Determination girder distribution factor by using refined analysis

Steel Girder

While undertaking Double tee girders design and constructions, it is very crucial that the maximum moments of the double tee girders are considered thoughtfully since it plays a very significant role in determining the section and strengths of a box girder bridge.  This is particularly critical since a three-dimension double tee bridge introduces a complicated aspect and behavior that is linked with the transfer of massive loads from the slabs and concrete to the steel girders. It is important that the strength of a double tee bridge is highly dependent on the fervor of the girders and any extra weight that tends to reduce the strength must always be addressed conclusively especially with a three-dimensional double tee bridge. The AASHTO is a bridge specification body that has suggested a varied approach used in the refined analysis of double tee girders such as the finite element analysis, the grillage method, and finally using load distribution factor (LDF) calculations.

In fact, AASHTO LRFD Specifications insists that the most appropriate method to address the challenges brought about by distribution factors while dealing with three Dimension double tee girders is that refined analysis must be pursued. This helps in dealing with the distribution factors of the box girders bridge or beams when the requirements and the ranges of use of the equations are not valid or not adhered to.

Refined Analysis

The refined analysis methodology according to the AASHTO specifications is one of the methods that are very handy when other code specifications methods do not apply appropriately in one way or the other. It is, therefore, important to devise another method that can be used to determine appropriate distributions levels.  The aim of the refine analysis methodology is to determine the slab thickness, stiffness, weight, spacing and beams strength. AASHTO specifications have presented very many methods that can be used in performing refined analysis including finite element analysis, numeric analysis, among many other refine methodology that can effectively be used to determine the distribution factors. The most important factors to determine in these methods include both the moment and shear distributions factors especially in three-dimensional bridges

Despite the effectiveness of all the methods, and AASHTO specification seems to favor both numeric and grillage methods especially in addressing three-dimensional double tee girders bearing both steel and concrete slabs. The two methods are very effective in determining the factors and providing a detailed view of the shear distribution as well as the generation of the final shear distribution. The most significant bit of the numerical analysis of the refined method is the complexity associated with it in that the modeling can be increased to include three-dimensional elements such as solid and finite slab which have been used in the past to determine and understand the bridge complexity during modeling of the bridges in the lab.


  1. The resulting beams must be augmented: the concrete I-beams, Concrete Box Beams, and the Precast Beams. The following must be acknowledged in step 1.
  2. The roadway must be divided into ten ft to determine the fractional traffic on each lane.
  3. Each value resulting from the above division should be reduced by a factor obtained through the linear interpolation to determine the intensity of the traffic which in turn provides the load in a unit time that the bridge is expected to carry.
  4. The total number of lane should be split as per the number of lanes in each beam so as to determine the load per beam or the distribution of the load which leads to shear or moment distribution factor per beam
  5.  Determine the Superstructure Type Modification and the slap concrete factor 
  6. Determine the Skew Angle as well as the SPM(Span Length Modification )

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