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Rigid bolted cold-formed steel connections have been very popular for their better distribution of straining actions and for better full-filling serviceability requirements. However, the behavior of every single element in the connection as well as the interaction between them is not clear due to its complex behavior. A nonlinear finite element parametric study was conducted on the apex knee connection of a portal frame constructed with cold-formed back-to-back double-lipped C sections for both the rafter and the column. These back-to-back lipped C sections, connected by gusset plates are used as beam and column members. Only the webs of lipped C sections are connected to the connection gusset plates with bolts. The section flanges are not connected to the columns for ease of construction. The study focused on investigating the effect of different parameters on the ultimate capacity and the failure mode of the eave connection. Studied parameters include different bolt configurations, bolt diameters, gusset plate thickness, the bolt pre-tensioning force, and the edge distance. The effect of bolt bearing on the C-section web was considered in the calculation of the capacity of the connection. Five modes of failure were identified namely; lateral out-of-plane buckling of the gusset plate, local buckling of the connected section, bolt bearing of C-section web and the gusset plate, tearing of C-section web. It was observed that bearing failure is the dominant mode of failure. However, in some cases, the lateral-out-of-plane buckling of the gusset plate takes place and causes the premature failure of the connections at low values of applied loads. The local failure of the connected sections occurs at the ultimate loading phase. Several observations were found and some useful recommendations for optimum connection configuration are suggested. A simple interaction design procedure for special configuration to calculate the resistance of the joint connection under moment, shear, and normal forces based on the bolt bearing failure for different bolt pitches is proposed.

RESUMEN: Tradicionalmente, los puentes pórtico o los marcos de hormigón armado, y en general las estructuras de este material, se han calculado a estado límite último (ULS) despreciando cualquier resistencia a tracción del hormigón, dimensionando las secciones del lado de la seguridad. De unos años a esta parte, han ido apareciendo numerosos estudios sobre el posible aprovechamiento de la resistencia a tracción del hormigón aun cuando está fisurado produciendo relevantes cambios en el cálculo a estado límite de servicio (SLS) y numerosas normativas incorporaron formulación acerca de ello. El objetivo de este estudio no es otro que evaluar y cuantificar ese margen que se está despreciando cuando no tenemos en cuenta la colaboración del hormigón tras la fisuración, comparando 4 secciones (con la misma geometría, solo modificando la ley tensión deformación del hormigón) por caso: sección bruta, sección donde se desprecia por completo la resistencia a tracción del hormigón, sección donde se tiene en cuenta la rama lineal hasta alcanzar la resistencia a tracción y por último, sección con la colaboración del hormigón a tracción teniendo en cuenta el fenómeno de estudio: el concepto de “tension stiffening” (TS). De este modo se podrían detectar posibles amenazas en estructuras existentes debido a un dimensionamiento inadecuado o conseguir ahorrar material en zonas sobredimensionadas para proyectos venideros. Para afrontarlo, se analizan una serie de casos y se revisa la amplia bibliografía en cuanto a la modelización de la fisuración en el hormigón, por ser éste altamente no lineal en su comportamiento a tracción. El análisis se lleva a cabo mediante un software de código libre de la Universidad Técnica Checa de Praga llamado OOFEM en el que se comprueba la influencia de distintos parámetros en la respuesta de la estructura frente a una carga puntual en centro de luz del dintel del pórtico. En los modelos de análisis se tiene en cuenta la colaboración del hormigón a tracción entre fisuras conocida como tension stiffening, fenómeno de estudio en este trabajo fin de máster. Como conclusiones, y a modo de resumen se obtiene que el no tener en cuenta la colaboración del hormigón a tracción en un cálculo en servicio, produce una sobreestimación de las flechas de hasta en un 300% y una rigidización del dintel de tal forma que los negativos aumentan en un 65% y el positivo se reduce en un 12%, por lo que la integridad de la estructura puede verse gravemente afectada debido a un mal armado en el nudo al tiempo que podría producirse un ahorro de material en la zona de centro de vano. Las longitudes de negativo y positivo también se ven alteradas, y pueden cambiar hasta en un 15 % (del total de la longitud del dintel), implicando ciertos cambios en la disposición de la armadura y su anclaje. Además, la colaboración del hormigón ayuda al acero, descargándolo, de tal forma que la tensión se reduce en más de un 50% para la cuantía del 2%. Acerca de la utilización del programa OOFEM, se puede concluir que la discretización de la sección en capas y el material que tiene en cuenta la no linealidad de la fisuración del hormigón devuelven resultados coherentes y competitivos pero deben ser contrastados con otro software comercial. Al ser un software de código libre, presenta algunos fallos de código que debe ser depurado al tiempo que su ausencia de interfaz gráfica en la entrada y salida de datos provoca una demora del tiempo de análisis que le resta atractivo. Como líneas futuras de investigación se propone el contrastar los resultados de OOFEM con una serie de campañas experimentales además de con otro software de elementos finitos más comercial. También una posible comparación con lo recogido en la norma española acerca del fenómeno de la colaboración a tracción sería interesante. ABSTRACT: Traditionally, the concrete frames and in general, the concrete structures have been designed neglecting the tensile strength of concrete . However, since 70s, several studies have emerged introducing the tensile behaviour of reinforced concrete in terms of load-deflection and a few years later, some design codes came up with a set of recommendations for service load level. The main scope of this document is to evaluate the difference in terms of bending moment, stresses and deflections when tensile contribution of concrete after cracking is taken into account or it is totally or partially neglected. For that purpose, 4 different sections per each case are designed (keeping the same cross section, only modifying the constitutive law of concrete): gross section, cracked section but concrete has zero tensile strength resistance, cracked section but we account for the linear branch of tensile strength and cracked section considering tension stiffening (TS). In this way, some threatens of existent structures (designed with old codes or neglecting the tensile behaviour of concrete) could be noticed or substantial quantities of steel could be saved for future projects. In order to achieve that, along this document, a series of cases are analysed in terms of concrete cracking, due to the nonlinear response of this material when it is submitted to tension forces. Also, a large review of the existent bibliography is carried out as a State of Art. The analysis is done using a free-code software called OOFEM, developed at the Czech Technical University of Prague, in which the influence of several parameters on the structural behaviour are checked. In these models, the concrete colaboration between cracks is taken into account by modifying the constitutive law of concrete in tension. In this way, the tension stiffening effect, which is the matter of study of this Master’s degree in Civil Engineering thesis, is considered. As some remarkable conclusions, when the steel ratio is analysed, neglecting the tensile contribution of concrete produces an overestimation of the deflection up to 300% at the same time that a global stiffening is achieved with a 65% more of negative bending moment and 12% less of positive moment. This situation could lead to a fatal failure near to the joint because there is not enough reinforcement, combined with a small saving of steel at midspan. Also, in terms of reinforcement lenght and anchorage variations up to 15 % (of the total beam length) are reached. About the use of the OOFEM software, the layered model and the material model taking into account the capacity of concrete of carrying tensile stresses after cracking give competitive results but they should be checked with some other commercial and more extended softwares. The fact of being a free-code, noncommercial software, without a graphic interface for input or output and some bugs makes it a tough tool with high analysis times that tarnishes the many possibilities that it offers as the parametrization. As future lines of research, it is proposed to contrast the OOFEM results with a series of experimental campaigns as well as with other commercial finite element software. A possible comparison with the Spanish standard on the phenomenon of the tensile collaboration of concrete would also be interesting Máster en Ingeniería de Caminos, Canales y Puertos

