Analysis on the diaphragm design as one storey stiffening method against lateral loads

A certain building typology, the one storey steel framed buildings with purlins as a secondary structure, is a popular solution for industrial and storage buildings. In order to avoid stability problems in the portal frames, a proper stiffening of the frames and purlins is necessary. The diaphragm design is one of the available methods to stiffen buildings globally against lateral loads, as wind or small cranes. The diaphragm design uses the roof as a deep plate girder to transmit the external forces on the side of the buildings to the stiffened portals, normally placed at the end of the building, reducing the horizontal load in the frames and its side displacement. This transmission is done by means of two processes: loading at shear the corrugated metal sheets and compressing and tensioning the purlins of the secondary structure. A typical metal diaphragm includes several corrugated steel sheets, fastened to each other and to the secondary structure of the building. This dissertation covers three big issues in order to evaluate the suitability of this method: An introduction to the calculation methods for corrugated sheets in bending, a needed step to understand the behaviour and limitations of the corrugated metal sheets. On a second step, the whole theory of Bryan/Davies is developed, which is the tool to calculate the strength and flexibility of a diaphragm in a shear field. There is a detailed explanation of the formulation presented in the ECCS recommendation. A comparison with the Schardt/Strehl theory is also developed. The second objective was to point the limits of the diaphragm capacities. A whole set of calculations with different metal sheets, purlins spacing, buildings dimensions, fastening patterns and fasteners has been performed to give the brighter scope possible on all the conceivable configurations. A detailed analysis on the data obtained has been performed. Two objectives were pursued: set limits and reference values for flexibility and strength of the shear cells and identify which are the relevant factors and its influence in the design. The third part of the dissertation analyses the interaction between the diaphragm and the building. A procedure, in order to calculate the effects of the external loads in the building, was done for two different configurations: symmetric rigid frames in both ends or just one rigid frame in one of the building’s ends. Resume tables are given for both cases, so that the designer is able to calculate on a given case the load share on the diaphragm and frames, and its deformation. The load distribution is controlled by one parameter : the relative flexibility between the diaphragm and the portals. With the results of the previous analysis a range of maximum bays on the building and maximum external loads is proposed. Analisis del efecto diafragma como arriostramiento de naves mononivel frente a cargas laterales Autor: David Fabregas Cabrera Tutor: Alfredo Arnedo Pena