BRIDGE LIVE LOAD DISTRIBUTION

Muhammad Furqan Ahmed

INTRODUCTION

Live load distribution on highway bridges is a key response quantity in determining member size and, consequently, strength and serviceability. It is of critical importance both in the design of new bridges and in the evaluation of the load carrying capacity of existing bridges.

Live load distribution is a function of the magnitude and location of truck live loads and the response of the bridge to these loads.

METHODS SELECTION

The selection of the analysis methods is very important in bridge designing. AASHTO LRFD Chapter.4 describes the acceptable methods that satisfy the requirement of equilibrium.

The following methods are mainly used for analyzing

bridge LL.

- Grillage/ Finite Element modelling- Approximate Methods ( Simplified Methods by derived equations as per code)

Note.This presentation will only covers the approximate methods as per code.

METHODS SELECTION

The selection of the analysis methods is very important in bridge designing. AASHTO LRFD Chapter.4 describes the acceptable methods that satisfy the requirement of equilibrium.

The following methods are mainly used for analyzing

bridge LL.

- Grillage/ Finite Element modelling- Approximate Methods ( Simplified Methods by derived equations as per code)

APPROXIMATE METHOD

There are so many researches done on the evaluation of these formulas by comparing its results with an accurate analysis, like Finite Element or Grillage analysis.

A database of several hundred actual bridge decks was also prepared, and analytical models were developed for all bridges in database for comparison purpose and found the results of the code equations are within 10% difference.

The key parameters for each bridge type used in these formulas are girder spacing (S), span length (L), girder stiffness (Kg), and slab thickness (t)

APPROXIMATE METHOD

There are so many researches done on the evaluation of these formulas by comparing its results with an accurate analysis, like Finite Element or Grillage analysis.

A database of several hundred actual bridge decks was also prepared, and analytical models were developed for all bridges in database for comparison purpose and found the results of the code equations are within 10% difference.

The key parameters for each bridge type used in these formulas are girder spacing (S), span length (L), girder stiffness (Kg), and slab thickness (t)

DISTRIBUTION FACTORS(AASHTO LRFD & NCHRP)

FOR INTERIOR GIRDERFOR INTERIOR GIRDER

FOR EXTERIOR GIRDER

EXTENTIONS

In addition to base formulas, several extensions were also investigated. This includes.

- Continuity Effect. (Use Span length as an average of adjacent spans)- Edge Girders. (Depending upon the edge distance)- Skew effect. (Angle)

For most of the cases the results of the base formulas can be adjusted by a correction factors which is given in detail in AASHTO LRFD. By using this factor ‘fs’, the final modified distribution factor would be.

gs = fs go

EXTENTIONS

In addition to base formulas, several extensions were also investigated. This includes.

- Continuity Effect. (Use Span length as an average of adjacent spans)- Edge Girders. (Depending upon the edge distance)- Skew effect. (Angle)

For most of the cases the results of the base formulas can be adjusted by a correction factors which is given in detail in AASHTO LRFD. By using this factor ‘fs’, the final modified distribution factor would be.

gs = fs go

CONCLUSION/ RECOMMENDATIONS

The simplified formulas as discussed above results within 5-10% of results of Finite-element deck analysis.

But still, a grillage or finite element analysis is recommended for verification of the skewed and curved bridge cases where these simple formula methods may not applicable.

CONCLUSION/ RECOMMENDATIONS

The simplified formulas as discussed above results within 5-10% of results of Finite-element deck analysis.

But still, a grillage or finite element analysis is recommended for verification of the skewed and curved bridge cases where these simple formula methods may not applicable.

THANK YOU!!!

REFERENCES:

1- AASHTO LIVE LOAD DISTRIBUTION SPECS.2- AASHTO LRFD CHAPTER 4.3- NHI LECTURES, CHAPTER-6