Follow along with the video below to see how to install our site as a web app on your home screen.
Note: This feature may not be available in some browsers.
3DDave (Aerospace) said:The I-35W gussets had been known to be buckling for some time and were just being observed. I think the accidents reports indicated they weren't specifically analyzed by the original team who considered the frame members to be the load path which is why no one noticed they were undersized.
waross said:Over tensioning a PT rod is not dangerous.
waross said:I assume that this was an experienced crew. They were sent to work alone without direct supervision.
waross said:Normally a structure is supported by some type of false work until post tensioning is completed.
waross said:We may never know truthfully what the instructions and warnings were given to the crew before the work began.
It is doubtful that the crew was aware of the reduced safety margin.
waross said:It appears that one worker falls from his safety harness. If true, that says something about this workers profesionalism.
waross said:The man who will take shortcuts with important personal protective equipment may also be the man who will overtension a PT rod without a second thought.
waross said:I believe that the root cause of the wye may be a tiny grain of sand or other foreign matter jamming the threads of the PT nut and the following cause was over tensioning the rod in an effort to free the jammed nut.
Ingenuity said:Over stressing PT is a big deal. However, it is somewhat moot in this discussion, because the PT bar did not fail in tension, so therefore it was NOT overstressed.
epoxybot said:FDOT reasonably distances itself from this project because Florida International University received Local Authority Program (LAP) Certification to oversee the project
jgKRI said:Off topic, but I'd love to see the PT procedure for a 1,000+ foot length tendon.
jgKRI said:Is relative strength of the components an active design consideration when designing PT members with relatively thin sections and a low number of tendons?
In other words- for the web members of this concrete truss, they are relatively slender and with a low quantity of PT tendons. We know the level of strain applied to the PT tendons was insufficient to load them to UTS, because they did not fail. But was the level of strain applied to the PT tendons high enough to fail the concrete member in compression? Does that extra 10% applied during the destressing procedure ever run the risk of cracking or otherwise damaging the PT member?
dik said:[That's where I have a problem...
minimum design standards as you have noted... could have been more than AASHTO required, or, may not have been designed in conformance with AASHTO... It should have been, but, just don't know this fundamental information...
Safety factor predicated on AASHTO design... see note above.]
Yes, we do not know what the bridge was designed for - but we know for sure that it collapsed, and based on this fact, we know that there was some major error somewhere leading to the catastrophe. I have seen already another design driven by the appearance, without any understanding of the V-pier design.
And I have seen indiscriminate application of the code, without understanding of the meaning of these factors, and resulting safety of the design.
No code, or design guide are perfect - and there are many variables which should be taken into consideration, like the percentage of the particular loads in the overall design forces. And perhaps the codes should specify the limits, to which particular formulas applies. ("when DL/PL ratio is >2, use DLx1.6" - just example)
Almost anybody, with some basic computer skills, could operate Midas, or Bridge Designer, or Robot program, to name just few. One could model a bridge, and size it without even knowing how the structure works.
And I really hope, that that's not the case, and the cracking moment and shear for the span was checked, and compared with the design loads and minimum sensible safety factor achieved.
And I also hope, that somebody designing another heavy pedestrian bridge is reading this, and will alter a design to achieve minimum 1.7 SF.
Archie264 said:we may have built better in the days of the slide rule. Just my opinion.