May 21, 2014
By Maik Gehloff, Dipl.-Ing. (FH), M.A.Sc.
Widely used in conventional frame construction, wood is not new to the Canadian design community. What is ‘new,’ however, are changes to building codes that allow for taller structures to be constructed of wood, along with the introduction and development of new products like cross-laminated timber (CLT) and other massive wood panels manufactured from laminated veneer lumber (LVL) or laminated strand lumber (LSL).
The prospects could get even more exciting with composite materials, such as wood-concrete composites or the clever combination of wood and steel. An abundance of recent innovative buildings have been built using these newer products based on the time-tested material.1
In creating these impressive structures, various factors had to be considered and solved both in understanding the material with its beneficial and challenging properties. Wood not only provides a warm esthetic, but it is also strong, easily workable, and sustainable. It is both a renewable resource, and it can significantly reduce a building’s carbon footprint.
On the other hand, wood is a hygroscopic, non-homogeneous, and un-isotropic material with inherent weaknesses like its low compression and tension perpendicular to the grain strength and its low capacity in longitudinal shear. All these challenges, however, can be overcome by:
Quite likely, the most challenging endeavour is the connections in these wood or timber structures.
Most building structures can be thought of as collections of beam and plate-like elements held together by connections. In wood and timber projects, however, the opposite is true: they can be viewed as collections of connections held together by beam and plate-like elements.
A lot of energy has been spent on the constant improvement of glued-laminated timber (glulam), CLT, and other engineered wood products like LVL, LSL, and parallel strand lumber (PSL). In Canada and the United States, however, the development of connections has not kept up with these changes. North American material codes still look at nails, timber rivets, screws, lag-screws, bolts, and threaded rods, as well as shear-plates and split rings.
All these connectors still have their place in timber engineering and construction, but to really push the envelope for wood buildings like the currently under-construction Wood Innovation and Design Centre (WIDC) in Prince George, B.C.,2 new and innovative connectors and connection systems have to be explored and used.
One of the most prevalent innovations in timber engineering was the development of engineered self-tapping wood screws (STS).3 These screws are a far cry from the decking screws and conventional wood screws to which many are accustomed.
Modern STS do not require pre-drilling or pilot holes; they work well in most materials and are designed to ‘cut’ or form threads into the material as they are driven in. Some STS tips look similar to a drill bit, but they lack flutes—in other words, they do not remove material from the hole. Instead, the tips slightly loosen the material to reduce friction and drive-in torque, which is especially beneficial for the fully threaded varieties of self-tapping wood screws.
Self-tapping wood screws come in countless different diameters, lengths, and head-types, along with fully threaded or partially threaded versions. Some of the main characteristics of STS are their generally smaller core diameter with larger thread wings, leading to less splitting and more bite (and therefore high withdrawal capacity). The high capacity in withdrawal is also achieved through the high tensile capacity of the screw’s steel due to hardening and quenching process of the screws during manufacturing.
The partially threaded version of the screws also feature an improved shank cutter that further reduces the friction during installation of the screw, lowers the risk of clogging of the shank cutters, and creates a hole larger than the core diameter of the screw to allow for shrinkage and swelling of the wood without getting hung up on the screw’s shaft. All these characteristics combined create an efficient and economical connector in timber connections that is easy to apply, without the need of pre-drilling and has a high withdrawal and tensile strength. Their availability in Canada is generally not a problem.
Self-tapping screws and timber construction
When looking at the possible STS applications, it becomes clear why they have helped revolutionize the field of timber engineering and construction. Self-tapping wood screws can be used by themselves as primary fasteners or reinforcement, or in combination with other parts (e.g. steel plates and weldments or standardized connectors). The principle on which they work remains their incredible capacity in withdrawal.
An example for the use of these screws as a primary fastener could be a common wood-wood (-wood) / steel-wood (-steel) / wood-steel (-wood) connection with one shear plane or two shear planes respectively. That type of connection is usually achieved by bolting the individual members together and transferring the forces through shear in the bolts.
The same connection could also be done using STS by installing the screws at a 45-degree angle, and then transferring the loads through the screw’s axis. A connection using screws would yield a higher capacity with fewer or smaller screws without the need of pre-drilling.
In many cases, timber members have to be over-sized to allow for the end and edge distances required for a bolted connection. Due to the smaller diameters and reduced number of screws, the timber member in most cases would not need to be oversized anymore. This results in an added economic benefit.
The same can be said for screws used as reinforcements. Instead of being over-sized due to notches or protrusions, beams can be reinforced with self-tapping screws; in most cases, they can be kept at the cross-section of a beam without such notches. For example, in the case of a notch or a bolted connection, the wood would be loaded in tension perpendicular to the grain—one of wood’s inherent weaknesses. Concessions would have to be made to transfer that load over a larger area. When a self-tapping wood screw is used, the force transfers along the screw’s axis, again employing the high capacity in withdrawal—the beam or connection geometry does not need to be altered.
