Preflex beams: A method of calculation of creep and shrinkage effects

This paper presents a method of calculation of creep and shrinkage effects for composite beams. It is particularly applicable to Preflex and Flexstress beams, which are composed of a steel I-girder with the bottom flange encased by concrete. The concrete is prestressed by predeflection of the steel beam and the subsequent release after hardening of the concrete flange or by means of prestressing cables. The presented approach using concrete age-adjusted modular ratios allows the calculation of time-dependent stresses in the concrete flange due to creep and shrinkage, with sufficient accuracy for practical applications and without carrying out cumbersome numerical computations. The results can be extended directly to the analysis of ordinary steel-concrete composite beams. The main goal of the present paper is the calibration of the parameters which must be introduced to simplify the equations describing the system. This calibration is discussed and its sensitivity to some calculation inputs is presented. The conclusions are very encouraging and the simplified approach seems to agree very well with the results of the numerical approach.