huangyhg

basic masonry bridge building in developing countries
i'm a civil (mainly environmental office work) engineer who has just found myself in africa building small bridges. what's around are bricks and cement, and the odd steel beams.
having never partaken in this type of work before, i would welcome any tips or comments.
i intend to build masnry walls (columns)either side and span a few beams... not the most technically designed piece, but have very little access to info from here and internet doesn't seem to be helping me too much. the bridges are generally about 7 metres in length and about 3 metres high.
any comments would be appreciated.
this sounds scary. i can see all kinds of problems from the foundation to the deck.
do you have good foundation material? if you do, can you use masonry arches? this would be uncommon in the developed world because it is labor intensive but i assume labor is cheap there. if an arch has a good foundation and has enough opening to pass floods it will be very strong and last a long time.
be upfront with your office and ask them for someone with more experience to have your work checked time to time and for guidance. some disaster may bring you bad name, but, i bet, honesty wont.
a few thoughts to consider:
1. break out your structural text and study the sections on small structures.
2. define the loads your bridges are to carry. are they for foot traffic, cart traffic, car traffic, or truck traffic? keep in mind that if the bridge deck is wide enough for a car or truck, they will try to cross it, even if it is only designed for foot traffic.
3. make a resonable and conservative assessment of the soil conditions at each site. select a soil bearing capacity for the foundations. a guess, site unseen, would be between 1,000 psf to 3,000 psf. unless you are on rock.
4. keep the bridge design as simple as possible, simple span, simple supports. is lifting equipment available or is everything installed by hand?
5. design your framing system, using the available materials, with an eye toward what deck materials you will be using.
6. don't forget a curb and railing!
best of luck! you have an opportunity to make a real difference!
thanks for the replies.
isntbard:thanks for the cautionary note. the ground is generally clay and sand! not ideal. wouldn't masonry arches be quite difficult to construct? even rock is difficult to come by, and i wouldn't be quite sure of driving some of our ten ton trucks over it.
i was thinking of building big, to ensure safety factors.
however, on your advice, i will look into this option further.
flame: dont worry, i have never been one to feign knowledge. i have informed others, but out here, there isn't much support.cheers.
jheidt: thanks for the considered reply. all is hand work and most of your points are extremely relevant. i appreciate the sentiment.
brian,
before you go for a design, why don't you check with some government agencies for what moving load they have adopted, which design code they follow... and so on. or have you already done that? if they don't have such standards then you may follow some appropriate design code, so that you could defend your design.
good luck.
i agree with all of the points made by jheidt2543 and wished to communicate my support of the same. it also appears that anbesaw might have some good reference material.
as mentioned earlier, site unseen, i would have estimated bearing to be around 2000 psf. thus a fairly large foundation may be required especially since masonry will be heavy compared to smaller concrete supports. however, one point not raised is scour. the devastating forces of water should not be underestimated. having said that you'll need to embed your foundation quite abit to achieve stability during a flood.
while we're on the issue of floods another force to be reckoned with is waterborne debris. you'll need to make sure you have enough freeboard (usually 2') to the lowest point of the superstructure. naturally the question would be freeboard for what flood occurance.... i would simply look for highwater marks and go from there unless you have better data. ultimately it sounds like you have an abundance of natural materials and cheap labor. so a detailed analysis for somethings can be dimissed based on conservative estimates.
i suspect your deck will be made of wood planking. yet, i'm not sure how it would be fastened. on the other hand maybe its made of concrete if available. you could use arch bridges with closed spandrels and then pave a new roadway ontop.
also, there are an abundance of bridge sites on the internet though certainly many of them don't go into much detail for design. try
brianireland,
how did you find yourself in this situation?
as a structural engineer it makes me a little nervous. but as a engineer tired of designing commercial & highrise structures for ultra-wealthy clients while barely scraping by as a design professional it sounds exciting.
i've lost any passion for this work and would love the opportunity to do some work in a developing part of the world. let me know how you got into this? and keep us posted on how it goes.
brick bridge abutments have been around for many many years overseas. we have some on our project (the existing carriageway) that have been in place for over 25 years and in making the new carriageway, we have not redone the old ones. for the small span bridge types you are doing, you can also use mortar rubble - the walls would be about a metre to metre and a half thick for moderate heights - someone told me once what the ratio (thickness to height) but i've forgotten. in mortar rubble, you use rock fragments (min size about 150mm) and you then mortar them together with 2:1 mortar (sand/cement and water). they work fine. simple seating of top slab - in fact, our slabs were precast rcc placed on simple seats - not even on bearing pads but on the abutment itself. worked okay. othere comments about flooding, etc. are common to most bridges. i see no need for an arch bridge for a 7m span. we did some stone arch culverts in china but they were a pain to do - much easier to do the mortar rubble wall and then use simple rcc spans across. i'll look up some information for you.
thanks all,
the info. supplied will certainly help me out a lot (and give me more confidence at the end of the day).
anbesaw: will try to get my hands on the doc. you have recommended. thanks.
ctseng: a bit like yourself, i found myself getting tired of office work (5yrs) in an environmental office. i was project manager on a number of large environmental projects. however, i found myself getting far too mature, far too fast so quit and have taken a year out doing volunteer development work in the congo. that's why i needed to ask the previous questions as i have had no experience in bridge building. i am also involved with other types of work but it is the bridges that i'm worried about.
i am working for an ngo (non-governmental organisation) and there are many job oppurtunities available by searching the web. it is extremely satisfying (but difficult) work.
in case it sways you, some ngo's do pay good money even for first-timers.
bigh: thanks for the info. will wait for more. thanks.
brian,
you must be bewildered by now with all the bridge design tips. for an engineer who has never designed a bridge it will be confusing where to start and what to look for and what is the sequence. because there are some unique parameters such as flood and water-way, scour depth, moving vehicle loads etc. let me summarize what you need to do:
1. collect information about highest flood level and width of the river during that flood. also find out what kind of debris the river brings during floods (trees, boulders, etc)
2. find out or determine the standard design vehicle for moving load and width of carriageway
3. determine what material you are going to use for the deck, sub-structure and foundation
4. now you can proceed forward to the design. the first thing to do is determine the length of the bridge. then according to available material and technology fix the number and lengths of spans
4. in a bridge, the deck (superstructure) is designed first. then you go for the substructure and foundation. design of the abutment/piers and foundation is also influenced by flood forces and scour depth along with the forces from the superstructure.
5. lastly, you may need to design some river training structures such as guide bunds, floor protection, check-dams etc.
now get a good book and go ahead. good luck!

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