Some manufacturers have also created self-tapping wood screws with specific features tailored to specialized uses. For example, this author knows of two companies who produce screws with two different pitches, allowing them to draw a pair of members tightly together as long as the change of pitch is located right in the interface between the two members. One company has developed screws with a progressively changing pitch, allowing it to tightly draw together multiple members without the need to ensure a proper interface placement.
Companies have also developed screws with an additional threaded part under the head. The thread under the head has the same pitch as the regular thread—this ensures a certain amount of spacing between two elements without compressing insulation, for example, in between. Instead of transferring the load from one element to the other relying on the material between them, the loads are transferred through the screw shaft.
There is another product that technically is not a screw, but rather a tight-fitting dowel able to drill through the wood and up to three steel plates. It has a short-threaded part under the head to secure the dowel from falling out.
Manufacturers have also engineered various methods for shear transfer between wood and concrete in wood-concrete composites. In some cases, the screw has a distance marker to ensure it is driven to the correct depth before the concrete is poured; in other examples, a placeholder is cast into the concrete allowing for precast elements. Onsite, screws are later installed through the placeholders, ensuring proper shear transfer between the concrete and wood.
There is even a screw that has threads for both concrete and wood. When installing sill plates, a hole can be drilled through the wood into the concrete, and then the screw driven to connect the two. The benefit is the perfect alignment of the hole in wood and concrete as they are drilled in-situ at the same time.
When it comes to the length of self-tapping wood screws, some manufacturers offer 1-m (3.3-ft) and 1.4-m (4.6-ft) models. These long screws can be used in part as replacements for glued in threaded rods in moment connections, but it is recommended a small pilot hole be pre-drilled to set the installation angle and to reduce the friction and drive-in torque.
Timber moment connections can also be achieved using common connectors augmented with the STS as reinforcements to increase capacity. There are also steel plates that, together with their screws, can be used in a system to create a timber moment connection.
Further developments in STS
When it comes to using self-tapping wood screws with other systems, the sky seems to be the limit, reaching from simple steel plates, custom weldments of any shape imaginable, or standardized system connectors. The principle, however, remains the same by using the STS in withdrawal by ‘hanging’ these system connectors off or on them. Figure 1 shows a proprietary product, a form-fitting connector employing the time-honoured shape of a classic dove-tail, being installed on a glulam beam in the plant with STS, before being shipped to the WIDC site where the two parts simply slide together.
The advantage of the various proprietary connection systems lies in the fact they are standardized, allowing designers to simply pick from a catalogue based on the required capacities, minimizing time for design and drafting, for example, custom steel weldments. Additional benefits of standardized systems are higher quality control and reduced assembly times due to a higher grade of pre-manufacturing, as their individual parts are shop-installed.
Some of these standardized connection systems provide capacities up to 600 kN (134,885 lb), and simple steel plate tension splices in excess of 1 MN have been achieved solely by virtue of self-tapping wood screws (granted, a lot of them).
While STS development keeps progressing and more new ways of using them are being explored, another modern connection system is holding entry into the North American market: epoxy. The principle itself is not new, but there is innovation in using an existing and developing technology in timber engineering and engineered timber construction.
Two technologies in particular have successfully been used in North America. One system is a wood-concrete composite using a glued-in steel mesh as shear connector between the wood and the concrete, whereas the other is a steel mesh welded to a steel connection plate and then the mesh glued in the wood for a steel-wood composite that can now be joined to other materials. The great advantage of these systems is their high strength and stiffness while still being ductile. The system can be used to create strong and stiff moment connections, and has been used in the feature staircase of University of British Columbia’s (UBC) Earth Sciences Building (ESB) in Vancouver.
When it comes to the design and engineering of connections using self-tapping wood screws or standardized connection systems, only one manufacturer currently has a Canadian Construction Materials Centre (CCMC) report to use alongside Canadian Standards Association (CSA) O86, Engineering Design in Wood. However, this does not mean the screws of other manufacturers cannot be used.
The CSA standard has a provision in 3.3.2 for new or special systems of design and construction. Based on this, such systems can be used without a CCMC report if they are following engineering principals and/or reliable test data. In the case of STS and any of the other standardized systems, the engineering can be done following some adopted provisions of the EuroCode 5 (EC5) and the manufacturer’s European Technical Approvals (ETA).
To gain ETAs, the manufacturers must go through a battery of tests fulfilling the CSA O86 provisions for reliable test data and applied EC5 and the ETAs to show the design follows engineering principles. The design process itself is not difficult; most distributers offer design tables to further simplify the process. Still, caution should be taken regarding the constraints of these design tables like densities and load duration factors—such constraints are usually explained in the footnotes.
