ISO-SAFETY DESIGN OF FLAT SLABS IN ACCORDANCE WITH EUROCODE 2

This research work focuses on the development of Iso-safety design charts for flexural design of flat slabs at predefined reliability levels in accordance with Eurocode 2 (2004) design criteria. Constitutive models for reinforcing steel and concrete were selected in accordance with the Eurocode 2 design requirements and subsequently the flexural limit state function was derived. Charts were developed for the flexural design of rectangular reinforced concrete sections with respect to the position of neutral axis ( ) for each concrete grade (fck) and steel grade (fyk). Uncertainties in loading and geometrical properties were obtained and a program was developed taken into consideration EC2 design requirements, a safety index β value of 1.81 was achieved for various points on each of the generated curve using First Order Reliability Method (FORM). Reliability-based design charts called Iso-safety charts were produced to target safety indices; βT of 3.3, 3.8 and 4.3 as the minimum recommended for the three failure consequence classes by Eurocode 0 (2002). This recommendation shows that Eurocode 2 design of flat slabs considering flexural failure with safety index value of 1.81 provides designs that are below the recommended target safety indices. A flat slab was there after designed using the charts and was shown that for the same loading and geometrical considerations, the area of flexural reinforcement required increased by 40%, 55% and 75% over Eurocode 2 design for corresponding target safety indices of 3.3, 3.8 and 4.3 respectively. Sensitivity analysis of these provided reinforcements was carried out on other flat slab failure modes and was observed that at low reinforcement ratios punching shear safety is dependent majorly on the effective depths rather than the flexural reinforcement.

Flat-slab system of construction is one in which the beams used in the conventional methods of constructions are done away with. The slab directly rests on the columns and load from the slab is directly transferred to the columns and then to the foundation (Anitha et al., 2007).

The Common practice of design and construction is to support the slabs by beams and support the beams by columns. This may be called beam-slab construction. The beams reduce the available net clear ceiling height. Hence in warehouses, offices and public halls sometimes beams are avoided and slabs are directly supported by columns. This type of construction is also aesthetically appealing. The slabs which are directly supported by columns are called flat slabs.

For many years, it has been assumed in the design of structural systems that all loads and strengths are deterministic. The strength of an element is determined in such a way that it exceeded the load with a certain margin. The ratio between the strength and the load was denoted as the safety factor which is considered as a measure of the reliability of the structure. In codes of practice for structural systems, values for loads, strengths as well as safety factors are prescribed (Sorensen, 2004).

The safety factors are traditionally determined on the basis of experience and engineering judgment. However, in recent codes such as Eurocode 2 partial safety factors are used. Characteristic values of the uncertain loads and resistance are specified and partial safety.