Pedestrian Suspension Bridge

I'll not advise, for fear of being wrong.

There are plenty of DIY articles on the subject.

This guy seems to know what he's doing. His bridge is 80 Feet long, but have a look.

http://www.wildcatman.com.

Good luck.

Welcome to the forum.

This should work for you....

https://www.youtube.com/watch?v=rYOC_3H27Yg

I'm nervous already

I'm highly reluctant but I will point out a few key points you should consider before taking this project on. a suspension bridge is designed to transfer weight to anchor points, usually the ground. in the pic the thick black line is taking the weight of the Deck and transferring that load to the red towers, the yellow anchoring points are equaling important. I fear this project is over your head proceed with great caution

Nah, as long as he has red and yellow paint surely he'll be fine!

Del

I don't think this is an especially difficult project, if undertaken with the proper planning.

The weight of the supported materials can be figured and the strength of the main cables and support cables is known.

The main cable anchors can be concrete and are just a matter of using good materials.

I would not recommend anchoring the cables to trees, without taking care to keep the tree from growing around them.

Thank you. I picture the towers as being ends of the suspended portion. From the towers out, the 'deck' would be on the ground. Is the portion of the cable(s) from the top of the towers out, a mirror of the cable arc from the towers in? This would mean the main cable came down to the deck level in the center. How do you get the right balance in these arcs, so you are not pulling the tops of your towers in or out? Especially as the load in the center of the span changes with pedestrians.

All of my anchors will be poured concrete cylindars with their tops at or slightly below ground. My towers will be 4x6 wood, and I am thinking 12 - 14 ft tall above the deck/ground, to be calculated.

I am thinking in addition to the cable which carries the deck, I would put in another cable at waist level to be a handrail. This cable would also be attached to the vertical support cables as needed.

I am also planning a large turnbuckle in each main cable to make adjustments possible.

Thanks in advance

Yes, the towers, or king posts are at the end of the suspended portion.

"Is the portion of the cable(s) from the top of the towers out, a mirror of the cable arc from the towers in?"

No. See below.

The anchor cables carry all the tension/load from the top of the king posts to the ground.

in the video the treads were poorly attached to the cables.I'd like to see them drilled and sleeved with metal tubing or pipe. then the cable run through them. the design in the video would be an immediate problem and hazardous if a tread flips when bearing weight. once all the treads were installed with the cable in place kick them together with and end strip like a breadboard edge on woodwork. it will look clean and prevent treads from spinning on the cable

In the video, I didn't see any anchors and stays beyond the kingposts. There could be a large bending moment where the posts meet the ground.

Starts about 2:18....eyebolts buried in concrete in the ground....

this really intrigues me. what exactly do you plan to anchor your main cables to?

The "pull" on the guys at each end will be at least 10 x 600lbs and when a person walks on the bridge, this will increase to 20 x 600lbs. The actual tension will depend on how low the bridge is slung.

Is that pull for EACH guy on each end, or combined for all four guys, or??

If the king posts are 14 ft tall, and 40 ft apart, with the main cable hanging down 12 ft below the top of the king posts, and the guys going down fairly steeply, (rather than going out 20 ft from the king posts), does that change the numbers much?

Look up the equations for a catenary. I've been away too long--don't remember if they include provisions for a concentrated load. What I used back when was all uniform loads anyway. Don't forget the safety factor; sometimes called a factor of ignorance! Tension in the cable will depend a LOT on the sag you use. Don't forget ice and snow loading on the deck & cables. Also wind which will produce transverse loads. Then there is resonance when the person(s) walk in time with the natural frequency of the assembly; this can be destructive--I think that one photo was of "galloping Gertie" which was caused by low speed wind.

