Threaded Rod Hanger 4

[BriceLee]

Hi all,

Ive got the following situation:

A 4.3m threaded rod is anchored to a composite concrete slab and this is divided into two sections of 2.15m connected by a M10 coupler. Assumming this rod is of Grade 8.8 and is also M10, how should I proceed to calculate its capacity? Would I have to consider the tensile load resistance, the concrete pull-out resistance and/or the coupler's resistance? (Both Preloaded and Non-Preloaded Conditions are to be considered. This is for the retrofitting of a BallRoom ceiling expected to carry gymnasts and lighting systems, etc) - in this system, every 1.5m there will be a hanger and this will go 'round' a square.

I did my years in academia under the Eurocode, but now working in Asia, we follow quite a few codes, so any help is welcomed (if explanation could be in metrics would be preferable).

Thank you all in advance.

Attached are the photos of the ballroom:

http://files.engineering.com/getfile.aspx?folder=cbc72e66-a4dc-46ac-964c-dae3de724f22&file=BR1.PNG

http://files.engineering.com/getfile.aspx?folder=a1aa4534-4d49-472b-91e8-ad6bf168661e&file=BR2.PNG

http://files.engineering.com/getfile.aspx?folder=b0c72632-61f4-4f80-853b-c8935ea27ce4&file=BR3.PNG

http://files.engineering.com/getfile.aspx?folder=c9f50c9b-39b6-4f60-ae09-9d59c42c10dc&file=BR4.PNG

http://files.engineering.com/getfile.aspx?folder=91fe9ebb-5a02-4f93-b0eb-32d9191b31eb&file=BR5.PNG

http://files.engineering.com/getfile.aspx?folder=dd6c72e3-c67d-46bd-9004-0520fde1cee5&file=BR6.PNG

 

[KootK]

I agree with the failure modes that you've suggested. I suspect that concrete breakout will govern overwhelmingly unless your embedment is very deep. I also don't think that you'll get too much capacity out of this system. The gymnast business might be a stretch. I'd add slab flexural capacity to your list of checks. And the rest of the supporting structure of course.

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

 

[jayrod12]

Depending on what is above, whether it is roof or another floor, you might be better off bolting right through the concrete slab above and using steel plate on the top side to utilize more concrete area. Then your problem just lies in the rod and coupler strength. I thought that coupler (and regular nuts) depths are determined such that they fully engage the capacity of the bar but I've been wrong before.

As KootK mentioned, this might not work as gymnast supports but it will likely be alright as lighting supports.

What kind of gymnastic stuff are they planning that they need to be hanging off of the ceiling? If it's the rings (the only one I can think of right now) they will need an infinitely stronger solution that what you are proposing.

 

[Archie264]

The gymnast business might be a stretch. I'd add slab flexural capacity to your list of checks

Bravo! Who says engineers can’t write?

Reminds me of…

All this climbing’s getting me down.
--Fairchild/Cochran

The sunshine bores the daylights out of me.
--Mick Jagger

Pasta Maker Uses It’s Noodle to Whip Competition
--Wall Street Journal headline

Brice: listen to KootK, or anyone else who can write like that.

 

[BriceLee]

Cheers guys.

The gymnasts I'm referring to are actually 'aerial silk performers/acrobats'.

About the connection to the concrete slab these are not bolted right through it. The client is not sure but it is either a mechanical or chemical fixing.

Said that, the client only wishes to determine the current capacity, from which they will decide if they are going to further reinforce the supports with more hangers or substitute the current ones with bigger sizes rods. (The photos show there are threads that have been bent due to the installation of piping after initial handover.)

 

[dhengr]

BriceLee:
This sounds to me like a particularly difficult assignment to answer with any degree of certainty. You don’t seem to know much about the slab above. The client (or you) don’t know what kinds of anchors you have, mechanical or epoxy, what size, depth of embedment, etc. You’re not too sure about the strength of the hanger rods or coupling nuts, but you can at least measure them. Do you know anything about the location of the hangers, and how does that relate to what you are trying to do? Maybe you should test all of them and use the lowest failure (pull-out) value to base a cap’y. on. All-n-all, you don’t have any info. on which to even begin to base a capacity number. Then, overhead epoxy anchors are notoriously difficult to do correctly and are prone inferior cap’y. Google, ‘Boston, Big Dig Tunnel Roof Failure.’ There was also an instance of a fairly dramatic failure of a hanging system like you are suggesting, where 6 or 8 acrobats all fell from some considerable height, all hanging from the same ring/frame, within the last year, when the securement system above failed. I would think all you can do is educate the client about the difficulty of the problem, the/a range of cap’ys., and the uncertainties of hoping for a exact cap’y answer. Show him some cap’y. tables for various anchors, concrete strengths, hanger rod strengths, and let him pick a value. Or, start over, with a design intended for the purpose, all loads known, etc., and a sensible redundancy and factor of safety. Also remember, they don’t usually call you when they want to hang double the weight from the system. They just do it, assuming you way over designed it anyway. They will call you, and it will be assumed to be your fault when it fails. One of our duties as Structural Engineers is to protect the life, safety and well being of the people who use our structures.

