Design for Two-Way Spanning Cross-Laminated Timber

Mass timber panels are commonly used in single-span floor and roof systems w在这里 support beams or walls are perpendicular to the strong axis spans of the panels. 然而, a unique attribute of cross-laminated timber (CLT) is that panels have structural capacity in both the major and minor strength directions. Capacity in the minor strength direction is typically less than in the major strength direction, 这使得CLT是正交异性的. 尽管如此, the capacity of CLT in the minor strength direction can be utilized to create a two-way or flat-plate style of construction. This allows for framing systems with no support beams, w在这里 the CLT panels span in both directions between columns. One of the most notable examples in North America is 布洛克Commons Tallwood House, a student residence at the University of British Columbia with 17 stories of mass timber over a concrete podium. Designers of the Brock Commons project used 5-ply CLT panels spanning over a grid of columns spaced approximately 9 ft x 13 ft apart.

因为这种结构构型并不常见, the project team undertook several extraordinary tasks to be confident in the design. 第一个, the engineering team at 快速+ Epp developed a custom CLT layup with the appropriate shear and minor direction bending strengths. The team then worked with researchers at FPInnovations to perform structural testing of the CLT and connections to verify the two-way panel and connection behavior. The column and panel joints were detailed with steel components to directly transfer column axial forces between levels and prevent loading floor panels in compression perpendicular to grain. Further information on this process can be found on the 快速+ Epp website 在这里.¹

类似的, the 1510年韦伯斯特 project in Oakland uses two-way CLT with structural composite lumber laminations over a grid of columns set 12 ft x 15 ft apart.

A more common structural configuration that uses the two-way strength capacities of CLT is to have panels cantilever in two directions off the corner of a building, 创造无梁屋檐. Another is to use CLT over corridors in a multi-family building. 在传统的双层走廊里, mass timber floor panels span over the dwelling units from unit demising wall to unit demising wall, 平行于走廊. 在走廊上, one option is to turn the CLT panels so they span from corridor wall to corridor wall. 然而, 创建更统一的面板布局, an alternative is for the long direction of the panels to be 平行于走廊. Depending on the width of the corridor and minor axis strength, it may be possible to achieve a beamless support condition, w在这里 panels span from corridor wall to corridor wall in their minor axis. 在更开放的平面图中类似的配置, 比如办公室, is to have a colonnade of point-supported CLT panels in a central position of a floor plan, creating a beamless zone through which MEP trunk lines and fire sprinkler mains can traverse the building without needing to penetrate dropped beams.

当看到双向跨CLT结构, at least three potential capacity limits may need to be considered:

1. 面板的抗弯和抗剪能力
2. 支座承载能力
3. 支座处局部冲剪

面板的弯曲和剪切能力 

Manufacturers of CLT panels compliant with ANSI/APA PRG 320: Standard for Performance-Rated Cross-Laminated Timber provide third-party certified flexural and shear strength (and stiffness) of their panels in both the major and minor flatwise bending directions. Design of wood structures in jurisdictions of the International Building Code should generally follow the American Wood Council’s 国家 Design Specification® (NDS®) for Wood Construction. Chapter 10 of the NDS includes provisions to calculate ASD and LRFD design capacities to be compared against the building code-defined loads. 对于双向行为, 然而, the NDS does not address interaction effects between the major strength and minor strength bending and shear checks of CLT panels. Designers must t在这里fore rationalize the interaction effects based on engineering fundamentals, 如NDS 2015 C3中描述的.9.2-2. A common design approach in Europe apportions load to the major and minor strength directions based on relative stiffness of the orthogonal layup and then conducts the respective flexural and shear design checks. This common state of practice in European design is described in Section 5.11 of Cross-Laminated Timber Structural Design: Basic Design and Engineering Principles According to Eurocode.²

