MDX Software - FAQ #16<BR><BR> 

How do the performance ratios in the Maximum Performance Ratios 
table relate to the inventory rating shown in the rating table? <BR>
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<UL>The first method we used in the program for flagging an overstress was to 
  calculate the ratio of actual stress over allowable stress. An overstress 
  occurs when this ratio is greater than unity, and the maximum ratio for each 
  type of stress is tabulated in a table labelled Live Load Capacity.  At some later time we 
  were requested by a number of users to develop a table for inventory and 
  operating ratings based on the AASHTO rating equation (6-1a) in the Manual for 
  Condition Evaluation of Bridges. This equation indicates an overstress when 
  the value is less than unity. <BR><BR></UL>