# Flexural Performance of Novel Nail-Cross-Laminated Timber Composite Panels
## 1 Introduction  
Cross-laminated timber (CLT) is a wood panel product made by gluing layers of solid wood together. Each layer is orientated perpendicular to adjacent layers and glued on the wide faces of each panel, usually in a symmetric scheme so that the outer layers have the same orientation [1]. Regular timber is an anisotropic material with physical properties depending on the direction at which a force is applied [2]. By gluing layers of wood at perpendicular angles, the panel can achieve structural rigidity in both directions. CLT panels are distinct to glued laminated timber, a product having all laminations orientated in the same direction [3]. Because of its structural properties, ease of prefabrication and lightweight compared with other construction materials, CLT panels have gained wide use in high-rise buildings [4].  
CLT panels were first developed in Europe, then quickly promoted in other countries. Several studies on CLT panels have been conducted in Canada, Finland, the United States, Germany, and other countries [5; 6]. Moreover, CLT panels have been used in diverse projects, which greatly stimulated further development of this industry. [PERSON] et al. [7] studied the application of CLT panels in shear wall systems and proposed factors that affect its performance. [PERSON] and [PERSON] [8] introduced the unidirectional bending state of CLT panels and proposed a structural optimization scheme. [PERSON] et al. [9] further studied cross-laminated timber panels under bending and compression. Their results showedthat the bending stiffness and ultimate load resisting capacity of CLT panels can be predicted by numerical models.  
CLT panels have numerous advantages, making it attractive as a building material [10; 11; 12]. This includes design flexibility since they can be applied in walls, roofs, ceilings, etc. The thickness of the panels can easily be increased by adding more layers, and the length of the panels can be augmented by joining panels together. Other benefits include eco-friendliness, prefabrication, and thermal insulation. Yet, CLT panels have shortcomings including: (i) most glues are susceptible to environmental exposures, which compromises service life [13]; (ii) large amounts of glue are used, causing environmental pollution [14].  
Therefore, Nail-Cross-Laminated Timber (NCLT) panels are proposed in this study as an alternative. Compared to CLT panels, NCLT panels have numerous benefits including: (i) longer service life; (ii) high yield; (iii) pipelines can be integrated; (iv) superior recovery ability after large deformation; and (v) NCLT panels can be used with many cavities, which requires less raw materials [15]. Several studies have previously shown that steel-CLT composite connection panels have good connection properties and long service life in buildings [15; 16]. The orthogonal staggered laminated staple structure of the glulam-CLT shear connections with double-sided punched metal panel improved the shear load carrying capacity [17; 18]. There are still no detailed experimental studies in the open literature to demonstrate this concept. Moreover, there is no analysis or discussion on the extent to which different nails and other design parameters affect the NCLT panel structural performance.  
Accordingly, the present study reports experimental results of six groups of NCLT panels subjected to four-point load bending. The flexural performance of the NCLT panels was discussed, and the effects of influential parameters including the nail incidence angle, nail type, total number of nails, and number of layers have been explored, discussed, and analyzed. Based on the test results, a modified formula for predicting the flexural bearing capacity of NCLT panels was proposed and proven accurate. This work could blaze the trail for future research on NCTL panels and its introduction in building codes and full-scale construction.