Patent Publication Number: US-10787832-B2

Title: Connector for use in inter-panel connection between shear wall elements

Description:
RELATED APPLICATIONS 
     This application claims the benefit of the filing date of U.S. provisional patent application No. 62/505,036, filed May 11, 2017, entitled “Connector for Inter-panel Connections between Shear Wall Elements”, the entire contents of which are incorporated by reference under 37 C.F.R. § 1.57, and is a divisional application of co-pending U.S. patent application Ser. No. 15/801,237, filed Nov. 1, 2017, entitled “Connector for Use in Inter-panel Connection between Shear Wall Elements”. 
     This application is related to U.S. patent application Ser. No. 15/786,141, filed Oct. 17, 2017 entitled “Method and Apparatus to Minimize and Control Damage to a Shear Wall Panel Subject to a Loading Event”, the entire contents of which are incorporated by reference under 37 C.F.R. § 1.57. 
    
    
     TECHNICAL FIELD 
     Embodiments of the invention relate to building products. In particular, embodiments of the invention relate to a connector to connect a shear wall to an adjacent shear wall in a single or multistory building. 
     BACKGROUND 
     A factor behind the increasing use of mass timber panels, such as Cross-Laminated Timber (CLT) panels, vertically laminated veneer (LVL) panels, and parallel strand lumber (PSL) panels, in construction projects is the accelerated construction timeline compared to using traditional building materials and processes. When designed correctly, it is possible to erect an entire structure for a multiple story building in a matter of weeks instead of months. An additional factor that is driving the increased demand for mass timber panels in building projects is the difference in types of on-site field labor required. Erection of a structure using mass timber panels requires carpenters or general laborers, while traditional multiple story building projects that use concrete and steel construction require concrete finishers and iron workers typically at higher labor rates than carpenters and general laborers. Finally, the environmental benefit of sequestered carbon associated with timber construction versus steel and concrete construction, and the utilization of small-diameter trees in mass timber panels, provides additional motivation to use mass timber panel in construction projects. 
     One of the current issues in using mass timber panels in low-rise to mid-rise buildings is the lack of information associated with the performance of such panels in regions with higher seismic hazard. While quantifying the seismic design parameters for mass timber panel-based buildings is progressing in the building industry, currently there are no inter-panel connectors that are qualified or certified for use in high seismic regions other than standard hardware bolt-, nail, or screw-type connectors. Most of the connectors used in current construction of mass timber panel-based building projects are not capable of handling the reversed cyclic load deformations associated with earthquakes. Mass timber panels are relatively stiff and thus energy dissipation must be accomplished through the ductile behavior of connections between different shear wall elements. Therefore, new high load deformation capacity-connectors that provide high ductility/hysteretic energy dissipation are needed to achieve acceptable performance of mass timber panel-based buildings during events such as earthquakes and high wind loads. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example, and not by way of limitation, and can be more fully understood with reference to the following detailed description when considered in connection with the figures in which: 
         FIG. 1A  illustrates an elevation view of two mass timber wall panels interconnected according to an embodiment of the invention. 
         FIG. 1B  illustrates an elevation view of two mass timber wall panels interconnected according to an embodiment of the invention. 
         FIG. 1C  illustrates an elevation view of two mass timber wall panels interconnected according to an embodiment of the invention. 
         FIG. 1D  illustrates an top view of two mass timber wall panels interconnected according to an embodiment of the invention. 
         FIG. 2A  illustrates a front view of an inter-panel connector in accordance with an embodiment of the invention. 
         FIG. 2B  illustrates a perspective view of the inter-panel connector in accordance with an embodiment of the invention. 
         FIG. 3A  illustrates an elevation view of a means for fastening an inter-panel connector to adjacent mass timber wall panels in accordance with an embodiment of the invention. 
         FIG. 3B  illustrate a plan view of a means for fastening an inter-panel connector to adjacent mass timber wall panels in accordance with an embodiment of the invention. 
         FIG. 4  illustrates a top view of an embodiment of the invention. 
         FIG. 5  illustrates a flow chart in accordance with an embodiment of the invention. 
         FIG. 6  illustrates a load-deflection curve for a hysteretic response curve in accordance with an embodiment of the invention. 
         FIGS. 7A, 7B and 7C  illustrate various aspects of an embodiment of the invention. 
         FIG. 8  illustrates an inter-panel connector in accordance with an embodiment of the invention. 
         FIG. 9  illustrates a model of the inter-panel connector in accordance with an embodiment of the invention illustrated in  FIG. 8 . 