支座承载能力  

NDS Chapter 10 includes design provisions for the bearing capacity of CLT panels loaded on the CLT panel face. The underlying reference design stress for the CLT panel for compression perpendicular to the grain, F_c⟂, is based on the constitutive lumber in the panel. These values are sometimes omitted in third-party CLT product evaluation reports aside from references to the species group and grade of lumber used in the CLT. 在这种情况下, the perpendicular-to-grain reference design stress may be obtained from the NDS Supplement for the species and grade of lumber in the surface layer of the CLT or from the manufacturer. Example connection details of CLT supported on columns can be found in Table 3 of the WoodWorks 大型木材连接索引.³ T在这里 are reinforcing detailing strategies that have been used to strengthen the face bearing capacity of CLT panels. 但在国家安全局的管辖范围之外, such reinforcing strategies use fully-threaded self-tapping screws to distribute stresses into the CLT panel. 使用示例请参见 Technical Engineering Bulletin SDCF Timber-CF as a Compression Reinforcement.⁴

支座处局部冲孔剪切

在双向CLT面板的支持, the shear capacity around the perimeter of the support point needs to be verified. This check is performed using the reference flatwise shear capacities V_s,0 和V_s,90 根据ANSI/APA PRG 320开发. 虽然在NDS中没有明确提到, a European approach has been developed by Bogensperger et al. (2016).⁵ A controlling perimeter line is found by projecting into the panel from the edge of the support at 35 degrees from vertical to the center of the panel. The shear capacity of the panel around this perimeter line is then checked against the applied design loads. More recent information on punching shear research can be found in Slotsboom et al. (2023).⁶ 和Shahnewaz等人. (2023).⁷

Since two-way bending interaction effects and punching shear are not explicitly covered, 应采用谨慎的工程设计. Acceptability of the approach is at the discretion of the designer, 建立官方, 和管辖. 直到设计方法与美国标准一致.S. standards are developed and verified for CLT punching shear, it is recommended that punching shear design verification be based on test results, 由合格专家进行同行评审, 或两个.

双向弯曲性能建模与分析

A robust method to evaluate CLT panels for two-way bending behavior is though finite element method (FEM) plate analysis of the CLT and support system coupled with experimental validation of the model. While t在这里 are many ways to do this through structural analysis packages, it is important to keep in mind that CLT is not an isotropic plate, meaning that flexural and shear stiffness is different in the major and minor strength axis of the panel and these properties can very across and within manufacturers. Adequate FEM analysis packages need to support orthotropic plate analysis, ideally with shear effects as in “thick plate analysis” using Mindlin plate analysis or similar. Such modeling can be found in the examples in WoodWorks’ U.S. 大质量木地板振动设计指南.⁸

¹ 快速+ Epp. 点支持的CLT测试.

² Wallner-Novak, M.科佩尔胡贝尔，J.波克，K. proHolz奥地利. (2014). Cross-Laminated Timber Structural Design: Basic Design and Engineering Principles According to Eurocode.

³ 木制品-木制品委员会. (2021). 大量木材连接的木工指数.

⁴ 辛普森强关系. (2023). Technical Engineering Bulletin: SCDF TIMBER-CF as Compression Reinforcement.

⁵ Bogensperger T.约翰斯蒂尔，R.奥古斯丁，M. (2016). Concentrated Load Introduction in CLT Elements Perpendicular to Plane – Experimental and Numerical Investigations. 世界木材工程会议. 奥地利的维也纳.

⁶ Slotsboom C.迪克夫，C.杰克逊，R。. (2023). A Comparison of Punching Shear Design Approaches for Point Supported CLT Panels. 世界木材工程会议. 挪威奥斯陆.

⁷ 马里兰州Shahnewaz.迪克夫，C.甘加利，H.斯洛布恩，C.谭，M.杰克逊，R。.波波夫斯基，M.坦纳特，T. (2023). Experimental Research on Point-Supported CLT Panels: Phase 1: Rolling Shear Strength. 世界木材工程会议. 挪威奥斯陆.

⁸ Breneman,年代.齐默尔曼，R.格伯，A.艾普，L.迪克夫，C.泰勒，A.W .洛斯比.麦克唐纳，E.斯洛布恩，C.麦卡琴，J.维舍尔，R. 木制品-木制品委员会. (2023). U.S. 大质量木地板振动设计指南.