Risk-Informed Selection of Steel Connections for Seismic Zones

The fi ndings about the fragile behavior of steel welded connections after the Northridge 1994 earthquake, specially for frames designed to withstand lateral force, has brought an amount of new attention to the design and safety issues of the welded connections for structures located on seismic zones. In México, practitioners and designers are wondering about the seismic e¿¿ ectiveness of the several kinds of connections as used in steel structures. A decision must be made to balance the safety required with the costs incurred after exceeding the serviceability limit state. Structural reliability techniques provide the proper framework to include the inherent uncertainties into the design process. Registered motions after the 1985 Mexico City earthquake are properly scaled according to the seismic hazard curve for soft soil in Mexico City. Earthquake occurrence is modeled as a Poisson process and the expected life-cycle cost is taken as the decision criteria. Parametric analyses allow the identifi cation of dominant variables and ranges where one option is more recommendable than the other one. The proposed formulation may support designers and builders for the decision making process about the selection of the convenient connection type for the seismic zones with soft soil in Mexico City

Selección de conexiones de acero para zonas sísmicas con base en criterios de confiabilidad

Los hallazgos encontrados a raíz del terremoto de Northridge en 1994 han atraído actualmente la atención de los especialistas, sobre todo en cuanto a los problemas de diseño y seguridad de las conexiones soldadas para estructuras ubicadas en zonas sísmicas. Para equilibrar la seguridad requerida deben tomarse decisiones en relación con los costos para prevenir la posibilidad de excederse algún estado límite de servicio. Las técnicas de confiabilidad estructural proporcionan el marco adecuado para incluir las incertidumbres inherentes en el proceso de diseño. La formulación propuesta permite apoyar a diseñadores y a constructores para la selección del tipo de conexión más conveniente para las zonas sísmicas.Los hallazgos encontrados a raíz del terremoto de Northridge en 1994 han atraído actualmente la atención de los especialistas, sobre todo en cuanto a los problemas de diseño y seguridad de las conexiones soldadas para estructuras ubicadas en zonas sísmicas. Para equilibrar la seguridad requerida deben tomarse decisiones en relación con los costos para prevenir la posibilidad de excederse algún estado límite de servicio. Las técnicas de conÀabilidad estructural proporcionan el marco adecuado para incluir las incertidumbres inherentes en el proceso de diseño. La formulación propuesta permite apoyar a diseñadores y a constructores para la selección del tipo de conexión más conveniente para las zonas sísmicas

Evaluation of the Response of Posttensioned Steel Frames with Energy Dissipators Using Equivalent Single-Degree-of-Freedom Systems

The hysteretic energy (EH) dissipated in posttensioned steel frames (PTSF) with hysteretic dampers is calculated by using equivalent single-degree-of-freedom systems (ESDOFS), where the nonlinearity of both the steel bars (beam and columns) and the connections of the structural frame is separately considered. Five multi-degree-of-freedom (MDOFS) PTSF and their corresponding ESDOFS are studied under the action of 30 seismic motions recorded in soft ground, scaled in terms of the spectral pseudoacceleration evaluated at the fundamental period of the structures. Several simple mathematical expressions are proposed. The first expression is useful to calculate modifying factors (FMEH) to relate EH of the ESDOFS with that of the MDOFS; the second is to determine the interstory drift (γ) as a function of the seismic intensity. The third equation is to calculate the factor of the relative participation of the energy that the connections dissipate with respect to the total energy EH and the fourth equation, which is function of the γ, is to obtain the distribution factors of EH through the height of the structure. The methodology proposed can be used for the design or the structural revision of PTSF with dampers

Maximum inter-story drift demands of steel frames in terms of the intensity measure INp

In the present work, a new equation to predict the maximum inter-story drift demands of mid-rise steel framed buildings is proposed in terms of a new ground motion intensity measures based on the spectral shape. For this aim, the maxim\um inter-story drift of steel frames with 4, 6, 8 and 10 stories subjected to several narrow-band ground motions is estimated as a function of the spectral acceleration at first mode of vibration SaT1, which is commonly used in earthquake engineering and seismology, and with a new parameter related to the structural response known as INp. It is observed that the spectral-shape-based intensity measure INp is the parameter best related with the structural response of the selected steel frames under narrow-band motions. For this reason, an equation to compute the maximum inter-story drift demand of mid-rise steel frames as a function of INp is proposed. The equation is useful for the rapid seismic assessment

Reglas de combinación de los efectos de las tres componentes de terremotos y respuesta crítica

