Cable analysis on series of posts

[trishamae]

Hi everyone!

i need some help in analyzing series of posts with cables and guy wires. This is an adventure facility called aerial walk and I wonder if anyone has tried doing an analysis of this kind of adventure structures. I am using STAAD but it seems I don't get the results that I need.

I wanted to check the stresses per wood poles. The poles have several attached cables to it, including the lifeline, top belay and elements.

Please help, asap.


Thank you,


trisha

 

[bridgebuster]

The attached might be of some help. It's a brief guide to the design of wire rope bridges, i.e., an unstiffened suspension bridge, which is similar to your situation.

 

[SlideRuleEra]

Download (free) the "USS Wire Rope Handbook" from this page of my website:

http://www.slideruleera.net/miscellaneous.html

It will not have a direct solution to the problem, but does have the needed basic mathematical formulas for you to develop one.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[UcfSE]

Can STAAD analyze cable elements? If not, use SlideRuleEra's link. You can calculate forces in the ropes - total tension and horizontal force - and apply them to your structure. You should be able to get pretty close unless your ropes are coupled, i.e. forces in each cable depend on forces in other cables, or if your structure moves significantly.

 

[trishamae]

Hi everyone! Thank you for your replies.

bridgebuster: For some reason, the link you gave isn't working, it prompts an internal server warning. I would really appreciate if you can post the pdf file or provide a working link...

SlideRuleEra and UcfSE:
In STAAD the cable members should be specified with an initial tension, unfortunately STAAD gives me several instability point warnings. A single person can walk on these wires. It is a moving point load but since STAAD only treats moving point load for vehicles, I simply placed a concetrated load at the midspans of the cable to get the maximum stresses. I also wonder if I can make my model simplier... If someone can take a look at my model, I can provide the staad file.

Thank you for the link @SlideRuleEra.

trisha

 

[UcfSE]

Can you post some screen captures?

 

[trishamae]

Hi UcfSE, sorry it took a while I cannot upload any file from the engineering.com. Please try to access thi one...

http://www.2shared.com/photo/4PLQDMNM/aerial_3d.html

http://www.2shared.com/photo/jkqUeHyq/aerial_walk_staad.html

Let me know if it works.

Thank you

 

[trishamae]

Hi UcfSE, the link is now working...

 

[bridgebuster]

Trishamae - I did post a PDF. I'll see if I can locate the site that I got it from ( it was a few years ago).[COLOR=][tt][/tt][/color]

 

[trishamae]

Hi bridgebuster, I got the file now. Seems like the weblink was down last night. But I was able to download it already. Thank you.

Hi UcfSE, did you get the files that I posted? I hope someone can help me with the modelling.

Thanks!

trisha

 

[Denial]

Following on from UcfSE's 11:44 post above, if you are prepared to look at a cable in isolation and are happy that the cable's end points will not move as the loads change or move, then there is a cable analysis spreadsheet available from my website (

http://rmniall.com).

[ ] This spreadsheet allows for a single concentrated load anywhere along the cable, and it accommodates the case where the cable's endpoints are at different levels.

 

[trishamae]

Thank you Denial, yes the cable end points are tight and are fastened to the wood posts by lag bolts. For the zipline cables, I am maintaining static belay grade of 5% and dynamic belay grade of 10%.

I have already downloaded the spreadsheet and will look into my post by post elements. I guess I need to do such calculations instead of STAAD....

Thank you,


Trishamae

 

[dhengr]

Trishamae:
You have a might interesting design problem there, and I’ll bet we won’t find much coverage for “an adventure facility called aerial walk” in any of the building codes. You will have to weave together bits of codes like the IBC and various fall protection provisions from OSHA, etc. The design of the columns/poles, and the design of the cables and their fittings should not be too difficult, pretty std. design methods exist for this. The tough part will be determining/setting the design loads. How many people can be on each area or piece of equipment at one time, and who is going to police this in operation? Remember, people who like this kind of activity will try almost anything they can get away with, and then want to blame someone else for not keeping them safe when things go wrong. It appears that in some cases one post supports several different pieces of equipment which might interact, multiply the post loading, when they are in use at the same time. Look over the links Bridgebuster and SlideRuleEra offered, and note that a cable stretched from point A to point B, with a 200lb. load hanging off of it (at whatever factor of safety you use) can induce 1000's of pounds of horiz. force at their reaction points. And, these will produce some very large guy wire loads and dead man weights for these guy wires.

