Truss design represents one of the most elegant and efficient solutions in structural engineering, using triangulated frameworks to span large distances with minimal material. A truss is a structural system composed of straight members arranged in a triangular pattern, with loads applied primarily at the joints (nodes) and each member carrying primarily axial forces — either tension or compression. This efficient load transfer mechanism makes trusses ideal for roofs, bridges, towers, and other long-span applications where weight and material economy are critical. The history of truss design spans centuries, from ancient timber roof structures to modern steel and timber trusses that span auditoriums, sports facilities, and industrial buildings. Understanding the principles of truss design, the various truss configurations, and the construction practices for different truss types is essential knowledge for structural engineers and construction professionals alike.
To build on this knowledge, explore our detailed guide on Design Principles Of Steel Trusses for more in-depth insights into related framing and construction concepts.
Fundamentals of Truss Behaviour
Understanding Roof Trusses Selection is a critical component of effective structural framing and construction planning.
The fundamental principle underlying truss behaviour is that when loads are applied at the joints of a triangulated framework, the individual members are subjected only to axial forces — pure tension or pure compression — with no bending or shear forces. This axial-only behaviour is the key to the truss’s efficiency, as members can be sized for their axial capacity without the additional material required to resist bending moments. In practice, loads are often applied between joints (particularly on the top chord of roof trusses where roof sheathing is attached), creating secondary bending stresses that must be accounted for in design. However, for properly detailed trusses where primary loads are applied at panel points, the bending stresses are small relative to the axial stresses, and the truss behaves essentially as predicted by idealised analysis. The three fundamental assumptions of classical truss analysis — pin-connected joints, loads applied only at joints, and members straight between joints — provide a conservative basis for design when combined with appropriate safety factors and consideration of secondary effects.
The terminology used in truss design describes the various components that make up the truss system. The top chord (or upper chord) forms the top edge of the truss and is typically in compression under gravity loading. The bottom chord forms the bottom edge and is typically in tension under gravity loads. Web members connect the top and bottom chords and are arranged in a pattern that determines how loads are transferred through the structure. Panel points are the joints where web members connect to the chords. The span is the distance between supports, typically measured from bearing point to bearing point. The depth (or height) of the truss is the vertical distance between the top and bottom chords, measured at the deepest point. The span-to-depth ratio is a critical design parameter that affects both structural efficiency and economy — typical ratios range from 8:1 to 12:1 for roof trusses and 10:1 to 15:1 for bridge trusses. For a detailed look at steel truss design principles, see our guide on design principles of steel trusses.
Truss analysis determines the forces in each member under applied loads, using the method of joints (equilibrium of forces at each joint) or the method of sections (equilibrium of a cut section through the truss). Modern truss design uses computer software that performs matrix structural analysis, automatically determining member forces for any loading condition and truss geometry. The design of each member then proceeds based on the maximum axial force (tension or compression) it must carry, with compression members checked for buckling capacity and tension members checked for net section fracture at connection locations. The connection design at each panel point must develop the member forces through welded, bolted, or connector-plate details that provide adequate strength without introducing excessive eccentricity or secondary stresses that could reduce truss capacity.
Timber Truss Systems
For professionals tackling similar framing challenges, learning about Timber Roof Trusses provides valuable context and practical solutions.
Timber trusses have been used for centuries in roof construction, evolving from traditional mortise-and-tenon joinery to modern metal-plate-connected wood trusses that dominate residential and light commercial roof construction. Metal-plate-connected wood trusses — commonly called truss plate trusses or simply roof trusses — are prefabricated in factories using dimensional lumber (typically 2×4 or 2×6) connected at the joints by galvanised steel truss plates with integral teeth that are pressed into the wood on both sides of the joint. The engineering design of these trusses is performed by the truss manufacturer using proprietary computer software, with each truss designed for specific span, spacing, loading, and deflection criteria. The completed trusses are shipped to the construction site as individual components that are lifted into place and braced according to the manufacturer’s erection plan. The efficiency of factory production, consistent quality control, and reduced on-site labour have made metal-plate-connected wood trusses the dominant roof framing system for residential construction in North America.
