Structural reliability of light-frame wood systems with composite action and load sharing

In a majority of light-frame wood buildings, studs and joists are the basic structural components in walls and floors. The walls act as bending and compressive panels and transmit lateral wind and gravity loads to the foundation. Joists are used in floor systems, and together with the sheathing member act as an orthotropic plate in supporting live and dead loads. Mechanical fasteners form most of the joints and provide semi-rigid connection between the framing members and the sheathing. Current design methods do not incorporate the system behavior within the wall or floor, that is, both the composite action of the framing with the sheathing and load sharing between the framing members. Yet, system strength and stiffness rely on structural interaction. This study of light-frame wall and floor systems introduces a probability-based evaluation, including the interaction of components as well as the nonlinearities of materials and connectors. Load sharing in the wall and floor systems was modeled by a series of elastic springs with the sheathing as a distributor beam. Stochastic distributions were used to represent certain properties and loadings. Reliability analyses of wall systems under bending and compressive loads were conducted. It was found that composite action and load sharing contribute to a reduction in failure probability. Reliability studies verified the hypothesis that wall systems are highly reliable and can sustain loads exceeding those expected under 50-year and 100-year wind loads. Reliability levels were examined for floor systems in a bending limit state with 16-in. and 24-in, joist spacing, under a Type I extreme value distribution for live load. Floor system capacity was sensitive to the variability of the lumber modulus of rupture. The degree of load sharing was grade dependent; No.1 joists exhibited higher load sharing than No.2 joists as the coefficient of variation in strength of No.2 joists was much higher than No.1 joists. Based on these results, the 15% increase in the allowable bending stress for repetitive light-frame members as specified by the National Design Specification for Wood Construction appears to be conservative

Dispersion Stability and Electrokinetic Properties of Intrinsic Plutonium Colloids: Implications for Subsurface Transport

Subsurface transport of plutonium (Pu) may be facilitated by the formation of intrinsic Pu colloids. While this colloid-facilitated transport is largely governed by the electrokinetic properties and dispersion stability (resistance to aggregation) of the colloids, reported experimental data is scarce. Here, we quantify the dependence of ζ-potential of intrinsic Pu­(IV) colloids on pH and their aggregation rate on ionic strength. Results indicate an isoelectric point of pH 8.6 and a critical coagulation concentration of 0.1 M of 1:1 electrolyte at pH 11.4. The ζ-potential/pH dependence of the Pu­(IV) colloids is similar to that of goethite and hematite colloids. Colloid interaction energy calculations using these values reveal an effective Hamaker constant of the intrinsic Pu­(IV) colloids in water of 1.85 × 10^–19 J, corresponding to a relative permittivity of 6.21 and refractive index of 2.33, in agreement with first principles calculations. This relatively high Hamaker constant combined with the positive charge of Pu­(IV) colloids under typical groundwater aquifer conditions led to two contradicting hypotheses: (a) the Pu­(IV) colloids will exhibit significant aggregation and deposition, leading to a negligible subsurface transport or (b) the Pu­(IV) colloids will associate with the relatively stable native groundwater colloids, leading to a considerable subsurface transport. Packed column transport experiments supported the second hypothesis