Lumber Beams Research Articles

Analytical cracking load estimation of Laminated Veneer Lumber (LVL) beams with holes

Predicting the load-carrying capacity of timber beams with holes cannot be performed by usual analysis methods if the failure of the beam is governed by the crack initiation and propagation around the hole at low load levels. Predicting the cracking load is an important design issue because it corresponds to the load-carrying capacity of the timber beam before the crack propagation. One of the models that can be used for the fracture formulation is that of a beam on elastic foundation. In this model a part of the beam is assumed to follow the conditions of beam with elastic foundation which has spring stiffness equal to the fracture properties of the material in the crack surface. Based on beam on elastic foundation model, the cracking load prediction is the target of the paper. Some closed form solutions for the beam with hole are presented. The formulation has been derived for two cases of pure shear and pure bending moment. Finally a semi-empirical formulation for combination of shear and bending moment in the section is presented. The model predictions are compared with the results of an experimental program showing good correlation. The analytical model can therefore be proposed for future revisions of codes of practice such as the Eurocode 5.

Neutral Axis of Full-size Lumber With Multiple Knots

The neutral axis (NA) for a beam with isotropic, homogeneous material coincides with the geometrical centroid of the cross section. For wood beams, it is often assumed that the NA is at the centroid of the beam as well. Wood, however, is neither isotropic nor homogeneous. A multitude of anatomical features such as grain direction, grain pattern, knots, and so on provide wood with its orthotropic properties and in turn cause deviations in the location of the NA from the centroid of the beam. Knowledge of the true location of the NA would help one gain a better understanding of the mechanical behavior of wood beams. The objective of this study was to continue developing a protocol to optically analyze the NA in wood beams. Moreover, this study characterized the changes in the location of the NA with the presence of single and multiple knots in the beam. To characterize the location of the NA and its subsequent movement due to the presence of knots, a digital image correlation technique was applied to full-size lumber beams, which were loaded in flexure. The NA location was determined using longitudinal strain plots. The location and move- ment of the NA varied greatly between specimens and was affected by different patterns and types of knots in those specimens. In this testing, the average location of the NA in a nominal 2 � 4 clear beam was 50.7 % of the depth above the tension side. With multiple knots, the results revealed more complex, qualitative behav- ior. In some beams the knots in tension and the knots in compression had very nearly equal effects on the NA and, in fact, offset each other with very little movement in the local location of the NA. In other cases, when the knots were placed close together longitudinally, the NA showed little to no movement. This work marks significant progress in developing a protocol for optically understanding the effect of knots on the NA and can be used for other beam sizes and species.

Splitting of timber beams loaded perpendicular to grain by connections – Combined effect of edge and end distance

The paper presents experimental results of tests on splitting of laminated veneer lumber beams subjected to loading perpendicular to grain by single bolts with small edge and end distances. Two different edge distances were used, and a total of eight different combinations of edge and end distances were tested. Material properties such as perpendicular-to-grain tensile strength and mode I fracture energy of the tested beams were also determined by testing. The experimental results are compared with a simple analytical model based on quasi-nonlinear fracture mechanics, which takes into account the effect of the edge distance as well as the end distance. A generalized model is proposed, which applies to small and large edge distances and to small and large end distances, and which contains as a special case the linear elastic fracture mechanics model, which is currently used as the basis for calculation of the splitting capacity in the Europeans and Canadian timber design codes.

Charring rates for double beams made from laminated veneer lumber (LVL)

SUMMARYThis paper examines charring rates for different cross‐sections of single and double timber beams made from laminated veneer lumber, with nailed, screwed or glued connection types for the double beams. Charring rates and burning characteristics were examined both in a small furnace and in a larger pilot furnace. The bottom charring rates were sometimes greater than the side charring rates for very narrow beams dominated by corner effects and for double beams where the two components could separate during the fire exposure. The nail‐connected double laminated veneer lumber beams experienced the largest separation, leading to charring between the two components. The best performance was from the glued connection, which showed similar charring rates as a solid timber beam. Both the large‐scale and small‐scale testing showed that suitably placed screws (preferably full‐length threaded) can be used to give almost the same performance as a glued connection. Experimental findings were compared with results from a finite element analysis. There was reasonable agreement while the char layer was small, but less agreement in later stages as the char layer increased in thickness. Experimental findings were used to modify a spreadsheet design tool that predicts the fire resistance of a timber–concrete composite floor. Copyright © 2012 John Wiley & Sons, Ltd.

Effect of Hole Location on the Load-Carrying Capacity of Laminated Veneer Lumber Beams

Predicting load-carrying capacity of timber beams with holes requires a model capable of accounting for the microscopic material behaviour that influences crack initiation and propagation. The complex stress distribution around the periphery of a hole causes additional tension perpendicular to grain stresses, which can change the failure mode of the beam. This situation can also be affected with a change of hole location within the beam depth because stress intensity factor will be increased by tensile stresses and decreased by compressive stresses. This is not an unlikely situation as services often have to pass through beams at different depths. This paper investigates the effect of changing the hole location through the depth of laminated veneer lumber (LVL) beams utilising an experimental and numerical investigation. Experimental tests to failure of LVL beams and numerical simulations using finite element methods show that for a hole eccentricity of less than 20% of the beam depth, the load-carrying capacity of the beam does not change significantly. For uniformly distributed loading, a linearly decreasing stress intensity factor from the support to mid-span is exhibited, showing an increase in load-carrying capacity as the opening approached mid-span.

