Equivalent Force Control Method for Real-time Testing of Nonlinear Structures

The equivalent force control (EFC) method replaces numerical iteration with a feedback control strategy to solve the nonlinear equations of motion using an implicit integration method for real-time substructure tests (RSTs). The method, however, requires the conversion of the equivalent forces to structural displacements using a conversion matrix. It is demonstrated in this article that with the use of a proportional-integral (PI) controller for the EFC, one has the convenience of choosing the initial stiffness matrix of a structure to construct the conversion matrix regardless of the properties and degree of nonlinearity of the system. The stability condition of the EFC using a PI controller has been derived with the Routh stability criterion. Methods for designing and tuning a PI controller for RST using EFC have been presented and excellent system performance has been obtained from numerical simulations and actual tests. The simulation results showed that the EFC method using a PI controller and the initial stiffness matrix to construct the conversion matrix can deliver excellent performance even for structural systems that develop a severe strain-softening behavior. Its superiority over iteration method proposed by Jung et al. [2007] Jung, R. Y., Shing, P. B., Stauffer, E. and Thoen, B. 2007. Performance of a real-time pseudo dynamic test system considering nonlinear structural response. Earthquake Engineering and Structural Dynamics, 36: 1785–1809.  [Google Scholar] was demonstrated through numerical simulation. This provides an efficient means to test nonlinear multiple-degrees-of-freedom structures.     

Development and evaluation of geochemical methods for the sourcing of archaeological maize

Strontium (Sr)-isotope values on bone from deer mice pairs from 12 field sites in the Chaco Canyon area, New Mexico, were compared with isotope values of synthetic soil waters from the same fields. The data indicate that mice obtain Sr from near-surface sources and that soil samples collected at depths ranging from 25 to 95 cm contain Sr that is more accessible to the deep roots of maize; thus, synthetic soil solutions provide better data for the sourcing of archaeological maize. However, the Sr-isotope composition of mice may be more valuable in sourcing archaeological remains of animals such as rabbit, turkey, and deer.     

Diversification,Intensification and Specialization: Changing Land Use in Western Africa from 1800 BC to AD 1500

Journal of World Prehistory - Many societal and environmental changes occurred between the 2nd millennium BC and the middle of the 2nd millennium AD in western Africa. Key amongst these were...     

The cleaning of burned and contaminated archaeological maize prior to Sr/Sr analysis

Accurate trace-metal and strontium-isotope analyses of archaeological corn cobs require that metal contaminants be removed prior to chemical analysis. Archaeological cobs are often coated with construction debris, dust, or soil which contains mineral particles. In addition, most archaeological cobs are partially or completely burned and the burned parts incorporate mineral debris in their hardened residual structures. Unburned cobs are weak ion exchangers and most metals within a cob are not firmly bound to cob organic matter; therefore, immersing cobs in acids and rinsing them in deionized water to remove mineral contaminants may result in the undesirable loss of metals, including strontium, from the cob.     

Garu Kime: A Late Borno Fired-Brick Site at Monguno,NE Nigeria

Abstract  

This paper primarily presents analyses from geophysical data and archaeological remains collected at one of a few known sites to the west of Lake Chad associated with fired-brick structures. It reports on previous fieldwork at the location, considers its alleged relationship with the early history of the Borno kingdom and then turns to present new data that provide fresh insights into the physical appearance, dating, material remains, economy and environment of the site. A brief discussion of the historical purpose of the fired-brick structures and the site itself concludes the paper.     

Who provided maize to Chaco Canyon after the mid-12th-century drought?

Between A.D. 1181 and 1200, in the early part of a climatically wet period, corn was imported to Chaco Canyon from a region outside the Chaco Halo (defined in this paper as the region between the base of the Chuska Mountains and Raton Wells). Strontium-isotope (⁸⁷Sr/⁸⁶Sr) analyses of 12 corn cobs dating to this period match ⁸⁷Sr/⁸⁶Sr ratios from five potential source areas, including: the Zuni region, the Mesa Verde-McElmo Dome area, the Totah, the Defiance Plateau, and Lobo Mesa. The latter two areas were eliminated from consideration as possible sources of corn in that they appear to have been unpopulated during the time period of interest. Therefore, it appears that the corn cobs were imported from the Zuni region, the Mesa Verde-McElmo Dome area, or the Totah area during a time when the climate was relatively wet and when a surplus of corn was produced in regions outside Chaco Canyon. Based on proximity to and cultural affiliation with Chaco Canyon, it is hypothesized that the corn probably was imported from the Totah.