The following describes the design based on equations that would allow the engineer to be free of constraints and set all parameters as required for the project at hand. For STS loaded in shear, perpendicular to the length axis, it can be done with the yield equations given in CSA O86. For screws loaded in withdrawal, parallel to the axis of the screw, the main direction of loading in most cases uses the EC5 equation in Figure 2.
When using this EC5 equation, special consideration has to be given to the characteristic (5th percentile) density. The densities for local wood species are given in CSA O86 as relative specific gravity (G) at a wood moisture content (MOC) of oven-dry. However, the densities used in EC5 are given as 5th percentile values at an MOC of 12 per cent.
To use the CSA densities, some statistical ‘soft’ conversions implying certain assumptions have to be done. The conversion of relative gravity (a value relative to water with 1000 kg/m3 density) to density is straightforward, and the value can simply be multiplied by 1000. To convert the mean value to a 5th percentile value, the former is multiplied by 0.84, as shown in the equation. As a last step, the MOC has to be taken into consideration. The Wood Handbook by the Forest Products Laboratory of the U.S. Department of Agriculture (USDA) Forest Service gives great information of the effect of moisture content on the density and offers the following formula for conversion:
ρk,0% = ρk,0% * (1+M/100) with M = 12%
Putting it all together the following equation can be used to convert the CSA gravities (G) to EC5-compliant densities:
ρk = ([G *1000] * 0.84)* (1+M/100)
When it comes to establishing the factored design capacity, the provisions given in CSA O86 can be used or the EC5 provisions applied, since both codes are semi-probabilistic and use load resistance factor design with similar factors. The material safety factor for connections in EC5 (ƳM) is 1.3 whereas a common factor φ of 0.7 can be used for connections utilizing self-tapping wood screws.
When looking at 1/ƳM = 0.769, it is slightly higher than the 0.7 used in CSA, but when looking at load duration factors and how they are established (which is beyond this article’s scope), the 1/ ƳM can be normalized to φ = 0.7 in combination with the use of the common Canadian load duration factors (KD).
The engineering design of standardized connection systems from Europe using STS can be done the same way, but most manufacturer provide characteristic (5th percentile) values for their systems in their ETA as the number of screws per connector is fixed. In that case, only the safety factors have to be applied to the tabulated values.
It is worth noting the tabulated values are, in almost all cases, tabulated solely for the reference density and the standard connection with screws at a set angle to the grain. With a system connector used in connections that are inclined or oblique, the designer has to calculate the capacity as the angle between screw axis and wood grain can differ from the standard situation and impact its capacity.
This also means competing connection systems have to be checked separately as comparison of the standard situation capacity difference between systems may not be linear; it could also be different for inclined or oblique connections. For example, if System A is 10 per cent stronger than System B in a standard situation, it may in fact be weaker in other scenarios.
Innovation almost always moves faster than building codes can react. Codes, however, give provisions for the use of emerging new systems and manufacturers do their due diligence in research and development.
All system types listed in this article, and more coming, are readily available in North America; in most cases, they are actually imported to the continent, but more specifically distributed from Canada. All these distributers offer technical support from within the country, and most also offer engineering support with knowledge of the prevailing codes. The respective technical support can be enlisted in helping with engineering design, ideas, and offer proper installation and application guidance and instructions.
1 Numerous Construction Canada articles have focused on these innovative wood projects. Examples include “Why Wood Works: Designing the Richmond Olympic Oval,” by Jim Taggart (November 2009), “Building the Earth Sciences Building at the University of British Columbia,” by Eric Karsh (August 2013), and “Overcoming the Learning Curve: Design and Construction of the UBCO Fitness and Wellness Centre,” by Patrice R. Tardif (October 2013). (back to top)
2 For more on this project, see the article “Constructing an All-wood Building,” by Werner Hofstätter, which appeared in the April 2014 issue of Construction Canada. (back to top)
3 They should not be confused with self-drilling screws, which are connectors that physically remove material and are used predominantly in the metal industry—they do not work well in wood. (back to top)
Maik Gehloff, MASc, Dipl.-Ing. (FH), is the founder and owner of Gehloff Consulting Inc., providing services including technical support for timber connections for many years. Gehloff holds a degree in wood science and technology, specializing in timber engineering from the University for Applied Sciences in Eberswalde, Germany, as well as a degree in timber engineering from University of British Columbia (UBC) in Vancouver. His research projects focused on self-tapping, structural wood screws, as well as other modern wood connectors. Maik is a member of the Timber Framers Guild of North America and the Timber Frame Engineering Council. He can be reached at firstname.lastname@example.org.
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