Diaphragm (structural system) | EPFL Graph Search

In structural engineering, a diaphragm is a structural element that transmits lateral loads to the vertical resisting elements of a structure (such as shear walls or frames). Diaphragms are typically horizontal but can be sloped in a gable roof on a wood structure or concrete ramp in a parking garage. The diaphragm forces tend to be transferred to the vertical resisting elements primarily through in-plane shear stress. The most common lateral loads to be resisted are those resulting from wind and earthquake actions, but other lateral loads such as lateral earth pressure or hydrostatic pressure can also be resisted by diaphragm action. The diaphragm of a structure often does double duty as the floor system or roof system in a building, or the deck of a bridge, which simultaneously supports gravity loads. Diaphragms are usually constructed of plywood or oriented strand board in timber construction; metal deck or composite metal deck in steel construction; or a concrete slab in conc

Seismic performance of structures incorporating seismic isolation with lead-rubber bearings

Elias Merhi

The emergence of new high-performance materials and equipment, as well as advancements in numerical calculation techniques, have allowed base isolation to take its place among the strategies used by engineers in earthquake resistant design. Despite the enormous advantages of implementing seismic isolation, its applications have so far been limited to critical infrastructures (hospitals, re stations, etc.) located in earthquake-prone areas. The relatively high costs associated with the installation of an isolation system constitute a major constraint when choosing the structural system of a building. In this Master's project, a 4 storey composite-steel administrative building located in Sion (Switzerland) is designed, according to several importance classes, in order to study the relevance of the installation of a base isolation system. Two dierent design methodologies are adopted to determine the dimensions of the foundations and the main load-bearing elements of two buildings, one with a xed-base and the second with a seismic isolation system. The results of the various design procedures clearly indicate that the choice of installing isolating supports is not obvious. Indeed, when the building under study is considered as a critical infrastructure, signicant savings, up to more than 20% in terms of building materials, can be achieved for an isolated structure. On the other hand, for an ordinary structure, the additional costs associated with the installation of isolated supports cannot be oset by savings in construction materials. These results reinforce the perspective that, in the presence of an ordinary building, the traditional xed-base solution remains the most ecient. However, despite its relatively high cost, a base isolated building performs better during a seismic event, with structural demands and damages signicantly mitigated compared to a xed-base building. To compare the responses of the two structural systems, two non-linear models are created on OpenSEES. Two dynamic analyses are then conducted for two groups of 40 seismic records, calibrated according to two characteristic intensities. Based on the results obtained, it is clear that the xed building undergoes higher demands (in deformations and accelerations in particular) than its isolated counterpart. These demands are then used and transformed into a decision variable (in this case, repair costs) through loss analyses conducted on the EarL software. These analyses show that after the occurrence of an earthquake, the repair costs of a conventional building can reach 25 % of the initial construction costs, almost 10 times the value obtained in the case of a base isolated building. This Master's project demonstrates that despite the high initial cost of base isolation technologies, their implementation can not only be amortized through possible material savings, but also, through the reduction of repair costs due to a better performance of the building over its lifecycle.

2022

Ductility reduction factor formulations for seismic design of RC wall and frame structures

Katrin Beyer, Matteo Zerbin

Seismic design of standard structures is typically founded on a force-based design approach. Over the years this approach has proven robust and easily applicable by design engineers and - in combination with capacity design principles - it provides a good protection against premature structural failures. However, it is also known that the force-based design approach as it is implemented in the current generation of seismic design codes suffers from some shortcomings; among these is the fact that the base shear is computed using a pre-defined force reduction factor, which is constant for a given structural system. Thus, for the same design input, structures of an identical type but different geometry are subjected to varying ductility demands and may perform differently during an earthquake. The objective of this research is to present an alternative formulation for computing force reduction factors for RC wall and frame structures, using simple analytical models which only require input data already available at the beginning of the design process. Such analytical models allow to link global to local ductility demands and therefore to compute an estimate of the force ductility reduction factors that lead to equal local ductility demands and expected damage levels. A series of pushover and nonlinear time history analyses are run on simplified numerical models of a set of wall and frame structures. The results show that the proposed alternative formulation yields a more accurate ductility reduction factor than the current Eurocode 8 design approach.

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Seismic performance of structures incorporating seismic isolation with lead-rubber bearings

Elias Merhi

2022