Help for Honduras: Swinging Bridge Design

I would not dare to advise you on your question - way outside my area of expertise, but you are indeed welcome here. I am sure you will get the information you are looking for. Just keep asking.

welcome to the site. I would be better at trying to fix your car, than building a bridge for it. There have been a few threads on water wheels recently. Just do a search on the right side of this page. good luck.

Here is one thread about affordable water wheels.

http://cr4.globalspec.com/thread/33422#newcomments

Just as a common sense approach, I noted you report that these sorts of bridges are common in Honduras.

Why not just find out which ones there, do what you want to do, and then do that?

If I was an engineer, I would be asking, how long is this bridge you are designing? How wide, and what sort of traffic will it be asked to support?

What is the budget?

Are you restricted to local materials, and what are they?

Eighteen inches as a footer does not imply to me much of a foundation. From that bit of information I'm not seeing much more than 15 foot 4by4s and a 20 foot walk bridge over a creek, at best.

There may be some part of the UN that could be of help, though I wouldn't be surprised at all if that was untrue.

I as well welcome you to this forum. It has been a great hangout for me.

I would be extremely reluctant to provide recommendations for design of such bridges. The calculation of the cable tensions for gravity load should be fairly straightforward if you know the span, the elevation of the abutments, the load and the sag. You should use catenary formulas but would likely be close enough using parabolic drapes.

But it is not enough to simply carry gravity load. You must also provide stability against high winds. It is not a simple assignment.

A friction coefficient of 1.0, I believe, is much too high for the abutments. It is probably in the order of 0.5 or less (depending on the soil), but you could benefit from passive soil pressure on the vertical face of the footings. The abutments must be properly anchored to the ground at each end.

You may find some useful information at some of these sites. Good luck!

Thanks to all for the welcome! I'm glad I found the site.

What we're doing here is basically looking at improvements to the existing designs; the new designs are very much based on the existing, with a few improvements. I'm not sure anyone has ever done any calcs on a lot of what is currently there, so I'm checking what we're doing to get some idea of where we are on safety factors. The typical spans we're looking at are from 80 to 120 feet, width about 5 feet for a pedestrian walk-bridge. The loading will presumably include a couple mules at a time loaded with a couple hundred pounds of coffee each.

We are for the most part restricted to local materials - cement, rebar and local-cut lumber. The notable exception is the cable, which is retired elevator and ferry cable donated from the US. I haven't seen much UN presence here; the project we are currently working on is in conjunction with a humanitarian facet of the EU.

When I said 18" footers, that was a mis-statement. The common approach here is to dig down several feet and pour an abutment of anywhere from probably 5 to 10 yards of cement. I suspect way overkill in some cases but I, too, hesitate to suggest changes without having a better idea of the forces involved.

ba/ael, can you elaborate on the abutments being properly anchored to the ground?

Hello montclair357,

The abutments are properly anchored to the ground if they are capable of resisting all forces applied to them with an adequate factor of safety.

De-stabilizing forces are primarily tension in the cables. Cable tension can result from wind and gravity loading.

Stabilizing forces or anchorage forces include:

1) Weight of concrete in abutment.

2) Shear between concrete and soil.

3) Passive pressure between abutment and soil.

4) Uplift and lateral capacity of deep foundations if applicable.

The factor of safety = Stabilizing Forces/De-stabilizing Forces and should be at least 3 under ideal conditions (having a geotechnical investigation). Under actual conditions, it should likely be higher. Nickname suggests it should be >10 which would likely be difficult to achieve.

ba/ael - I think this is exactly what I'm looking for. Forgive my ignorance, but a few questions -

- Shear between concrete and soil - are you talking about shearing between the soil and the concrete along the sides and bottom of the abutment that resists the pulling forward of the abutment by the cables? If so, this would be the equivalent of a friction coefficient I referred to earlier. How does one calculate this?

- Passive pressure between abutment and soil - is this the pressure against the front of the abutment as the cable tension is trying to pull it forward? Again, how does one calculate this?

- Uplift and lateral capacity of deep foundations - how deep is 'deep'? Does this have to do with hydrostatic pressure?

Again, pardon my ignorance here, but we parted ways with the CE's in engineering school after about statics, and my knowledge of soils and concrete foundation design is pretty nil. Is this basic foundation engineering? And, are there any publicly available sources of info on this that I could study up on?

1) Yes, shear force between concrete and soil, friction if you prefer. Shear strength may be expressed as s = c + σ*tanΦ where c = cohesion (a property of the soil), σ = effective normal stress and Φ is the angle of friction (a property of the soil). I consider shear on the bottom of the footing, not on the sides because shrinkage can cause a gap between the concrete and the soil on the sides.

