Engineered trusses, made from a variety of materials, have a wide range of applications in both commercial and residential construction. Metal plate-connected wood member trusses are commonly used in light-frame construction, such as residential. They make it possible to span large distances more efficiently, with less materials, labor, and expense. Building material suppliers have developed specialized software for designers to quickly and accurately design and specify member sizes and connector plates required to safely carry the design loads mandated by the applicable model building codes.

There are a number of conditions that can result in failure of a metal plate-connected wood truss, or other engineered trusses: improper design and sizing of the truss members, manufacturing errors, mishandling during construction, improper loading, and environmental conditions can all result in damages that jeopardize the stability and safety of the structure. Understanding the principles of wood truss design is essential to accurately diagnose the cause of damage and formulate recommendations for repair or other mitigative actions. Donan Engineering is often consulted to determine the cause of truss failures that have resulted in significant collateral damages to a structure. We are prepared with the tools, experience, and abilities to effectively evaluate and analyze the conditions leading to a truss failure.

Structural engineers at Donan Engineering are equipped with state-of-the-art analytical software to quickly model a truss, allowing virtual application of design loads for the purpose of pinpointing areas of stress concentration.

Truss Altered for Clearance

Resulting Building Collapse

For example, the pin-connected wooden bow-string truss below was improperly altered to provide additional clearance to accommodate a new tenant.

Graphic Analysis Output Quickly Identifies Overstressed Members in Red

The graphic output of the program quickly identifies in red that the members of the altered design were overstressed and incapable of supporting the required design loads. Other advanced software features allow quick visual verification of model geometry.

Picture View of Model Allows Quick Verification of Model Geometry

Forensic analysis is also useful in identifying the cause of failures that are not quite as obvious. As shown below, the vertical member at the peak of this metal plate connected scissor truss pulled out of the metal plate connector. Determining the cause of the failure requires knowledge of the forces that the members were subjected to. Without this information, it cannot be conclusively determined whether the truss was overloaded, designed improperly, manufactured improperly, or subjected to loads that exceeded its design capacity.

Quick, efficient and cost effective modeling of this truss allowed us to conclusively determine (by independent analysis) that a manufacturing defect was the cause of the failure. In this case, the correct metal truss plate connector was specified, but not properly positioned to resist the intended design load.

As shown below in the visual graphic output, the vertical member is subjected to 3.12 kips (3,120 lbs) of tension. The truss plates were not positioned properly resulting in an inadequate number of teeth embedded in the vertical member.

Results View Shows Deflected Shape of Truss and Design Forces

Donan Engineering’s structural engineers are familiar with the principles of modeling trusses in accordance with the National Design Specifications (NDS), Truss Plate Institute (TPI), and applicable model building codes. Our expertise with state-of-the-art modeling software allows us to quickly and accurately analyze an “as-built” truss to determine if deficiencies in design, manufacturing, installation, or environmental conditions affected the performance of a truss.

Donan Engineering has the tools, personnel, qualifications, and credentials to make accurate, defensible, cost-effective conclusions regarding the cause and origin of damage to trusses.