Standard LLDF for Prestressed Concrete Beams Used in Multi-Beam Decks
Date | March 2025 |
Product | AASHTOWare BrDR |
Version | 7.6 |
Description | Standard LLDF for Prestressed Concrete Beams Used in Multi-Beam Decks do not use lane reduction factor
|
Fixed | Not applicable |
There were three interrelated issues discussed by the Analysis TAG:
1. BrR is not distinguishing between one lane or multi-lane when calculating the Live Load Distribution Factors (LLDFs).
2. BrR is not implementing multi-lane reduction factors when the number of lanes > 2
3. Load effect from adjacent vehicle is affected by LLDFs and multi-lane reduction factors.
Bridge model: 17x36 PS Box Beams, 15 beams, width = 45 ft, total number of traffic lanes = 3, span = 60 ft
1. BrR is not distinguishing between one lane or multi-lane when calculating LLDF's.
Results when using the “Compute from typical section…” button in BrR:
Standard Specifications
LRFD
2. BrR is not implementing the multi-lane reduction factors when the number of lanes > 2.
3. Load effect from Adjacent Vehicle is affected by LLDF’s and multi-lane reduction factors.
BrR uses Eq 4.6.2.2.5-1 when an adjacent vehicle is used.
Analysis TAG conclusion:
The Analysis TAG has discussed this issue on 03/19/2025 and has concluded that BrDR is calculating the standard live load distribution factor (equation 3-11) for prestressed box beams used in multi-beam decks correctly. The NL value is the total number of traffic lanes from article 3.6 which defines it as the total number of design lanes and not as number of loaded lanes. Hence, the computed value should be the same irrespective of the number of loaded lanes during rating and the reduction factor is not applicable to this equation. The distribution factor essentially represents the presence of a single line of wheel load on the beam. The user may manually override the standard LLDF, if necessary for a rating analysis.