Stability of pre-tied bridge column steel

[KNOBHEAD2]

We recently had an accident where the 2nd lift of pre-tied rebar on a 12ftx4.5ft bridge column collapsed when a guy cable broke (do not yet know what broke the cable). The pretied column simply bent over (staying tied to the first pour projecting steel) like a limp weiner and stayed completely intact. The DOT, while reviewing the possible cause, has now said that pre-tied cages for columns are unstable because the individual bars are unstable and tie wire cannot be designed or proven to brace the bars (we're talking #9's x 36 LF with #5 bands at 6"). Since our company has been pre-tieing these cages for 60 years, we obviously differ on this subject, but are having trouble finding data to back up the rigidity of tied cages. (I know bumblebees fly, but I can't prove it mathematically) The Engineer who is arguing that cages are unstable has never been in the field. Any thoughts?

 

[ishvaaag]

You surely being in the job you are know that in structuress such slurry walls some inclined bars are included just to control distortion upon placement, so some practice of conforming reinforcement acknowledges some steel just for bracing issues.

Anyway, everything is a matter of degree and with cages so heavy as you are showing us most likely the knot action on deformed rebar shows enough resistance to distortion as to not be any problem most of the times.

How to prove it? Difficult thing in from of someone that is not wanting to hear and is just seeking some alternative practice. Most of the problem is that you can't generalize the kind of interlock actually in place.

For some limited kinds of cages, experiments could prove that: produce them at standard practice and subject them to forces akin to those at placement; but this is precisely the everyday everytime practice gives and the objecting party is not wanting to see, or as the old spanish saying goes

No hay peor ciego que el que no quiere ver.

There's no more blind than who doesn't want to see.

 

[hokie66]

Not much help, but I can't see why a pre-tied cage would be any more unstable than one tied in place. You apparently had recognized the inherent instability of tall cages by using guy wires, which seems a sensible approach. What does the DOT suggest as a better way of making the cages stable?

 

[KNOBHEAD2]

The only thing that DOT would approve was to set the form in place and hold on to the form while a second line lowered the pre-tied cage into the form. At that point, the form is lifted a few feet to allow the cage to be tied to the dowels of the previous pour. This probably is more dangerous than about anything else we could do. DOT thinks that the cage is somehow stable when it is hanging, but unstable when it is tied off to the previous pour's dowels and guyed off. ????

 

[youngstructural]

I don't envy your situation, but I would guarentee you I would not do what you are saying they've suggested. That sounds like an OSH nightmare to me, and just begging for trouble!

You might want to try and make an argument for your procedure based on CP-230. This is a document talking about checking existing structures against new codes, however the principle is the same: You've proven that this technique works through doing it in the field, ergo it has been field tested.

Good luck,

YS

B.Eng (Carleton), P.Eng (Ontario), MIPENZ (Structural-New Zealand)
Working in Canada, and missing my adoptive New Zealand family... at least I brought the little Kiwi with me!

 

[boffintech]

am inspecting on a job right now where nearly the same thing happened: tall heavy column cage set on footing, column cage got torqued from self weight and inability to hold shape and leaned/fell over, dowels from footing bent over.

ishvaaag is on the right track with "some inclined bars are included just to control distortion upon placement" but with the size of your column you'd need some pretty big diagonals.