In this publication, I reviewed glued parallel chord truss joists (or glued-joint open web wood trusses) in the August 2021 issue and glued roof trusses in the April 2023 issue. This article focuses on parallel chord glued trussed floor joists utilizing the new G-technology, https://www.gjoist.com/. [For all images, See PDF or View in Full Issue.]
G-Joist has some unique patented features, web fingers extending completely through the chord, as shown in the image. This web-chord interface maximizes the glue area by utilizing the entire overlapping area between the chord and web with longest web fingers exceeding 2”. The three critical failure modes in a glued truss are knot failure in the web finger, splitting failure in the web, and chord splitting along the grain. These failures are eliminated when employing the G-technology. Knot failure in the finger is prevented using large fingers, which accommodates chord knots and small web knots, while large web knots are graded off. Web and chord grains cross each other throughout the joint, reinforcing the grains against web and chord splitting analogous to the reinforcement seen in plywood. With the G-joint, only two types of failures are possible: web finger cut and glue line failure. Finger cut failure is unlikely when the webs are graded to exclude major knots. Glue line failure is also improbable due to the low mean glue line shear stress of 70–100 psi (0.5–0.7 N/mm²). By keeping all webs identical, and with no verticals, all joints are similar, ensuring uniform resistance throughout the joist. The truss can maintain structural integrity even when installed upside down.
Joint routing is done using a saw blade at two blade inclinations to create tapering fingers, as shown in the figure. This two-step routing process is somewhat slow, but it offers multiple advantages: the cutting surfaces are ideal for adhesive bonding, maintenance of the saw blades is inexpensive and simple, the saw blade works well even with knotty timber, the cost of the saw blade is low, and when assembled, there are no open routings, water pockets, or cavities.
The web-chord interface (WCI) joint behavior is similar to, and as reliable as, the web-flange interface (WFI) commonly employed in the manufacture of wood I-joists. The historical data and low coefficient of variation (COV) of this technology eliminates the need to proof test each component, requiring only intermittent random testing.
Any adhesive approved for structural applications can be used. In the Finnish pilot production https://tk-palkki.fi/, a one-component PUR adhesive is utilized. Although this adhesive is somewhat costly, the expense is negligible at +/- 2% of the overall cost of the finished product. Moreover, this adhesive is easily employed and forgiving of voids, moisture, and temperature variations.
The G-Joist truss can be made using either a single-finger or a two-finger web end joint. The robust truss, with 2x4 (1-1/2” x 3-1/2”) chords and 1x3 (1” x 2-1/2”) webs, employs the two-finger web end. The lighter truss, with 2x3 (1-1/2” x 2-1/2”) chords and 1x3 (3/4” x 2-1/2”) webs, employs the single finger per web end. The web joints (node points) are spaced 24” apart. The figure below shows the basic truss design.
These trusses can be cut arbitrarily and installed either top chord bearing or bottom chord bearing. If the truss is cut anywhere other than a node point, a reinforcement is needed. In the center of the truss span, one or two webs may be removed to create a trapezoidal chase opening (…44”*free depth*…18”). In this middle section where shear forces are minimal, the node points may be more than 24” apart (depending on top chord loading) creating an adaptation zone that allows both ends to match node points without requiring reinforcements.
Top chord bearing
Joists are normally bottom chord bearing (BCB), as top chord bearing (TCB) is more expensive, but top chord bearing is a growing trend. In Finland, metal web joists are widely applied and virtually all these joists are supported at the upper chord, though these joists, as well as steel connector plated joists, have limited resistance when supported this way. In these joists, the upper chord acts as an overhang on the support, inducing moment and shear stresses. In contrast, the G-Joist’s web grips the chord above the support, creating an ideal truss joint: web-support-chord. This design prevents the chord from being cantilevered, thereby eliminating moment and shear stresses. All laboratory tests for the G-Joist are conducted using this support option and the design values are based on this support option. Using steel girder-hangers adding considerable material and labor costs that can be reduced. In wall support applications, multiple advantages are realized: a rim board is not needed, building wrap or membranes can be installed reliably, and building envelope forces can be transferred more simply and reliably between the upper and lower walls. Some builders in Finland have decided to use only TCB trusses for their floors. This figure shows the upper chord support of the G-Joist, but the joist can be turned upside down and installed with the bottom chord bearing and the top chord cut back and attached to a ledger on the inside face of the upper wall. The load bearing chord may protrude into the wall, no reinforcements are needed. These options again create a superior thermal and structural diaphragm design.
