There is interest in the lumber and truss industry to supply and use fingerjoined lumber for metal plate connected wood trusses. To support this, it is necessary to provide evidence that fingerjoined lumber meeting the requirements of a recognized fingerjoined lumber product standard can be used with the lumber design provision provided in the governing wood engineering design code.
In consultation with the truss and lumber industry, it was agreed that fingerjoined machine graded lumber meeting the requirements of the National Lumber Grades Authority (NLGA) Special Product Standard 4 (SPS 4) would be assessed for truss applications. The assessment would need to show no issues with applying the lumber design provisions in Clause 5.5.13 of CSA O86, the Canadian Engineering Design in Wood Code, to NLGA SPS 4 fingerjoined lumber. This is necessary because Clause 5.5.13 was originally developed for non-fingerjoined lumber and applies specifically to the design of lumber in truss applications.
The tests carried out under this program included bending test specimens with 1 to 4 joints per specimen tested to failure under three different bending moment configurations, and single fingerjoints tested to failure under pure axial tension or compression, and then under eccentrically applied axial tension or compression to induce bending in addition to the axial loading. All test specimens were prepared using a 2100f-1.8E grade spruce-pine-fir lumber and because the test to failure was typically less than 5 minutes, polyvinyl acetate (PVA) adhesive was used to bond the fingerjoints to facilitate joint fabrication.
Additional testing was also carried out to extend the testing protocol developed in 2008-09 for assessing fingerjoint adhesives under sustained tension loads. Samples bonded with a known performing adhesive, phenol resorcinol formaldehyde (PRF), were substituted with samples bonded with PVA, a known poor performer under sustained loads.
In the bending test, test span configuration and characteristic number of joints showed strong effects on the average bending capacity of the fingerjoints. While more joints in the region of maximum bending moment were expected to contribute to lower bending capacities, this was not as evident in this study. This is likely due to the small sample sizes and the tight control over the joint strength (i.e. low strength variability). Instead, having one or more fingerjoints in the maximum moment zone but near the load points appeared to have a stronger effect. The bending strength reductions were on the order of 5 to 10%.
In the combined loading test, loading eccentricity showed a strong effect on the capacity of the fingerjoints in both tension-bending and compression-bending. The tension-bending interaction should be noted for those evaluating online or offline tension test results. Both the tension-bending and compression-bending results are consistent with the assumptions in the CSA O86 design code.
This benchmarking study aims at providing the Canadian industry, agencies and governments with the necessary understanding of the knowledge and perception of wood roof trusses among specifiers in selected urban regions in China for ongoing and future promotions of wood roof trusses in China.
The objectives of this project are the following:
1. Assess current awareness, knowledge and perception of wood roof trusses in multi-family housing among specifiers (architects, engineers and builders/developers);
2. Examine how decisions on roofing/building systems and materials are made;
3. Determine best ways to transfer knowledge about wood roof truss systems to specifiers.
Two separate surveys were carried out for benchmarking wood use in roofs in China. The first survey was part of a survey of Chinese building specifiers (Benchmarking Chinese Building Specifiers (Cohen and Ding 2004)) carried out in October/November 2003. A second survey was administered during the Conference on Hybrid Building Construction in China and Wood Roof Truss Workshops in Shanghai and Beijing in December 2003.
The report presents the verification of two truss analysis computer programs developed at Forintek. Validation consisted of comparing the computed truss deflections with the deflections measured from full scale truss tests conducted at Forintek. Issues which should be considered when modeling parallel chord trusses using either of the two computer models are discussed. A strategy for estimating lateral load sharing and composite action in flat roof and floor systems using these models with the Floor Analysis Program (FAP) is discussed.
Trusses - Computer simulation
Joints and fastenings - Strength - Computer simulation
Two of the major topics of interest to those designing taller and larger wood buildings are the susceptibility to differential movement and the likelihood of mass timber components drying too slowly after they become wet during construction. The Wood Innovation and Design Centre in Prince George, British Columbia provides a unique opportunity for non-destructive testing and monitoring to measure the ‘As Built’ performance of a relatively tall mass timber building. Field measurements also provide performance data to support regulatory and market acceptance of wood-based systems in tall and large buildings. This report covers vertical movement and roof moisture performance measured from this building for about three and a half years, with sensors installed during the construction.
The report first describes instrumentation. The locations selected for installing displacement sensors for measuring vertical movement comprised of the following: glued-laminated timber (glulam) columns together with cross-laminated timber (CLT) floors on three lower floors; a glulam column together with a parallel strand lumber (PSL) transfer beam on the first floor; and a CLT shear wall of the core structure on each floor from the second up to the top floor. Sensors were also installed to measure environmental conditions (temperature and relative humidity) in the immediate vicinity of the components being monitored. In addition, six locations in the timber roof were selected and instrumented for measuring moisture changes in the wood as well as the local environmental conditions. Most sensors went into operation in the middle of March 2014, after the roof sheathing was installed.
