Torsion of Exterior Girders of a Steel Girder Bridge During Concrete Deck Placement Loads: Field Test Report

Abstract

This report is the second part of a two part report. The first part is written and developed as a design aid to determine the torsion acting on outside steel bridge girders during concrete deck placement. This second part reports results of measurements taken from two bridges. The first bridge is located at K-10 highway over I-70 between Lawrence and Topeka, Kansas. The second bridge is located on southbound I-635 highway over Swartz Road in Kansas City, Kansas. During bridge construction, deck overhang loads occur on steel plate or rolled beam girders and are supported by cantilever brackets. In addition to supporting the weight of the placement screed, these brackets must also support the weight of the additional construction loads. The vertical loads applied on the deck are eccentric and generate large torsional moments at the intervals between cross bracing. The result of this loading effect is torsional moments that generate a combination of longitudinal stresses and loads from the cantilever brackets. Strain gages were installed on the Swartz Road bridge to measure these overhang loads. A "Multiframe 4D" computer model was made to compare the results measured in the field, with AISC recommendations, and with TAEG ( Torsional Analysis of Exterior Girders) results. The screed loads measured from the static load runs and analytical model were based on the locations of bogey and gang vibrators. In the analytical model, the loads were moved across the beam at quarter points beginning at midspan, then tabulated and plotted alongside the field results. After all of the moments representing the various load cases were compiled, an influence diagram was constructed from the loads measured in the field and the analytical model. Loads were analyzed for two cases using the AISC method outlined in the "Design for Concrete Overhang Loads". The first load case represented the static field test while the second represented the results measured the day of concrete placement. The same wheel loading for the analytical model was used for the AISC calculations. In some instances, the strains measured on the Swartz Road bridge were small. In these situations it can be difficult to guarantee the sensitivity and output of gage readings, however, the major axis moments measured on the Swartz Road bridge during static load testing were almost identical to the moments calculated with the Multiframe analysis. This shows that the loads that were used and how they were distributed in the Multiframe analysis were close to actual field conditions. This also shows consistent. and accurate behavior of the strain gages. No significant differences were found with the moments measured from the static load runs where blocking had been removed. More blocking had been provided than what was needed on the Swartz Road bridge, however, when concrete and live loads are added, the change in load response should be greater. Surveying prisms were used to measure deflections during the load tests. The recorded and predicted maximum vertical deflections on the Swartz Road bridge were consistently ~lose for all load runs. Horizontal deflections were not observed at any location. The Multiframe model used to calculate torsional bending did match closely with the moments measured in the field at midpoint between stiffeners but varied greatly between measured and analytical results for endpoint locations. The computer model used to calculate torsional bending did not match as closely with the moments measured in the field. The difference between measured and analytical results varied for maximum values but was in relative agreement for the trends of the moments. Most of the differences can be attributed to the lateral stiffness provided by a combination of deck formwork and a portion of concrete deck in place in the Northbound lanes. Unfortunately, the loose play of the form work connections to the girder makes the lateral stiffness difficult to measure. Some of the differences in the torsional moments that were calculated using the Multiframe model and the TAEG program can be attributed to some basic model assumptions. The Multiframe analysis was based on a non-prismatic girder section that was continuous over diaphragm locations. The T AEG program assumes a three span, prismatic member. A comparison of torsional moments calculated by T AEG show a large difference in results from field measurements, the torsional model, and AISC calculations. In some cases the differences are small and in others they are significant. For the static field tests and the Multiframe torsion models, the trends show close similarity, however the maximum loads for all locations do vary. The TAEG program was always conservative in comparison to the field results and the multiframe model. Since the T AEG program is intended to be used as an in-house design aid, this conservative approach is regarded as positive.