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live load on bridge

live load on bridge

live load on bridge

hi all,

I am a entry level bridge designer in Phoenix. I am having a difficulty on understanding the live load.
Especially i do not understand what is difference between lane load and truck load.
which case i should use lane load and which case i should use truck load to calculate critical live load moment and shear?

and how to deal with distributed load and concentrated load?
i am very confused... please help me. which case i should use distributed load and which case i should use concentrated load?

i did not take any class about truck live load. i only know the live load on building.
please help me. i am having a bad dream everynight.

Thank you so much.

happy new year.


RE: live load on bridge

This is an important area for bridge engineers; it is addressed in many texts on bridge design.  You should become familar with those texts.  Please see the Frequently Asked Questions (FAQ) section of this forum.  Of course AASHTO is imperative to understanding this concern.  Also check out at the library of the local university (should be no problem in Phoenix) the AISC reference: Moments, Shear, and Reactions for Continuous Highway Bridges.  This publication will make your life easier and it has examples to boot.

Having said that, consider:

First of all know the classes of live loading.  Most roads are designed to HS20-44 unless reduced to something lower by a client.  All interstate bridges are now designed using HS20-44 Modified or the HS25 vehicle.  The lower weight vehicles are H15 and HS15.  The difference between H and HS is the number of axles; HS has 3 while H carries load only on 2.  

As an example we're going to assume you're working with a HS20-44.

The axle loads for a HS20 vehicle are 8 Kips on the front axle and 32 kips on each back axle.  Thus the single wheel line (1/2 - axle) loads are 4 kips, 16 kips and 16 kips.  The wheel loads or axle loads should be spaced 14 feet apart.  This load configuration should be run over the length of the bridge at 10th or 20th points to ensure a the proper envelope of moments, shears and reactions.

The lane loading for the HS20 is 640 pounds per foot with additional concentrated loads (applied to effect the critical loading)18 kips for moment and 26 kips for shear.

You will apply both the truck loading and lane loading.  In general, truck loading will control for short/medium span bridges while lane loads control for long span structures.  In addition, you could find that lane load plus the AASHTO prescribed concentrated loads will also control the negative moment for continuous bridges in short to long spans.

Above I mentioned how to apply the truck loading (assuming you don't have the AISC reference).  Now the lane load (also an axle load) is applied like any uniform load and the concentrated loads are applied where necessary to achieve the maximum positive moment, negative moment, and shears.  Influence lines, by the way, are also useful.  To apply the concentrated loads, think about what response you are looking for and how to achieve it.  For example, for a two span bridge, the maximum negative moment can be achieved by placing the uniform load over the entire length of bridge and placing the concentrated loads at the 0.6 pt of the first span and the 0.4 point of the second span.  Other arrangements will produce the maximum shears and positive moment.

The results of the truck and lane loading are compared and an envelope is developed.  This envelope is then used to design the girders.

I hope this helps.  I really suggest that you consult the AISC reference or discuss this with a senior engineer.  Now is the time to bring it up and get solid answers.  If there is a problem later it will affect much more than the superstructure design.  

Good Luck!

RE: live load on bridge

First of all, thanks so much for your kind advise.
I would like to summarize what i have undertood and ask you further question if you do not mind.

To obtain max. moment, shear and reaction from moving load, we need to consider two kind of truck loading.

first is truck loading with axle loads. second is lane loading with an axle load.

axle loads(1/2 of an axle, here why we use half not full?) are moving along the bridge span with fixed width. so, to obtain the max. load case we move the fixed axle loads and find out which gives most criticle case. right? this is i understand. but what is "E"? is it same as DF? i am confused here a lot. and divide by that axle load. and why? this make me confused a lot. and still. some book said i need to only consider only one axle load to find out max load case. why is that so?please tell me about it.

lane load has uniform load with an axle load which will move along the span to produce max. load case. right?
why we use lane load? i mean we already consider axle load case. this one kills me. please tell me about it, sir.

another queation is that if there is two or three trucks are moving together(perpendicually) or moving with little bit different distance. i mean trucks are moving not one lane but little bit different distance and moving together. this case how we gonna consider its behavior? did we consider it or not?

and, to use influence lines, should i develop all of the case and use it or there are some other sourse for it? both loading case above, we have to use to find max. loading or only lane load case?

please help me about my confusion.

lastly i was looking at AISC books, i could not find for bridge but building. could you please tell me exact title of this manual?

I know i am asking you kind stupid questions, but i never had chance to even think of this kind of loading in my life. code and books are still hard to understand for my current knewledge. i could not sleep well due to this matter. please once again help me sir.

RE: live load on bridge

I will try to answer as many questions as possible but I do not know the nature of your bridge so the answers are going to be general at best.

The AISC book is "Moments, Shears and Reactions for Continuous Highway Bridges".  It is(was) published by AISC.  The last date that I know of is 1989 but I'm sure it is still available at AISC and should be in many libraries.

Why do we use only 1/2 of the axle loading?  Well, we're trying to simplify the analysis.  When designing a girder/slab bridge or a section of slab for concrete frame bridges we don't use plate theory, we simply make some assumptions about the behavior of the individual sections (tributary sections) of the bridge slab.  This way we can analyze one girder or one foot section of slab.  For live load this means that the axle load has to broken up as the actual distribution to the girder or section of slab is not all going to 1 girder or a one foot section.  This is where the distribution factors come into use.  Distribution factors (DF) are used when there is another component available to resist the load other than the concrete deck.  When the concrete deck is the only resisting element then AASHTO prescribes the use of "E".  They are similar but are for different applications.  For more information on distribution factors and live load distribution see Bahkt and Jager's Bridge Analysis Simplified.  Incidentally, the analysis of live load distribution is based on plate work and influence lines done by Newmark back in the 30s at Univ. Illinois.

Lastly, another live load that must be accounted for on interstates is the military loading.  This load governs on short structures but must be checked as it could govern specific responses on medium spans as well.  AASHTO gives a description of this loading.

I hope this helps.

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