The monitoring showed that the wood inside the building reached an average moisture content (MC) of about 5% in the winter heating seasons and about 8% in the summer, from an initial MC of about 13% during the construction. Glulam columns were extremely stable dimensionally given the changes in MC and loading conditions. With a height of over 5 m and 6 m, the two glulam columns monitored on the first floor showed very small amounts of vertical movement, about 2 mm (0.04%) and 3 mm (0.05%), respectively, over a period of about three years and a half. Assuming the two monitored columns are representative of the other columns along the column line, the cumulative shortening of the six glulam columns along the height of the building would be about 12 mm (0.05%), not taking into account deformation at connection details or effects of reduced loads on upper floors. The CLT wall was found to be also dimensionally stable along the height of the building. The measurements showed that the entire CLT wall, from Floor 1 to Floor 6, would shorten about 19 mm (0.08%). The PSL transfer beam had a reduction of about 12 mm (1%) in the depth, i.e., along the building height. The CLT floor panels also showed considerable vertical movement of about 5 mm (3%) in the thickness direction. All the differential movement was expected and taken into consideration in the design and construction of the building.
In terms of the roof performance, two locations, both with a wet concrete layer poured above the plywood sheathing, showed wetness during the construction but continued to dry afterwards. The satisfactory drying performance can be attributed to the interior ventilation function designed for the roof assemblies by integrating strapping between the sheathing and the mass timber beams below.
The intent of this project is to systematically build the infrastructure to expand the end uses of fingerjoined lumber products. Through fingerjoining lumber, the lumber industry may be able to produce a product that helps the truss industry remain competitive. While not all of the advantages of steel can be countered, fingerjoining should provide some flexibility to lumber producers so that they can continue to meet the needs of the wood truss industry. Based on the results of the first phase and on the discussions that followed, the IWG suggested a number of other potential issues to be further examined for particularly long-span trusses in a follow-up two-year project.
A research program aimed at examining some issues arising from use of fingerjoined lumber for long-span metal plate connected trusses was carried out in two phases. The first phase examined the effect of truss plate over-embedding and fingerjoint offset on plate capacity in small wood specimens. The results indicated that the presence of joint offset had a significant effect on plate capacity, and that over-embedding the plate into the joint bonded with polyvinyl acetate had significant effects on joint strength.
In the second phase, two follow-up studies were conducted to evaluate: i) the effect of truss plate over-embedding on strength of fingerjoints bonded with phenol-resorcinol formaldehyde, and ii) the effect of joint offset on plate capacity in tension perpendicular to grain and in tension parallel to grain in large wood members.
The present over-embedding study corroborated the results obtained in the first phase, i.e., over-embedding had a significant effect on fingerjoint strength. In bending, the severity of effect depended upon plate location. Where plates were applied on edge face, over-pressed plates gave 27% lower bending strength (MOR) compared to normally pressed plates and 35% lower than control (not plated), and normally pressed plates showed 11% lower MOR compared to control. Where plates were applied on flat face, over-pressed plates yielded 20% lower and normally pressed plates 14% lower MOR compared to control, but there was no significant difference between over-pressed and normally pressed plates. Plate location had no significant effect on MOR of normally pressed plates and that of control, but did have a significant effect on over-pressed plates. In the latter, plating on flat face gave 23% greater MOR compared to that on edge face. In tension, the severity of over-embedding effect also depended upon plate location. Over-pressed plates gave 29% and 16% lower ultimate tensile strength (UTS) compared to normally pressed plates when plates were applied on edge and flat face, respectively. On both faces, over-pressed plates yielded 18% lower UTS compared to control, but there was no significant difference between normally pressed plates and control.
In the fingerjoint offset study, the presence of offset did not have a significant effect on truss plate capacity in tension perpendicular to grain. However, the joint control without offset yielded significantly greater (about 6%) ultimate tensile load (UTL) compared to solid horizontal control. Plate location also had a significant effect on plate capacity. Plating on flat face gave 13% greater UTL compared to that on edge face. This was similar to that observed in the plate over-embedding study. Also, the presence of joint offset did not have a significant effect on plate capacity in tension parallel to grain in 2 x 4 wood members. The present results showed similarities to and differences from those obtained in the first phase depending upon plate grip location.
One negative implication is that over-embedding truss plates over fingerjoints should be avoided. This should be inspected as part of truss fabrication quality inspection. On the positive side, generally the NLGA 1/16-inch maximum fingerjoint offset should not impact truss plate capacity in tension perpendicular or parallel to grain.
Under this project, a set of guidelines for using fingerjoined lumber for metal plate connected trusses has been prepared. This project is a result of several projects undertaken over the last 10 years on trusses, trusses and fingerjoined lumber, and the development of new structural wood adhesive standards.
The draft guidelines are based on findings from previous Value-to-Wood studies, and on information gathered from truss fabricator interviews and surveys carried out by FPInnovations Industry Advisors of the past year. The guidelines also assume that the fingerjoined lumber is produced to the recently updated National Lumber Grades Authority Special Products Standard (SPS) 4, which was revised to reference the latest edition of the Canadian wood adhesive standard CSA O112.10. While use of this standard permits the fingerjoined lumber industry to use a wider range of structural wood adhesives, it does require that the lumber be marked as “Dry Use Only”.
Given that NLGA SPS 4 is the first standard on commodity structural fingerjoined lumber to have a “Dry Use Only” label, the development of these guidelines is timely. The guidelines will not only help the truss industry and builders to better understand and properly use the product, but it will also help in code acceptance of the NLGA SPS 4 standard.