International Concrete Abstracts Portal

Deflection of reinforced concrete (RC) beams is affected by the mechanical properties of concrete, which directly affect the structural stiffness of the element and indirectly define the moment redistribution due to cracking. Therefore, it is important to incorporate uncertainty of the mechanical properties of concrete in deflection calculations for robust prediction of RC deflection. In this study, inherent variations of mechanical properties of concrete are evaluated using the finite element (FE) method. Considering concrete as discrete particles of aggregate and cement paste connected by interfacial transition zone (ITZ), a non-linear representative volume element (RVE) of concrete is developed based on concrete section images. Tension and compression behaviors in concrete are simulated by modeling the cohesive response of ITZ and considering contact mechanics within the RVE. The concrete RVE is validated with a theoretical concrete constitutive model based on compressive strength. The proposed RVE model is then used to describe the constitutive properties of concrete. The mechanical properties of cement paste and ITZ are used as sources of uncertainties in concrete. The homogenization approach allows for considering uncertainties due to concrete microstructure randomness. These uncertainties are reflected in the macro properties of concrete derived from the RVE. The deflection variations of RC beams are then propagated from the variations in macro properties of concrete using Monte Carlo (MC) simulation based on a non-linear FE beam model incorporating cracking and tension stiffening.

The drift of multi-story flat plate buildings due to lateral loads of wind or earthquake affects the serviceability and the strength. The analysis for the drift has to be preceded by designating the components of the lateral force-resisting system (LFRS). Only the shear walls may be designated as the LFRS; alternatively, the LFRS can comprise all monolithic components of the building: shear walls, flat plates, columns and outriggers if any. This alternative is here recommended in the design of wind or earthquake. The basis and justification of this recommendation are given. The presented study employs linear analyses accounting for cracking as permitted by ACI 318. The work may be considered a pilot study; more extensive non-linear time history analyses need to be carried out before proposals for code changes can be made.

This paper is concerned with the effects of high construction loads on long term deflections, and evaluates the ACI 318 deflection control requirements. Practical applications involving two building construction cases are presented. In these cases, the concrete slabs developed extensive cracking and excessive deflections soon after the slab construction and formwork removal. Additional deflections were developed following the installation of interior partitions. The paper also investigates the effects of the shoring/reshoring operations and the construction load history, as well as the effects of improper deflection control design procedures on the slab deflections. The ACI 318 slab deflection control requirements, along with other published proposed methods on deflection control are applied and compared. The conclusions of this study indicate that the ACI’s minimum slab thickness requirements for deflection control are not sufficient since they do not account for the creep and shrinkage effects, as well as the effects of early-age high construction loads, and the resulting initial cracking of the concrete members.

Today’s slab designers face a quandary – economic efficiency demands the use of very thin slabs; however the deflection performance of these slabs is difficult to predict. Exacerbating the situation is that the construction methods and sequences to be utilized are unknown at the time of design. This paper looks at the predictions of several deflection calculation methods and then compares them to actual measured deflections in both the laboratory and field. The variability of deflections and the reasonable range of predictions are also discussed concluding with recommendations for design practice utilizing off-the-shelf software.

The thickness of reinforced concrete slabs is usually governed by the need to limit deflections in service to avoid damage to finishes and partition walls. Deflection control has become more important over recent years due to requirements for longer spans and more economic use of materials as well as design method developments. It is well established that long-term slab deflections can be increased by early age construction loading and cracking induced by restrained shrinkage. The author has previously shown that long-term slab deflections in the European Concrete Building at Cardington were governed by construction loading. This paper examines the influence of short term peaks in loading on long term curvatures in slabs subject to sustained loading. A simplified method is proposed for taking account of the effects of construction loading in deflection calculations with Eurocode 2. The method is validated using laboratory data from tests on simply supported slabs carried out at Imperial College London.