Load on the bridge

[Nowaday]

Hello professionnals,

I just want to have a quick and simple way of loading a bridge beam. I am sure that other than the somewhat complicated writings of AASHTO, they should be nowaday tables of live load in kip/ft + kip(for concentrated) for bridge beam.

So let me ask this as an example:
-If the bridge were simple span(3 spans), 2 lanes, 5 beams under. What should be the distributed load on each beam? or do I need any concentrated load for that? Please can you answer me this with an example.

-If the bridge were continous (2 spans), 3lanes, 7 beams under.

Do you guys know that in mathematical point of view, so many loadings prescribed in AASHtO cancelled each other sometimes, that designing the bridge beam for dead load and traffic load(vehicle, truck) it is already enough unless we are dealing in a earthquake or windy area?

Thanks guys

 

[graybeach]

The solution to your problem is experience. If you have experience designing dozens of bridges, you might be able to make such estimates of the top of your head. If you do not have experience, you need to go through the design process.

"Moments, Shears and Reactions for Continuous Highway Bridges" ASCE 1966, might be helpful if you can find it.

 

[MiketheEngineer]

Follow AASHTO - there are no quick and dirty answers unless you just want a SWAG

 

[msquared48]

"California Bridge Design Manual", 1963, is my bible here in conjunction with AASHTO.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[BridgeEI]

Caltrans has a summary for simple spans using the HL-93 loading. The article graybeach listed is really god but it uses the old HS20, you an still find it on the aisc website.

 

[Nowaday]

Some of the answers didnt help at all. The reason I am asking this is because I am new in the field. If I had experience, I wouldnt come up and ask.
We all know that we dont come here to get final answers, but just an estimate so to be sure that we are on the right path.

Mr BridgeEI, is that okay for example for a 3 span bridge, 3 lanes, 7 bridges under

-I load each beam with 640 Ib/ft + 18 kips of concentrated load on the middle span, to get maximum positive moment? ( I am talking here for trafic load only(truck or vehicle)). Just wanna know that if I ll be safe with this method or close to safety when comes about failure.

-It is that true that if one designes bridge beam for dead load and live load(moving truck or vehicle), that the bridge wont fail, regardless if other extra load is not applied correctly(like wind on vehicle, braking force, etc..)?

Thank you guys in advance

 

[bridgebuster]

Wind on live load, wind on superstructure, breaking, force, etc are not applied to a stringer or girder.

640plf is applied over a 10' width.

AISC Moments, Shears and Reactions for Continuous Highway Bridges" is the best thing to use if you have a continuous bridge; otherwise just down load a table a live live rmoments, shears, and reactions for simple span bridges. You're also overlooking impact and distribution of live load. If you want to be conservative just for add 33% for impact. Distribution will vary based on member spacing and deck type - be conservative add 50%.

 

[BridgeEI]

I'm not sure what the 7 bridges (beams maybe?) under means. However, if you look at the AISC table, it should get you the moments/shears/reactions you are looking for, but this table is for only a single loaded lane of traffic.

Like bridgebuster said, you need to determine the distribution of live load. This depends on the beam spacing. Once you get that, then you know the number of lanes that each girder will support.

I'm sure there are plenty of examples out that do it line by line, just search for it on the internet.

 

[rapt]

I will be one of the unhelpful posters and agree with graybeach.

The way to get experience is to do the calculations yourself. After doing it several/many times you get a feel for what you have to check. You cannot get this experience using the modern method of googling it and becoming an instant expert. You have to do it yourself. How can you know that any answer you get is correct. Spend the time over a few years doing it all and you will gain the necessary experience if you are any good.