Meatloaf,
I think what e-s is trying to tell you is that you are probably confusing “post tensioning” with “pre-stressed” concrete beams, slabs or other structural elements.
What you see going down the highway are pre-stressed elements in most cases. They have been cured with an accelerated method, the internal cables and reinforcing were tensioned as the concrete was placed in the factory forms.
With Post Tensioned concrete slabs and other structural members, the tension is applied after the concrete is placed and begun its curing process to a certain point in its design strength curve.
In the former, any intentional camber is often formed into the member, and in the later it is possible for the tensioning itself to add some camber to the element I believe.
To add a little more confusion, there are two types of cables being discussed.
One is post tensioning cables with a slab or element section. The other types of cables notably absent are the large suspension cables swung from the central structural tower to structural connection point on the bridge box section that tie into the sloping “X” shapes between the lower deck and the upper structure that suspend the bridge in the “cable stayed”. If a tower was built and a full half, one side, was suspended by cables, it would obviously impose a great moment load on the tower pulling it over to one side.
Editor-Surveyor, if that was not your point, you are welcome to correct me.
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Exactly.
Post tensioning is what lifts the bridge off of the falsework and allows its removal. All camber is achieved after passing the design test targets.
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Regardless of the confusion, something whether rebar or cables allowed the lower structure of the concrete bridge to be moved to be placed on the piers. I suspect we understand the lower part of the bridge below what would have been the neutral axis cannot be allowed to be subjected to a tensile load. I believe that something happened after the bridge was in place that created a tensile loading leading to the collapse.