Earthquake Spectra

A methodology for simulating seismic damage of unreinforced masonry buildings for seismic risk assessment of urban areas is presented in this paper. The methodology is based on the Italian vulnerability index and on the results of a post-earthquake damage survey study whose main result was an observed vulnerability function. The Monte Carlo method was then used to simulate damage probability matrices, fragility curves and vulnerability functions, all of which are the basis of a seismic risk study. The simulation process required the generation of thousands of hypothetical buildings, the analysis of their seismic behaviour and probabilistic studies of the computed results. As an example, probable damage scenarios were developed for an urban zone of Barcelona.

Inelastic time history analyses typically indicate that the traditional sub-assembly “capacity” approach used in the design of ductile moment frames grossly underestimates the maximum moments experienced by the columns during a maximum credible earthquake. In addition, these analyses predict that the maximum column demand moments often occur near the mid-height of concrete structures, whereas a conventional elastic analysis predicts maxima at the lowest levels of these structures. Incremental displacement analyses using modal properties and displacements predicted by a maximum credible response spectrum should be used to more accurately predict the maximum anticipated column demand moments in the analysis of existing structures or the design of new structures.

A strong motion database was compiled for California earthquakes of surfacewave magnitudes, M _s ≥ 6, occurring from 1933 through 1992. The database consisted of horizontal peak ground acceleration and 5 percent damped response spectra of accelerograms recorded on four different local geologies: rock (class A), soft rock or stiff soil (class B), medium stiff soil (class C), and soft soil (class D). The results of analyses of the database within each of these site classes were used to derive a set of site-dependent spectral amplification factors for oscillator periods between 0.1 and 4.0 sec and ground acceleration levels between 0.1 and 0.4 g. The amplification factors at 0.3 and 1.0 sec periods (designated as F _a and F _v , respectively) are generally within 20 percent of those recommended during the 1992 Site Response Workshop conducted by the National Center for Earthquake Engineering Research (NCEER). The F _a and F _v values recommended from our study and those from the NCEER workshop are intended for use by code committees making future revisions to the National Earthquake Hazard Reduction Program (NEHRP) seismic provisions and the Uniform Building Code.

The rapid availability of estimated shaking intensities, dollar losses and social impacts following the Northridge earthquake of January 17, 1994, proved to be a valuable resource to the California Office of Emergency Services (OES) and the Federal Emergency Management Agency (FEMA) in response and recovery decision making. These estimates were used to supplement standard reconnaissance procedures and expedite decisions which in previous disasters were delayed until observational assessments had been completed. Based on extensive, in-depth interviews with state and federal emergency managers, this paper will focus on how these estimates were used in making decisions. Methods and models employed in generating estimates will be addressed only briefly. Discussion will include an assessment of the potential benefits and pitfalls of near real-time loss estimation in emergency management.

A number of building authorities have included or are proposing to include loss in lateral capacity of the structural system caused by earthquake damage as a basis for requiring specific degrees of seismic repair and upgrades of the damaged members or of the entire structural system. Attempts have been made to apply this criteria through the size of cracks in reinforced concrete walls. This paper reviews experimental results which demonstrate that size of wall crack is not directly related to a reduction in wall capacity. The effectiveness of various wall crack repair techniques on restoring wall characteristics is discussed.

Evaluating and, if necessary, retrofitting seismically loaded Gravity Load Designed (GLD) infilled frames requires accurate analytical models. This paper gives an overview of experimental and analytical investigations conducted on GLD steel frames infilled with unreinforced concrete block masonry walls. The presented analytical models can reasonably predict the seismic response of such structures. The experimental study adopts both quasi-static and pseudo-dynamic methods. Detailed finite element models and a more practical compression-only strut model for the infills are the analytical tools considered in the present study.

