- Timber engineering is currently in the midst of a significant evolution due to the rise of mass timber products, like cross-laminated timber (CLT). Increasing numbers of structural engineers are facing the challenge of designing lateral-force-resisting systems (LFRS) for multistory CLT structures. LFRS solutions such as steel frames, concrete cores, or light-frame shear walls have been combined with CLT to create hybrid structures. However, there is great interest in utilizing CLT as shear walls. Researching and developing methods to incorporate CLT as an LFRS are attractive because of its structural, environmental, and societal benefits/impact. The high in-plane strength and stiffness of CLT makes it well suited for tall-wood buildings, and CLT has been promoted as sustainable and shown to positively impact users. The demand growth for tall-wood buildings in the United States has been greatest in the Pacific Northwest. This region of the United States has some of the highest seismic load demands in the world because of the Cascadia Subduction Zone. The current state-of-the-art for CLT shear walls has demonstrated that it performs well as an LFRS when coupled with robustly designed connections. These connections are the primary source of energy dissipation in a CLT lateral system and have a major role in the ability of a multistory timber structure to remain stable during and after an earthquake.
To date, the connections most investigated have been metal hold-down and angle connectors taken from conventional timber construction and applied to CLT shear walls. Studying these connectors has helped to develop some typical CLT wall details. Testing of various combinations of these metal connectors has demonstrated the importance of designing CLT shear wall connections to be ductile, capable of dissipating energy, and able to be modeled appropriately to accurately capture system performance. However, other connections for CLT shear walls have not been as thoroughly investigated.
Studies on a variety of connections are needed to expand the state-of-the-art and improve structural design flexibility for continued development of CLT shear wall use in multistory timber structures. This study covers substantial design, testing, and review of two promising CLT shear wall connections that provide different levels of structural seismic performance.
The first phase of the project studied the viability of using inclined self-tapping screws installed through a CLT wall into a CLT floor. These inclined screws create a toe-screwed CLT shear wall connection easily applied to typical platform construction methods. CLT connection assemblies representing typical CLT wall slide and uplift, using toe-screwed, fully-threaded (FT) and partially-threaded (PT) self-tapping screws (STS), were tested. The toe-screwed CLT assemblies were cyclically tested under shear and tension loading to determine failure modes and applicability as a seismic connection. Various mechanical properties and backbone curves were extracted for comparison with design performance estimates. The toe-screwed connections exhibited screw fracture, head-pull through, and pinched hysteresis loops. ASCE/SEI 41-17 idealized-component curve connection parameters were extracted for use in nonlinear-static pushover analysis. The proposed design method for toe-screws loaded in tension accurately predicts strength and displacement. Partially-threaded washer-headed screws were found to be superior to fully-threaded screws for seismic applications due to significantly greater deformation and energy dissipation capacity.
Because toe-screwed connection tests with washer-headed, PT STS exhibited promising characteristics, full-scale toe-screwed CLT shear walls were tested using the same type of PT STS from connection assembly tests. The tested walls represented common platform construction wall-to-floor conditions in multistory timber buildings. Three CLT shear wall connections — equally spaced toe-screws, grouped toe-screws, and a combination of toe-screws with hold-downs — were tested with static and pseudo-static cyclic loading. Toe-screwed CLT shear walls exhibited significant energy dissipation and head pull-through failure modes observed in previous testing. Wall tests were observed to have significant hysteretic pinching, localized CLT damage, substantial drift capacity, and good overall strength. Results indicate that washer-head, partially-threaded toe-screws are a viable connection in lateral-force-resisting systems offering comparable, or better, performance to other, similar connection options.
The second phase of the project tested CLT rocking walls with slip-friction connections (SFCs) used as a passive supplemental energy damper and yielding element. The slip-friction connections were a unique design using inclined STS to connect CLT to the SFC. Connection assembly and full-scale wall tests were conducted to better understand the novel application of multiple modern timber technologies. The full-scale SFC LFRS was designed as a slip-friction hold-down in a self-centering CLT rocking wall system. Component axial SFC tests demonstrated ductile and elastic-plastic characteristics in an otherwise non-ductile CLT connection condition. Because the internal forces were controlled by SFC yielding, no wood damage was observed during SFC sliding. The damping ratio was found to be 0.6 and close to typical idealized friction hysteresis models. Full-scale, 1.52 m by 3.04 m, CLT walls with SFCs were tested under static and pseudo-static cyclic loading. The SFCs reliably dissipated energy and protected the rocking walls from damage. Idealized rectangular or flag-shaped parallelogram hysteretic models accurately represented the LFRS behavior. Results indicate highly repeatable performance and structural protection beyond allowable loads. Various magnitudes of restoring forces were applied with results ranging from wall vertical ratcheting to wall re-centering.
Both the toe-screwed and slip-friction connection studies provided evidence of these connections as useful structural alternatives for use with CLT shear wall in multistory buildings. Future, multistory CLT seismic connection design is enhanced by this study because the state-of-the-art of CLT shear wall connections used in prescriptive- and performance-based design has been expanded for use by researchers, engineers, and code officials.