         FIG. 10  illustrates a load-deflection curve for an hysteretic response curve in accordance with the embodiment of the invention illustrated in  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention involve a connector to join two mass timber shear wall panels (or simply “mass timber panels”) that performs acceptably during a seismic event such as an earthquake or high wind load. Embodiments of the connector should be easy to install, and easily replaced after the building experiences a seismic event, to allow the building to be more easily erected and easier to repair following the seismic event. In one embodiment of the invention, the connector has high initial stiffness to minimize wall racking displacement under low and moderate intensity earthquakes. (Racking resistance of wood shear walls is a major factor in determining the response of the shear walls to wind and seismic forces; the less resistance, the greater the racking displacement. When a wall panel is subjected to a racking force, the connectors distort, and the racking force imposes a horizontal displacement on the lateral system). 
     One embodiment of the invention achieves a clearly defined load at which the stiffness of the connector changes from a high initial stiffness to a low stiffness to allow high displacement capacity of a wall comprising mass timber shear panels when the building is subjected to a significant seismic event. The clearly defined load is the proportional limit of the connector where the linear-elastic yield strain of metal is attained and beyond which non-linear inelastic strains develop. In one embodiment, the ideal performance of the connector yields an elastic (reversible)-plastic (irreversible) load-deflection curve for an envelope curve. A representative curve is illustrated in the chart  600  of  FIG. 6 . This curve was generated in a nonlinear numerical model of one embodiment of the connector during a cyclic racking (shear) deformation. The elastic range can be seen by viewing the straight line that begins at the origin of the chart and is a straight line up into the upper right quadrant of the graph. The proportional limit for the connector as modelled is at a force level of about 2 kips. From there the inelastic (flat horizontal line) range is achieved. (An object in a plastic deformation range will first have undergone elastic deformation, which is reversible, so the object will return part way to its original shape). Embodiments of the invention further should have the ability to sustain large displacements without metal fatigue, fracture, or unstable buckling to provide drift (lateral displacement/story height) capacity of 4-6%. Finally, embodiments of the invention should have hysteresis loops as large as possible, as illustrated in chart  600  in  FIG. 6 , with a minimum of pinching, in order to maximize their capacity for energy dissipation. The hysteretic energy dissipation is a measure of the area contained within the full loop of the curves as depicted in chart  600  in  FIG. 6 . 
     In structural engineering, a shear wall is a structural system composed of rigid wall panels (also known as shear panels) to counter the effects of in-plane lateral load acting on a structure. Wind and seismic loads are the most common loads that shear walls are designed to carry. Under several building codes, including the International Building Code (where it is called a bearing or frame wall line) the designer is responsible for engineering an appropriate quantity, length, and arrangement of shear wall lines in both orthogonal directions of the building to safely resist the imposed lateral loads. Shear walls can located along the exterior of the building, within the interior of the building or a combination of both. 
     Plywood sheathing is the conventional material used in wood (timber) stud framed shear walls, but with advances in technology and modern building methods, other prefabricated options have made it possible to insert multi-story shear panel assemblies into narrow openings within the building floor plate or at the exterior face of the floor plate. Mass timber shear panels in the place of structural plywood in shear walls has proved to provide stronger seismic resistance. 
     With reference to  FIGS. 1A, 1B, 1C and 1D , in one embodiment  100 , one or more ductile/dissipative inter-mass timber panel connectors (e.g., plates  101 A and/or  101 B) fasten a minimum of two mass timber wall panels  105 A and  105 B together along their respective abutted vertical edges  106 A and  106 B. The connectors  101  are suitable for use in platform- or balloon-framed mass timber construction methods. When subjected to actions from service level earthquake and less than ultimate wind events, the connector  101  is designed to maintain elastic stiffness so that adjacent panels  105  act, or move, together as a rigid or single body. When subjected to actions from design (Building Code Level), Risk-Targeted Maximum Considered Earthquake (MCE R ) events, or ultimate wind events, the connector  101  achieves a low stiffness plastic state which allows each individual wall panel  105 A,  105 B to rotate (rock) about a respective tie-down  110 A,  110 B resulting in a lower stiffness deformation controlled system suitable for seismic regions. 