Over 61 years of earthquake performance of steel transmission and distribution supply pipelines operated by the Southern California Gas Company are reviewed. The seismic record includes 11 major earthquakes with M _L ≥ 5.8 and epicenters within the transmission system. An evaluation is made of the most vulnerable types of piping, failure mechanisms, break statistics, threshold seismic intensity to cause failure, and damage induced by permanent ground displacement. The database assembled represents one of the most comprehensive and detailed records of seismic response in a large, complex gas transmission system.

The strong influence of lateral heterogeneities and of source properties on the spatial distribution of ground motion indicates that the traditional methods require an alternative when earthquake records are not available. The computation of broadband synthetic seismograms makes it possible, as required by a realistic modelling, to take source and propagation effects into account, fully utilizing the large amount of geological, geophysical and geotechnical data, already available. For recent earthquakes, where strong motion observations are available, it is possible to validate the modelling by comparing the synthetic seismograms with the experimental records. The realistic modelling of the seismic input has been applied to a first-order seismic zoning of the whole territory of several countries. Even though it falls in the domain of the deterministic approaches, the method is suitable to be used in new integrated procedures which combine probabilistic and deterministic approaches and allow us to minimize the present drawbacks which characterise them when they are considered separately. Detailed modelling of the ground motion for realistic heterogeneous media (up to 10 Hz) can be immediately used in the design of new seismo-resistant constructions and in the reinforcement of existing buildings, without having to wait for a strong earthquake to occur. The discrepancies between the ground responses computed with standard methods and the results of our detailed modelling cannot be ignored when formulating building codes and retrofitting the built environment.

Paper presents an analytical study on the possible application of commonly used retrofit schemes for an existing nine-story apartment building in Mexico City. The building, located in Mexico City's lake-bed region, was originally conceived as a RC waffle-flat slab structural system and was built in the late sixties. The building suffered important structural damage during the March 14, 1979 Petatlán Earthquake, primarily caused by structural pounding with neighboring structures due to its flexibility and resonant response with the ground. The building was retrofitted shortly after the Petatlán Earthquake with steel bracing in the direction of pounding. The retrofitted building survived the September 19, 1985 Michoacán Earthquake with light structural damage. Some evidence of pounding with one neighboring structure was again observed.

Because of the chronic pounding problems with adjacent structures, which are separated eight to fifteen cm from the apartment building, it is felt that an additional upgrade of the structure may be needed for the safety of the building if a strong earthquake similar to the 1985 Michoacán Earthquake may strike Mexico City soon again. Different retrofit schemes were studied, which can be grouped as follows: a) weight reduction, b) column and waffle-slab jacketing, c) addition of energy dissipation devices, d) removal of top floors, e) replacement of diagonal bracing with newer bracing and, f) a combination of previous solutions. The effectiveness of the studied retrofit schemes is discussed by comparing their dynamic structural response with respect to the response of the actual retrofitted structure using nonlinear dynamic analyses for a postulated M _s =8.1 earthquake, as well as construction process, property value, and building use considerations. It is concluded that many aspects need to be evaluated in a seismic retrofit plan for a given building structure and often, the best alternative from the structural viewpoint may not be the best alternative overall.

The influence of tying the walls with steel ties on the seismic behavior of existing brick-masonry houses has been investigated. Three models of simple two-story brick masonry houses with wooden floors, with or without wall ties, have been tested on a simple earthquake simulator. In addition, a model with identical structural configuration, but with r.c. slabs instead of wooden floors, has been tested for comparison. Whereas wooden floors with joists not anchored to the walls did not prevent separation and disintegration of the walls, rigid slabs and steel ties significantly improved seismic behavior. On the basis of test results, a simple method for designing the ties has been proposed.

Friction-Damped Moment-Resisting Frames are discussed as an alternative to steel ductile moment-resisting frames. The design involves replacing all full penetration welds with bolted friction connections, thereby eliminating the problem of failures at welded connections as exhibited during the Northridge earthquake. Energy dissipation is achieved at the friction interface in lieu of inelastic behavior of the frame. Analytical methods that perform an energy accounting are discussed.