Full Scale Testing of Overhang Falsework Hangers on NCDOT Modified Bulb Tee (MBT) Girders

Abstract

Today's bridges are being constructed with longer spans and higher girder spacing. As a result, the use of precast prestressed concrete modified bulb tee (MBT) girders has significantly increased. The MBT girders have a wider and thinner top flange than other conventional precast concrete cross-sections. The strength of the thin top flange has been identified as a concern for the North Carolina Department Of Transportation when edge of flange falsework hangers are used to support the overhang deck slab falsework system. In response to theses concerns, the NCDOT has funded research project 2005-18 "Full Scale Testing of Overhang Falsework Hangers on NCDOT Modified Bulb Tee (MBT) Girders" to study the behavior of the edge of flange falsework hanger systems. Standard falsework hangers manufactured by Dayton/Richmond and Meadow/Burke were tested as a part of this research.

This research consists of two parts, the experimental and analytical investigations. The effects of amount of the shear reinforcement, the effect of the adjacent loaded hangers and the type of hanger were investigated in the experimental investigation. The finite element models were generated to predict the behaviors of the falsework system such as the limit state, ultimate load, and the response of the system.

From the experimental result, the ultimate load of both hanger types, Dayton/Richmond and Meadow/Burke did not reach the ultimate strength provided by manufacturer. The Dayton/Richmond and Meadow/Burke hangers failed approximately 75% and 55% of the manufacturer ultimate strength by the spalling of the concrete under the hanger and punching shear failure. There were effects of the shear reinforcement and the adjacent loaded hanger observed.

From the results of finite element models, the initial stiffness of the applied hanger load and deflection at mid-span of the model was higher than the experimental results. The models predicted failure by flexural cracking and punching shear for the Dayton/Richmond and Meadow/Burke hangers respectively. When multiple hangers were loaded, the ultimate loads decreased for the Meadow/Burke model. A series of small finite element models were used to gain the better prediction of the local behavior at the hanger. The small finite element models were able to capture the initial stiffness of both hanger types, but only the mode of failure of the Meadow/Burke hanger was captured.

It was concluded that the shear reinforcement, number of loaded hanger and the type of hanger affect the ultimate strength and behavior of the structure. The ultimate load of the Dayton/Richmond hanger test is higher than the one from the Meadow/Burke hanger test. However, the observed ultimate loads were less than the manufacturer's ultimate load. The failure of hanger head for Dayton/Richmond hanger occurs because the rotation of hanger head. The bearing failure of Dayton/Richmond hanger test causes the excessive rotation of hanger head.

It was recommended that the safe working load of the Dayton/Richmond and Meadow/Burke falsework hanger embedded on the NCDOT modified bulb tee (MBT) girder be reduced, and the use of different type of overhang hanger system such as through flange hanger appears to be necessary to reach the 6,000 lb. safe working load.