loaded edges. The allowable shear stress values dominated the connection at the 4D loaded edge distance and were conservative in some cases. Comparing the TR-12 equations to experimental connection strength, there was little difference for the span/depth ratios and main member materials. Different loaded edge distances demonstrated significant differences between the TR-12 equations and experimental values. The choice of displacement limit for the capacity dowel bearing strength was identified as an important factor in the TR-12 equations.

Strength of Sawn Lumber and Wood Composite Dowel Connections Loaded Perpendicular to Grain. II: Fracture Mechanics Equations

Splitting failure modes of perpendicular to grain bolted connections have received little attention when compared to splitting modes in parallel to grain connections. Previous research by the authors tested a series of single bolt, double shear perpendicular to grain connections of machine stress rated (MSR) lumber, laminated veneer lumber (LVL), and parallel strand lumber (PSL). Many of the experimental tests resulted in failure by splitting. Two fracture mechanics based models for connections proposed elsewhere were applied. Input parameters for the models were generated by testing matched sections to the connection strength samples. Input properties for the Van der Put model included shear modulus and Mode I fracture energy. Inputs for the Jensen model included shear modulus, modulus of elasticity, Mode I fracture energy, and tension perpendicular to grain strength. Mode I fracture energy of PSL was markedly greater than the energy associated with MSR Lumber and LVL. The Van der Put model overpredicted the experimental connection strength by at least 77%. The Jensen model was found to most accurately predict the connection strength over the entire range of configurations tested. Comparing the Van der Put and Jensen models to the previous work using the national design specification, the Jensen model performed the best in terms of accuracy. The tension perpendicular to grain strength and the method of fracture energy calculation may be important parameters for the capacity of perpendicular to grain loaded wood members.

Application of Scaled World Model for Usability Testing of Fall Arrest System in Residential Construction

Falls from heights in residential construction continue to be a leading cause of fatalities and severe injuries. Field studies are difficult to implement in residential construction, because companies are hesitant to allow observers on site to collect data while tasks are being conducted. Use of scaled world models has been recommended for experimental contexts that require high levels of ecological validity as well as internal (control) validity . In this project, we explored several attributes to design a scaled world model of residential roofing that would support controlled trials to evaluate usability and to conduct load testing, in a realistic setting. Features of the scaled world model were selected to support validity and ease-of-measurement while participants conducted roofing tasks. Several validity constraints were explored, including ecological, structural, psychological and process validity. Possible in-lab roofing task attributes were discussed and heuristic guidelines for scaled world models were provided. Lastly, an example of a scaled-world model was assembled to test the fall arrest system. The design team continues to develop the model, which will be reviewed by residential roofing experts for realism and accuracy.

Comparison of Single- and Two-Bolted LVL Perpendicular-to-Grain Connections. I: National Design Specification for Wood Construction Equations

AbstractLittle research is available for perpendicular-to-grain connections in wood using multiple bolts. Experimental testing of single- and two-bolt connections was compared with predicted values from General Dowel Equations for Calculating Lateral Connection Values Technical Report 12. Two different laminated veneer lumber materials were tested with variable loaded-edge distances, and variable bolt sizes in both single- and two-bolt connections. Splitting failures, and crushing of main member and formation of a single plastic moment in the bolt were observed. Connection resistance increased as loaded-edge distance and number of bolts increased. Most design factors of safety representing the ratio of experimental strength to lateral design values were controlled by shear design provisions. Design factors of safety were greater at smaller loaded-edge distance with single bolt compared to other configurations. Further investigation of the design shear provisions may be needed. Yield theory overpredicted t...

Metal Plate–Connected Wood Trusses Exposed to Out-of-Plane Lateral Loads

AbstractOut-of-plane loading of trusses occurs when a horizontal force is applied to the truss perpendicular to the plane of the truss. Because trusses are not specifically designed for lateral loading, the behavior of trusses during construction before adequate bracing is installed is unknown. Examples of out-of-plane loading include wind and seismic forces, as well as forces from fall-arrest anchors attached to trusses during construction. The purpose of this paper was to measure the maximum load and rotation of a single truss exposed to a horizontal out-of-plane load applied by a fall-arrest anchor. A specialized testing fixture was used to apply a horizontal load to a single unbraced truss attached to two stem walls, representing a typical construction application. A set of monoslope queen-post trusses with two different pitches were used, and load was applied at both the truss eave and peak. Maximum loads for a single truss were much less than the expected 8 kN required to act as a personal fall-arre...

Bracing of Wood Composite I-Joists to Resist Lateral Buckling from Walking Loads

AbstractLateral buckling of unbraced beams during construction may be an important cause of fatalities and injuries. Temporary bracing to restrict lateral buckling is a potential preventive approach, but has received little study. Lateral acceleration, lateral displacement, and rotation of wood composite I-joists were measured under different bracing conditions while participants traversed the joists. Five different bracing stiffnesses and two different bracing placements were examined. Lateral displacement and rotation increased near the midspan of the joist, while acceleration remained consistent. Greater lateral displacement and rotation were found with increasing participant weight and lack of construction experience. Construction experience was found to be a significant factor in the amount of joist motion. A safety platform similar to the testing methods discussed may have potential as a training tool to expose workers to partially braced joists. The relationship of lateral displacement and rotation...

Mechanical Response of Unbraced Wood Composite I-Joist to Walking Loads

AbstractLateral buckling of unbraced wood composite I-joists is a form of instability that causes the joist to deflect laterally and rotate. This instability may be a cause of worker falls and should be explored to understand the range of loading and out-of-plane motion required to cause lateral buckling. While walking on unbraced joists is uncommon on construction sites, understanding the movement of unbraced joists is needed to model more commonly observed partially braced joists. This paper investigated the load and out-of-plane movement of several unbraced wood composite I-joists subjected to human walking loads. Vertical load at the ends of the joist, top and bottom horizontal loads at one end of the joist, as well as lateral deflection and rotation at the midspan and quarter-span were measured while participants traversed the joist. Lateral buckling of joists was observed due to walking loads. Total vertical load measured was similar to participant weight, while horizontal loads ranged from 9.6 to 1...

Comparison of Single- and Two-Bolted LVL Perpendicular-to-Grain Connections. II: Fracture Models

AbstractPrevious research of structural composite lumber connections loaded perpendicular to grain has shown brittle failure by splitting. In previous testing, two different laminated veneer lumber (LVL) materials with single and two bolts in a row loaded perpendicular to grain were compared with fracture mechanics models proposed by Van der Put and Jensen and splitting capacity equations from Eurocode No. 5. Connection strength values of two-bolt connections were larger than single-bolt connections owing to the increased distance to the farthest fastener. The first LVL material (LVL-1) had significantly larger predicted capacity strength values than the other LVL material (LVL-2) for the same configuration owing to larger mode I fracture energy and tension perpendicular-to-grain strength values. A rederivation of the brittle failure constant CI specifically for LVL materials would improve performance of Eurocode No. 5 predictions. The different fracture models had improved connection prediction values wh...