Building components · Frame Members
Truss Web Member
Explains the web member as the position that triangulates a truss, distinct from bracing that stabilises a structure and from struts that take load out of a roof.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a web member is
Web members sit between the chords of a truss, each connecting a joint on one edge to a joint on the other. Some are inclined, some vertical, and together they turn a pair of parallel or sloping edges into a set of triangles that cannot change shape without a member changing length.
Their duty is to carry force across the depth of the truss rather than along it. That force alternates in a regular pattern, so a web member near one end may be pulled while its neighbour is pushed, and the pattern reverses along the truss. It is why removing a single web member is never a local matter: the triangle it completed is the reason the assembly holds together.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a truss chord. The chords run continuously along the edges of the truss and carry its bending as a push and a pull; a web member sits between them, connects two joints, and carries the force across the truss rather than along it.
- It is not a bracing member. Bracing triangulates a frame against sideways movement of the structure as a whole and has nothing to do with carrying weight; a web member triangulates the truss itself and is part of the truss's ability to span at all.
- It is not a roof strut. A strut takes purlin load out of the roof plane and down to a loadbearing wall or binder beneath; a web member stays within the plane of one truss and works only between its own chords.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- web member
- lattice member
- internal diagonal
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- The joints at both ends, since a member connecting two nodes does everything it does through those nodes.
- Whether it is being pulled or pushed at that position in the truss, because a member in compression depends on being held in line and a member in tension does not.
- The joint geometry, since the arrangement assumes the members meeting at a node are meeting where the design placed them.
- The truss being treated as a designed whole, because the force in every web member follows from the arrangement rather than from what sits above it.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Force alternates between tension and compression along a truss, so two web members that look identical may be doing opposite things.
Web members cross the middle of a roof space or a floor zone diagonally, which is why they are the members services, ducts and conversions most often want to pass through.
A truss that has lost a web member or a node connection commonly shows it as distortion of the assembly rather than as failure of the member itself.
Which members a truss has, and what the joints at each node are, is recorded in the fabrication information rather than deducible from looking at the finished assembly.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Narrower forms of this element
Clarifications
Commonly misunderstood
- That a web member can be cut to make room in a loft or for a service run. Removing one changes the force in every other member of the truss.
- That the web members are the roof's bracing. They make the truss able to span; the roof still needs separate bracing between the trusses if the structure is to be stable sideways.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- Is this web member in tension or in compression at this position in the truss?
- What are the node connections at each end, and are they intact?
- If a service has to pass through the truss, where does the design allow it?
- What provides bracing between the trusses, separately from the members within each one?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- This entry describes a position within a truss. It states no node plate size, fastening pattern or capacity at any node, and none can be inferred from it.
- Cutting, drilling or removing a truss member is structural alteration requiring qualified design, and its effects are not confined to the member altered.
Structural Frame Members
The columns, beams, joists, rafters, plates and bracing that make up a load-carrying frame, identified by the position each occupies.
Browse all frame members entries →