Tacoma narrows

The tension must be worked out for a concentrated load in the centre of the bridge. This is the worst loading. The answer you get needs to be supported by two guys. (Forget the other end). Divide the tension by two. Since the guys are at 45 degrees, they will be each have 50% more tension. A Load of 6,000lb accounts for walking, jumping, safety factor.
The first thing that will happen is 20 kids will jump across the bridge and it will collapse like the bridge at the Olympic Games.
That's why you should double the cable size and especially the anchors in the ground. Normal safety factors are between 2 to 5.

good link

Bungie cords...seriously...of the ejection tower or bridge jumper variety.

This design is especially aesthetic for a suspension bridge, IMO. The Ravenel Bridge in Charleston, SC. For a 40-ft span, one central tower would suffice and the triangular construction imparts greater strength whilst using lighter-weight materials. The largest unsupported span would be only twenty feet, furthermore. A very graceful design all in all.

Yes, beautiful bridge, of a general design similar to the new Tappan Zee bridge under construction, both of which are not suspension bridges, rather cable-stayed bridges, now generally more popular than suspension bridges for various reasons, not the least of which being no need for massive anchorage requirements at the ends of suspension cables. I still think a bungie bridge in the back yard would be the thing; lot of bounce.

Very aesthetic example! I don't know when this type originated, but the "Cable Bridge" between Kennewick and Pasco, WA, USA was a fairly early instance.

Why not just make a Tarzan swing....more fun, cheap, easier to maintain....plus you get to do the Tarzan call...

Oh yes!! Just like the ones we Boy Scouts built back in the 1950's when I was a Scout. Here is a link to some pictures at various Boy Scouts of America Jamborees http-::scoutpioneering.com:tag:boy-scout-bridge:

Agreed.

600 pounds' rating could easily be exceeded by two or three large people, and a 40-foot span could accommodate many people of ordinary weight, far surpassing a 600-pound design limit - as would a heavy, wet snowfall.

Designing for 6000 pounds would I think be far more realistic. Best to err on the side of safety.

If the OP builds the bridge, he should get lots of liability insurance no matter what. Legally speaking, it would pose an "attractive nuisance." People (kids, most likely) could climb the railing and fall, what, eleven feet, to their deaths or suffer severe, lifelong injuries.

IMO, I would not even consider building this bridge. Not in litigious, lawsuit-happy America I wouldn't. People have lost everything over much less - McDonald's hot coffee, anyone?

"attractive nuisance."Jeez the lawyers have really screwed up your heads...Attractive nuisance... hmmm, I've met a few women like that...
Del

Yeah, my neighbor has a couple teenage daughters. Very attractive nuisances!

Lawyers have screwed up a lot more than just the language.

We had to demolish a spring house because the insurance companies called it an attractive nuisance and would not cover it. It was a historic building, although not registered, (I didn't register the house or spring house because I didn't want the associated restrictions) and the historic Society was upset when they found out afterwards.

As to this bridge, Op doesn't seem to understand that the end spans must balance the inner span cables.

The bridge is in the lads back garden, assuming that is, it belongs to him. So anyone using it as a short cut is trespassing and is not legally covered by insurances.

Besides catering for stupid people, insurances, lawyers, criminal and civil liabilities, just how do you there, manage to actually do anything, build anything or enjoy anything in life without being sued, jailed, hung drawn and quartered.

Really starting to realise how oppressed this nation is!

Yeah, as a Brit it I find it mind boggling that you can have guns but need insurance if you want to build what is no more dangerous than a climbing frame in your back garden.

Next thing you know the car thieves will be pleading that the shiny new sports car was an "attractive nuisance".

About time there was some common sense.

I don't have any public liability insurance for anything on my property. If someone insists on climbing in and slipping on the tiny bridge over my garden pond and breaking their leg... they can't come running to me for compensation.

Del

"they can't come running to me for compensation."

Nor can they out run the arrows.

