Distribution Tab (LFD)

When you click Analysis Factors, the Analysis Factors screen opens with the Distribution Factors screen tab displayed as shown in Figure 1.

Figure 1 Analysis Factors | Distribution Tab

Distribution Factor Computation Options

Precast/Prestressed Girder allows you to specify how dead loads are to be distributed to different beams. You can either choose to distribute the dead load equally to all beams (as AASHTO allows) or distribute them based on tributary fraction (more accurate in cases where beams are not spaced uniformly).

The program calculated distribution factors appear in the text boxes on the screen. You can select the Computed option to allow the program to calculate the factors or select the Manual option to have the program use customized or user-defined factors. For customized or user-defined factors, you must select the specific span # and beam #, and check the Manual option and overwrite the calculated values in the text boxes. For more information about the calculation of distribution factors see Distribution of Loads (LFD) and Distribution of Loads (LRFD).

Check the option to Use Composite DC Distribution as per FDOT to compute the dead load Comp. DC factors as per Florida SDG 2.8 (Barrier/Railing Distribution for Beam-Slab Bridges). All DC comp loads will be distributed using the alternative distribution factors when this check box is checked.

For impact factors, if the Computed option is selected, then the impact factor will be calculated according to Art. 3.8. If the Shear if same as Moment check box is checked then the impact factor for shear will be taken equal to the impact factor for moment.

Note: The live load distribution factors in this screen are the axle load distribution factors. The axle load distribution factor is half the wheel load distribution factor. The results of individual items in LRFD are multiplied with corresponding distribution factor only.

Live Loads

Precast/Prestressed Girder allows users to manually enter the live load distributions factors or to allow the program to calculate them based on one of the following options:

Use Code Equations
This option calculates the distribution factors based on the code equations as described in the Theory section.

Distribution Factor for each Dead Load

Only one dead load distribution factor was available in Precast/Prestressed Girder LA v1.2. However, different agencies required dead loads of different kinds to be distributed with different distribution factors. Precast/Prestressed Girder LA v1.3 and higher has improved features that allow users to specify different distribution factors for different dead loads.

For LFD version of the program, the following dead load distribution factors are allowed in the current version.

DL on Composite
ADL on Composite

However, there is no change in the results. The results for LFD are shown which in include results of DL and ADL with appropriate distribution factor multiplied and then reported as a summation of DL+ADL results.

Note: In LFD, there are two loads available to input dead loads; DL (dead load) and ADL (additional dead load). These loads may be either on the precast, composite. When the code is toggled across the LFD and LRFD specifications, the following will take place:

Toggle from LFD to LRFD

All DL on Precast is retained as Precast-DC
All ADL on Precast is retained as Precast-DW
All Composite superimposed DL is retained as Composite-DC
All Composite superimposed ADL is retained as Composite-DW

Toggle from LRFD to LFD

All Precast-DC is retained as Precast DL
All Precast-DW is retained as Precast ADL
All Composite-DC is retained as Composite DL
All Composite DW is retained as Composite ADL

The results for individual load case results for LFD are available under load case results. However, when program reports results for envelope of forces and moments and then stresses etc, it reports the results as summation of DL+ADL. There is no change in results in LRFD specs.

Apply Distribution Factor Settings to All Beams

When the Apply to All Beams button is clicked, the Apply Current DF Settings screen displays. This screen allows users to select how to apply the current distribution factor settings to other beams.