2) Pressure at rest is the lateral pressure in soil under normal circumstances. It is sometimes labeled K₀. For granular soil, K₀ = 1 - sinΦ but for clay soils, it is considerably more complex. It is the lateral pressure which exists in soil in its natural state. So, if a soil has a density of gamma, then at depth z, the pressure will be gamma*z and the lateral pressure will be K₀*gamma*z.

Active pressure K_a is substantially less than K₀ and represents the pressure against a wall when the wall is being pushed by the soil (like a retaining wall) and is capable of deflecting away from the soil to reduce the pressure.

Passive pressure is substantially larger than K₀ and represents the pressure against a wall which is moving toward the soil mass and trying to displace it. There are several theories on passive pressure. According to the Rankine theory, K_p=tan²(45+Φ/2). Coulomb's expression for passive pressure is considerably more complicated.

I am not an expert in geotechnical engineering. When I design a foundation such as this, I always seek advice from someone with geotechnical expertise.

3) Deep foundations are normally considered to be piles. There are many types of piles...screw piles, friction piles, belled piles, Raymond piles, compaction piles, driven precast concrete or structural steel piles. They are deep compared to footings which are near the surface. In my area, the shallowest friction pile used for a residential foundation would be twelve feet in depth (in order to get well below frost penetration) but screw piles could be as shallow as ten feet. Many piles are twenty or thirty feet deep, depending on the soil strata at the site. In some cases, piles could be as deep as fifty or sixty feet, perhaps even more.

Rock anchors or prestressed anchors are another type of deep foundation.

The capacity of piles to resist lateral or vertical load depends on the type of pile, the size of pile and the characteristics of the soil. Recommendations of this sort would have to come from a geotechnical investigation.

I am not aware of any publicly available source of info that you could study up on. Soil mechanics is a complex field.

You should look at the safety coefficients which are in the case of such design > 10.

The anchors have to take into consideration the ground behaviour under unusual weather conditions as heavy rains which reduce by hydrostatic pressure the force between concrete massifs and supporting ground. This leads to a loss of "friction" and a possible sliding or an overload on the parts taking in charge the horizontal forces.

Are there not regulations in Honduras? There is also an empirical approach. You make an analysis of several existing bridges which are ok since several years in use and determine by those analysis the values to be considered for new designs.

I would like to add a comment. Bridges as you want to build as subject to transversal loads from wind and have a tendency to swing since if the structure is stable on the vertical it is quite deformable on the horizontal. I would suggest that you look at the possibility to have a transverse stabilisation symmetrical with cables using same approach as for the vertical loads. If it is not clear enough I can make a sketch.

With respect to safety, it is a usual coef. for transportation on the vertical of human beings as in elevators or other devices especially in those hanging on a cable. If you consider this value as too important I would suggest you use the backwards engineering I mentioned and base your new designs on the results of previous realisations. Experience is always better than estimations.

Something from the past to consider;

I remember a proposal by Pier Luigi Nervi some 30 years ago for a suspension bridge over the Strait of Messina showing basically a trapezoidal cross section of the bridge in order to give lateral stability i.e. the main cables set wider apart than the deck dimension.

Any comments?

Cheers

Vince

It is a very good suggestion since the same cables are used for the 2 functions:

- carry the load

- transverse rigidity

But it implies a more complex computation and may be more expensive anchor massifs. Due to the angle the local loads on the cable are amplified with 1/ cos(angle) which could lead to bigger cables. In fact it will be a problem of optimal design since the lateral forces are I presume low but a too small angle will not improve in an important way the transversal stability.

Hi!

Cable-stayed bridges are the simplest and most time-tested examples of cross drainage works, particularly in mountainous and inhospitable regions and where economy and ease of construction and maintenance are of primary importance.

The applied and computed forces generally dictate the sizing of the cables and the stringers to support the intended carriageway.

It must be remembered that cable-stayed bridges require sound and strong abutment conditions - free from loose or weathered rock and other unstable foundation conditions.The abutment size is essentially dictated: (a) by the width of the carriageway and (b) by the nature of the crossing - whether intended as a temporary measure or as a permanent structure for use over long durations in all states of climatic conditions.

Therefore, the abutment width is the sum of the carriageway width plus the width of the sidewalks, if there are any proposed. The height of the abutment is mainly dictated by the end conditions of the cable termination levels and the space required for mounting the cable-stayed mechanism - usually embedded anchor support systems. The structural stability of the abutments is computed, applying the usual factors of safety as for a traditional gravity structure built of the naturally available stones - in preference to the more expensive concrete construction.

Trust, this gives you some practical ideas of application in your area?

Regards.

PLAN

Hi and a warm welcome to the forum, I would like to thank you for the good work you are doing out there .