What the heck is wrong with an American Citizen owning a firearm? It's a guaranteed RIGHT under our Constitution, covered under the 2nd Amendment. The Right to bear arms has nothing to do with Militias and hunting. It has everything to do with preventing a TYRANNICAL Government. Just in the last century, over 300 MILLION people died at the hands of tyrannical governments after the populace surrendered their firearms: starting with 8 million ethnic Armenians in the Ottoman-Turk Empire; nearly 30+ million Ukrainians and Russians at the hands of Stalin; over 40 million Chinese at the hand of Mao; and the list goes on and one........

The UK should go back to allowing citizen ownership of firearms. Ever since the great gun grab in the UK, the instance of violence in the UK has more than doubled. The Bobbies can't be everywhere at once to save your fanny. They'll be along shortly after your demise to draw a chalk outline of your corpse, then photograph it. Think about that real hard. Ditto for what has happened in Oz too.

Just saying.....

I totally agree with you, they have every right. Just when are they going to use them to rid the world of the tyrannical govt. Maybe tomorrow the voter will shaft the lot and not one shot will be fired.

But the UK, well, the police would need to do nothing as the public would be knocking off each other between tea breaks. And the NHS could not cope so they would go on strike. Ambulance drivers would need hazard pay, there would be a massive influx of foreign nurses, they would be taken out on arrival, there would be no where to put the dead as the open space of graveyards has flats built on it now or it is a car park.

And besides, all that smoke from smoking firearms would not sit very well with the Environ lads, so they would be issuing fines and complaining about CO2 emissions. But HSE would make a fortune running safety programs and writing safety procedures. So it has a up side, which is fair enough.

But I don't see it working in the UK at all. By the time one has read the safe work method, followed the step-by-step instructions it would be tea time. Besides imagine if the cell phone rang at that vital moment. Fire. Answer the phone. Hmmm! Someone has to go on hold then.

No it wont work in the UK.

The UK had better get it's head out of their collective arses in a jiffy, because they have a growing Islamic problem there, just like the rest of Western Europe. The complacent government won't be there to protect it's citizens when the SHTF (Jihad - Stage 3)

Afraid it maybe too late for the Brits. They're in what is called "Jihad - Stage 2". They have only themselves to blame for allowing the mass influx due to their very Liberal immigration policies...

Truly sad.

http://www.youtube.com/watch?v=_hUVVlgEqxY

If they are chased out, who is going to live there? The place will be empty and who will tend the lawns, drive the buses, clean up the street, cook the curries, clean the offices, pick up the rubbish, and do customs duty at the airports and build stuff and play football for them?

You are reading it all wrong!

I'm not arguing against the GUNS...
I'm arguing against the damn LAWYERS.

e.g Your lawyers aren't worried by any risk from a gun, but say you need to be insured against a stranger falling off your bridge...

I think maybe you are knee jerking because you expect me to be anti fire arm??!!

Del

(BTW, I've been off line, family reasons)

Get hold of your local overhead line team who look after the distribution lines. You need some stay wire, which is galvanised steel, 7 strand. 200 feet. 4 x Stay anchors which will have the turn buckles. 8 x dead end grips, new, not used and a dynamometer to tension your stay wire, four eye bolts, 12 D shackles (5kN) and 2 x Come-alongs and a chain block . You will have no sag, it will not stretch, it will not give way using the dead ends grips, it will support way over 600lbs of weight.

In fact, it could easily be completed using only two wooden power poles, cut in half, for that short span length.

1 newton is equal to 0.224808943871 pounds.

Most of these items are usually thrown away when they replace poles, so you may get the poles, the eye bolts, anchors with turn buckles for free. The stay wire you can buy new along with the dead-end grips. And I am sure one of the linesmen will show you how to apply the dead end grips. (Easy when you know how).

You will have the suspension and two wires to attach the wooden lattice treads to and you can then install you side stringers as you please. That is, if you want a wire framed suspension bridge.

Go Big or go home.....powered zipline!

https://www.youtube.com/watch?v=aWSXTvrINKg

Normally, I would admonish you to "hire competent engineering help".