The selection of a roof truss configuration depends on the building’s architectural requirements, span conditions, and load requirements. Common configurations include Fink trusses (W-shaped web pattern, most common for residential roofs with spans up to about 40 feet), Howe trusses (vertical web members in compression, diagonal web members in tension, suitable for medium spans), Pratt trusses (vertical members in tension, diagonals in compression, efficient for longer spans), scissors trusses (providing a vaulted ceiling by sloping the bottom chord), and hip trusses (used at the intersection of perpendicular roof planes to create hip roofs). Mono trusses (single-slope configuration) are used for additions, porches, and roofs that slope in one direction. Each configuration has specific advantages in terms of material efficiency, production cost, and architectural suitability, and the truss designer will select the most economical configuration that meets the project requirements. For a comprehensive guide on roof truss selection, see our article on roof trusses selection.
Timber trusses are also used in heavy timber and post-and-beam construction for commercial, institutional, and high-end residential projects where exposed wood structure is desired. These heavy timber trusses use larger members (typically 4×6, 6×8, or larger) connected with steel gusset plates, bolts, split-ring connectors, or custom fabricated steel connections. The design of heavy timber trusses follows the National Design Specification (NDS) for Wood Construction, with connection design following NDS provisions for bolted connections, timber rivets, or other connector types. Glued laminated timber (glulam) is often used for heavy timber truss members, providing longer lengths, larger cross-sections, and greater strength than solid-sawn timber. The visual appearance of heavy timber trusses is an important design consideration, with exposed connections, wood grain, and member proportions contributing to the architectural character of the space.
Steel Truss Systems
Steel trusses are used extensively in commercial and industrial construction for long-span roof and floor applications where the capacity of timber trusses is insufficient or where non-combustible construction is required. Steel trusses can be fabricated from hot-rolled wide-flange sections, double-angle sections, hollow structural sections (HSS), or cold-formed channel sections, depending on the span, load, and architectural requirements. The most common steel truss configurations for building construction include the Pratt truss (with diagonal members sloping toward the centre for gravity loading), the Howe truss (diagonals sloping away from centre), the Warren truss (equilateral triangle pattern with alternating tension and compression diagonals), and the Vierendeel truss (rectangular panels with rigid joints that resist bending, used where diagonal members are undesirable for architectural reasons). The selection of truss configuration and member types depends on the span length, loading conditions, available depth, fabrication costs, and erection considerations.
Steel truss design follows the AISC Specification for Structural Steel Buildings, with members designed for axial tension or compression and connections designed for the forces transmitted at each panel point. Compression members are designed for column buckling capacity (including flexural buckling, torsional buckling, and flexural-torsional buckling depending on the cross-section shape), with the effective length determined by the panel point spacing and the degree of end restraint at connections. Tension members are designed for gross section yielding and net section fracture at the connection, with the net section accounting for holes for bolts or the reduced area at welded connections. The slenderness ratio of truss members — the ratio of unbraced length to radius of gyration — is limited by code to prevent excessive flexibility and vibration, with typical limits of 200 for tension members and 300 for compression members (though compression members are typically governed by strength rather than slenderness limits). For more information on timber roof truss systems, see our guide on timber roof trusses.
Long-span steel trusses — spanning 100 feet or more — are used in体育馆, convention centres, hangars, and industrial buildings where column-free space is required. These trusses are typically designed with deeper sections (span-to-depth ratios of 8:1 to 12:1) and may be fabricated in segments for transportation and field-spliced during erection. The design of long-span trusses must consider not only strength but also deflection under gravity loads and drift under lateral loads, with pre-cambering used to offset dead load deflection and provide a level appearance under service conditions. The dynamic response of long-span trusses to wind, seismic, and live loads must also be considered, with particular attention to vibration serviceability for trusses supporting occupied spaces or sensitive equipment. The fabrication and erection of long-span trusses requires careful planning of lifting points, temporary bracing, and connection sequences to ensure structural stability at each stage of construction.