By the way, we frown on double posting the same question, so please don’t post this on a third forum.

 

[drawoh]

dhengr,

I was the one who suggested he post here, instead of in forum404. Judging from the responses, I think I was right.

--
JHG

 

[Ron]

If your hanger rods are existing, I would suggest you load test a representative number of them rather than guess at the capacity. An M10 rod is a bit small to be hanging anything substantial....like moving people.

 

[BriceLee]

First of all, thanks dhengr.

ALl you have pointed out are part of my concerns and I've made it clear to my client that it is not sth they would like messing around with. Since it is a 10 years old facility, where many other things have been implemented (e.g. ducts, pipes, etc.) having bent/deformed some hangers, they would certainly have to consider changing up the current system.

Nonetheless, being a big (big) company which slogan is the 'Wynning Way' (no, is not a typo and no...) I suppose more should be invested here.

Could anyone advice on whether the tensile cap check is alright? It seems too high when I take into consideration the Cap/m (18.56/1.5=12.37kN/m & 37.12/1.5=24.75kN/m).

From a preliminar analysis I believe the weakest link is the tensile capacity of the rod (M10 is not structuraly efficient for the application in cause)

Calcs are as follow:

http://files.engineering.com/getfil...f71-b7f0-127d912f3c35&file=WM-BR-CN-001-1.jpg

http://files.engineering.com/getfil...2d8-9a84-2dcd2ba37d47&file=WM-BR-CN-001-2.jpg

 

[racookpe1978]

Why are you looking at capacity per meter (per unit length)?

You have a series of vertical rods, each with a specific - very limited capacity. And each with a specific limit for the coupler nut and the mounting method into the concrete, which has a specific limit at each anchor.

Nothing "per length" there at all.

If, on any given horizontal rod of total length, you are assuming IT can evenly distribute ITS load uniformly onto VERY vertical rod uniformly and evenly as the gymnasts twirl below, you will find by accident and by injury which rod exceeded its load margin first. And which second. Third. Fourth. Etc.

 

[desertfox]

Hi Bricelee

I am also confused as to why you have checked bolt capacity per metre of length perhaps you could xxplain why your doing that?

The values for the fastener grades you quote in terms of yield stress appear correct to me but when I look at the pictures posted in the mechanical forum the screwed rod doesn't appear perfectly straight,to me they appear to be moved to one side or another to avoid existing piping etc and given the lengths you quote I doubt they would ever be perfectly straight.
So whilst your tensile capacity calculations appear okay to me,they not okay in practice because the rods are not straight and will therefore suffer eccentric loads and bending.

You also mention control tightening, I would take that to mean the rods will be tensioned by a tensioning device as opposed to a torque wrench, if your considering using torque values for control I would say forget it, torque setting can be out by upto 25% either way.

 

[jayrod12]

And I still find it unlikely that the rod itself will be insufficient when compared to the anchorage into the slab above.

Provided it isn't for some reason a 10" thick slab above providing 8" of embedment; my money on failure mode would be anchor pull-out.

 

[BriceLee]

Thank you all for your input!

The reason I have considered the distributed capacity of the hanger system is simply because I have considered that every two hangers each would provide half of their capacity to 1.5m, hence, per linear meter, this system has a capacity of 12.37kN/m or 24.75kN/m.

Am I thinking right? Also, a 5% pull-out has been suggested to check the full capacity of the anchorage.

About the concrete checks, I will do it for: 1) pull-out failure; 2) concrete cone failure; 3) splitting failure.

Looking at the steel elements, how am I to proceed checking the coupler's capacity (not from MDS)?

 

[kingnero]

machinery's handbook, google, most mechanical engineering books, ...
This has been covered here before, and is likely to be found in the faq of eng-tips.

 

[TXStructural]

I would not allow use of the existing couplers without removing them and verifying they are all the same, all can develop the full capacity of the rods, and are all installed properly. The risk it too high. I would also require that they have jamb nuts or another way to assure that they do not come uncoupled over time (think vibration and cyclic loading.)

Can you verify that all anchors and rods are the same, and were installed by the same method/people/person? Probably not, so random sampling may or may not detect a likely failure. Seldom is a system set up to allow sudden failure of one or two adjacent hangers without progressively failing the rest.

As you cannot even tell what method of anchorage was used, I would propose to replace them all with proper hardware. It really is a trivial amount of money for a gaming company like that. I would suggest that your name and the lives of the people on or under the system deserve better than "we think this will hold."

(My undergraduate degree and first 10+ years of practice was Safety Engineering. Does it show?)