The seismic force-resisting system of a building structure is composed of vertical elements, horizontal elements, and the foundation (Fig.1.1). The vertical elements extend between the foundation and the elevated levels, providing a continuous load path to transmit gravity and seismic forces from the upper levels to the foundation. The horizontal elements typically consist of diaphragms, including collectors. Diaphragms transmit inertial forces from the floor system to the vertical elements of the seismic force-resisting system. They also tie the vertical elements together and thereby stabilize and transmit forces among these elements as may be required during earthquake shaking. Diaphragms are thus an essential part of the seismic force-resisting system and require design attention by the structural engineer to ensure the structural system performs adequately during earthquake shaking. The horizontal elements can consist of truss elements or horizontal diagonal bracing and diaphragms are constructed as essentially solid, planar elements made of wood, steel, concrete or combinations of these.
Fig.1.1 Isometric view of a basic building structural system comprising diaphragms, walls and frames, and foundation 1.1
Diaphragms serve multiple roles to resist gravity and lateral forces in buildings (Fig.1.2). They include: •
Resist gravity loads.
Provide lateral support to vertical elements against buckling, and tie these together so that they complete the three-dimensional framework to resist lateral loads. •
Resist out-of-plane forces developed by the exterior walls and cladding as a building responds to an earthquake, and also by the wind pressure acting on exposed wall surfaces. •
Resist large horizontal thrust from inclined columns, acting within the plane of the diaphragms due to gravity and overturning actions. •
Transfer lateral inertial forces to vertical elements of the seismic force-resisting system. •
Transfer lateral shears through the diaphragm one vertical element of the seismic force-resisting system to another (especially at discontinuities in the vertical elements). •
Support soil loads below grade – For buildings with subterranean levels, soil pressure bears against the basement walls out-of-plane. The basement walls span between diaphragms, producing compressive reaction forces at the edge of the diaphragms.
Fig.1.2 Roles of diaphragms
Collectors are tension and compression elements that gather (collect) shear forces from diaphragms and deliver the force to vertical elements. Collectors also deliver forces from vertical elements into the diaphragm as shown in Fig.1.3. This type of collector, referred to as a distributor, is required where forces are redistributed among vertical elements. Collectors can be in the form of beams or a zone of reinforcement within a slab such as shown in Fig.1.4. Wide sections of slabs used as collectors are referred to as distributed collectors.
Fig. 1.3 Distributor
Fig.1.4 Collectors (Plan)
In some cases, tension and compression collectors can be fit within the width of the wall, in which case all the tension and compression force is transferred into the wall at the wall boundary. In this case, only uniform diaphragm shear is transferred through shear-friction to the side of the wall. In other cases, the collector has to be wider than the width of the wall, so only part of the collector force is transferred directly into the wall boundary, with the rest being transferred through shear-friction to the side of the wall. Significant force transfer often occurs at ground level slabs over one or more basements. At these slabs, forces are distributed out of vertical elements such as shear walls and transferred through the diaphragm to the basement walls. The flexibility of the diaphragm will greatly reduce the force that is distributed out of the walls, thus reducing the backstay effect. It will also reduce...
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