Flexural Behavior and Strength of Cold-formed Steel L-Headers

Cold-formed steel framing of residential and light commercial buildings continues to grow in popularity due to its structural and material advantages. The North American steel industry is actively performing research studies and developing design standards to assist in the cost-effectiveness of cold-formed steel in these markets. Cold-formed steel L-headers are structural components used over wall openings to transfer the loads to adjacent king studs. Recently, there has been an increased interest in L-headers among homebuilders primarily due to their ease of installation and low material cost. Design of the L-headers in North America is currently governed by the North American Standard for Cold Formed Steel Framing – Header Design, in combination with the North American Specification for Design of Cold Formed Steel Structural Members. However, the design provisions in the AISI - Header Design Standard are particularly limiting. For instance, the method for evaluation of span deflections for both single and double L-headers, and uplift flexural strength for single L-headers is currently not available primarily due to lack of research on the issues. Presented in this thesis are the findings from an extensive laboratory testing program of full-scale single and double cold-formed steel L-headers. The objective of the research was to investigate the structural behavior of L-headers under both gravity and uplift loads. From the analysis, improved ultimate flexural strength design expressions and new vertical deflection expressions for single and double L-header assemblies were developed. The concept of semi-rigid members was introduced to evaluate the flexural behavior and deflection performance of L-header assemblies

Alpine Crossroads or Origin of Genetic Diversity? Comparative Phylogeography of Two Sympatric Microgastropod Species

The Alpine Region, constituting the Alps and the Dinaric Alps, has played a major role in the formation of current patterns of biodiversity either as a contact zone of postglacial expanding lineages or as the origin of genetic diversity. In our study, we tested these hypotheses for two widespread, sympatric microgastropod taxa – Carychium minimum O.F. Müller, 1774 and Carychium tridentatum (Risso, 1826) (Gastropoda, Eupulmonata, Carychiidae) – by using COI sequence data and species potential distribution models analyzed in a statistical phylogeographical framework. Additionally, we examined disjunct transatlantic populations of those taxa from the Azores and North America. In general, both Carychium taxa demonstrate a genetic structure composed of several differentiated haplotype lineages most likely resulting from allopatric diversification in isolated refugial areas during the Pleistocene glacial periods. However, the genetic structure of Carychium minimum is more pronounced, which can be attributed to ecological constraints relating to habitat proximity to permanent bodies of water. For most of the Carychium lineages, the broader Alpine Region was identified as the likely origin of genetic diversity. Several lineages are endemic to the broader Alpine Region whereas a single lineage per species underwent a postglacial expansion to (re)colonize previously unsuitable habitats, e.g. in Northern Europe. The source populations of those expanding lineages can be traced back to the Eastern and Western Alps. Consequently, we identify the Alpine Region as a significant ‘hot-spot’ for the formation of genetic diversity within European Carychium lineages. Passive dispersal via anthropogenic means best explains the presence of transatlantic European Carychium populations on the Azores and in North America. We conclude that passive (anthropogenic) transport could mislead the interpretation of observed phylogeographical patterns in general