LIVE LOAD DISTRIBUTION FACTORS 1

[sponcyv]

I am using the Standard Specifications for Highway Bridges 17th edition in the design of a concrete bridge with precast concrete girders. Section 3.23 discusses distribution of wheel loads (Truck Load) to stringers, but I do not see any mention of lane load to stringers. Am I missing something? Do the equations used to find DFv for truck loading apply when lane loading controls?

 

[asixth]

I thought wheel loads and lane loads need to be applied simultaneously.

Lane loads are a uniform load and will be distributed to the girders more evenly than large point loads from traffic. I don't know of any but I am sure that there are tables out there for bridge deck design that can calculate the distribution from lane loading, but I would not use the same factors for point loads because this will give you very high results.

I have never used tables or distibution factors for prestressed girder calculations. I prefer to model the structure with the deck slab and let the software calculate the load sharing effects.

Do you have access to a F.E program?

 

[Qshake]

The lane load is uniform in the longitudinal direction owning to the fact that it is modeling trucks idle on a bridge bumper to bumper. As such, in the transverse direction, the direction for distribution, the trucks still have the same axle configuration as a typical truck.

IMHO, a FE program is not a tool to use for someone starting out in bridge design and with little FE experience. The stiffness is affected by many factors pending the actual configuration. And while these factors may not all be applicable for prestressed concrete I-girders it is important to make note of them. Such factors include: connection of the girders to the deck, connection of the girders cg to the cg of the deck (several ways to model), differing modulus of elasticity, cracked sections, etc.

It's better to go back to AASHTO and understand what and how the formulas are to be applied.

There are several good examples available on the FHWA website concerning prestressed girder design which should provide some insight on the application of the formula.

Good Luck.

Regards,
Qshake

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[miecz]

sponcyv

You're right, 3.23 doesn't say what to do with lane loads. I think the key is to look at 3.7 where AASHTO says the standared loading is "standard trucks or lane loads that are equivalent to truck trains." The footnote to 3.7.1 discusses how lane loads were developed to be a simplification of truck loads. As such, I believe the distribution factors of 3.23 apply to lane loads as well as truck loads. Of course, the values of table 3.23.1 are for wheel loads, not axle loads, so those values must be factored by 2.

 

[pozze]

hello
i dont know for what codes you talk, but as we use Eurocodes, i think that they have clearly explained that problem
thnx