Timber Connection Systems
Timber is an important material for building, can be used for many different structural forms, including beams, columns, trusses, and girders. It can also be used for piles, deck members, railway sleepers, and concrete formwork. However, for timber to be used coherently to form a structure, there must be a way of joining timber components to form a structurally stable unit, and this is what we’re about to discuss.
Joints and connections are essential components in timber structures, responsible for maintaining the structural integrity, stability, and performance of the assembly. These connections can be achieved through various methods, such as traditional carpentry techniques, including mortise and tenon, dovetail, and scarf joints, or by employing modern engineered connectors, like metal plates, brackets, and screws. The choice of joint or connection depends on factors like load-bearing capacity, ease of assembly, aesthetics, and the specific requirements of the project. Proper design and execution of these connections are crucial, as they ensure the efficient transfer of forces within the structure, preventing failures or deformations. Additionally, well-designed joints and connections can accommodate the inherent properties of timber, such as moisture-induced movement and natural variability, contributing to the long-term durability and performance of timber structures.
Fasteners Used in Timber Connection Systems
Timber connections play a critical role in joining structural elements in wood-based constructions, and fasteners are key components utilized in these connections.
A fastener is a hardware device used to mechanically join or affix two or more objects together, creating a non-permanent joint that can be disassembled or adjusted without damaging the connected components. Fasteners come in various types, sizes, and materials, with specific designs tailored for different applications and load-bearing capacities.
There are several fasteners used in timber connection systems. They are;
Nails
Screws
Dowels
Bolts
Glue
Nails
The most used fastener in wood construction is the nail, which comes in a variety of lengths, cross-sectional areas, and surface treatments. The most popular kind of nail is a smooth steel wire nail with a circular cross-section that is made from wire coils with a minimum tensile strength of 87,000psi. It is offered in a typical range of sizes up to a maximum of 5/16 inch and can be either plain or corrosion-resistant, such as by galvanizing.
Nails can be driven manually or with portable pneumatic devices. There is a risk of excessive splitting happening when nails are pushed into dense timbers. To prevent splitting, the pointed end of the nail can be blunted so that it slices through the fibers of the wood rather than separating them, or a pre-drilled hole that is less than 80% of the nail diameter can be drilled into the wood to. Timbers having a lower characteristic density of 32lbs/ft3 are often not pre-drilled.
Screws
In addition to being useful for timber-to-timber joints, wood screws are also well suited for steel-to-timber and panel-to-timber joints. Typically, these screwed joints are created as single shear joints. In order to enter screws, they must be turned, which can be done either by hand or with a power tool, depending on the circumstance. A major advantage that a screw has over a nail is it’s withdrawal capacity.
Dowels
Dowels are circular rods with a minimum diameter of a quarter of an inch, and are made of wood, steel, or carbon-reinforced polymers. Dowels are inserted into pre drilled holes on the wooden members. In timber construction, joints made using dowels are utilized to transmit strong forces. Dowels joints are affordable and simple to create.
Bolts
Bolts are nut- and head-equipped dowel-type fasteners. Typical bolts consist of common machine bolts (M12-M14 with a coarse head) with washers that have a side length of around 3d and a thickness of 0.3d, where d is the diameter of the bolt.
Bolts will be applied through pre-drilled holes that are 1/25 inch to 1/16 inch larger, and the bolt and washer will be tightened as needed to ensure that the connection’s parts fit snugly together. When the timber reaches an equilibrium moisture content, bolts may need to be retightened if necessary.
There are four major types of bolts namely;
Carriage bolt
Hexagonal head bolt
Square head bolt
Lag screw
Glue
A glued joint is a rigid type of joint that is formed using an adhesive. Compared to mechanically secured connections, structural glued joints are typically more rigid, require less wood, and look nicer.
Timber Connectors
Timber connectors are load-transferring devices that hold the joint assembly together with bolts or lag screws. They increase the area of wood across which a weight is distributed, making them more structurally effective than bolts or lag screws when used alone. They are typically not protectively coated and only need to be galvanized if used with preservation-treated wood or in wet service conditions. They are primarily used to transfer loads in heavy timber or glulam members, such as in roof trusses. The bolt's placement and specification are crucial because they bind the joint together, enabling the connector to function properly.
There are three types of timber connectors in use. They are
Split Ring
Shear Plate
Toothed Plate
Split Ring
This is used for only timber to timber connections and is installed in pre-cut grooves.
Shear Plate
This type of timber connector is used for both timber to timber and timber to steel connections. It is installed in pre-cut grooves
Toothed Plate
This type of timber connector is used for both timber to timber and timber to steel connections. It is pressed into the timber.
Types of Timber Connection Systems
Mechanical Connections
Joinery
In wood structures, joinery and mechanical connections are the two main categories of connections. Many variations fall under each of these categories, offering a vast variety of choices to fit almost any architectural design.
Mechanical Connections
There are three main forms of mechanical connections used in wood structures: dowel, metal connector plates with integrated teeth, and shear. There are also a lot of proprietary connections that mix features from all of these different types.
Dowel
Most typical mechanical connections use dowel-type fasteners to join wood parts together because they effectively transfer loads and are reasonably quick and easy to install. They come in a variety of shapes, and the National Design Standard (NDS) for Wood Building can be used to calculate their strength properties. When loads are relatively light, as in multi-family and small commercial structures, nails are typically used. While the NDS does not publish design values for staples, similar capacities must be found when using staples in place of nails. Under certain circumstances (such as exposure to moisture), screws may be more effective than nails because they tend to work less loose and typically have strong wind pullout resistance. High-strength timber rivets are a type of dowel fastener that are used with specially made metal plates.
