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Minimum number of girders of prestressed bridge

Minimum number of girders of prestressed bridge

Minimum number of girders of prestressed bridge

(OP)
Hi,

I am designing two lanes 30m span prestressed bridge and the width of the bridge is 8.3m for south bound lane for expressway. I have put four girders with girder spacing of 1.8m. But my boss is not happy that I am using four girders he is asking me where in the code it mentions minimum number of girders is four. I have found in Texas Design Specification that minimum number of girder is four. But can anyone direct me where I can find that the minimum number of girders is four.

Thanks,

RE: Minimum number of girders of prestressed bridge

i'd suggest that if your boss is unhappy with four girders then he's got his own reasons (why 4 isn't desirable), and it's probably "better" for your future prospects if you go along with his preference. unless you can build a comprehensive case as to why 4 is "better" ... cheaper manufacturing and assembly ?

do you guys have to consider failed structure ? ie would you have to carry load on only 3 of the 4 (or 4 of 5) girders ?

another day in paradise, or is paradise one day closer ?

RE: Minimum number of girders of prestressed bridge

I'm not aware of any AASHTO requirement and a search of the current LRFD spec didn't come up with any minimum. NYSDOT prefers a minimum of four girders; however, this isn't an absolute. Although I'm sure two would not be permitted except for a pedestrian bridge. Three girders were always considered to be the "magic number" for redundancy.

RE: Minimum number of girders of prestressed bridge

Some DOT's prefer at least four girders because it can make future deck replacements easier in two stages, leaving half the bridge open. It's difficult with three girders since a joint is required over the center girder which isn't flexible in layout and is weak during construction. It also reduces the amount of transverse steel in the deck which has cost and durability advantages (reduces the exterior span lengths). Distribution is also improved though I don't know if it's based on that. It's dictated by the client/DOT and as bridgebuster says not necessarily an absolute requirement. If you want to pursue it you could talk to your client/DOT.

RE: Minimum number of girders of prestressed bridge

i thought he'd proposed 4 girders and his boss wanted more ...

another day in paradise, or is paradise one day closer ?

RE: Minimum number of girders of prestressed bridge

Sounds to me like the boss wants less than 4. Oh, well.
CooperDBM makes several good points.

www.PeirceEngineering.com

RE: Minimum number of girders of prestressed bridge

In the UK precast manufacturers of bridge beams distribute span tables in their literature which are helpful for detailed design. Note sure if theres anything similar in the US though, the tendon arrangement is usually designed by the precaster as well.

RE: Minimum number of girders of prestressed bridge

PEinc ... true enough ... boss may want less ... usually boss gets his/her way ...

another day in paradise, or is paradise one day closer ?

RE: Minimum number of girders of prestressed bridge

(OP)
His reasons are there are bridges with three girders and the cost of saving a girder will be alot. But in my opinion girder height will increase(for 3 girders) and the cost will not be that of difference. If we are saving a penny today and spending pound tomorrow for deck replacement then there is no use. Its hard to work with people when its my way or highway.

RE: Minimum number of girders of prestressed bridge

yes, the three girders would be bigger than a four girder design but installation time would be less.

yes, the road deck might not be as durable (with three girders) but the penny you are saving today is yours (or maybe the current politician's if you're bidding for the work now) and the pound you're spending tomorrow is someone else's ! an important business/politics consideration !!

and, yes, I know that's not how an engineer is supposed to think but that's pretty much how the world thinks today.

this usually leads me back to thinking about ...
a) the definition of a horsepower ... more than double what any reasonable horse can produce (Watt didn't want to be beaten by some overly ambitious nag), and
b) the London sewerage system ... built so over-spec that it still works pretty well today ... these days any highway is usually undersize when it's completed (we don't build in much room for growth).

another day in paradise, or is paradise one day closer ?

RE: Minimum number of girders of prestressed bridge

rb1957 any client with a sizable bridge inventory, like a DOT, wouldn't consider future rehab costs someone else's problem. These clients are already dealing with expensive rehabs today that resulted from poor decisions made thirty years ago. They're pretty motivated to pay for better designs today for their own bridges which they will still be responsible for in the future. To save costs today they may compromise on things like aesthetics. A good consultant would have that in mind.

struct123456 if your client (is it the Texas DOT?) has a minimum girder requirement then they should be part of the decision on economics in the end. You'll need their approval anyway.

RE: Minimum number of girders of prestressed bridge

(OP)
No its not Texas DOT. It is not in the US. Current trend in the country is three girders but we are pushing for four girders.
Also, please pardon my question. The RCC piers are almost 45m in height. I have not designed over 20m piers. Any thoughts?

RE: Minimum number of girders of prestressed bridge

I can see the point from both sides of the fence. On one hand I agree with your boss, but as others have suggested there is a penalty with respect to long term serviceability. The question I have is whom has met with the client at this point? That is the real boss. I am assuming your firm does a lot of bridge design based on the project scope and I would presume your boss is an experienced bridge engineer such as bridgebuster. It seems to me you are in the initial design phase so I wonder if your boss wants to present more than one option to the owner when they meet with a 50% design package? I am also curious how you reacted when you met with your boss. Based on the focus of your OP I guess you were argumentative (if not, I apologize). Years ago I had two engineers working for me. Both had impeccable analytical skills, but one would always argue endlessly when asked to evaluate multiple options. Guess whom might have been let go if times got tough? The other option is he is racking up fees. There is nothing wrong with that if it is justifiably. He/she likely has someone to answer to as well.

Now if your boss is an accountant, all bets are off.

RE: Minimum number of girders of prestressed bridge

ah, be fair to the accountant. his interest is the company dollar bills. it's his job to make sure the company spends what it needs to, to cover it's committments. it's the CEO's job to realise that "over-building" has value in the long term and to approve additional expenses. unfortunately most CEOs appreciate the short term value in "under-building" ...

it's also the client's position to value long term benefits over short term costs (and so not give the contract to the lowest bidder) ... but that means that one group of politicians pays for it, and another group (presumably the ones who pushed the others out of office 'cause of "inflated" spending) who'd reap the benefit.

however not giving the contract to the lowest bidder would probably attract a law suit (in the US) ...
company ... "you favoured another proposal, even though our's was cheaper"
politician ... "yes, but there design was 'better'"
company ... "your RFP didn't express the 'value' of these features. our design met the contract requirements and was cheaper"
politician ... "yes, but their's was 'better'"
company ... "ours would have been 'better' if you'd included those features in the RFP"
sigh

another day in paradise, or is paradise one day closer ?

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