Force Measurement of Cross-Laminated Timber Building Construction
Structural health monitoring (SHM) is defined as a process of implementing a damage identification strategy for manufactured building products or civil structures. SHM incorporates three major components: a sensor network installed on a structure, a data processing system including data acquisition, transmission, and storage, and an evaluation system such as algorithms for damage detection. Data generated from SHM projects are often reused in a variety of ways, including design validation of new systems, numerical model validation, or service-life management.
Cross Laminated Timber (CLT) walls are part of the structural health monitoring (SHM) method used to evaluate the performance of new building structural systems and critical infrastructure. With mass-timber building construction on the rise, SHM programs have emerged to document hygrothermal, static, and dynamic behavior of these structures.
CLT shear walls are a seismic force resisting system that has put wood back into the spotlight as a sustainable construction material. CLT shear walls’ prominence in North America is fundamentally changing approaches to design, manufacturing, and construction of commercial and residential buildings.
A CLT shear wall is an engineered wood product consisting of layers of kiln-dried dimensional lumber oriented at right angles to one another then glued to form structural panels. By gluing layers of wood at right angles, the panel delivers excellent structural rigidity in both directions. CLT is strong, with its crosswise design giving it exceptional structural stability. CLT walls can be used as an alternative to concrete for making the walls, roofs, floors and ceilings of a building, and is particularly well-suited to multistory taller wood construction. When subject to in-plane lateral loads, CLT walls primarily engage in rocking behavior due to the high in-plane shear strength and stiffness of the panels relative to the flexible connectors used to adjoin them.
Image 1: George W. Peavy Forest Science Complex, Oregon State University.
The George W. Peavy Forest Science Complex at Oregon State University is a mass-timber building that is the subject of a structural health monitoring (SHM) program to create a comprehensive building performance dataset. The building substructure consists of cross-laminated timber (CLT), concrete composite floors, a mass plywood panel (MPP) roof system, and the world's first application of CLT post-tensioned (PT) self-centering shear walls. The tension sensor in the CLT wall is a load cell.
Image 2: Load cell with readout meter and cables.
Riggio Mariapaola, Associate Professor of Wood Design and Architecture at Oregon State University is the project manager for this building. Stress-tek was contracted to design and manufacture tension monitoring load cells. 1” diameter rods within the CLT wall can be ideally torqued and subsequently monitored for the tension profiles of the rods within the building walls. A Micro Measurements T-rosette strain gage was used within the custom load cell assembly.
Image 3: Loads cells designed to support a 1” diameter building tension rod.
Image 4: CLT load cell assembly with Micro Measurements strain gages, terminal strips and wire.
In CLT shear wall systems, the walls may be post-tensioned to create continuity in components vertically along the height of the building. The continuity of these high-strength steel rods offers self-centering tendencies that serve to pull the building back to its original position after a seismic event.
Cross-laminated timber (CLT) is gaining popularity in residential and nonresidential applications in the European and North American construction markets. Many notable projects have been completed in the United States, illustrating that CLT is a revolutionary building material for the construction industry with many economic and environmental benefits.
Wood offers distinctive value and versatility from the natural world. From its smaller carbon footprint and prefabricated advantages to its nimble assembly. Building professionals are tapping into new opportunities in wood construction and design as wood finds the past, present and future together in the throes of imaginative designs.
Indeed, all building products, or civil structures degrade, including when advanced engineered wood products are used. It’s an expected result of aging and environmental effects. Moisture trapping, mold, corrosion, loads beyond the design criteria, vibrations, settlement, extreme events such as weather (extremely hot nor extremely cold) or impact, or poor design, material choice, wood treatment, or implementation during construction can all cause potential failures on short and long term performance of the structure. Whilst failures cannot always be reliably predicted (extreme weather events, accidental constructions machinery impacts), there are many instances where failures are the result of poor maintenance, advanced treatments or predictable structural degradation such as fatigue. Estimates of life based on typical load predictions can help, but in reality, conditions, humidity and loads change over time and, therefore, a reliable way is needed to predict maintenance based on actual use and conditions as well as extreme events. Strain gages load cells (or strain gage force sensors) are a natural choice to measure and predict the true performance of a structure. Mounted in various locations around the structure, the data accurately represents the structural response and can be used to compare to the design and material limits, and can also be fed into digital twins (computer models designed to run alongside physical objects and systems). Strain gage force sensors can be integrated during any phase of a design and construction, or added to existing structures using a variety of methods from custom-built force sensors, as in the example above, or even discrete adhesive-bonded or weldable strain gages, or embedment strain gages in concrete (discrete or installed on rebar). With an integrated approach in combination with modern and novel materials and construction methods, including 3D printed structures, strain gages or strain gage force sensors are an essential tool in this construction revolution.