Using a combination of dowel bearing and bending of the dowel fastener, dowelled connections transfer force between members. For flooring, a glued-nailed technique is recommended. With this technique, sheathing is glued to the substrate element, whether it be wood or an I-joist, to reduce squeaks and increase stiffness because of the action of the T-beam. The last nail must be driven in before the glue hardens. Due to the lower ductility, glueing is not advised for bonding wall or roof sheathing to framing. Adhesives are not permitted in Seismic Design Category D, E, or F, although being permitted in Seismic Design Categories A, B, and C with a decreased R=1.5.
Metal Connector Plate With Integrated Teeth
Metal connection plates are used mostly with manufactured light-weight wood trusses and partially pierce the wood members because they have many rows of teeth built right into them. These connectors allow loads to be transported close to the wood member’s surface.
Shear Connectors
Shear connectors, also known as bearing connectors, are frequently employed to support greater loads. They consist of shear plates, split rings, and toothed shear plates. They are often built of cast iron or light metals and are capable of carrying loads entirely due to the bearing and shear resistance of wood in directions parallel to the grain or perpendicular to it. They may be hidden or clearly visible, and they can be used to join wood to steel members or wood to wood members.
Split rings that have been profiled using specialized machining tools are often installed in a radial groove on the meeting side of the timber components. The split in the steel rings ensures that the wood members and split ring remain in contact by allowing the gap in the ring to shut or open if the wood members shrink or swell. The joint assembly is held together by a bolt that is inserted through the middle. Shear plates require grooves to be precisely cut with specialized tools, recessing the wood so the shear plates fit flush with the surface. In structures located in high seismic regions, shear plates and split ring connections should be avoided unless the designer uses elastic seismic design.
Proprietary Connections
Some of the most cutting-edge connection systems are proprietary, meaning that they are created solely for a certain structure. The usage of connecting systems and products created for use in larger, more intricate buildings and designed to take advantage of the advantages of wood from an economic, aesthetic, and environmental standpoint has experienced significant expansion in recent years.
For instance, two framing components can be connected in a single piece using custom-fabricated structural frame connectors. They are typically made of bent or welded steel and transfer load directly from the supported part into the supporting member (by hanger flange bearing, fastener shear or a combination of the two).
The self-tapping or self-drilling screw is one example of a proprietary product that is increasingly in use, particularly in cross laminated timber (CLT) and glulam buildings. They are made by numerous companies in North America, Europe, and Japan, and come in a variety of sizes, forms, and features. They come in a variety of capacities for varied purposes and have increased hardness for stronger lateral load capacity. The key benefit is that they can be drilled into wood with a standard handheld power drill without the need for pilot holes. This reduces the possibility of errors occurring in the field and boosts efficiency and dependability.
Some of the various proprietary connections include;
Castings
Castings, which are often constructed of ductile steel, are used as wood connections in buildings. They offer an economical, versatile and elegant way to achieve beautiful architectural connections.
Tight-fit Bolt And Pins
Tight-fit bolts are just standard bolts that are inserted in bolt holes that are drilled considerably more precisely in joining steel plates and timber. According to the Eurocode, tight-fitting bolts must have a bolt hole that is either the same diameter as the bolt or up to 0.5 mm smaller. The bolt diameter must not be more than 1.0 mm bigger than the bolt hole in the steel. The same specifications apply to tight-fit pins, which typically come in the form of headless stainless steel shafts with slightly chamfered ends and are frequently used for high-end exposed connections.
Ring Nails
Timber rivets are replaced by ring nails in Europe. They have a circular head and are shiny, giving them a neater appearance than timber rivets. Based on empirical data, the Swiss code offers detailed recommendations for developing ductile connections. The Gunnebo nail from Sweden is one type of exclusive ring nail attachment system.
Self-tapping Screws
Self-tapping screws are specialized, self-drilling screws that range in size from 3/16 to 12 inch (5 mm to 12 mm) in diameter and 3 to 23 inches (8 cm to 60 cm) in length. They are made of high strength (roughly 115ksi or 800 MPa) steel.
Self-tapping screws come in three different main categories. In wood-to-wood connections, heavy tension loads are transferred without the use of a washer plate using fully threaded screws. Steel bearing plates are fastened with partially threaded screws, which also have the ability to distribute shear. They can clamp things very tightly. In order to align the panels and transfer longitudinal shear, variable pitch screws are frequently utilized in edge-to-edge connections between solid wood panels.
HBV And HSK System
HBV is a connector used to create composite wood-concrete floor systems. It comprises of an expanded steel mesh that is cast into the concrete above and cemented into a saw cut on top of the timber beam or solid wood panel using a specialized adhesive, rigidly joining the two together.
The HSK technique Is similar, except it’s used to join steel pieces to wood or to join two separate wood elements. It comprises of a 3/32 inch perforated steel plate that is cemented into a kerf in the timber element and welded to a steel portion in the case of a steel-to-wood connection, rigidly joining the two members.
Joinery
For traditional joinery connections (sometimes referred to as carpentry connections), such as mortice/tenon and scarf joints, connected elements are generally given notches, holes, and tongues to help them interlock. In these kinds of connections, forces are transferred in compression and bearing. Wooden, metal, or keys are needed to secure interlocked connections in stress and prevent separation.
While very popular in single-story residences, businesses, and recreational facilities, joinery connections are rarely used for contemporary, multi-story heavy timber structures. One explanation is that these connecting systems require highly developed skills that are typically only found in seasoned carpenters. These connections are labor-intensive as well, making industrial manufacturing uneconomical.
Got an engineering project you need professionals for? Let's help you get started on it! You can speak with our professionals at JPC Design Consortium to get you started on your project today. Contact us right away.