(2) Qshake (Structural)
7 Jan 01 20:26
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.