Cantilevered bolted connection

[EspElement]

I have an application that i have been trying to use calculations from a different post

http://www.eng-tips.com/viewthread.cfm?qid=262972

and am having some trouble.

I have a customer that is wanting to remove a leg from their platform to allow for a conveyor to travel under it. See attached file. There are 2 angles as a beam across the platform (6 x 3 1/2 x 51/6 and 3 1/2 x 3 1/2 x 5/16) inertias came to 10.9 + 2.45 (modulus 5.42 + 2.47). Used a beam calc to determine stress and deflection. Total load on platform at one time will be 1500 + SW (550lb to Solidworks) I add 25% to SW to be safe. Total of 2188 lbs.
Thru beam calcs, I determined I needed to add one C6 to keep a stress SF of 10. Deflection was a non factor.

My major concern is the bolts. The pull stress of the 6 bolts holding this side up. Obviously there will be some stress transferred to the other leg, however I want to be sure there won't be any issues. I am looking at this if i were to be cantilevering with only the 6 bolts, worse case.

No one at my company knows how to do this, I need to prove this before giving the go ahead.

I am trying to use BD #2 on

http://school.mech.uwa.edu.au/~dwright/DANotes/threads/fatigue/exampleB.html#top

and i cant seem to determine where they are getting some of their numbers.

Any help would be greatly appreciated.

Thanks

 

[desertfox]

Hi EspElement

Sorry I am struggling as to what your set up is, I can't relate the little red diagram on the right of your model showing the load,pivot point etc to the model of the platform.
Can you provide some more sketches of your situation and either label the relevant components on the platform model.

desertfox

 

[GregLocock]

Agreed. I was frightened to comment before!

Using a bolted on staircase to prop up a cantilevered platform is not something I'd allow.



Cheers

Greg Locock


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[EspElement]

Sorry, the angle that is facing the right side that has 8 red bolts in it, that is bolted to a frame. That frame is made of 4 x 4 x 1/4" tubing. That is what is holding this platform along the entirity of the angle.

Desertfox
I went to your diagram used on that other forum. I couldnt find where you determined µ.. but i found you set your P load and set it = to the sum of the distance of each bolt from roational point. Then you took the farthest bolt and multiplied the distance times µ and it gave you the load.

I have been trying to use ur other refrence with the bolt deformation. That is the the calculation i have been having trouble with. FBD #2. How did they work that out?

if i took worse case, 2188 @ 37" out.. thats 21880 * 37 = µ3(1^2)+µ3(3^2) µ=26986 so 3 * 26986 = 80958lb load. Max for 1/2-13 is around 10000 @ 70,000psi.

That is with a SF of 10. Meaning that i need to add bolts to improve the SF.

 

[EspElement]

In the diagram i was representing a uniform load of 2188 * 10 SF. How would i calculate this in a unform load?

 

[desertfox]

Hi EspElement

Right your talking the 37" as being the cantilever length and there are 6 bolts to hold the angle to whatever your fastening it to correct?
I think the pennies dropped, so the leg in the red box on the left of the model you want to take out?
If that is the case then don't, it won't work.
What you might be able to do is put a diagonal brace/s under the cantilever and fasten the brace to the same vertical support that you fasten the angle to.
In answer to your other question convert the uniformly distributed load into an equivalent point load which gives the same maximum bending moment as the former load.

desertfox

 

[EspElement]

Desert fox,
Yes, that is correct. Boxed leg is the one they want to remove.

You are saying uniform is the same as taking my load at 37" out? I was thinking it would be half that cause the average would focus in the middle of the frame.

---------------

Keep in mind, i am adding self weight into that number at 37" away from fixed point.

I did beam stress and deflections thru a canilevered formula fixed from one end - S=Wl/Z 21880 * 37/5.42 + 2.47 + 4.38 (added c6 x 8.2) = 65979 psi steel ~ 33,000 psi. Which is clearly under rated. I will need to use 3 channels or use a stronger channels. Deflection - Wl^3/3EI (2188)(37^3)/3(30,000,000)(10.9 + 2.45 + 13.1 + 13.1 + 13.1) = 0.023" (this is without calculating SF)

Now that I look at it again, This is going to be a problem. I will definitly have to add 3 channels total just to get close to where i want to be.

Does anyone agree with the above?

 

[desertfox]

hi EspElement

The maximum bending moment on a cantilever beam is w*L^2/2 for a uniformly distributed load, so to generate the same bending moment on the cantilever beam with the equivalent point load therefore, would be w*L as a point load and placed halfway along the beam and in your case it would be placed at 37"/2.
Can you sketch out exactly what your new proposal is,obviously your very familiar with your problem but we are not, for instance where the fixing holes are, what are you fixing them too? Also how are you attaching channels underneath.
Again a sketch alongside your calculations would help greatly in our understanding of your problem, ie put the sketch and calc on the same sheet of paper and upload a file
see we can see then exactly what your trying to convey.

regards

desertfox

 

[EspElement]

See new attachment. The PDF has 2 pages. I hope that it shows more of what is happening.

