Column Rebar Cage Overturning calculation
Column Rebar Cage Overturning calculation
(OP)
we have to do a field check for overturning on a 25' tall rebar cage with 9'-6" column dowels extending from the footer. The force required to overturn the column is 775 lbs but I need to check what force it would take to make the column rotate over the 9'-6" dowel. The Cage weight is 5536 lbs, and is 42" wide. Can anyone give me an easy check on how to calculate this force?






RE: Column Rebar Cage Overturning calculation
RE: Column Rebar Cage Overturning calculation
RE: Column Rebar Cage Overturning calculation
One of the big assumptions in this analysis is that the longitudinal bars act compositely - not a great assumption to make if the cage is made with standard tying practices. Granted the cage is 3.5' wide and only 25' tall, but you still need to verify the assumption.
RE: Column Rebar Cage Overturning calculation
As for the dowel embedment into the footings, that sounds like an Appendix D check to me unless your able to transfer the dowel tension to some other rebar nearby. Never fun.
WinelandV sounds as though he's actually done this kind of thing before. I have not. Weight our recommendations accordingly.
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Rebar Cage Overturning calculation
I agree with everything you're saying - this isn't something that you do a cursory check on. However, I do understand the desire to not guy a 25' tall column (especially if there's a forest of them, and the 9' tall dowels exist). It's likely, however, that the analysis will show the need to guy the rebar anyway. Either way, all the issues being looked at in your link still need to be addressed.
Awesome presentation, btw - lots of great info. Going to save that in my stash of info.
RE: Column Rebar Cage Overturning calculation
Koot's linked presentation is the result of Caltrans-funded research by Dr. Itani and his students. They did thorough experimental work and the findings correlate well to field findings.
RE: Column Rebar Cage Overturning calculation
http://www.asce.org/Product.aspx?ID=2147487569&...
WinelandV is on the right track, if you want to try and make the cage stand without external support. There are going to be two big problems though. One, is the connection between the dowels and the cage -- a tie wire connection isn't going to do it. Also, ASCE's guide only allows you to cantilever a cage with these proportions if you have a mechanical connector of some sort that's rated for the appropriate load. And the second problem is that the wind can blow from any direction, and you can't count on a typical cage to act compositely without a lot of internal bracing, so you end up needing a mechanical connector on every couple of bars -- which gets really expensive.
Now, with a 25' by 3.5' cage, you probably don't need to use guys -- pipe braces (like for precast construction) on deadmen are almost certainly easier, and probably about the same cost. I currently have a half dozen cages of similar proportions braced up this way.
And as a final note, I want to stand fully behind what TXStructural is saying.. Rebar cages aren't to be treated lightly. Once you figure them out, they aren't too technically challenging -- but a lot of the implicit assumptions that we use in other structural analysis just don't apply. People do die from these. That's not hyperbole.
Lo
RE: Column Rebar Cage Overturning calculation
The greatest trick that bond stress ever pulled was convincing the world it didn't exist.
RE: Column Rebar Cage Overturning calculation
RE: Column Rebar Cage Overturning calculation
I have seen cages that were embedded collapse above the embedded length because the longitudinal bars were not stiff enough on their own. I have studied cases where cages that seemed stable became unstable very quickly under lateral loading, such as being bumped by formwork being moved into position or sudden winds. In one case last year, cages (shorter than 25') seemed OK when they left the job on Friday and several were on the ground on Monday morning.
The linked article appeared in a February 2012 Ironworker Magazine. Note that even the ironworkers recognize that they are not qualified to assess cage stability and safety without engineering support, and those guys do it all day, every day.
Quote: "Guying and bracing systems must be designed by a qualified person"
https://www.impact-net.org/uploads/docs/news/OnSaf...
RE: Column Rebar Cage Overturning calculation
A #11 bar sounds good... but when you consider that they act on their own (not composite with all the others) and you're talking about a KL of 40-50' -- think about how squirrely that is with a radius of gyration that small.
Now, if you can hold rigged up to the cage with a crane while you bring the forms in, and then have the forms braced adequately for wind loads -- that's another solution that's well and good. But that depends on your form system and how it gets installed.
KootK -- it's a dollar to make the chalk mark, and the rest is knowing where to make the mark. But we can generally get drawings and calcs for a bridge worth of columns (say 6 to 10 with similar shapes and differing lengths or ground conditions) out in about a day's work. That's assuming it's a job where we've already worked out with the field engineers what methods and materials they have available, and where there is enough work to be worth the liability.
RE: Column Rebar Cage Overturning calculation