     The mass timber wall panels  105 A,  105 B stand on a base support  120 , e.g., a top edge of a lower story wall (such as a mass timber panel), or a foundation, for example, a foundation wall, a ground level floor, or upper story floor. The mass timber wall panels  105 A,  105 B are each connected to the base support  120  by a respective tie-down  110 A,  110 B. In one embodiment, the wall panels extend vertically one or more stories or levels from base support  120 . Generally speaking, in one embodiment, the wall panels are rectangular, with dimensions greater in height than in width. In one embodiment, the wall panels  105 A,  105 B are centrally supported on base support  120  at the location of a tie-down  110 A,  110 B. In other words, each wall panel  105 A,  105 B is coupled to the base support  120  by a tie-down  110 A,  110 B, and the tie down is located equidistant from the left and right vertical edges of the wall panel. Essentially, the wall panel is balanced on the supporting tie-down. During a low intensity seismic or other loading event the adjacent wall panels can rock to one side or the other, and back again as a rigid unit (as illustrated in  FIG. 1B ), under the influence of an imposed cyclic lateral or horizontal force. During a high intensity seismic or other loading event the adjacent wall panels can rock to one side or the other, and back again in an independent manner, under the influence of lateral or horizontal force. In either case, wall panels rock from side to side about their point of attachment to the base support, that is, about their respective tie-downs to the base support. The independent wall rocking allows for motion dampening/energy dissipation at the inter-wall panel connectors, as discussed below. 
     A “service level earthquake”, or service level earthquake shaking, may be defined as ground shaking represented by an elastic, 2.5%-damped, acceleration response spectrum that has a mean return period of 43 years, approximately equivalent to a 50% exceedance probability in 30 years. As for “ultimate wind events”, over the years, wind speed maps have changed from fastest mile to 3-second gust and then to “ultimate” 3-second gust wind speeds. A comparison of American Society of Civil Engineers (ASCE) 7-93 (fastest mile) wind speeds, ASCE 7-05 (3-second gust) ASD wind speeds, and ASCE 7-10 (3-second gust) ultimate wind speeds is provided in Table C26.5-6 of the ASCE 7-10 commentary. 
     Regarding the embodiment illustrated in  FIGS. 1A-1D , it is understood that one connector  101  may be larger or smaller, and the various length, width, depth/plate thickness dimensions of the connector may vary according to different embodiments, for example, the number of connectors installed between two adjacent wall panels, the height, width, thickness, and weight of the wall panels, etc., without departing from embodiments of the invention.  FIG. 7A  illustrates a connector in accordance with an embodiment of the invention  700  and as dimensioned, fabricated and tested by the assignee of the present invention. The connector was dimensioned and fabricated for easy handling and installation in 2 foot sections. 
     In one embodiment, an interlocking shear key  706 A,  706 B is located at the lower left and right corners of the connector  700 . A connector can be stacked on top of/above another connector, so that shear keys  706 A,  706 B of the connector on top fit into recesses  707 A,  707 B located at the upper left and right corners of the connector below. The keys interlock the stacked connector plates together to increase stiffness/performance as if it were one continuous steel plate element.  FIG. 7B  illustrates typical hole spacing in the connector, according to one embodiment  705 . Fasteners may be inserted through the holes and into the wall panels to affix the connector to the wall panels.  FIG. 7C  illustrates the shear key dimensions, according to one embodiment  710 . 
       FIGS. 8 and 9  illustrate a connector  800 , and a corresponding finite element model of connector  800 , in accordance with another embodiment of the invention, as modeled by the assignee of the present application. In particular, a finite element model  900  of a steel plate connector  800  was generated in ABAQUS, a software suite for finite element analysis and computer-aided engineering, available from Dassault Systèmes.  FIGS. 8 and 9  illustrate tapered leaves in the steel plate connector to provide relatively high stiffness initially, then as the connector is deformed (top displaced parallel to the base), the leaves begin to buckle and yield to provide a low stiffness and large displacement capacity. 
     The connector  800  was modeled using ABAQUS in an iterative procedure, with several refinements to improve the overall performance. It is believed that the performance of the connector is dependent on the thickness of the steel plate, the overall length of the individual leaves  805  (4 inches in  FIG. 8 ), the ratio of the base of the leaves  810  to throat of the leaves  815  (1 and 7/16-in/½-in in  FIG. 8 ), and the modulus of elasticity (MOE) and yield strength (σ y ) of the steel. The load-displacement response of the connector is shown in  FIG. 10 . The decrease in load resistance illustrated in the larger displacement demand cycles are due to the connector leaves buckling as well as yielding. The model does not include stain hardening or failure characteristics in the material characterization at this time. When the connection is tested on mass timber shear wall panels, the buckling performance will change since the steel plate will only be able to deflect in one direction (away from the panel) in reality, and the model currently does not restrict this deformation. 
