LATERAL LOAD RESISTANCE BEHAVIOR OF WOOD-PLASTIC-TO METAL SINGLE-BOLT CONNECTIONS IN OUTDOOR FURNITURE

The lateral load resistance behavior of an unconstrained, two-member, single-bolt connection in outdoor furniture applications was investigated. The unconstrained connection consisted of a wood-plastic composite (WPC) main member fastened to a metal plate as a side member through a 6.35-mm-diameter bolt without a nut or washer used. Experimental results indicated that unconstrained WPC-to-metal single-bolt connections had a significantly higher lateral resistance load if the WPC main member is loaded in the direction perpendicular to the WPC material extrusion direction than the parallel direction. Tested connections failed with bolts having one plastic hinge bent, which occurred at the interface between the metal plate and WPC main member, accompanied by the WPC main members having a compressive yield fracture at their sides close to the metal plate, but no obvious compressive mark was observed at the opposite sides. Proposed linear and yield mechanical models were verified experimentally as a valid means for deriving estimation equations of lateral resistance loads of unconstrained WPC-to-metal single-bolt connections

Dominant Species in Subtropical Forests Could Decrease Photosynthetic N Allocation to Carboxylation and Bioenergetics and Enhance Leaf Construction Costs during Forest Succession

It is important to understand how eco-physiological characteristics shift in forests when elucidating the mechanisms underlying species replacement and the process of succession and stabilization. In this study, the dominant species at three typical successional stages (early-, mid-, and late-succession) in the subtropical forests of China were selected. At each stage, we compared the leaf construction costs (CC), payback time (PBT), leaf area based N content (NA), maximum CO2 assimilation rate (Pmax), specific leaf area (SLA), photosynthetic nitrogen use efficiency (PNUE), and leaf N allocated to carboxylation (NC), and to bioenergetics (NB). The relationships between these leaf functional traits were also determined. The results showed that the early-succession forest is characterized with significantly lower leaf CC, PBT, NA, but higher Pmax, SLA, PNUE, NC, and NB, in relation to the late-succession forest. From the early- to the late-succession forests, the relationship between Pmax and leaf CC strengthened, whereas the relationships between NB, NC, PNUE, and leaf CC weakened. Thus, the dominant species are able to decrease the allocation of the photosynthetic N fraction to carboxylation and bioenergetics during forest succession. The shift in these leaf functional traits and their linkages might represent a fundamental physiological mechanism that occurs during forest succession and stabilization

CHARACTERISTICS OF SCREWDRIVING TORQUES IN WOOD-PLASTIC COMPOSITES

The characteristics of torque magnitudes for driving screws into wood-plastic composites (WPCs) were investigated through evaluating effects of different factors on seating torque (SET) and stripping torque (STT) commonly used to characterize the process of driving screws into amaterial. The factors were embedded screw orientation in WPCs, pilot-hole diameter, screw penetration depth, and screwdriver air pressure. Recorded torque-time curves of driving screws into WPCs evaluated in this study indicated that the complete screw driving process can be described as a three-phase process of thread forming and screw seating, clamping, and screw stripping WPCs. Mean SET values for driving screws into WPCs can range from 0.47 to 1.83 N-m, STT values from 1.54 to 4.87 N-m, and their corresponding STT-to-SET ratios from 1.0 to 4.4. Statistical analyses indicated that mean SET and STT values of driving screws into WPCs increased as screw penetration depth increased from 12.7 to 25.4 in increments of 6.35mm. The significance of pilot-hole diameter effects on mean SET and STT values is influenced by screw penetration depth. Mean SET values at 0.45 MPa air pressure level were higher than those at 0.62MPa, whereas the significance of screwdriver air pressure effects on mean STT values is influenced by pilot-hole diameter and also screw penetration depth. The significance of embedded screw orientation effects on mean SET values is influenced by screw penetration depth and pilot-hole diameter and on mean STT values by screw penetration depth, pilot-hole diameter, and screwdriver air pressure

Distribution and Source Apportionment of Polycyclic Aromatic Hydrocarbons (PAHs) in Forest Soils from Urban to Rural Areas in the Pearl River Delta of Southern China

The upper layer of forest soils (0–20 cm depth) were collected from urban, suburban, and rural areas in the Pearl River Delta of Southern China to estimate the distribution and the possible sources of polycyclic aromatic hydrocarbons (PAHs). Total concentrations of PAHs in the forest soils decreased significantly along the urban–suburban–rural gradient, indicating the influence of anthropogenic emissions on the PAH distribution in forest soils. High and low molecular weight PAHs dominated in the urban and rural forest soils, respectively, implying the difference in emission sources between the areas. The values of PAH isomeric diagnostic ratios indicated that forest soil PAHs were mainly originated from traffic emissions, mixed sources and coal/wood combustion in the urban, suburban and rural areas, respectively. Principal component analysis revealed that traffic emissions, coal burning and residential biomass combustion were the three primary contributors to forest soil PAHs in the Pearl River Delta. Long range transportation of PAHs via atmosphere from urban area might also impact the PAHs distribution in the forest soils of rural area