Thanks for the help fox

 

[desertfox]

Hi EspElement

Okay I see what your doing, [μ] was the unknown in the previous thread that your referencing and was defined as the (load/per unit length). Once I found [μ] I can calculate the maximum bolt load as you describe.
In your case here I am not convinced its a good idea, but you can use the same method assuming the resultant load and self weight of the platform act at a distance from your pivot point and balance that against the number of bolts at each individual distance^2 from the same pivot point, similar to your second to last post on the 16th, except your load of 2188 appears to have become 21880 which I think is a typing error.
I think on your latest post with the uploaded pdf's you appear to have a bolt at 5" from the pivot on the first page but I cannot see it on page 2 please clarify.
As previously stated I don't think this will work, because you seem to have a large overhang load and a relatively small span for the bolt positions and as you are already finding the bolt loads are tending to be rather large.
The self weight of the frame would act at its centre of gravity and not at the end of the 37" as you have indicated.
To sum up what your doing here, your trying to transfer some very large loads through some relatively small bolts back into a structure, I think you should be trying to transfer those loads through structural steel members and back into the structure at some point without the bolts.

desertfox

 

[dvd]

Bolting through a tube seems like a bad idea. If the deflection of the tube changes dimensions of that tube you could lose tension on the bolts.

You need to buy a copy of "design of weldments" by blodgett, it is for sale at the Lincoln educational bookstore online. It is cheap. You can learn a bit about bracing a cantilevered frame in that book.

You really should consult with a structural p.e. about these types of issues, however. Don't let your management convince you that you should make decisions about these issues unless you are sure that you can make the right decision, including citing structural codes as needed. You will be the one sitting in the witness chair.

 

[GregLocock]

Is there no possibility of introducing a diagonal brace back from the end of the cantilevered part back to the remaining leg?


Cheers

Greg Locock


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[EspElement]

Thanks for the replies, I was also thinking since this machinary is in the plant. I can have them weld the platform to the tubing and lose the bolts. It was bolted for shippig.

I have that book! i just recently bought it cause it was recommended. I love it! got a lot of good information. I havnt gotten to get thru it all but i will look up that section.

I can possibly do an angled brace from where the leg was to another frame. Since im intending to weld together and ditch the bolting, I can probably do a little of both (angled and channels across)

Fox, I was taking 1500+ self weight = 2188. Since its holding personell. I want a strength factor of 10, sometimes i hear its even more depending on the application. I want to ensure we dont have failures. Thats way I have been using 21880 for strength. and just 2188 for deflection, since the load should never get above that. Does this sound reasonable?

Reguardless of what I was going to do, i still wanted to get an understanding of how to calculate stress on the bolts in a cantilevered design, now i know.

Thanks again!

 

[desertfox]

Hi EspElement

I think an angled brace and welding the components together might be the better option, but of course if its needed to be transported to site you might want to weld on site or weld some of components on site.
Well I think you at least got how to stress the bolts under cantilever action good luck.

desertfox

 

[EspElement]

See attachment, This is what i have decided to do. It will be welded into place.

Thanks fox, i appreciate it!

 

[cooperjer]

Hello EspElement,

In addition to the welding book by Blodgett please consider Fundamentals of Machine Component Design by Juvinall and Marshek. There is an entire chapter dedicated to design with threaded fasteners, with an example that matches the information given in the DANotes you linked above.

I'm in agreement with all above. You run a risk of significant deformation in the square tube you're securing to. Even if you change the fastening type to a weld instead of a bolt you would probably want to identify how the square tube with the bolts running into it will deform in rotation.

Finally, if you change to using welds please consider calculating the stress in the welds.

Thank you,

Cooperjer
Mechanical Engineer

 

[EspElement]

The above picture is what my engineering manager told me to do. I went to him and told him i wasnt comfortable removing the leg, he said to add that cantilevered weldment shown above and told me that was it. He told me that "no body" calculates platforms and that i was wasting my time. I said, okay. Then you can be liable for it. He said thats fine, you will be fine adding that.

I learned alot doing this and i will continue to do what i do, but for this particular issue its out of my hands because im dealing with a moron.

I greatly appreciate all of your guys time and help. Im still going to school and one day have a degree. But in the mean time this is helping me alot.

Thanks

 

[williedawg]

Make sure HE's the one that signs off on it.

 

[desertfox]

Hi EspElement

Well I can assure you people do calculate stresses and deflections on platforms regularly.
I recently designed a platform for supporting two generators on exploration vessel, which also had personnel access for maintenance with flooring supported off cantilever arms running back and welded to the main frame.
I have a question about your cantilever support you have uploaded, which end of the platform is the support situated?
Is it directly under the corner where the leg (now removed) was or is it at the other side?

desertfox

 

[dvd]

Careful about calling your boss a moron. It is a lot easier on the psyche to assume he's just trying to do his job. You will end up happier if you don't work for a moron.