But, "This will be in my backyard" leads me to say, go for it, with proper preparation and planning.

The liability issue does create some problems and if your insurance carrier is like the other 99.99% of them, they'll want their pound of flesh, now or later.

If you have sufficient property, you might be able to bury any potential litigants and deny ever seeing them.

After you have taken out additional insurances, had the all clear from HSE, received certification of all welds, bolts, wires, 7 & 28 day cure tests on any concrete foundations, fully secured your property with high fences, secured gating, installed anti vehicle devices, put in place all the safety notices, taught you alligators to not eat anyone who jumps, falls, slips, dives, levitates off your bridge, provided full Work Methods, complete all daily site safety surveys, toll box talks and ensured all safety equipment is to standard, all cranes and lifting equipment is safe to handle the loads and that all sanitation requirements are in place, (environmental issues), and all waste is controlled and removed to a licenced dump site, you do not burn you hands when drinking a hot cup of coffee. Full PPE is worn at all times

Let us know if you actually take on this mammoth task some time next year when planning permissions, licenses and authorisation are in place.

And you thought this was a weekend job!

I can resist no longer.

I am the least qualified person on this site to give you advice on building your suspension bridge. But, what if your suspension bridge is actually supported from the bottom, and the supports are hidden with vegetation?

Concrete posts buried in the center of the center of the width of the bridge, every 8 feet along the length of the bridge. An iron beam connecting the concrete posts, with pressure treated 2x6 on top of the beam, going side to side. A series of 2x6's screwed below the deck on the outer edges.

Any cables at that point would be only decorative. The sides would help hide the real bridge supports. Just my thoughts. Hope it helps some.

Some advice and enlightenment:

You have come to the wrong place, CR4, for detailed information for a 40' pedestrian suspension bridge. Perfect examples why not to are the Silver Bridge, The I-95 Bridge in CT, the Tacoma Bridge and numerous others, and they were designed by professionals! Most of us are not bridge designers although some may try to sound like one.

Something with the disastrous potential as this could kill people if it was designed wrong or spec'd wrong. You don't sound like a bridge professional so get one to do this design work. The life you the engineer saves could be your own or that of a family member.

I built a 24' bridge in my back yard, across a stream, with the major components being pressure treated glulams. I designed it, the engineer said I overdesigned it and why. I took his advice but still over built some parts to make it safer than minimum. Last torrential rain we had I was happy it was better than necessary. Next guy upstream from me made the engineers bridge and my bridge held up when his bridge fell apart and crashed into mine. No structural damage to mine except for a few dings and scrapes. I am extremely happy I had him design it professionally and me over-building it.

Wife hated the idea when I started planning for it but she now loves it because I made room for some flower planters on the sides!

Good Luck, Old Salt

Your story is great, but it only really illustrates that the engineer didn't know what he was doing...

He was the one going to kill people. After all, engineers designed the Tacoma narrows bridge.

Del

For a long time after that failure, it was over-simply described as a resonance phenomenon (as if wind gusts were timed to match the bridge's natural frequency). Only later did vortex shedding come to be understood.

The bridge's principal designer, Leon Moissieff, died about three years after this incident.

Yeah, I know it's a special case and to some extent I'm playing Devil's advocate.
One could argue that commercial engineers are designing to a budget and are thus more likely to under design than an amateur. After all, infrastructure bridges that fail were designed by civil engineers!
Del (tongue in cheek)

In terms of static design and "simple" wind load, the bridge probably had adequate safety factors; i.e., not "cheapened" in design. Instead, a new phenomenon was involved. In this case, fortunately, no injuries or fatalities ensued. Some other bridge failures owe to poor inspection and maintenance rather than bad design, but I don't know of the statistics on that.

Poor bryngary, scared off. I suggest you simply build a small cable car system. I am sure you will have less hassles. Heck just put in a zip slide. You will be quicker and have more fun when crossing your gulley.