Effect of hole location on the load-carrying capacity of laminated veneer lumber beams

Effect of hole location on the load-carrying capacity of laminated veneer lumber beams

Suitability of live and fire-killed small-diameter ponderosa and lodgepole pine trees for manufacturing a new structural wood composite

Finding alternative uses for raw material from small-diameter trees is a critical problem throughout the United States. In western states, a lack of markets for small-diameter ponderosa pine (Pinus ponderosa) and lodgepole pine (Pinus contorta) can contribute to problems associated with overstocking. To test the feasibility of producing structural composite lumber (SCL) beams from these two western species, we used a new technology called steam-pressed scrim lumber (SPSL) based on scrimming technology developed in Australia. Both standing green and fire-killed ponderosa and lodgepole pine logs were used in an initial test. Fire-killed logs of both species were found to be unsuitable for producing SPSL but green logs were suitable for producing SPSL. For SPSL from green material, ponderosa pine had significantly higher modulus of rupture and work-to-maximum load values than did SPSL from lodgepole pine. Modulus of elasticity was higher for lodgepole pine. The presence of blows was greater with lodgepole pine than with ponderosa. Blows had a negative effect on the mechanical properties of ponderosa pine but no significant effect on the mechanical properties of SPSL from lodgepole pine. An evaluation of non-destructive testing methods showed that X-ray could be used to determine low density areas in parent beams. The use of a sonic compression wave tester for NDE evaluation of modulus of rupture showed some promise with SPSL but requires further research.

Construction time and cost for post-tensioned timber buildings

This paper describes post-tensioned timber alternative designs for a six-storey office building in New Zealand. The building has long-span timber–concrete composite floors supported on laminated veneer lumber beams, columns and walls. Lateral loads are resisted with moment-resisting post-tensioned timber frames in one direction and post-tensioned cantilever timber walls in the other direction. This paper compares the time and cost of timber construction with the concrete building actually being constructed. Construction times of the timber and concrete buildings are found to be almost identical. The cost of the timber alternative is approximately 6% more than concrete or steel designs. This paper demonstrates the opportunity for post-tensioned timber to compete with concrete and steel for multi-storey long-span office buildings, providing a more sustainable alternative.

Process-based analysis of added value in forest product industries

Manufacturing products with greater added value is increasingly viewed as a strategic goal of forest products industries. Added value is defined here as the difference in economic value between the physical inputs and outputs of a production process, and is generally analysed at the firm or national economy level. In this study we identify and discuss issues involved in quantifying added value at the industrial process level, and develop a bottom-up method to estimate the value added by forest industry processes. We calculate the value added by 14 traditional and emerging processes within the Swedish forest products industries, and express the results using various indices. We find that the type of biomass input strongly influences the potential for adding value, with sawlogs allowing more added value and being less sensitive to input price fluctuations than pulpwood and forest residues. Structural wood products such as lumber and glue-laminated beams are found to give the greatest value added. Co-production of multiple products from a single raw material increases total value added. Integrating the value chain of pulp and paper production significantly increases the value added to pulpwood. Multiple conversion processes exist for using forest residues as fuel, with a range of potential added value. Consideration of the climate benefits of forest product use, through the application of a carbon tax, significantly increases the added value.

Efficient utilization of plastic waste through product design and process adaptation: A case study on stiffness enhancement of beams produced from plastic lumber

AbstractThe aim of the present work is the development of a method for structural reinforcement of beams obtained by in‐mold extrusion of plastics from solid urban waste. The beams obtained by in‐mold extrusion are commonly used for outdoor furniture and structures. The material used for such applications is mainly composed of low‐density polyethylene derived from bags and films, with small amounts of high density polyethylene and polypropylene, as well as traces of polyethylene terephthalate. This material is usually referred to as “plastic lumber.” Plastic lumber products have a low stiffness, which results in high deflections under flexural loads, particularly under creep loading. In this study, reinforcing rods of high aspect ratio were incorporated into plastic lumber beams in specific positions with respect to the cross section of the beam. The reinforcement of the plastic lumber beams with fine rods is introduced in the typical intrusion process used for the production of unreinforced beams. Glass fiber reinforced pultruded rods were chosen for this purpose from a preliminary evaluation of different materials. Different diameter glass fiber reinforced pultruded rods, including surface‐abraded systems to increase the roughness, were used for the reinforcement of plastic lumber beams. The reinforced beams were tested in terms of flexural stiffness, creep resistance, and pullout resistance of the embedded rods. The results obtained from the mechanical tests showed a significant enhancement of flexural stiffness and creep resistance behavior. The performance at higher stress levels was shown to be significantly dependent on the interfacial adhesion between rods and polymer matrix. © 2009 Wiley Periodicals, Inc. Adv Polym Techn 27:133–142, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20131

