stress analysis
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Re: stress analysis
Re: stress analysis -- JMarks		Top of thread	Forum

Posted by: swearingen ® 03/30/2006, 07:31:54 Author Profile eMail author Edit

The reason you're having so much trouble is there is one variable that you're leaving out - deflection. The amount of load required depends on how much the net deflects when the load is placed on it. Once you know that (or guess a few - use Excel with what's below to come up with different scenarios), you can calculate what the load is. The key here is that the more deflection that occurs when the load is there, the less load is in your cables. I'm going to do the calculation only for the 150lb load - you must go find the weight of your net and use catenary equations to calculate the lateral load of it and then add that to the lateral load of the 150lb to get your total load. 1. Assume a deflection, d. 2. The lateral load at one post is 18(ft)/d(ft) * 150/2 (lb) 3. One cable takes half of this = 18*75/(2*d) = 675/d 4. In plan view, the load along one cable would be 675/(d*cos(30)) 5. Bringing that to the elevation view, the actual load in the cable is 675/(d*cos(30)*cos(60)) = 2700/(d*sqrt(3)) So, if your cable deflects 1' from 150lbs, the load in the cables due to that 150lbs would be 1559lbs! A 2' deflection gives you 779lbs and a 6" deflection gives you 3118lbs! A 1' square plate 3' down in sand is only going to give you maybe 400lb of resistance maximum, so you may have a problem considering these loads don't count the tension in the net itself. Back calculating, if you only have a 400lb capacity on your anchors, the net tension can't be over 346lbs with no extra load in the center. May I ask why you want to hang 150lbs from your net?

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Re: stress analysis
Re: Re: stress analysis -- swearingen	Post Reply	Top of thread	Forum

Posted by: JMarks ® 03/30/2006, 13:32:02 Author Profile eMail author Edit

Very good. Thank you very much. Now I see how that works. As for your question why I want to hang 150 lbs. from the net, I don't want to hang anything from the net. But there is always someBODY getting angry and for what ever reason they grab the net and yank as hard as they can which can certainly bring down the whole system. I've seen that very thing happen at a pro tournament in New York back in the late eighties. The anchors pulled and the ref and stuff when flying in all directions. I use the court for beach volleyball clinics I conduct and I sure wouldn't want someone getting knocked on the head by a falling post. So here's my first thoughts on changes. I can make the angle of the cables to the posts, looking in the top view, 45 deg. instead of 60 deg. And in the plane of the cables to the post I can do the same thing, make that 45 deg. also. That should lessen the force on the anchors, right? Aside from those angle changes I'd have to include some force absorbing machanical element like a spring assy on the cables that would give when a force over the ability of the anchors to hold in the sand was exceeded. What do you think? JMarks Modified by Administrator at Fri, Mar 31, 2006, 08:30:30

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Re: stress analysis
Re: Re: stress analysis -- JMarks	Post Reply	Top of thread	Forum

Posted by: zekeman ® 03/31/2006, 00:41:59 Author Profile eMail author Edit

If you are worried about that , the force is not 150 for someone weighing that much; you have to find the energy he (or she) comes down with.before impact. You know why Zekeman.. Admin Modified by Administrator at Fri, Mar 31, 2006, 08:28:36

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Re: stress analysis
Re: Re: stress analysis -- zekeman	Post Reply	Top of thread	Forum

Posted by: JMarks ® 03/31/2006, 08:53:34 Author Profile eMail author Edit

Oh how right you are. Some of these guys can weigh 250 lbs or more on the pro tour. I'm sure they could tear the whole thing down even if it was set in concrete. Well, maybe not, but I know what you mean. Young children like to grab on to the net and hang so I still need to know the forces generated without the dynamic application. Maybe I can apply some of the methods of analysis used in rock climbing to determine load forces on protection during a fall. Those forces can reach a couple thousand pounds or more. For now I'll settle to determine the forces required to pull my court down and see if I can protect it. Thanks... JMarks

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Re: stress analysis
Re: Re: stress analysis -- JMarks	Post Reply	Top of thread	Forum

Posted by: swearingen ® 03/31/2006, 10:33:30 Author Profile eMail author Edit

Increasing your elevational angle will certainly help. Dereasing your plan angle will help as well, but it means that you're giving up some out of plane stability. I wouldn't narrow that angle much more than you have it already. You might add a third line in the center parallel with the net... Shock absorbers are a great idea because they help take out those dynamic loads.

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Re: stress analysis
Re: Re: stress analysis -- swearingen	Post Reply	Top of thread	Forum

Posted by: JMarks ® 03/31/2006, 11:15:34 Author Profile eMail author Edit

Thanks. Adding a third line sounds like a do-able solution without getting too cumbersome. It's got to be portable, after all. I'll get to work on some sort of shock absorber. I think I've see something with a compression spring with the cables running up the center of the spring and attached on opposite ends so when there is a load placed on the assy. the spring compresses. I really appreciate the help with this. JMarks

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Re: stress analysis
Re: Re: stress analysis -- JMarks	Post Reply	Top of thread	Forum

Posted by: swearingen ® 03/31/2006, 13:40:55 Author Profile eMail author Edit

You could also do a bypass spring: Cinch a piece of rubber or multilpe bungee cords to one point on the cable and attach the other end to a point on the cable farther away than the length of the rubber/bungee. Now the force goes through the cable, then the rubber/bungee, then back to the cable with a loose piece of cable hanging beside the rubber/bungee. When the load increases, the bungee elongates and is loaded. If it is overloaded, you always have the loose piece of cable there when it's fully stretched out. The loose piece will get tight and start to take the load.

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Re: stress analysis
Re: Re: stress analysis -- swearingen	Post Reply	Top of thread	Forum

Posted by: JMarks ® 03/31/2006, 15:49:35 Author Profile eMail author Edit

Oh man, that's even better. I like that idea cause, like you said, you don't loose the integrity of the cable throughout the system. That would give you exact control of the amount of deflection in the net cable before the excessive load reaches the anchors. Say you could allow it to deflect 2 feet before the cable comes taught against the anchors. Thanks. I'll try to come up with a design incorporating these features. JMarks

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