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Optimizing Fire Resistance in Largescale Steelwood Truss Construction
Large steel-wood truss buildings are increasingly common, but their fire performance depends on assembly details that are easy to overlook. Designers must reconcile strict fire-resistance requirements with structural performance, often by relying on certified assemblies and understanding precisely which changes are permitted.
Core challenge: balancing compliance and structural performance
Metal-plate-connected wood trusses are widely used in modern large building projects. Building codes set explicit fire-resistance ratings, and assemblies must meet recognized standards, including ASTM E119, while still serving the design intent. That makes selection a technical exercise as much as a code-compliance one.
A fire rating is not an isolated product attribute. It is the result of how materials and components interact within a tested assembly. Even a seemingly minor design change can affect overall fire performance, so any alteration should be reviewed with a professional engineer, the architect and code officials.
Certification bodies and reference resources
Several organizations publish the listings, directories and design manuals that engineers and code officials use to verify fire-rated assemblies.
| Certification body | Primary role and resources |
|---|---|
| Gypsum Association (GA) | Publishes the Fire Resistance Design Manual |
| Underwriters Laboratories (UL) | Publishes the Fire Resistance Directory |
| Intertek Testing Services (ITS) | Provides the Listing of Building Products, formerly Warnock Hersey |
| Factory Mutual (FM) | Provides industrial fire certification and testing support |
The golden rules for design changes
When working with certified fire-rated truss assemblies, the following general rules can reduce compliance risk:
- Dimensional redundancy: Listed truss depths and wood member sizes are minimums. In practice, deeper trusses or larger members are generally allowed because they tend to improve fire performance.
- Application conversion: A parallel-chord truss certified for floor-ceiling use may also be used in roof-ceiling applications if the minimum end-support depth is satisfied.
- Insulation flexibility: If an assembly listing describes insulation as optional, it does not have to be installed. If the original certification did not include insulation, however, a compliant retrofit is required.
Compliant paths for adding insulation
When insulation must be added to a fire-rated assembly that was not certified with it, the retrofit must not undermine the original rating.
Option one: structural depth compensation
Increasing the depth of a flat or pitched assembly allows insulation to be installed above the original design depth. The added physical separation helps preserve the integrity of the fire-protective layer.
Option two: added protective layer
For certain UL- or GA-certified assemblies, insulation of any depth may be added at any location if an additional layer of gypsum board is installed at the ceiling. The extra covering offsets potential changes in heat transfer caused by the insulation.
A disciplined selection process
Some fire-rated assemblies are tested with specific branded products and cannot be substituted without approval. Certified assemblies that use generic materials are generally preferable because they reduce dependence on a single supplier. Tables summarize only key data; full technical details remain in the original reports issued by the relevant organizations. Where documentation is ambiguous, project teams should resolve the question through the applicable certification body or a qualified fire-protection engineer before construction.
