DesignExample2 - 1.054/1.541 Mechanics and Design of...

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1.054/1.541 Mechanics and Design of Concrete Structures Spring 2004 Prof. Oral Buyukozturk 1 / 12 Massachusetts Institute of Technology 1.054/1.541 Mechanics and Design of Concrete Structures (3-0-9) Design Example Failure Investigation of A Prestressed Concrete Bridge Girder Objective : To investigate the failure of a prestressed girder in accordance with ACI 318-02 . Problem : A highway overpass consists of 3 parallel continuous prestressed concrete beams. The length of the overpass structure is 292.8 ft, with a width of 47 ft (Fig. 1 and 2). Each prestressed beam had 5 strands of prestressing steel. There were 22 wires in each strand and each wire had a diameter of 0.6 in. The end of each prestressed beam was supported by a corbel, which was inclined at an angle with respect to the bearing plate (Fig. 3, 4, and 5). Construction proceeded as planned: the beams were cast-in-place, and after the concrete hardened, they were post-tensioned. Minutes after the prestressing operation, 4 out of the 6 corbels broke (Fig. 3). The State Transportation Authority decided to determine the responsible parties involved in this failure case. Task : You are hired to be the expert witness on the case. The following information were established: (a) The reaction force (R) at each end of the beam right before the collapse was estimated at 275 kips. (b) The horizontal restraint offered by the bearing (i.e., the Teflon disk) is negligible. (c) Normal weight concrete was used with the compressive strength of f c = 5000 psi. (d) Yield stress for normal reinforcement was f y = 60 ksi. Using the above information and the attached drawings, you are asked to assess and testify on the following questions: (1) Was the design (Fig. 6) adequate in accordance with ACI code requirements? (2) It was reported that the elastic shortening of the beam due to the initial prestressing was 0.9 in (Fig. 7). Check the design adequacy for this situation. (3) It is postulated that the workmen might have placed the Teflon disk in the wrong position initially. Together with the elastic shortening due to prestressing, the final position of the Teflon disk was as shown in Fig. 8. Check the design again using the ACI code.
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DesignExample2 - 1.054/1.541 Mechanics and Design of...

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