La precisión de las reglas de combinación del 30% y SRSS, y la orientación crítica de las componentes de terremotos se estudian en este trabajo. Se analizan modelos estructurales complejos que representan edificios de baja y mediana altura. Se realiza un análisis estadístico de la precisión de las reglas de combinación aplicadas a parámetros de respuesta individuales y múltiples. El efecto de la correlación entre las componentes de los terremotos en dicha precisión también se estudia. Finalmente, se realizan análisis con varios ángulos de incidencia de las componentes con la finalidad de encontrar la orientación crítica. Los resultados muestran que las reglas subestiman la carga axial en columnas, pero sobreestiman razonablemente los cortantes de entrepiso. Ambas reglas son más conservadoras cuando los modelos se excitan por las tres componentes. Los efectos individuales pueden estar altamente correlacionados, incluso para componentes principales no correlacionadas. Las reglas no siempre son precisas para valores pequeños de coeficientes de correlación, y valores altos de éstos no siempre están relacionados a una estimación imprecisa de la respuesta combinada. La precisión de las reglas de combinación depende del grado de correlación de las componentes, del parámetro de respuesta, de la localización del elemento considerado y del nivel de deformación estructural

Improving the structural reliability of steel frames using posttensioned connections

In this paper, various moment-resisting steel frames (MRSFs) are subjected to 30 narrow-band motions scaled at different ground motion intensity levels in terms of spectral acceleration at first mode of vibration in order to perform incremental dynamic analysis for peak and residual interstory drift demands. The results are used to compute the structural reliability of the steel frames by means of hazard curves for peak and residual drifts. It is observed that the structures exceed the threshold residual drift of 0.5%, which is perceptible to human occupants, and it could lead to human discomfort according to recent investigations. For this reason, posttensioned connections (PTCs) are incorporated into the steel frames in order to improve the structural reliability. The results suggest that the annual rate of exceedance of peak and residual interstory drift demands are reduced with the use of PTC. Thus, the structural reliability of the steel frames with PTC is superior to that of the MRSFs. In particular, the residual drift demands tend to be smaller when PTCs are incorporated in the steel structures.Peer ReviewedPostprint (published version

Ductility and Strength Reduction Factors for Degrading Structures Considering Cumulative Damage

The effect of cumulative damage on the strength requirements of degrading structures is assessed through the evaluation of the target ductility and corresponding strength reduction factors of simple degrading structures. While the reduction on ductility is established through the use of Park and Ang index, the suggestions given by Bojórquez and Rivera are used to model the degradation of the structural properties of the simple systems. Target ductilities and their corresponding reduced strength reduction factors are established for five sets of ground motions; most of them are recorded in California. The results given in this paper provide insight into all relevant parameters that should be considered during seismic design of earthquake-resistant structures. Finally, some recommendations to evaluate the effect of cumulative damage on seismic design are suggested

Prediction of Inelastic Response Spectra Using Artificial Neural Networks

Several studies have been oriented to develop methodologies for estimating inelastic response of structures; however, the estimation of inelastic seismic response spectra requires complex analyses, in such a way that traditional methods can hardly get an acceptable error. In this paper, an Artificial Neural Network (ANN) model is presented as an alternative to estimate inelastic response spectra for earthquake ground motion records. The moment magnitude (MW), fault mechanism (FM), Joyner-Boore distance (dJB), shear-wave velocity (Vs30), fundamental period of the structure (T1), and the maximum ductility (μu) were selected as inputs of the ANN model. Fifty earthquake ground motions taken from the NGA database and recorded at sites with different types of soils are used during the training phase of the Feedforward Multilayer Perceptron model. The Backpropagation algorithm was selected to train the network. The ANN results present an acceptable concordance with the real seismic response spectra preserving the spectral shape between the actual and the estimated spectra

Probabilistic seismic response transformation factors between SDOF and MDOF systems using artificial neural networks

An approach to obtain with acceptable accuracy probabilistic response transformation factors by training an artificial neural network (ANN) model is presented. The transformation factors are defined as the ratio of the seismic response of multi-degree-of-freedom structures and their equivalent single-degree-of-freedom systems, associated with a given annual exceedance rate. The approach is used for predicting the seismic response of steel framed buildings. Equations useful to obtain probabilistic response transformation factors for maximum ductility and inter-story drift, as functions of their mean annual rate of exceedance, and of the fundamental vibration period of the structure, are proposed. It is shown that artificial neural networks are a useful tool for reliability-based seismic design procedures of framed buildings and for the improvement toward the next generation of earthquake design methodologies based on structural reliability

Reduction of Maximum and Residual Drifts on Posttensioned Steel Frames with Semirigid Connections

The aim of this paper is to study the seismic performance of self-centering moment-resisting steel frames with posttensioned connections taking into account nonlinear material behavior, for better understanding of the advantages of this type of structural system. Further, the seismic performance of traditional structures with rigid connections is compared with the corresponding equivalent posttensioned structures with semirigid connections. Nonlinear time history analyses are developed for both types of structural systems to obtain the maximum and the residual interstory drifts. Thirty long-duration narrow-banded earthquake ground motions recorded on soft soil sites of Mexico City are used for the analyses. It is concluded that the structural response of steel buildings with posttensioned connections subjected to intense earthquake ground motions is reduced compared with the seismic response of traditional buildings with welded connections. Moreover, residual interstory drift demands are considerably reduced for the system with posttensioned connections, which is important to avoid the demolition of the buildings after an earthquake