I had just typed in a lovely reply and was reviewing it when the system hung, so I'm having to start all over again, and hope that I remember most of it.

My advice is caution - simple suspension bridges are far from simple! As others have noted they rely entirely on their abutments (v difficult) and their cables (easy but exposed to elements). There is a massive leap from a simple rope swung between trees or similar abutments for foot traffic, to laden vehicles suspended from wires anchored in the ground!

Are you familiar with the FMEA concept / process? Basically it formalises consideration of risk and the chance of detection and consequences of failure. Consider the scenario - a bridge in such a place will get overloaded to its maximum physical capability, maintenance and inspection will be poor, therefore failure at some point is likely. For a suspension bridge the process of failure is likely to be immediate and catastrophic, either cable breakage (all on bridge dumped in ravine/water) or abutment failure (one or both cables slacken rapidly, all on bridge dumped into ravine/water), therefore the likelyhood of detection of imminent failure is low. Lastly what will be the consequences? for foot traffic an unpleasant experience, bruising and a dunking , for a mule the same (as long as its load doesn't drag it down), for a motorcycle, not much worse unless the riders get tangled up in it, for an open vehicle there is the chance of being crushed by it, for a closed vehicle then being trapped and drowned is a distinct possibility, as well as the loss of a valuable asset plus goods.

So a simple rope bridge can work and provide a low-cost, low tech sensible means of crossing for foot traffic, but a larger scale bridge for vehicular traffic needs a lot more care. Even rock abutments need careful consideration, never mind slabs of concrete poured into unknown soils.

Look at experience and what do we see? Over the centuries and all over the world the next steps from simple rope bridges are always trussed timber (or bamboo etc) frames or stone arches. There is probably a reason for this! Only when and where a complex society is present do we see the suspension structure starting to be used again for big bridges.

Sorry to be a wet blanket......

All the best and good luck, DP

Dear Montclair 357,

Firstly let me express my admiration for the work you are doing. Elsewhere in this region of the world my most trusted Friends did similar altruistic work many years ago . Since , I have never forgiven myself for not helping them more at the time . Such self-lessness is highly commendable and greatly rewarding . On or off the thread I will endeavour to help you as much as I can.

I do have some experience of the suspension bridges you are calling our attention to and I must say that the comments you have received so far are extremely sharp and to the point . What I would add at this stage is that : in hilly ( and generally wet terrains ) never trust an abutment that is purely poured concrete into such soil . Invariably (more so if you have earthquakes or more solicitation by way of high and cross winds ) these will affect adherence which otherwise you could factor if the foundations were supporting say : a truss bridge. Far better to anchor your abutments with simple wooden piled foundation. Simple pile driving methods are available and use of local hardwood piles ( yes even bamboo as used in: Indonesia is good to anchor abutments). So you do not need big piles but many small ones will suffice to give you peace of mind and longevity to your construction.( If you are not sure simply ask )

For simplicty of design and to complement what has been said to you, you should differentiate between : the main wires supporting the walk- way and those which can steady the bridge. A simple (again timber frame or even two timber frames on each side of the bridge can be built ) and at both extremities . These frames ( much wider than the bridge deck) will carry wire cable ( one on each side ) that are purely to stay the bridge. The wider these are from the center line of deck , the better as with smaller section connecting wires (adequately spaced ) they will steady the deck and stop it from swaying . Normally these two wires controlling the sway will actually be attached to the center of the bridge deck and on both sides thereof .Simple means of tightening these main staying cables are at hand : block and tackle, chain hoists, tire-forts etc. but again make sure that their anchoring is simply but soundly constructed and if possible : piled.

Best Regards,

Labor Omnia Vincit.

if you do ask for help from a school remember to send pictures when building and when done, this way they will have some return for their effort and be willing to help the next guy , who might just be you the next time.

Cant advise you on bridge design, but welcome to the site. Having been to Honduras numerous times doing water missions in rural villages near San Francisco de La Paz in Oloncho Department, I know the type of bridge of which you are speaking. Honduras is a lovely country.

Thanks for the reply, it's good to meet you. What sort of water projects have you worked on? I have a couple applications here that I may be interested in some input with.

Hello everybody:

Thanks a lot montclair357 for living and work in our beautiful country. I am honduran and I am thankful to you and to the fellows of this forum by the support they are offering you in your search for information.

I am Mechanical Engineer but, my field of work is the small hydroelectric power plants and the internal combustion engines. For that reason, I regret not to be of help to you in your bridge project but, any way, I will be ready to assist you in whatever I can.

So far, I only can say "DO NOT GIVE UP montclair357, GO AHEAD, noble people endorse to you".

"NO DESMAYES montclair357, ADELANTE, un pueblo noble te respalda".

Good luck!