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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
Controlled interfacial assembly of 2D curved colloidal crystals and jammed shells
Assembly of colloidal particles on fluid interfaces is a promising technique
for synthesizing two-dimensional micro-crystalline materials useful in fields
as diverse as biomedicine1, materials science2, mineral flotation3 and food
processing4. Current approaches rely on bulk emulsification methods, require
further chemical and thermal treatments, and are restrictive with respect to
the materials employed5-9. The development of methods that exploit the great
potential of interfacial assembly for producing tailored materials have been
hampered by the lack of understanding of the assembly process. Here we report a
microfluidic method that allows direct visualization and understanding of the
dynamics of colloidal crystal growth on curved interfaces. The crystals are
periodically ejected to form stable jammed shells, which we refer to as
colloidal armour. We propose that the energetic barriers to interfacial crystal
growth and organization can be overcome by targeted delivery of colloidal
particles through hydrodynamic flows. Our method allows an unprecedented degree
of control over armour composition, size and stability.Comment: 18 pages, 5 figure
Direct Visualization of Dislocation Dynamics in Grain Boundary Scars
Mesoscale objects with unusual structural features may serve as the analogues
of atoms in the design of larger-scale materials with novel optical, electronic
or mechanical behaviour. In this paper we investigate the structural features
and the equilibrium dynamics of micron-scale spherical crystals formed by
polystyrene particles adsorbed on the surface of a spherical water droplet. The
ground state of sufficiently large crystals possesses finite-length grain
boundaries (scars). We determine the elastic response of the crystal by
measuring single-particle diffusion and quantify the fluctuations of individual
dislocations about their equilibrium positions within a scar determining the
dislocation spring constants. We observe rapid dislocation glide with
fluctuations over the barriers separating one local Peierls minimum from the
next and rather weak binding of dislocations to their associated scars. The
long-distance (renormalised) dislocation diffusion glide constant is extracted
directly from the experimental data and is found to be moderately faster than
single particle diffusion. We are also able to determine the parameters of the
Peierls potential induced by the underlying crystalline lattice.Comment: 11 pages, 4 figures, pdf forma
Novel Pickering Emulsifiers Based on pH-Responsive Poly(2-(diethylamino)ethyl methacrylate) Latexes
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 PuÂ(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 PuÂ(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 PuÂ(IV) colloids in water of 1.85 Ă 10<sup>â19</sup> 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 PuÂ(IV) colloids under typical groundwater aquifer conditions led
to two contradicting hypotheses: (a) the PuÂ(IV) colloids will exhibit
significant aggregation and deposition, leading to a negligible subsurface
transport or (b) the PuÂ(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