     The above described embodiments, place the connectors on opposing outside faces of the mass wall panels. Under small to medium racking deformations the plate metal elements are stabilized from rotating or buckling out-of-plane by bearing against the wooden panels. At large racking deformations and high strains, the individual metal plate elements are allowed to rotate out of plane. These connectors are depicted as relatively thin, perforated, metal sheets that are attached to the wall segments (i.e., nailed, bolted, or screwed, etc.), at a plurality of locations or otherwise attached or adhesively bonded to adjacent wall panels  105 A and  105 B. In one embodiment, the metal sheets are comprised of sheet steel product manufactured to ASTM A1011, but the steel alloy can be changed and the relative dimensions of the connector can be modified to compensate for the change in mechanical properties. 
     An alternative embodiment  200  of a mass timber-to-mass timber wall connector  101  is illustrated in  FIGS. 2A  (front perspective view),  2 B (perspective view),  3 A (elevation view), and  3 B (plan view). The alternative embodiments sandwich the ductile/dissipative connector  101  between plywood (or similar) cover panels  115 A,  115 B (not depicted in  FIGS. 2A and 2B ) on opposing sides of the adjacent panels  105 A,  105 B. The panels  115 A,  115 B are through-bolted to each other at  116 . In such an embodiment, these cover panels  115 A,  115 B are thought to restrain out-of-plane connector plate buckling, while at the same time float within the plane of the cover, such that they do not affect the strength/stiffness of the connector  101 . A low-friction material, such as Ultra-High-Molecular Weight (UHMW) Polyethylene sheets may be introduced in the sandwich to help reduce friction, for example, between the connector  101  and the cover panel  115 . One advantage of the buckle-restrained embodiment illustrated in  FIGS. 2A, 2B, 3A and 3B  is that any non-linear energy dissipation is more stable and deterministic. 
     In another embodiment  400 , with reference to  FIG. 4 , one or more mass timber-to-mass timber wall connectors  101  are embedded within, and span between, mass timber wall panels  105 A,  105 B. To accommodate embedding of a connector  101 , a volume of panel material at least the dimension of that portion of the connector that is embedded into a respective mass timber wall panel is removed from the mass timber wall panel. In one embodiment, the volume of panel material removed is greater in width, and length of that portion of the connector inserted into the mass timber wall panel, and the depth of the area removed is equal to or greater than the thickness of the connector, to allow for placement of the assembly and to allow for rocking of the mass timber panels while at the same time minimizing deformation or buckling to the connector, for example, during a significant seismic or wind load event. In this embodiment, at small, medium, and large racking deformations the connector elements are prevented from buckling/rotating out-of-plane by being restrained by the wood panel itself, on both sides. 
     According to one embodiment  500 , with reference to  FIG. 5 , a method of manufacturing the connectors is described below. Initial steel sheet is purchased and manufactured into the connectors at step  505 . A sample of the connectors is then tested by itself in a universal test machine to quantify the actual load-displacement curves and hysteresis performance of the connector, at step  510 . If the sample passes the performance testing, further test sample connectors in a 2-panel mass timber-to-mass timber wall specimen in full-scale at step  515 . In one embodiment, this uses several of the connectors to be tested on the wall. It is envisioned that the overall wall specimen would have 8 connectors (4 on each side of the panels  105 A,  105 B). In one embodiment, the number of connectors is not as significant as the total length of connector per story height of the mass timber wall panels. 
     A connector according to an embodiment of the invention is envisioned to be developed like a widget, similar to products manufactured by Simpson Strong-Tie. The manufacturer of the connector will pre-qualify through testing a range of suitable connectors. A designer first designs a wall for a building and determines the mass timber panels require a certain amount of shear force capacity on the inter-panel seam for the wall. The designer then specifies how many connectors and what size are required to meet the wall design. It is envisioned that the connectors in various sizes and shapes are available for viewing via website or catalog, and the designer selects a number of connectors of appropriate size and shape. These connectors are then attached to the two panels in the field as the building is being erected. In one embodiment, one or more connectors are attached according to such factors as the dimensions and strength of the connectors, and the dimensions of the mass timber wall panels. In one embodiment, a minimum total cumulative length of the attached connectors, in a vertical direction, is met or exceeded, based on such factors as the dimensions and weight of the mass timber wall panels, and various building codes and zoning codes. 
     Although embodiments of the invention have been described and illustrated in the foregoing illustrative embodiments, it is understood that present disclosure has been made only by way of example, and that numerous changes in the details of implementation of the invention can be made without departing from the spirit and scope of embodiments of the invention, which is only limited by the claims that follow. Features of the disclosed embodiments can be combined and rearranged in various ways.