Estimating the Error in Calculated Deflections of HPC Slabs: A Parametric Study Using the Theory of Error Propagation

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Title: Estimating the Error in Calculated Deflections of HPC Slabs: A Parametric Study Using the Theory of Error Propagation

Author(s): M. M. Reda Taha and M. A. Hassanain

Publication: Special Publication

Volume: 210

Issue:

Appears on pages(s): 65-92

Keywords: deflection; modulus of rupture; serviceability; slab; theory of error propagation

Date: 2/1/2003

Abstract:
Although limit state design blends both serviceability and strength limit states, most engineers tend to be less confident in serviceability limit states than in strength limit states, especially when deflections of reinforced concrete slabs are considered. A major source of the lack of confidence is the existence of many uncertain variables in calculating slab deflections of reinforced concrete slabs are considered. A major souce of this lack of confidence is the existence of many uncertain variables in calculating slab deflections such as concrete properties (e.g., modulus of elasticity, modulus of rupture), creep coefficient, curing regime and duration, and the existence of construction loads. The absence of any reliability ocefficients in deflection calculations or deflection limits gives the impression that engineers are expected to evaluate the exact deflection that will take place on site. To make matters worse, the introduction of high-performance concrete (HPC) has increased the uncertainty about concrete properties. While HPC enhances the overall material's performance, it is usually reported to have higher shrinkage strains, and it is more susceptible to plastic shrinkage than normal-strength concrete (NSC). The possible reduction of curing periods from the recommended one week to two or three days due to tight construction schedules can result in substantial microcracking which would significantly reduce the concrete modulus of rupture. Therefore, serviceability performance is dependent on many inter-related factors that the engineer cannot control in the design assumptions. Unless the band of errors in deflection calculations is known to the engineer, the lack of confidence in deflection claculations will always be there. This paper describes a mathematical model utilizing the theory of error propagation to predict the error in the calculated deflections of simply-supported, one-way reinforced concrete slabs. Parametric studies have been carried out to examine the effect of changing the concrete properties as a result of changed site conditions on the accuracy of the estimated deflections.