Bridge loads 1

[DTS419]

Can anyone explain the difference between a lane load and a truck load?

The verbage on how to apply the 640 plf lane load in IBC is not clear at all.

How is a bridge beam to be designed for the max loading? And what's the difference between a lane load and a truck load?

 

[cntw1953]

DTS419:

This question belongs to college classroom, or work place. Lane load is uniform in nature, and truck load is series of concentrated loads.

Try to get your hands on a copy of design textbook, or AASHTO code, that would explain/answer your questions. Be ready for more questions. Good luck.

 

[DTS419]

My question was: What is the difference in how they are applied.

The code does not explain how to go from what it says to a structural model of a loaded member.

 

[cntw1953]

Code require (a form of instruction) to use whichever produces the maximum .....

 

[ImminentCollapse]

DS419,
I don't want to come down too hard, but Cntw did explain the difference, which is what you explicitly asked in the first post. It does come off as a very simplistic question, maybe you just worded it different from what you meant.

How to apply them is a slightly different question, but AASHTO is pretty clear in my opinion on how to apply the loads. I'm not sure I understand why you reference IBC.

IC

 

[bridgebuster]

If you have a copy of the AASHTO Standard Specs, go to Appendix A - Table of Maximum Moments etc.

It gives you the range of span lengths for which truck and lane govern for SIMPLE SPANS. Generally, for continous spans it's a copmbination of both lane and truck. Lane load is often used to produce maximum negative moment; truck used for maximum positive. However, for long spans you need to compare both.

follow cntw's advice.

 

[DTS419]

Yes, it is a simple question I realize that. I will elaborate...

My training was specific to buildings and my background is concrete structures such as tanks and dams. I never had to apply and AASHTO load to a bridge in the conventional way.

Every time I have designed a slab, culvert, etc. for a vehicle load I simply applied the wheels as point loads at the locations that would cause controlling moment/shear.

In reading the AASHTO section on loading (which I do not have a current copy of at my office), it strikes me as very vague considering the possibilities of members that it could be applied to.

I don't find it clear how to load a 6 lane highway bridge that spans 80' versus a one-lane fire trail bridge that crosses a 15' creek.

I also find the verbage that explains the "lane load" to be vague. It is a lineal load and it says to apply it over a 10' line that is normal to the center line of the lane. Ok, is that it? I just put one 640 plf load across the lane and I'm done?

When is this lane load applied instead of or with the truck load? Seems to me that the load on a large span bridge would need to be a function of the span of the bridge, or the quantity of possible traffic loading.

Unfortunately like most things in this profession anymore, I find that the trend is to rely on computers. I have asked several friends and colleagues of mine who do bridge design for a living and have found that they really don't understand that actual application of the loads because the computer does it for them.

So would anyone care to explain how exactly the AASHTO loads are to be applied now that we're passed the fact that this is a stupid elementary question?

 

[Qshake]

DTS419 - The question you've asked in the latest post is not an easy one to answer as it entails several different sections of the AASHTO code. Not to mention which AASHTO code because the new LRFD code now requires that the truck load be combined with the lane load whereas the older code allowed the maximum of the two.

That said, lane loads are not used for fatigue design either and so again it points to the overall complexity of the matter.

If your friends make their living designing bridges and can't explain the matter verbally, why do you think some is going to sit down and write a thesis for your question?!

My best offer would be to check out the following:

Bridge Engineering by Tonias or
Theory and Design of Bridges by Xanthakos or even
FHWA website and view some examples.

The short answer to a six lane highway is that both the truck and lane loads are applied to all lanes and the resulting reactions (Moments, shears and reactions) are multiplied by a distribution factor (accounting for many, many things) and again multiplied by a multiple presence factor to account for the probability that not all lanes will be loaded at the same time. Then the reaction (moment, shear, or reactions) can be plotted for a single girder line and that girder designed. A single lane structure would obviously only have one lane loaded with the truck and lane.

I hope this helps you understand that it's lengthy and to be fully understood should be experienced first hand under the tutelage of a mentor.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[hokie66]

A lane load is applied per foot of span, not per foot of width. The 10' width in your example defines the structural width that resists the lane load.

 

[IJR]

DIS419

You sound a bit bored or frustrated from reading some texts on bridge design.

Qshake and hokie66 have described the issues better(and they are really great engineers).

Some basics you should be able to visualize though loads are highly statistical and design vehicle loads are not really physical, they are "design" loads. A lane is simply a width of roadway where a number of vehicles may be found back to back(think of a lane at jammed traffic). Code says this is 10ft, no matter what. Isnt it logical that some line load can be assigned over the length of the bridge for this width? Next you have say a military truck crossing the bridge. Isnt it logical that you have point loads to consider too.

Which one is critical for a long bridge, a series of vehicles per lane jammed in traffic or 3 to 4 military trucks permitted to cross at late night?

It always helps to use some common sense first. And it never helps to argue against a code.

IJR

 

[crossframe]

DTS419,

Simplistically, think of the lane load as a uniform 64psf, applied over a 10 width (which gets you the 640plf) for the entire span length. There are also associated concentrated loads placed along the span to generate the maximum moment and shear effects. This applies to any and all lanes on the bridge.

The truck load consists of one or more three axle trucks moved along the bridge span to determine the maximum moment and shear effects. This also applies to any and all lanes. Remember influence lines? This is why your colleagues use a computer.

The respective lane and truck loads are combined or not, factored or not, as described above and in the code.

All of this is explained, including how to go from what is said to a structural model, in the AASHTO code. This seems to be the big missing piece of your puzzle.

 

[miecz]

DTS419-

I know what your going through, as I made the transition from Wastwater Treatment Facilitites to Bridge Design a few years back. As you say, many bridge engineers have been using canned software for so long they can't explain how to do it by hand.

Analyzing a simple span bridge for live load is essentially a two step process. First, analyze a single imaginary beam for the lane/axle loads. Then, apply a series of factors to account for impact, load distribution, and load case.

From your posts, I'm guessing that your struggling most with getting the maximum moment and shear on the single beam for the standard truck load and for the New Jersey Permit vehicle. It's plain to see where the truck needs to be positioned to give maximum shear, but for permit vehicle moment, it's trial and error.

Keep in mind that the maximum moment will occur beneath one of the axle loads when the center of the span is halfway between that load and the resultant of all the axle loads. Using that principle, you can usually find the truck position that gives the maximum moment pretty quickly.

 

[bridgebuster]

DTS419 - perhaps the attached will help. it's from an old California DOH Manual.

miecz - Occasionally, I still use the influence lines in the old AISC publication "Moments, Shears, and Reactions",