Column Rebar Cage Overturning calculation 1

[ducksfan]

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?

 

[WinelandV]

Any chance you could provide a sketch? When you say "column", do you mean just the rebar cage? Rebar cage and associated formwork? Rebar cage and dowels? What exactly is the 775# rotating? In the past when I would check rebar cages, while global stability is important, the real issue was trying to get the longitudinal bars to act compositely - so that you have more than one bar resisting the lateral force. For tall cages, you usually have to provide some kind of internal frame to ensure that you're engaging enough bars, otherwise you're just sticking strings of spaghetti into the air.

 

[ducksfan]

By column I mean just the rebar cage. Our GC wants to make sure the dowels that are embedded into the footer will be enough to keep the cage from tipping over. The 775 lbs is the force we came up with that would take to get the rebar cage to start rotating. I guess the real question is how do I check to make sure the 9'-6" embed dowels are enough to resist the rebar cage from overturning.

 

[WinelandV]

I would suggest treating the system as a cantilever column - determine what the lateral loads on cage are, determine the moment demand at the base, and then resolve that into a force couple: and tension and compression force into your dowels (assuming only 2 dowels effective in resisting the moment). Then you need to check connection between the dowels and cage, and also the buckling capacity of the dowel in compression (assuming the material strength of the dowel in tension is adequate). Then you need to check the embedment of the dowels into whatever is underneath to resist the tension and compression. And don't forget the factor of safety.

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.

 

[KootK]

A few years back, I attended a presentation by Harris rebar on this topic. I've attached the slides. Based on that discussion, I question whether or not you want to cantilever a large cage up from the foundation. All the examples discussed during the presentation were tied with guy wires at some location above the dowels. And, even at that, P-delta and guy wire stability issues were a big deal. Granted, we were discussing VERY large column cages.

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.

 

[WinelandV]

Koot,

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.

 

[TXStructural]

You need to guy the cages. I cannot emphasize that enough. Rebar cages act more like a host of single bars than as a unit. The tension on dowels due to overturning is NOT the issue. THIS IS SERIOUS SAFETY HAZARD that needs to be handled properly; people have been killed when the cages collapse.

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.

 

[Lomarandil]

ASCE has recently published a guide that will help you with a lot of the basic questions for supporting a rebar cage:

http://www.asce.org/Product.aspx?ID=2147487569&ProductID=179122182

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

 

[KootK]

Nice. I'll have to get a copy of that. Out of curiosity Lomarandil, if I asked you to design one of these systems for me, how many hours would you bill me for? 30 min? 1/2 day? Some extra purely to account for liability?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[ducksfan]

Thank you all for the information and the advice. I have spoken with a few of our guy onsite and they said once we get one stood and see how it acts with the 9'-6" #11 dowels and #11 verticals bars in the column then we will see what supports we may need to secure it until the forms are attached.

 

[TXStructural]

The guys in the field cannot adequately determine the safety of the cage. Without proper assessment and engineering, the risk is quite high. Tie wires DO NOT provide reliable resistance to movement but they will make cages SEEM stable for a while. This is precisely how mishaps occur. I would rather not be argumentative, but I feel quite strongly, as a structural engineer, a safety professional, and a representative of the industry.

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/OnSafeSide_RebarColumn_Feb.pdf

 

[Lomarandil]

Agreed. Ducksfan, what you've laid out is asking for serious trouble. Cages aren't going to give lots of nice ductile yielding to let you know they're about to come down -- buckling failure is going to be swift and severe. I can't stress this enough -- call your guys in the field back.

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.

 

[ducksfan]

I have passed this information along to our field crew as well as our qualified person who will design the supports. I appreciate the help with this and will make sure that we brace all the rebar cages until the forms are installed and braced per the manufacturers design.