David B. Garber

Experimental Investigation of Prestress Losses in Full-Scale Bridge Girders

An experimental study was conducted in which 30 full-scale, precast, pretensioned bridge girders were constructed and instrumented with the intention of investigating prestress loss. Several different precast beam fabrication plants were used to investigate the influence of different concrete materials and construction techniques. The constructed girders were conditioned in several different climates for up to 3 years. During this period, prestress loss was measured by using vibrating wire gauges (VWG) embedded in test specimens. Following the conditioning period, the girders were flexural service load-tested to quantify the prestress loss at the time of testing and in turn verify the losses measured using VWGs. Prestress losses were found to be heavily influenced by concrete stiffness, which was dependent on coarse aggregate type and quantity. The measured short- And long-term prestress losses were compared to those determined using several different estimation procedures, suggested by ACI Committee 423.

Prestress loss database for pretensioned concrete members

A comprehensive experimental database containing 237 specimens from 27 different research studies was assembled during a larger research effort undertaken at the University of Texas at Austin to investigate three different short-term loss and four different longterm loss estimation procedures. The assembled database was filtered to eliminate specimens with incomplete information or those not adequately representing those found in the field (as a result of either being too small or overstressed at release). The specimens contained within this filtered database offered a diversity of characteristics well representative of beams found in bridges. This database was used to evaluate several different short-and longterm loss estimation procedures. The results of these evaluations can help to aid the designer in selection of an appropriate procedure for estimating prestress loss.

INVERTED-T BEAMS: EXPERIMENTS AND STRUT-AND-TIE MODELING

Contrary to rectangular deep beams, inverted-T beams are loaded on a ledge at the bottom chord of the beam. This loading configuration induces a tension field into the web and the resulting complex strain distribution renders sectional design provisions inadequate. The applicability of strut-and-tie modeling (STM), developed for rectangular deep beams and simpler, two-dimensional designs, was evaluated. An experimental study was conducted in which 33 tests were performed on 22 large-scale reinforced concrete inverted-T beams and the effects of the following variables were investigated: ledge geometry, quantity of web reinforcement, number of point loads, member depth, and shear span-depth ratio. It was concluded that strut-and-tie modeling, although developed for much simpler structural components, offers a simple and accurate design method for the more complex strain distributions in inverted-T beams. The STM provisions developed for rectangular beams accurately captured both failure mode and ultimate capacity and are recommended for use in inverted-T beam design, as a major conclusion of this research.

Simplified Element-Based Model to Estimate Strain-Related Prestress Loss in Pretensioned Simply Supported Bridge Girders

While models to study prestress losses based on cylinder data are valuable for their flexibility and ability to investigate micro-phenomena, calibration of models based on girder data is also valuable for some applications. More specifically, data obtained from the monitoring of girders captures the effects of physical phenomena taking place within a girder section, effects that are not accurately captured with cylinder testing (e.g. irregular sectional geometry and variable stress history). While general methods are aimed at a broad range of cases, it is intuitive that less complexity is necessary in a method aimed to a singular type of structure. The development of such a method, for use on pretensioned, simply-supported girders is briefly summarized herein. This model provides simple numerical estimates for strain-related prestress losses, and retains the transparency of basic creep and shrinkage models. The model was calibrated using full-scale structural test data.