Introduction Earthquakes

The analysis of structures that resist wind loads could it be similar to earthquakes loads, because both are mainly lateral loads. However, deep differences are present in their design.

Design for combined Gravity and wind load is based in verifies that loads are less than capacity of structural elements, trying to keep an elastic behavior. However, design of structures for combined gravity and earthquake is based in deformations and damage rather than only strength. Inelastic behavior is expected, and the design allows reach less strength than an elastic analysis. Moreover, design lead to ductile structures and a design by displacements instead of strength.

Ductility can be defined as the ratio between dmax and the drift expected. The ductility will keep constant in an Elastic behavior. Because the inelastic behavior is complex, then it is assumed that ductility of structures are defined always in the elastic range.

Due to the ground accelerations, then inertia forces appear in the structure, which distribution depends of mass distribution through the structure. Magnitude of the response is depending of the dynamic properties of the structure. However, the drift between floors could generate plastic hinges, which modify the structure stiffness, thus the dynamic response of the whole structure is modified.

Capacity Design principle leads to ensure enough dissipation of energy under large earthquakes, and this system has to be reliable for the duration of the Earthquake. Capacity design ensures not collapse of the structure. Therefore CD assume that establishing strength hierarchies within the structure and detailing weak zones to respond in a ductile way, then the structure will remain stable during a large earthquake.

• Selection of suitable structure for inelastic response, i.e. regular structure
• Selection of suitable location for inelastic deformations.
• Insurance, that inelastic behavior does not occur in undesirable locations.

Prescriptive Criteria

A new approach is moving from prevent collapse to controlled damage, because of the huge costs involved in repairing or rebuild structures, which will increase dramatically.

A design based in performance can be stated as how much damage can be resisted by a given building in a given level of earthquake. This depends of the performance that we want for a certain building. We have to link the performance of a building for a certain level of earthquake. To control the damage, we have to understand that displacements are more important than just keep a strength performance. Topics like limit the drift between floors, controlled deflection to reduce damage of non-structural elements who cannot resist vertical loads has to be analyzed.

Performance Design, procedure: Select performance objectives, Develop preliminary design, Asses this against performance objectives. The performance objectives can be presented in the matrix as follow: