Comparing and evaluating monitoring instruments by testing a laboratory scale Warren truss

A wooden truss in a laboratory surrounded by testing equipment.
The scale-model Warren truss was constructed, tested, and monitored in laboratory setting. Photo: Henry Helmer-Smith

The Department of National Defence maintains approximately 70 timber Warren truss Air Force Hangars that were built during the Second World War. Throughout the almost 80-year lifespan of these immense buildings, several repair and rehabilitation efforts have occurred to preserve these heritage structures.

However, due to their age and numerous service factors, it has been mandated that all truss structures located at Canadian Forces Base (CFB) St-Jean, Québec must be evacuated during snow events greater than 10 cm.

Read this open access paper on the FACETS website.

These evacuations limit the operational capabilities of CFB St-Jean and the day-to-day functions of building occupants. In an effort to extend the lifespan of these structures and increase occupant safety, the RMC Green Team, from the Royal Military College of Canada, was tasked to develop, install, and maintain a Structural Health Monitoring (SHM) program within Hangar 1 at CFB St-Jean.

The program consists of instruments that monitor building deflections, environmental conditions, and member stresses. In addition, the program can communicate with building occupants if certain limits (deflections, strains, etc.) are surpassed. The aim is provide building occupants an insight into the health of the structure and serve as an alarm system if evacuations are truly required.

Before installation, to optimize the monitoring program, a scale-model Warren truss was constructed, tested, and monitored in a laboratory setting. The 8-m long truss was approximately one-quarter the size and was constructed with standard 2 inch × 4 inch members and bolted connections.

Testing was conducted by loading the truss joints with weight plates to simulate distributed loading. Several instruments shown in Figure 1 were used to monitor the truss including an Automatic Total Station (ATS), Linear Variable Differential Transducers (LVDT), Light Detection and Ranging (LIDAR), Electric Strain Gauges (ESG), and distributed Optical Fibre Sensors.

A close-up of the truss. Inset photos of testing euiqment point to area of the truss where that equipment was tested.
A close-up of the truss. Inset photos of testing euiqment point to area of the truss where that equipment was tested.
Figure 1: Various forms of instruments used to monitor the laboratory Warren truss. Photos: Henry Helmer-Smith.

These instruments were then compared and evaluated in terms of overall reliability and effectiveness in capturing truss deflections and stresses under several loading scenarios. In addition, the respective capabilities and limitations of the instruments were determined and discussed within the paper. Critical monitoring locations and members of interest along the truss were also determined to develop a more effective monitoring solution. Finally, a weighted options analysis was used to compare and evaluate each instrument based on several factors such as accuracy and reliability, ease of use and effort, operational ability on-site, cost, and training requirements.

The program must serve as an alarm system; therefore, the instruments are evaluated based on how efficient they are in providing data continuously, in real-time, and in the long term as well as how well the instrument software can communicate with the users. It was determined that the ATS (deflections) and ESG (strains/stresses) were the optimal monitoring instruments for emergency monitoring in Hangar 1.

Overall, the laboratory testing and instrument comparisons discussed in this paper were vital in developing an optimized monitoring solution. In addition, this paper presents the methodology employed in the initial development of this larger-scale SHM project.

Read the paperComparison of multiple monitoring techniques for the testing of a scale model timber Warren truss by Henry Helmer-Smith, Nicholas Vlachopoulos, Marc-André Dagenais, and Bradley Forbes.