Experimental Behavior of Prestressed LVL-Concrete Composite Beams

The paper investigates the stiffness and strength of composite laminated veneer lumber (LVL) and concrete beams intended for use in long-span flooring systems. Quasi-static bending tests and impact tests were conducted on four, 6 m long specimens to observe the failure mechanisms and to estimate the static and dynamic properties of the systems. One specimen, with a plain LVL member, a strong shear connection, and a concrete slab, was used as a control. The other three specimens investigated the use of (i) a proprietary, lightweight nonstructural concrete panel as a slab; (ii) a straight prestressing tendon and a strong shear connection; and (iii) a draped prestressing tendon and a weaker shear connection. All four specimens had a T-shaped cross section, with the slab supported by either one or two LVL beams. The stiffness of control beam, which is mostly related to the stiffness of the shear connection, is shown to be almost three times that of a bare LVL beam, but the beam is only 74% stronger than the bare beam. The prestressing tendons are shown to have little effect on the stiffness and strength, but reduce the deflection due to permanent load, particularly when they are draped. The proprietary lightweight concrete panels are shown to provide little structural benefit.

Connections for composite concrete slab and LVL flooring systems

Composite concrete slab and timber flooring systems are commonly used in many parts of the world to exploit the high strength-to-weight ratio of timber and the good acoustic separation provided by concrete floor slabs. This paper describes the results of an experimental programme that investigated the suitability of a range of connectors to transfer shear between a concrete slab and a laminated veneer lumber (LVL) beam. Shear tests on reduced scale specimens were performed with the aim of comparing the strength, stiffness, and post-peak performance of different connectors such as round and rectangular concrete plugs with and without screw and steel pipe reinforcement, proprietary (SFS) screws, coach screws with different diameters, sheet brace anchors, and framing brackets. The rectangular concrete plug reinforced with a coach screw was found to provide the greatest stiffness and strength, as well as favourable post-peak behaviour. Such a system can be used for cost-effective composite floor systems due to its efficient cost-to-capacity ratio, which reduces the number of connectors needed along the beam axis to achieve the composite action.

A proposal of short road bridges composed of coupled sawing lumber beams made of Japanese cedar

A proposal of short road bridges composed of coupled sawing lumber beams made of Japanese cedar

Influence of cross-section on the strength of timber beams

This paper investigates the strength and deformation characteristics of lightweight timber composite beams manufactured with six different cross-sectional profiles in comparison with readily available laminated veneer lumber (LVL) and glued-laminated (Glulam) beams. All engineered profiles comprised solid timber or LVL flanges and three-ply plywood webs. The number of webs varied from one to four. The beams had an overall depth of 290 mm and were either 88 mm or 106 mm wide. A study was conducted to provide a comparison of the beam designs and to determine possible effects of cross-sectional configuration and connection details on the structural properties of the beams. To enable a realistic analysis, 12 beams were replicated for each design. The individual components of the beams were tested prior to assembly to obtain the modulus of elasticity and shear modulus and were grouped to provide an even distribution of the material properties. The addition of extra webs to the I-beam profile significantly enhanced the bending and shear capacity of the beam while maintaining a high strength to weight ratio. The boxed I-beam proved to be the most efficient to manufacture and displayed superior structural performance compared with the rest of the profiles in terms of flexural stiffness and bending and shear capacity. The experimental results confirmed the significant contribution of the shear deflection to the total deflection of the I-beams, box beams and even solid section beams.

Experimental Study on the Creep Behaviour of Structural Component of Traditional Wooden Buildings

The purpose of this study is to identify the bending creep behaviour of sawn lumber beam of Japanese Cypress and the compressive creep behaviour of full-size masugumi, which is one of the most important components in old wooden Japanese buildings in terms of both structure and decoration. As the former, practical size sawing (120×240×6000mm) was applied, and bending creep test was carried out for 4 years. As a result, approximate formula for bending creep behaviour was obtained. According to this formula, total deflection after 100 years later is estimated to be about two and a half times as big as initial deflection. As the latter, by using full-size masugumi component, which was modeled on that in national treasure building “Toshodaiji Kondo”, compressive creep test was carried out for 4 years. As a result, approximate formula for compressive creep behaviour was obtained. According to this formula, total compressive deformation after 100 years is estimated to be about five times as big as initial deformation.

Modelling of knots in logs

A geometrical model was derived to describe knots in logs and on the surface of lumber beams sawn from those logs. Each knot is defined by 7 parameters related to the shape and position in the stem. A computer simulation program was written to study knot shapes on a variety of observation planes. An initial investigation on the shape of knots in Scots pine logs and lumber beams suggests that the model is sufficiently accurate to describe knottiness in this species. Potential applications of this model include automated lumber grading, computerized log reconstitution and yield optimization studies.

Design of Timber Beams: A Graphical Approach

A graphical approach for the design of timber beams is presented. The proposed design charts and the method enable the selection of allowable bending stress that meets the bending, deflection, and shear requirements recommended by the National Forest Products Association. A number of illustrative examples are shown to show the step by step design approach. The method is applicable for both glulam and sawn lumber beams.