Seismic behavior of interior connections in existing waffle-flat-plate structures

This paper investigates the seismic behavior of interior connections in reinforced concrete waffle-flat-plate structures subjected to moderate gravity-load levels like those acting during an earthquake. The scope is the existing buildings designed and detailed according to construction practices used over 10 years ago in the moderate-seismicity Mediterranean area. The results are useful for both assessing the seismic vulnerability of older structures and orienting seismic retrofit alternatives. To this end, a 3/5 scale test model representative of an interior waffle flat plate–column connection in a prototype six-story building was subjected to gravity and cyclic loading until failure. The experimental data are discussed in conjunction with the results of previous tests and ACI 318-05 code idealizations. The specimen exhibited a “strong column–weak plate” type of flexural-yielding mechanism. The test results indicate that: (a) the effective slab width and the fraction of unbalanced moment transferred by flexure, γfγf, that ACI 318-05 code prescribes in Section 13.5.3.2. are adequate for predicting the strength of this type of connection, while the variation of γfγf permitted by ACI 318-05 code in Section 13.5.3.3 is not recommended; (b) the lateral drift at the first yielding of the longitudinal bars is 3 times higher than the 0.5% admitted by Eurocode 8 to satisfy the “damage limitation requirement”; (c) the ultimate drift ratio (4.4%), associated with a strength reduction of more than 20%, is approximately at the average of the values reported for flat slabs with shear reinforcement in previous studies; (d) the ultimate energy dissipation capacity of the connection is about one-fourth of the minimum value reported for well-behaving reinforced-concrete elements with flexural behavior.