LLDF using refined and simple line-beam analysis

[Shz713]

Hi all,

Can someone explain to me how live load distribution factors can be obtained using refined analysis (like grillage/FE) and simple beam analysis?
I would like to compare the LLDF obtained in such manner with that of AASHTO specifications (only for my own understanding!).
Any illustrative example for any kind of bridge deck is deeply appreciated, I just need to know the concept.

g = DF (refined)/ DF (simplified)


Shoot for the Moon, even if U miss, U still land among Stars!

 

[yakpol]

I guess there're a few ways to accomplish it.
1.

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Perform moving load analysis on your refined 3d multi-beam structure. Use multi-lane coefficients where appropriate.
2. Report maximum moment or shear for one particular beam, call it M3d
3. Built 2d continuous beam model and find forces due to a single lane moving load.
4. Report maximum Moment or shear, call it M2d
5. Distribution factor DF = M3d/M2d

So the next time you want to skip 3d analysis, you can find M3d = DF*M2d

Hope it helps!
Yakpol

 

[Shz713]

Thanks for comment. Based on your explanations:

-For simple line analysis (2D), do we consider the bridge to be simply supported (in case of single or continuous span) or we take into account actual support fixities of each girder?
-For FE analysis, is truck loading/unit loading positioned longitudinally or transversely for specific lane to get the adverse load effects for interior and exterior girders?
-And lastly, H20-S44 is considered as ML?

Btw, your spreadsheets are shibby specially Monty Hall Game . I cannot use Brudge2D as I don't have GT STRUDL

cheers

Shoot for the Moon, even if U miss, U still land among Stars!

 

[yakpol]

Shz,
Span restraining conditions for both models shall be the same. Bending diagram due to permanent loads shall closely match.
3d model shall be analyzed for several transverse locations of traffic lanes to maximize demands on interior and exterior beams.
Trucks shall move along the bridge.
Any vehicle is the moving load, it can be placed anywhere on the bridge.
Gobeam as stand alone spreadsheet with nearly the same capabilities as bridge 2d.

Regards,

 

[Shz713]

Your explanations are clear.
Just one more thing; in 2D analysis, what sectional properties (inertia and so on) will be used? Is it for whole girders or individual calculation for interior and exterior girders?

Shoot for the Moon, even if U miss, U still land among Stars!

 

[yakpol]

I would analyze a single line of beams and scale down substructure stiffness proportionally.

 

[Shz713]

Interesting, then in that case we can get the properties of each girder similar to grillage analogy .
But if in 2D analysis we're interested in Moment, shear or reaction, then the exact properties would it matter as long as the length, loading and support conditions are properly modelled?

Shoot for the Moon, even if U miss, U still land among Stars!

 

[yakpol]

The girder properties will matter in case of moment connection to the columns and if spans have different depth.

 

[Shz713]

Okay, got it now.
I need to take into account geometric variation of deck and when there's pier support in case of multi span decks.

Shoot for the Moon, even if U miss, U still land among Stars!

 

[Shz713]

Snap!
Just found there is LLDF as per Australian which is superseded long time but still pretty handy for grillage and line beam analysis. The awkward point that there is no article for DF in existing bridge codes in AS !
Attached table is from NARSA 1976.

Cheers,

Shoot for the Moon, even if U miss, U still land among Stars!