Brittle failures in RC frames designed according to old seismic criteria

Flavia De Luca¹, Gerardo Mario Verderame¹, Gaetano Manfredi¹

¹Università degli Studi di Napoli Federico II

Via Claudio, 21, 80125 Napoli

flavia.deluca@unina.it, verderam@unina.it, gaetano.manfredi@unina.it

KEYWORDS: RC frame, old seismic design, brittle failure, fragility curve.

TOPIC - Q - Sicurezza delle costruzioni esistenti, miglioramento e adeguamento sismico

ABSTRACT

Reliability analysis tool is employed to assess the seismic performance of an RC frame designed according to old seismic criteria.

It seems reasonable to assume that a building designed according to old seismic criteria has better performances than a structure in which seismic forces were not considered at all. On the other hand according to those old design criteria, seismic forces led to an higher percentage of longitudinal reinforcement in columns and at the same time transversal reinforcement in columns used to be not designed since shear design procedure was based on a threshold criterion and seldom shear stresses exceeded the threshold prescribed by old codes (eg. 5MPa according to Italian old code). A simple tool for the fast evaluation of the occurrence of brittle failures in existing RC elements was made up (De Luca et al., 2010) and highlighted how, given the shear span ratio, mechanical reinforcement ratios, both longitudinal and transversal, rules the occurrence of brittle failures. The study of construction drawings of real RC buildings designed according to old seismic codes showed that brittle failures can represent the main weakness of this kind of buildings.

An RC frame is designed according to the old practice employed in seismic zones in Italy between 70s and 80s in order to simulate the typical characteristics of RC buildings already observed in field. Reliability analysis procedure according to which it is possible to account for brittle failures (Jalayer et al. 2007) is employed to evaluate the fragility curves of this RC frame according to two parametric assumptions regarding the transversal reinforcement in columns and considering ordinary and near faults ground motions.

The comparison of the fragility curves in the different cases considered herein helps to clarify how the threshold criterion employed for shear stresses can represent one of the most critical issues in RC building designed according to old seismic criteria.

References

De Luca F., Verderame G.M., Manfredi G., Modelli Di Capacità A Taglio: Codici E Letteratura A Confronto. Atti del 18° Congresso CTE, Brescia, 11 -13 Novembre, 2010.

Jalayer F., Franchin P., Pinto P.E., A scalar damage measure for seismic reliability analysis of RC frames, Earthquake Engineering and Structural Dynamics, 36(2):2059-2079, 2007.