Posted by dpendzic on March 19, 2018 at 10:52:03 from (47.18.206.102):
Since many of you seem interested In this concrete bridge failure i will quickly go thru the basic points:
Deign---the span and loading are determined and the appropriate beam is selected--box beam,i beam,t beam,flat slab. Most highway bridges are built to support the maximum semi load of 40 tons per lane The beam chosen is anaylized as to how much prestreesing is needed and the number and location of the cables are placed Most cable patterns are on a 2 inch grid and the CG of the cables is toward the lower part of the beam--so to impart a high compression force in the bottom of the beam and minimal tension at the top.
Fabrication--a flat casting bed is used,either concrete or steel plate,4 to 8 ft wide, and 100 to 300 ft long. forms are made to form the sides of the beam. Rebars,called stirrups, are bent and place inside the forms. The number and spacing of the strands are placed inside the stirrups for the length of the bed,or beam, or multiple beams in a row. the strands are pulled to a calculated force, which was determine on length of beam,shrinkage,creep,strand relaxation,temperature drop,etc which all will reduce the strand tension over time--30,000 lb pull is fairly typical
Concrete of the 28 day required strength, 5000 typical,is pored into the form around the stressed strands. It is covered and stem cured overnight to get to 80% of the 28 day strength and check by breaking concrete test cylinders.
The design engineer has determined a cutting pattern and each strand is cut at each end of each beam simultaneously--even with that its amazing to see a 50 ton beam jump all over the casting bed, The high compression forces in the bottom of the beam and minimal tension on the top makes the beam arch upward in the middle--called camber,
The forms having been stripped already the beam is lifted gently and set in a storage area, covered so it can continue moist curing for 7 days and then not shipped until more concrete test cylinders show the 28 day strength
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