Fundamental Interpretation of GDFs 1

[PetieSmo]

Good day all,

I am a Mechanical Engineer working on an instrumentation and analysis project for live load testing on highway bridges. There are three bridges designed with identical substructure and superstructure (girders). However, the deck composition has been changed so that we may compare the three.

We have performed the live load testing and now we are analyzing the data. The Civil Engineers in the group have calculated GDFs for the girders in these bridges. I am attempting to assimilate this information into my analysis, but have yet to find a good source which explains how to interpret what is happening. I understand (I think) that they allow us to evaluate how much of the wheel load is being carried by the separate girders in the bridge (this bridge has 4 girders, 15m spans).

However, I have heard interpretations float around about "apparent transverse" and "apparent longitudinal" stiffness that may be derived from GDFs. This may mean nothing, but it could mean something. I would just like to know what everyone else has to say about how to interpret those numbers. Any help will be greatly appreciated.

Smo

 

[Qshake]

Being a mechanical engineer and one I assume to have some engineering mechanics background ( I'm not referring to statics and dynamics but advanced mechanics), you will note the simplicity of the Distribution Factors (DF).

My first note to you is to be wary of the overconservative nature of the DFs. From a mechanics point of view, the bridge superstructure can be treated as a plate (slab) reinforced with ribs (girders). However this is rarely ever done and with good reason - most superstructures don't fail by themselves. The origin of the superstructure analysis is noted in AASHTO and may be of interest to your studies. Also of interest is the results of a literature search on superstructure analysis and testing.

Much of the work behind the superstructure and distribution factors was done by Newmark in the 30s. You will see references to surface influence lines of German origin. Texts were written on the subject and other methods developed. Bakht and Jaeger come to mind as does NCHRP technical manuals.

There is a lot of information out there...good luck.

Regards,
Qshake

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[PetieSmo]

QShake,

Thanks for the sources. I will take some time to investigate them. I'm still a little unclear about how to interpret the numbers I'm seeing.

Most specifically, if I see a higher DF for one specific girder, what does that tell me about the nature of the deck and/or girder?

If I see a more even spread of the DFs across the girders, does that indicate that the deck is softer/stiffer? Is this softness/stiffness in the transverse or longitudinal direction?

Does a stiffer transverse deck give an "apparently" softer longitudinal response?

Am I way off-base?

PetieSmo