Wood Truss Failures
Wood Truss Systems Failures
The civil/structural engineers at Robson Forensic are frequently retained to investigate claims and injuries involving wood truss systems. This article focuses on modern wood truss systems and is the first in a two part series. The second article will focus more narrowly on wood bow-string trusses, which have become notorious for their propensity to fail.
History and Rationale
Wood trusses have been used in residential and commercial construction projects since the invention of the metal truss plate in the 1950s. Compared to conventional stick framing, engineered wood trusses can offer greater quality control, reduced construction time, and lower construction costs.
Wood trusses are used in a variety of applications, including roofing and flooring systems. They can also be connected together (multi-ply) to act as a girder truss to transfer loads. The diversity of wood truss applications combined with their various advantages over traditional framing has made them an increasingly popular choice among design and construction professionals.
Wood Truss Anatomy
The image below depicts a hypothetical wood truss. In real world applications, wood trusses are used as part of an engineered system and are only stable as part of that system. It is important to understand that out-of-plane bracing is required to tie each truss into a larger system of parallel trusses. In the two-dimensional illustration below, continuous lateral bracing is shown perpendicular to the truss and is item ‘G’. The bracing must be present at all times and must evolve from temporary during construction to permanent when the structure is completed. The engineer of record is responsible for all permanent bracing. The contractor is responsible for all temporary bracing.
- A. Span – the bearing to bearing unsupported distance (from the exterior wall or other supports).
- B. Truss Plates – the steel plates with multiple nails which join the wood truss members.
- C. 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.
- D. Bottom Chord – the member located at the bottom of the wood truss.
- E. Web Members – the diagonal or vertical members, spanning between the top and bottom chords of the wood truss.
- F. Panel Length – the distance between points where the web members intersect the top and bottom chords.
- G. Continuous Permanent Lateral Brace – the diagonal out of plane members (I.e. perpendicular to the plane of the wood truss) that provide lateral stability to the wood truss.
- H. Bearing – the point of contact where the wood truss is supported.
Wood Truss Mishaps & Failure Modes
A wood truss is an individual component that must work together in a wood truss system. The majority of claims and injuries associated with wood trusses occur during their handling, hoisting, or installation. Accordingly, standards exist in the wood truss industry addressing the proper installation and temporary stabilization to prevent catastrophic failures. 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.
A few conditions that commonly lead to failure are outlined below.
Wood Truss Handling & Hoisting
An individual wood truss, as a single unit, is inherently unstable and highly susceptible to out-of-plane buckling, a condition that may likely result in a collapse. As a result, proper care and attention is required when moving or hoisting them into place. If a wood truss is not properly hoisted, there is a likelihood that it will twist and bend (i.e. buckle), potentially damaging the wood truss and resulting in an unsafe condition.
Wood trusses can be hoisted by a crane individually, but are more stable when hoisted in bundles. Industry guidelines allow a single lift point for bundles of spans 45 feet or less, but 2 lift points are required for wood truss spans over 45 feet. Wood trusses 60 feet or longer require the use of three lift points. Care must be used to not overload a supporting element due to the concentrated load created by the wood truss bundle. Failure of a supporting element can cause hoisted wood trusses to fall or swing unpredictably.
- Concentrated Loads – As an example, a 60 foot long wood truss could weigh 1,000 lbs., weighing 500 lbs. at each end. If 10 wood trusses were bundled, a 5,000 lbs. load would be imparted onto the supporting structure at each end.
For single wood trusses, a single pick point (i.e. at the peak) by industry standards is not allowed. For wood trusses up to 30 feet, 2 pick points are required at top chord panel points, spaced up to ½ the wood truss’s length. For wood trusses between 30 feet to 60 feet, a spreader bar with 3 pick points is required which is ½ to ⅔ of the wood truss length and the end lines must “toe in” as shown in Figure 2.
Wood Truss Installation/Bracing
The contractor plays a vital role in ensuring that wood trusses are properly erected according to design specifications and applicable standards. Even when properly designed, a temporary wood truss bracing system can become dangerously unstable and unsafe if there is any deficiency in the temporary bracing methods.
The first few wood trusses set the precedence of all the proceeding wood trusses. Therefore, it is vital that the first wood truss be braced from the ground as shown in Figure 3, and the first three wood trusses be properly braced together prior to proceeding with the remaining wood trusses. Failure of a single wood truss in the early stages from overload or 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 there is no bowing (out of plane distortion) in excess of the allowable tolerances. Excess tolerances provide a primary failure mechanism for buckling. Buckling, or bowing, reduces a wood truss’s capacity to perform as intended and is a precursor for collapse.
Temporary Bracing Requirements
Proper temporary bracing is required throughout the construction process in order to prevent a failure of a wood truss system. Industry standards dictate that wood trusses cannot be released from the hoisting sling until temporary lateral restraints are in place. Temporary bracing must be in place at the top chord and web members before any loads can be applied to the system or workers are permitted to traverse and/or climb on the trusses for the sake of construction. An example of top chord bracing is illustrated in Figure 4.
The permanent roof sheathing is acceptable for the top chord lateral bracing in lieu of the top chord temporary lateral restraint (TCTLR). Criteria for temporary bracing regarding spacing and detail requirements are clearly established in industry standards. Deviating from the criteria set forth will likely result in a highly unstable and unsafe condition that can lead to partial or total collapse of the wood trusses.
Wood Truss Failure Investigations
Wood truss systems are an efficient, economical method of construction for residential and commercial buildings; however, we have seen many firsthand examples where carelessness, ignorance, or outright negligence have resulted in disastrous effects.
Investigation of these incidents commonly involves review of the project plans and shop drawings as well as the contractor’s means and methods. In addition to the sequencing, shoring, handling, temporary bracing and setting, forensic analysis must also consider jobsite safety requirements.