Wood Truss Failures Expert Article

This article provides an overview of wood truss systems with a focus on how and why these systems can fail during erection.

Wood Truss System Failures

The civil/structural engineers at Robson Forensic are frequently retained to investigate claims and injuries involving the collapse of wood truss systems during construction. Our experts determine causation by investigating the applicable standards of care, the contractor’s means and methods and the adequacy of temporary bracing.

Catastrophic collapses are often caused by a failure to provide adequate bracing in all the planes that are specified in the relevant standards and guides. This article focuses on failure investigations of metal plate connected wood truss systems as they are erected.

An individual truss is a two-dimensional structure that is strong in the vertical plane, specifically designed to carry roof and floor loads in that direction, but it has little to no resistance to out-of-plane forces. Wood trusses are typically used as part of an engineered system and are only stable when properly integrated into that system.

It is important to understand that out-of-plane bracing is required to tie each truss into a larger system of parallel and plumb trusses. To ensure stability, continuous lateral and diagonal bracing must be present at all times and must transition from temporary during erection to permanent when the structure is completed.

The bracing responsibilities for the project are generally divided into two separate responsibilities: the structural engineer of record is responsible for specifying the permanent bracing while the erector is responsible for the temporary bracing plan and its execution.

Standards exist in the wood truss industry addressing proper wood truss installation and temporary stabilization requirements to prevent catastrophic failures during the erection of the trusses. In addition, industry standards also require that an engineering professional is to design the temporary restraint/bracing system for any wood truss whose span exceeds 60 feet.

The predominant industry standard addressing the handling, installation, and temporary bracing of wood trusses is the Building Component Safety Information (BCSI) Guide, published jointly by the Structural Building Components Association (SBCA) and the Truss Plate Institute (TPI). The BCSI Guide is referenced by the International Building Code (IBC) as a recognized method for complying with safe truss installation requirements.

Wood Truss Installation/Bracing

The truss erector and the general contractor are responsible for ensuring that wood trusses are erected safely. Even when properly designed, a temporary wood truss bracing system can become dangerously unstable and unsafe if there is any deficiency in the execution of the temporary bracing procedures during erection.

The BCSI standard requires that temporary bracing be applied to all three planes of a roof truss system during erection: the roof plane along the top of the trusses, the ceiling plane along the bottom, and the vertical plane running between the trusses.

Each plane must be independently stabilized. A deficiency in any one of the planes can result in collapse.

Proper temporary bracing is required throughout the construction process to prevent a failure of the wood truss system. Standards require that wood trusses cannot be released from the lifting equipment until temporary lateral restraints are in place and properly fastened.

Figure 1 shows the primary components of a wood truss:

1. Span: the unsupported distance (from the exterior walls or other supports).
2. Truss Plates: the galvanized steel plates with protruding teeth used to join the wood truss members.
3. Top Chord: the member located at the top of the wood truss which forms the shape of the roof. The top chord can also be flat when used as a wood floor truss or flat roof wood truss.
4. Bottom Chord: the member located at the bottom of the wood truss.
5. Web Members: the diagonal or vertical members, spanning between the top and bottom chords of the wood truss.
6. Continuous Lateral Brace: out of plane members (i.e., perpendicular to the plane of the wood truss) that provide lateral stability to the wood truss.
7. Bearing: the point of contact where the wood truss is supported.

The first erected wood truss sets the stage for all the subsequent wood trusses. Therefore, it is vital that the first wood truss be adequately braced. One proven method of bracing the first truss is bracing them to the ground as shown in Figure 2.

Failure of a single wood truss during the erection process from improper lateral bracing can cause a ‘domino’ effect of failure across the entire system.

As the starter wood trusses are installed, installation tolerances must be strictly followed to ensure that the trusses are erected plumb and without any bowing, within allowable tolerances.

Exceedance of installation tolerances for spacing, plumbness, and/or bowing can lead to a failure of that truss.

Temporary bracing must be present at the top chord before any loads can be applied to the system or workers are permitted to traverse and/or climb on the trusses during erection. An example of top chord bracing is illustrated in Figure 3.

Source: BCSI

A critical and frequently overlooked requirement is that the lateral bracing members themselves must be diagonally braced. Lateral restraint members, the boards running perpendicular to the trusses that tie them together, maintain spacing between individual trusses in addition to providing a bracing mechanism for the truss.

However, without diagonal bracing, the entire group of trusses can simultaneously translate sideways as a unit and collapse together, producing the domino failure seen in many erection collapses.

For safe erection of trusses, an erector must use a combination of lateral restraint and diagonal bracing across the roof (top chord) and ceiling (bottom chord) planes. See Figure 4 for an example of Top Chord Lateral Restraint.

The permanent roof sheathing is acceptable for the top chord lateral bracing in lieu of the top chord temporary lateral restraint method.

Structural Failure Investigations

We have seen many examples where carelessness, ignorance, or negligence have resulted in catastrophic wood truss collapses. Investigation of these incidents commonly involves review of the project plans and shop drawings as well as the erector’s means and methods which include sequencing, temporary bracing layout and compliance with the standards. 

For more information, please submit an inquiry or call us at 800.813.6736.

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