Trimming
The truss can be made with a trimmable insert creating an I-joist at one or both ends. This configuration is straightforward if only bottom chord support is used. I’m also currently developing a detail allowing for either bottom or top chord arbitrary trim-ability.
Cutting
The G-Joist is the only truss that can be cut arbitrarily, thanks to two unique properties. First, every joint is reliable and strong, allowing any joint to carry the nominal load regardless of where the truss is cut. Second, chord splitting is not possible, which allows for easy and low-cost reinforcement. For simplicity, the reinforcement is uniform and valid for all cases, achieved by nailing plywood or OSB reinforcement to the chords on both sides using 3” nails 1” apart along the chord. The sheet must overlap at least one node point, meaning sheet length is 24” and depth matches the joist depth. In the upper chord support, a steel rail extending above the support is applied.
Staircase aperture
G-technology enables a new realization for staircase openings. The flush beams are inserted into the cut trusses and extended to the girder support trusses, as shown in the next figure. This detail brings multiple advantages. The opening can have any shape, including a circle. The cut joists can cantilever beyond the support beams allowing them to be arbitrarily trimmed. Custom engineering would not be needed as standard design details can be established to cover most cases. Since all trusses are uniform, ease of construction with low costs are realized even in complex cases.
Ecology, material, and flexibility efficiency
In some markets, green design with ecological considerations is an essential criterion for selecting materials and structures. Wood stands out as a superior material, with sawn wood being the best structural choice. Glued trusses have only about 15% higher CO2 emissions than normal sawn lumber, whereas glulam has about double the emissions, LVL about four times, and steel and concrete more than ten times the emissions. The truss is the most effective structural pattern for floors and roofs. It is also highly flexible, offering crosswise openings for MEP and girders. In addition, G-Joists can be supported at both the bottom and top chords.
Licensing and business opportunities
The G-technology can be introduced in the market in three ways:
- Custom manufacturing,
- Semi-commodity manufacturing for distribution and stocking dealer standard lengths with a trimmable ends option,
- Commodity manufacturing with stock lengths like I-joists.
Custom manufacturing
The single web row along the joist allows for customization in length, support type (lower or upper), and a chase opening in the middle without requiring reinforcements. The joist can be supported on a flush girder with full depth of the free space between chords fixed inside the joist, as webs can be spaced apart to accommodate girder width. If the joist has more than two supports, reinforcement may be needed, typically one sheet nailed to the chords and webs. The robust joist, with a chord width of at least 3-1/2”, may have two web rows, as shown in the figure. The web spacing is determined by resistance or opening requirements, meaning a specific truss pattern is not necessary. For example, an X web pattern can be used to create a large chase opening anywhere along the joist. This design allows the joist to have multiple supports without requiring reinforcement. Double webs at joist ends or point loads provide high resistance, making the overall structure very strong. Such a joist can replace robust girders like glulam, offering crosswise openings with typically less timber volume, lower depth, and the option for upper support.
Semi-commodity manufacturing
Semi-commodity manufacturing for standard length can be realized in various ways. The trimming option can be applied to one or both ends, and support can be provided at the bottom only or at both the lower and upper chords. The best option appears to be trimming on one end only, with support possible at both chords and fixed upper or lower support on the other end. A trapezoid chase opening is included in the middle.
Commodity manufacturing
The G-Joist is the only truss that can be manufactured as a 40’–80’ long continuous length, then cut into multiple working joists, trimmed, and supported arbitrarily. If the cutting point does not match a node point, reinforcement is needed. Two webs can be removed in the middle span to create a chase opening.
Manufacturing versatility
A production line that produces floor joists can also manufacture roof joists for both residential and commercial buildings. This dual capability significantly increases production volume.
Capitalizing on the off-site construction trend
Roof and floor pre-assembled cassettes are a growing trend in modern markets. G-Joists are ideal for off-site cassette construction methods delivering superior economical solutions. Wooden roof joists can technically span up to 100’ with spans up to 70’ being the most competitive against alternative options.
My team and I believe that glued truss technology will eventually emerge as a market-leading alternative in the wood truss and engineered wood joist market due to:
- Low material and manufacturing costs,
- Large web openings for MEP and girders,
- Reversable top or bottom chord support option,
- Simple and low-cost stairwell framing, and various manufacturing options.
We will continue to make advancements to faciliate the use of glued trusses and look forward to expanding the market for them.
A professor at Tampere University in Finland, questions about this article may be sent to Tuomo via email: tuomo.poutanen@tuni.fi.