Live Load Distribution Factors for concrete T girders

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YL0101
Live Load Distribution Factors for concrete T girders

Hello, I'm working on a load rating for a simple span concrete T girder bridge. It consists of 7 girders with shear connectors between girders, but no CIP concrete deck.

I noticed that the Moment LLFDs for interior girders are different when using different girder connectivity, as shown below.

For my understanding, the connection 1 should has greater LLDFs for both moment and shear, but the results below show the moment LLDF are smaller for connection1 and greater for connection 2. Could someone please can provide some explanations on that?

Thank you so much!
Yuan

Connection 1). Prevent Relative Vertical Displacement:

Strength/Service Fatigue/Special Permit Rating
+M -M V +M -M V
Girder A 0.450 ------ 0.450 0.375 ------ 0.375
Girder B 0.363 ------ 0.600 0.302 ------ 0.500
Girder C 0.363 ------ 0.600 0.302 ------ 0.500
Girder D 0.363 ------ 0.600 0.302 ------ 0.500
Girder E 0.363 ------ 0.600 0.302 ------ 0.500
Girder F 0.363 ------ 0.600 0.302 ------ 0.500
Girder G 0.450 ------ 0.450 0.375 ------ 0.375

Connection 2). Sufficient to make girders act as a unit:

Strength/Service Fatigue/Special Permit Rating
+M -M V +M -M V
Girder A 0.450 ------ 0.450 0.375 ------ 0.375
Girder B 0.466 ------ 0.520 0.311 ------ 0.433
Girder C 0.466 ------ 0.520 0.311 ------ 0.433
Girder D 0.466 ------ 0.520 0.311 ------ 0.433
Girder E 0.466 ------ 0.520 0.311 ------ 0.433
Girder F 0.466 ------ 0.520 0.311 ------ 0.433
Girder G 0.450 ------ 0.450 0.375 ------ 0.375

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Rick Brice
Live Load Distribution Factors for concrete T girders

My intuition is the same as yours, expecting a smaller load fraction for each girder where the girders are well connect and act as a unit. I would expect live loads to be shared by more girders and thus a smaller distribution factor.

The "prevent relative vertical displacement" case is based on the AASHTO Standard Specifications (See 17th Edition 3.23.4). This method seems to be based on articulate plate theory.

The "sufficient to make girders act as a unit" case is based on NCHRP Report 12-26.

Here is a link to NCHRP Report 12-26.
http://onlinepubs.trb.org/onlinepubs/nchrp/docs/NCHRP12-26_FR.pdf

Hopefully the NCHRP report provides the insight you seek.

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