Connections and Bracing in Truss Systems
Truss connections are critical to the performance of the overall system, transferring forces between members at each panel point and maintaining the geometry that enables the truss to function as intended. In metal-plate-connected wood trusses, the galvanised steel truss plates provide both the connection and the force transfer between members, with the plate teeth embedded in the wood on each side of the joint. The plate size, tooth pattern, and orientation are designed by the truss engineer to develop the required connection strength for each joint. The plates must be pressed into the wood using hydraulic or pneumatic presses in the factory to ensure consistent tooth penetration and connection quality. Field repairs or modifications to truss connections should never be attempted without engineering approval, as even minor changes can significantly affect the truss’s structural capacity.
In steel trusses, connections are typically made using bolted or welded gusset plates at each panel point. Gusset plates are steel plates, usually 3/8 to 3/4 inch thick, that are shaped to receive the truss members at their intersection point. The gusset plate is designed for the combined forces from all members framing into the joint, with bolt holes or welds sized to develop each member’s design force. The thickness of the gusset plate must be adequate to resist bearing, shear, and tensile forces at the connection, and the plate geometry must provide adequate edge distance and spacing for bolts or sufficient weld length for welded connections. Block shear failure — where a block of material tears out from the gusset plate around a bolt group — is a critical failure mode that must be checked in gusset plate design. For light-gauge cold-formed steel trusses, self-drilling screws, crimped connections, or powder-actuated fasteners may be used in place of bolted or welded connections. For more on trussed beam systems, see our guide on trussed beams in construction.
Lateral bracing of trusses is essential to prevent buckling of compression chords and to maintain the overall stability of the truss system. In roof truss systems, the top chord is braced laterally by the roof sheathing or by purlins attached to the truss top chord at panel points. The bottom chord requires lateral bracing to prevent buckling under compression that can occur in certain loading conditions or in continuous truss systems. Permanent lateral bracing, often called continuous lateral bracing, runs perpendicular to the trusses, connecting adjacent trusses at panel points to create a stable diaphragm. Temporary bracing during erection must be installed according to the truss manufacturer’s erection plan and must remain in place until the permanent bracing and roof sheathing are installed. Insufficient bracing — either temporary or permanent — can lead to buckling of truss members and catastrophic collapse during construction.
Erection and Construction Practices
Truss erection requires careful planning, proper equipment, and strict adherence to the manufacturer’s erection instructions to ensure safety and structural integrity. Roof trusses are typically lifted into place using a crane, with pick points determined by the truss manufacturer to avoid overstressing members during lifting. Long-span trusses may require multiple pick points or a spreader beam to distribute the lifting force evenly. The first truss lifted is typically braced temporarily in both directions before subsequent trusses are erected, with permanent bracing connections made as each truss is set in place. Trusses must be plumbed and aligned before bracing is completed, and all bearing connections must be properly seated and fastened before the truss is released from the crane.
The permanent bracing plan, prepared by the truss designer, specifies the location, type, and connection details for all lateral bracing, bridging, and anchorage required for the completed truss system. Bracing members are typically continuous lengths of lumber or steel that span across multiple trusses, connected at each truss panel point with nailed, screwed, or bolted connections. The bracing must be installed as the trusses are erected, never left for a later phase of construction. Permanent bracing at the ends of buildings must be designed to resist wind loads on the end walls, transferring these loads through the roof diaphragm to the lateral load-resisting system. The truss designer’s bracing plan should be reviewed by the project engineer and incorporated into the construction documents to ensure that the installed bracing matches the design assumptions.
Conclusion
Additional guidance on Trussed Beams can help you make more informed decisions throughout your framing and structural project.
Truss design exemplifies the power of geometric efficiency in structural engineering, using triangulated frameworks to achieve exceptional span capabilities with minimal material consumption. From the ubiquitous metal-plate-connected wood trusses that roof the majority of North American homes to the monumental steel trusses that span sports stadiums and convention centres, truss systems provide economical, reliable, and versatile solutions for a vast range of structural challenges. The principles of axial force transfer, panel point loading, and chord bracing that govern truss behaviour are fundamental knowledge for every structural engineer and construction professional. As design software becomes more sophisticated and fabrication technology continues to advance, truss systems will continue to evolve, offering even greater efficiency, longer spans, and more innovative configurations for the buildings and infrastructure of the future.