Development of design aids for the design of simply supported post-tensioned PC girder

Abstract

Post-tensioned PC I-girders are widely used in most of small to medium scale projects around the world instead of pretensioned girders because of economy and practicability of construction. Design aids for preliminary selection and design of girders are available for pretensioned girder types. However, it is observed that no such design aids are available for post-tensioned PC girders. This leads to either an inappropriate preliminary selection of girder type or lengthy and time-consuming design calculation procedure at the project planning stage which is often found impractical. In this context, an attempt has been made to develop some reliable design charts for post-tensioned PC I-girder to assist the designers for selecting the girder for different design parameters. The design aids are developed on the basis of AASHTO LRFD Bridge Design Specification, 8th Ed. (2017). Forty-five charts are prepared to aid designers to decide upon selection of post-tensioned PC I-girders based on different design parameters. The charts cover AASHTO I-beams Type III, IV, V, VI and custom Type VII. Design parameters which are covered in the charts are – i) girder span of 50 ~ 180 ft., ii) girder spacing of 6 ~ 12 ft, iii) concrete strength (f_c^') of 5 ~ 9 ksi, and iv) diameter of low relaxation strand (270 ksi) of 0.5 ~ 0.6 in. Comparisons of designs of post-tensioned girders based on the developed charts are compared with the corresponding designs of pretensioned girder. It has been found that prestressing steel requirement is higher for post-tensioned girder. This is due to increase width of the web of post-tensioned girder and some additional types of losses which are generally absent in pretensioned girder. Suggestions have been made to select the optimum girder depth for post-tensioned PC I-girders for a given span.