International Concrete Abstracts Portal

Recently in Mexico and other Latin-American Countries, reinforced concrete (RC) buildings with precast floor systems have been widely used, although such structural types are not yet included in the design codes. In order to understand the behavior of this type of building, the response of a RC building model to seismic loads was investigated by testing two full scale structural models. The building models were subjected to lateral aud torsional displacements. The models simulated the first story of a four-story building, and consisted of three dimensional one-story moment resisting single-span frames. The model was designed to assess the seismic performance of ductile frame structures following the Mexico City Building Code. The variable in the tests was the type of floor structural system: (a) the first model (CR model) was cast in-situ beam-slabs structure; and, (b) the second model (PCR model) had the same structural system, but with precast elements composing the floor structure. Both models were subjected to the same loading history and had similar instrumentation. This paper presents the results and conclusions obtained from the comparative study of the behavior of both models. One of the main conclbsions is that the presence of precast elements at the floor system was found to be an important parameter for the torsional stiffness of the whole structure.

The effect of varying cement source fresh and hardened concrete properties is studied under hot weather conditions. For the seven ASTM Type 11 cements studied here, the same mix proportioning was adopted at a mixing temperature of 95 F (35 C) with a constant dosage of water reductiag and air entraining admixtures. Properties of fresh concrete including slump loss over an extended mixing period (EMP) of 90 minutes, air content, and setting times are reported. Also, hardened properties including compressive strength development and rapid chloride permeability test data are reported. Results indicate that the rate of slump loss and setting times are affected by the cement compound composition, calcium sulfate content aad calcium sulfate type. The compressive strength, under hot mixing conditions, is found to be dependent on composition, fineness and morphology of cement compunds.

This paper presents strut and tie models that can be used to determine strength and deformation limits for RC members. Conventional strut and tie models have been used for strength design, but they cannot appropriately address the ductility of members that is important to insure safety under severe cyclic loads. This proposed strut and tie model is constituted to address the end region of flexural members, including plastic hinge regions, and will be the basis for the calculation of deformation and corresponding shear strength as well. The behavior of each component of the strut and tie model is evaluated based on the material properties and stress paths. The elongation of the tie element depending on crack spacing and width is obtained from the bond-slip relationship. The contraction of strut element is simply determined from the properties of concrete. The member deformation is obtained by combining the truss deformation determined from components behaviors with joint rotation, The strut and tie model in this paper will provide useful tools for both the design and evaluation of ductility-required RC members.

An ongoing study investigating the response of a reinforced concrete building rehabifitated after the 1985 Me&% City earthquake is presented. As part of this program, the behavior under cyclic loads of reinforced concrete columns rehabilitated with steel jackets made of angles and straps is king experimentally assessed. First results on the response of the budding and on the behavior of rehabilitated columns are shown. So far, Building BL has responded elastically, with no damage under recent events. It is apparent that BL response follows the soil fundamental frequency. A first look at the behavior of steel jackets made of angles and straps Wugh two column units, indicates that strength and energy dissipation capacities can be improved, especially in undamaged columns.

An exploratory study of the effect of foundation rocking on the shake table response of a quarter-scale two-column bridge bent was conducted. Several innovative details and designs were used in the columns and the cap beam but the footing represented typical substandard and retrofitted spread footings. The structure was tested in three modes: rocking with as-built footings, rocking with retrofitted footings, and fixed base. The foundation flexibility was modeled in the tests using elastomeric bearing pads that were designed to represent the stiffness of spread footings supported on medium soil. The rocking tests were conducted in the elastic range, but the fixed-base model was tested to failure. The Sylmar record of the 1994 Northridge earthquake was simulated on the shake table. The data showed that for the same input earthquake intensity the maximum steel bar strain near the base of the columns in an unretrofitted rocking footing was only 7 percent of the strain in the fixed kame. When the footing was retrofitted but was still allowed to rock, the maximum steel strain increased but was still only 16 percent of the strain in the fixed kame. The retrofit design implications of the results are discussed.