Trusses & Frames

Trusses and frames are structural systems that span large distances and support heavy loads using assemblies of individual members. A truss achieves this through triangulation — the most stable geometric form — while a frame relies on rigid connections between beams and columns. Both systems are fundamental to building construction, from the roof trusses in a single-family home to the moment frames in a high-rise office building.

A truss is a structural assembly of straight members connected at their ends by joints (nodes), forming a pattern of triangles. When properly loaded (at the joints only), the members carry only axial forces — pure tension or pure compression — with no bending. This makes trusses extremely efficient: they achieve long spans with minimal material.

Why triangles? A triangle is the only polygon that cannot change shape without changing the length of its sides. A rectangle can be deformed into a parallelogram by pushing on a corner (it has no resistance to racking). But a triangle is inherently rigid — it resists racking without rigid connections at the joints. This is why triangulation is the basis of all truss design and is also the principle behind braced frames.

Truss terminology:

• Top chord: The upper, typically horizontal or sloped member (usually in compression under gravity loads)
• Bottom chord: The lower, typically horizontal member (usually in tension under gravity loads)
• Web members: The diagonal and vertical members connecting the top and bottom chords (alternating tension and compression)
• Panel point (node): The intersection where members meet
• Panel: The segment between two adjacent panel points on the same chord
• Span: The distance between the truss supports

Pratt Truss: Vertical web members and diagonals that slope downward toward the center. Under gravity loading, the diagonals are in tension and the verticals are in compression. Since tension members can be more slender than compression members, the Pratt truss uses material efficiently and is one of the most common configurations.

Howe Truss: The opposite of the Pratt — diagonals slope upward toward the center. Under gravity loading, the diagonals are in compression and the verticals are in tension. Less common than the Pratt for building construction.

Warren Truss: Diagonals alternate in direction, forming a zigzag pattern with no vertical web members (or with verticals added at panel points where loads are applied). Efficient and aesthetically clean. Commonly used in bridges and long-span building structures.

Fink Truss: A W-shaped web pattern commonly used in residential roof trusses because it efficiently handles the triangular load distribution on a pitched roof.

Scissor Truss: Both chords slope upward from the supports, creating a vaulted ceiling profile below. Popular in residential and church construction for aesthetic reasons. The scissor truss is less efficient than flat-chord trusses because the geometry creates horizontal thrust at the supports.

Bowstring Truss: The top chord is curved (arched), creating an efficient form that approaches the ideal shape for a uniform load. Used for large-span roofs (gymnasiums, warehouses, arenas).

Parallel Chord Truss (Flat Truss): Both chords are horizontal, creating a rectangular profile. Used for floor systems (open-web steel joists) and flat roofs. The flat profile allows mechanical ductwork and utilities to pass through the web openings.

The method of joints is the fundamental technique for analyzing trusses. At each joint:

• All forces (member forces and applied loads) must be in equilibrium
• ΣFx = 0 and ΣFy = 0

By working systematically from joint to joint (starting at a support where the reactions are known), the force in every member can be determined. Each member is either in tension (T) or compression (C).

For construction professionals, the key takeaway is that every member in a truss is critical. Removing or damaging a single member changes the force distribution in the entire truss and can cause collapse. Trusses should never be modified in the field — no members cut, no holes drilled, no members removed — without engineering approval.

Open-web steel joists (bar joists) are lightweight, pre-manufactured trusses used for roof and floor support in commercial construction. They consist of angle or rod web members welded to angle or channel chords. Designated by the Steel Joist Institute (SJI):

• K-series: Standard joists for roof and floor, spans up to about 60 feet
• LH-series (Longspan): Deeper joists for longer spans, up to about 96 feet
• DLH-series (Deep Longspan): The deepest joists, spans up to 144 feet

Open-web joists are specified by series, depth, and load capacity (e.g., 24K9 is a K-series joist, 24 inches deep, with a specific load capacity). The open web allows mechanical ducts, plumbing, and electrical to pass through without penetrating the structure.

Manufactured wood trusses (using metal connector plates) have largely replaced traditional rafter-and-joist framing in residential roof construction. They are designed by the manufacturer using proprietary software and delivered to the site ready to install. Common configurations:

• Common truss: Triangular shape with a peak at the center. Available in virtually any pitch and span.
• Hip truss: A series of trusses of decreasing height used to form a hip roof.
• Valley truss: Used where two roof planes intersect to form a valley.
• Girder truss: A heavier truss that supports other trusses at a perpendicular intersection.
• Attic truss: Designed with a room-height open space in the center for habitable attic space.

Critical warning for wood trusses: Metal connector plates (also called gang-nail plates or truss plates) are the only connection at each joint. These plates have teeth that press into the wood — they rely on the wood fibers for their strength. Wood trusses must never be:

• Modified in the field (no cutting, notching, or drilling)
• Damaged during delivery or handling (bent connector plates compromise the joint)
• Improperly stored (storing flat without adequate support causes permanent damage)
• Loaded during construction before bracing is installed (trusses are laterally unstable individually)

A rigid frame (also called a moment frame) is a structural system where beams and columns are connected with moment connections — connections that transfer bending moment, shear, and axial force between the members. Unlike trusses, frames resist loads through bending of their members.

Portal frame: The simplest rigid frame — two columns and a beam (rafter), all connected with moment connections. A single bay of a rigid frame building. Portal frames resist lateral loads through bending of the columns and beam.

Multi-bay, multi-story frames: Office buildings and commercial structures typically have multiple bays (column spacings) in both directions and multiple stories. The moment connections create lateral stiffness — the frame resists wind and seismic forces through the bending strength and stiffness of its beams and columns.

Pre-engineered metal buildings (PEMB): These are portal frame structures manufactured by specialized companies. Tapered I-shaped columns and rafters are bolted together on site. The frames are designed and fabricated as a complete package including frames, purlins, girts, bracing, and cladding. PEMBs are the dominant structural system for warehouses, manufacturing facilities, retail stores, and other single-story commercial/industrial buildings.