PREDICTING THE STRENGTH OF WOOD-JOIST FLOORS.
Abstract : This study was intended to explore the relationships between strengths at first rupture predicted by linear and nonlinear models for wood-joist floors of a given configuration but having various combinations of joist material properties. The study produced two ways of altering linear models to account for the nonlinear behavior of nails in floors loaded to failure. The strength at first joist failure of 72 floors was analyzed using linear and nonlinear finite-element models and statistical comparisons. The results of the study are presented in two forms. The first is recommended factors by which the predicted failure loads from the linear analysis can be scaled to give failure loads consistent with the nonlinear analysis. The second is recommended substitute nail stiffness which can be used in a linear analysis to predict floor responses with near-equivalent accuracy as provided by a nonlinear analysis. The results presented are specific to the floor design used in this study, but similar techniques could be used to derive equivalent information for other floor designs.
Structural Implications of Increased Panel Use in Wood-Frame Buildings
To improve performance and lower cost, conventional wood-frame construction has evolved to incorporate increasing numbers of manufactured components. Through recent work at the Massachusetts Institute of Technology (MIT) to develop a panelized roof system, other, more general observations have been made about the structural implications of panel use, and these observations form the basis for this paper. A partial exploration of structural response in conventional wood-frame construction is included as background, with a particular emphasis on sheathing. Panelized and modularized buildings represent a significant shift from historical practice, with structural implications for floors, walls, and roofs. Without the structural redundancy and load sharing that continuous envelope sheathing provides, panelized wood-frame buildings require more and careful engineering to achieve performance comparable to conventional construction. Floors, walls, and roofs need panel-to-panel joints that resist differential transverse deflection and possess in-plane shear strength and stiffness comparable to site-build, staggered-sheathed assemblies. Wherever structural response includes diaphragm action, the disruption of sheathing and perimeter-chord continuity both have consequences for panelized building systems.
Pyrite multiple-sulfur isotope evidence for rapid expansion and contraction of the early Paleoproterozoic seawater sulfate reservoir
Abstract Earthʼs oxygenation is often described in terms of two unidirectional steps at the beginning and end of the Proterozoic Eon, separated by a long-lived intermediate redox state. Recent work defines a more complicated path to oxygenation, exemplified by an apparent drop in oxidation state following the early Paleoproterozoic Lomagundi carbon isotope excursion. The timing of this proposed drop in oxidation state is not well constrained, and it is not clear how it relates to redox conditions during the remainder of the Proterozoic. Here we present a study of pyrite multiple-sulfur isotopes, supported by Fe speciation and organic carbon isotopes, from early Paleoproterozoic black shales. We find evidence for the rapid expansion of the seawater sulfate reservoir during the Great Oxidation Event at ca. 2.3 Ga followed by a subsequent contraction in the size of the seawater sulfate reservoir at ca. 2.05 Ga. This scenario is consistent with the emerging view of a rise and fall in surface oxidation state during the early Paleoproterozoic. Comparison of our new data to other records of the seawater sulfate reservoir suggests that the elevated sulfate concentrations that characterize the early Paleoproterozoic did not return until the late Neoproterozoic.