Evaluación del coeficiente de disipación de energía, r, para algunos tipos de estructuras de acero
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Valencia Restrepo, Doralba
Valencia Clement, Gabriel F.
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2008
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En el presente artículo se cuestiona la validez del uso del coeficiente de disipación de energía tabulado en las Nor-mas Colombianas de Diseño y Construcción Sismorresistente, NSR-98, en la evaluación de las solicitaciones gene-radas por la acción del sismo de diseño en las estructuras de acero y las inconsistencias que se presentan al dimen-sionar la estructura para que cumpla derivas del 1%. Con tal fin se diseñan 45 pórticos del sistema de resistencia sísmica de cinco edificios: 15 pórticos resistentes a momento (PRM), 15 pórticos arriostrados concéntricamente (PAC) y 15 pórticos arriostrados excéntricamente (PAE). El diseño se realiza de acuerdo con los requisitos de las NSR-98 en cuanto a solicitaciones (evaluación de cargas y combinaciones) y rigidez (1% de deriva) y a los requisitos de las provisiones sísmicas de AISC-2005 en cuanto a resistencia. Se evalúa el coeficiente de disipación de energía de los 45 pórticos para diferentes niveles de desempeño, por medio de plastificación progresiva modal, encontrán-dose que tal coeficiente no es constante para ninguno de los tres sistemas estructurales estudiados (PRM, PAC y PAE), tal como lo sugieren las NSR-98, y además los valores del coeficiente de disipación de energía hallados en la presente investigación son mucho menores a los tabulados en dichas normas, que rigen la práctica común del dise-ño estructural, provocando graves errores en la evaluación de las solicitaciones de diseño, tanto de la estructura co-mo de los elementos de soporte (placas de base, fundaciones, pantallas) y estructuras adosadas a los pórticos del sistema de resistencia sísmico.
Response modification factor (R), tabulated in the Colombian Design Code as NSR-98, is used in this paper for eva-luating internal member forces produced by design earthquake action on steel structures and the inconsistencies pre-sent when designing structures when 1% drift limits must be complied with. The article presents the design of 45 frames corresponding to the seismic resistance system of 5 buildings: 15 special moment frames (SMF), 15 special concentrically-braced frames (CBF) and 15 eccentrically-braced frames (EBF). External loads and their combination were used in estimating internal loads and rigidity demands (1% drift) were evaluated in line with NSR-98 requi-rements. Member strength requirements were evaluated by using the AISC-2005 seismic provisions for steel structu-red buildings. Modal pushover analysis was used for evaluating the response modification factor for the 45 given frames at different structural performance levels. It was found that this factor was not constant for any of the three structural systems (SMF, CBF and EBF) suggested by NSR-98 and that the values of the response modification factor found in the present investigation were smaller than those tabulated in this design code governing everyday structural design. This would lead to significant errors being made in evaluating design forces, not only in the structures but in the support elements (base-plates, foundations, shear walls) and any structures attached to buildings constructed in line with the seismic resistance system.
Response modification factor (R), tabulated in the Colombian Design Code as NSR-98, is used in this paper for eva-luating internal member forces produced by design earthquake action on steel structures and the inconsistencies pre-sent when designing structures when 1% drift limits must be complied with. The article presents the design of 45 frames corresponding to the seismic resistance system of 5 buildings: 15 special moment frames (SMF), 15 special concentrically-braced frames (CBF) and 15 eccentrically-braced frames (EBF). External loads and their combination were used in estimating internal loads and rigidity demands (1% drift) were evaluated in line with NSR-98 requi-rements. Member strength requirements were evaluated by using the AISC-2005 seismic provisions for steel structu-red buildings. Modal pushover analysis was used for evaluating the response modification factor for the 45 given frames at different structural performance levels. It was found that this factor was not constant for any of the three structural systems (SMF, CBF and EBF) suggested by NSR-98 and that the values of the response modification factor found in the present investigation were smaller than those tabulated in this design code governing everyday structural design. This would lead to significant errors being made in evaluating design forces, not only in the structures but in the support elements (base-plates, foundations, shear walls) and any structures attached to buildings constructed in line with the seismic resistance system.