That you would think that you could find a single fancy computer program that would solve this problem for you, is probably dreaming, given the infinite plan and elevation variations which might exist, and the vast difference in the structural elements involved. And, that you would try to do this all in one big model sounds crazy to me. It might actrually say something about the fact that you could be in way over your head on this project. You should have a fairly good understanding of how to solve/design each of these separate systems longhand. Then when you get things sized and squared away, use some computer software to refine your designs and check more load conditions. Each activity system is sorta a separate project. I would be inclined to try to provide independent support poles and systems for each different activity area. Then find the worst loads and load condition for that support system and design it. Exactly how you interlace the various activity systems might be another issue.

 

[trishamae]

You are right @dhengr. I have already done manual calculations on each poles, given the properties of each and checking on maximum axial loads it can carry. This structure has an entrance and an exit, it won't be filled with too many people on it since the level of activity varies given also the elevation platforms they can walk on. Thank you for giving me some light that I am doing and should continue a simplier approach on this.

The big frustration I have in coming up and dreaming about this full 3d analysis of the structure, was being challenged and questioned by a structural reviewer to provide a whole simulation of the structure which he recommends doing a 3d. Seems to be a rocket science I think.

Anyway I deeply appreciate the comments you gave me. Thank you everyone.


trishamae

 

[bridgebuster]

Trishamae: I'm glad you got the file. It came from a site called

bridgestoprosperity.org

There's an updated manual but it's not posted. Their site says it can be requested by e-mail.

Good luck. It sounds like an interesting project.

 

[UcfSE]

The difficulty with this, given your picture, is the fact that the cable forces are so sensitive to the support movement. The supports will move, even guyed, especially where you have only one guy. The program is probably trying to choke having so many coupled members. I recently did something like this by hand and followed in up in SAP2000. I had difficulty with SAP for a bit but finally got it worked out. I had to specify maximum sag instead of minimum pretension. Are you doing a nonlinear analysis with this?

 

[trishamae]

Hi UcfSE,

I would like to do a nonlinear analysis with it however staad gives me a lot of instability warnings... I seek some technical support from Bentley and suggested to do a linear cable solution instead... Cables are to be truss members with the capacity to carry only tensile forces. However Bentley support told me that the consequence of doing so is that for all the cables that go into compression, STAAD switches them off and analyses the remainder of the structure for the same applied load until a convergence is attained. So, those members that get switched off will show up as having zero force.

SAP is interesting as I have read from the internet but I have zero experience with it... I am still exploring this software...

trishamae

 

[chicopee]

If I may offer a solution since I don't use STAAD. I would proceed with two poles with their guyed wires suspected to be in tension and omit any wire suspected to be in compression and determine your forces with the program and by hand. Initially I would omit pretension in the guyed and walking wires. Then when you have found your forces then I would increase the model to include an additional pole and repeat the analysis. That procedure I would think should give you a feel for the forces in the wires. Once the entire setup is analyze, I would repeat the entire analysis with the guyed and walking wires under pretension. This procedure should help you locate potential problem areas that gives headaches with the program.

 

[dhengr]

Trishamae:
It seems to me that STAAD might be working about right, since cables in compression aren’t worth a damn as a load carrying member. I don’t think you want any cables going anywhere near zero tension. And, if you are still trying to solve this entire system as one large 3-D system, I think you are spinning your wheels. Almost anything and everything is possible today, if you’ve got the time and the money, but some things are so impractical as to render their final, shaky, non-linear, highly highly indeterminate, design not a very rewarding experience or solution. If one post, or one cable system supports several different activity areas at the same time, I predict that some fat guy on one activity area may be annoying the hell out of another person three areas over because of the vibration he introduces into the entire system. I think each activity area has to be supported independent of the next area, so as to control their interaction and facilitate a safe design. I also have my doubts about wooden posts and lag screws for cable connections as wishful thinking.

 

[trishamae]

Hi dhengr,

We have height and weight limit for this kind of structures. I am placing here a link from youtube video which is of the same trade that we have also. We do use ACCT standards and PRCA standards. I wonder how to really convince the third party engineer who is new to this kind of facilities and was throwing back and forth questions which I think all I can say is it was taken from the 2 standards...

I was spending some time too looking for structural engineer who might be practicing this kind of endeavor... Anyway can I please print this thread so I can make this one as a report too? Thank you guys for being so supportive. This might be an interesting thesis with lots of headaches haha.

trishamae