The construction industry is a big contributor to greenhouse gas emissions, which has a negative environmental impact. Several studies have highlighted the possibility of using timber to reduce the environmental impact of construction. Most of these studies have focused on residential buildings, but little attention has been devoted to industrial buildings. In this paper, an attempt is made to compare the environmental impact of using timber, steel, and reinforced concrete in industrial buildings using life cycle assessment. The system boundary was set to cradle-to-gate with transportation to construction site due to the limitation of data, and only the quantities of the main structural system are considered. Portal frames with variable spans were designed using the three materials to meet similar load carrying capacity. Reinforced concrete was used in the foundation of all frames. The results of the comparative study show that timber has, by a good margin, better environmental impact than reinforced concrete and steel, due to the carbon stored in the wood. The results also show that reinforced concrete and steel alternatives have similar environmental impacts. The findings of this study agree with the findings of other studies on residential buildings.

The modern use of plated structures means that the cross-sectional and lengthwise profiles for a given member may be optimized by welding tapered plates together. The first figure of this chapter shows two ways in which a web-tapered member may be manufactured: the first is by welding two flanges to a tapered web plate, while the second shows a diagonal cut through the web of a hot-rolled I-section member, the rotation of the relative portions of the cut member, and its rewelding to produce a tapered profile. The concept of efficient steel design with tapered members is not new, and nowadays web-tapered members of this type find widespread contemporary use in steel portal-frame structures, since the deeper flanged portions may be utilized in the high moment regions (at the portal frame eaves) to economize the use of steel. However, because members with welded plates are relatively slender, their strength is often governed by lateral (or flexural–torsional) buckling.