When mill certifications are not available, material properties for a load rating will be based on recommendations for minimum material properties in the MBE. However, experience has shown that the default material strength values, especially for steel reinforcement and structural steel, are quite conservative when compared to results from mill certificates. (TxDOT maintains a large inventory of these historic mill test certificates.)

This observation initiated an effort to establish a minimum probable estimate of yield strength based on steel grade so that TxDOT could use a more representative minimum yield strength for load ratings when mill test reports are not available. TxDOT implemented a study to scan and analyze microfilm containing the mill certifications from 394 unique CSJs and establish a recommended yield strength based on statistical analysis. For reinforcing steel, a total of 3,976 yield strength data points for Grade 40, 90 data points for Grade 33, 1,587 data points for Grade 50, and 74 data points for Grade 60 were analyzed. For structural steel, a total of 548 data points for A373, 713 data point for A7, and 2,030 data points for A36 were also analyzed. The results are summarized in Tables 6-1 and 6-2 below.

Table 6-1 Summary of Mill Study Results for Steel Reinforcement

	Grade 33	Grade 40	Grade 50	Grade 60
Specified minimum yield strength (ksi)	33.0	40.0	50.0	60.0
Recommended yield strength (ksi)	36.0	43.0	50.0	61.0

Table 6-2 Summary of Mill Study Results for Structural Steel

	A373	A7	A36
Specified minimum yield strength (ksi)	32.0	33.0	36.0
Recommended yield strength (ksi)	37.5	36.4	38.0

The increased steel yield strength can be applied to load factor ratings on bridges which were designed using Allowable Stress or Load Factor methods. The increase in steel grades are not to be applied for bridges designed with the AASHTO LRFD method. The AASHTO MBE 3rd Edition states that the resistance factors for LRFD have been calibrated with the

consideration of a higher yield strength than that of the stated minimum grade. Therefore, the structural capacity calculated by AASHTO LRFD methods already includes the effect of higher yield strengths. The application of these higher yield strengths is limited to bridge elements in fair or better condition without significant distress which would otherwise cause the bridge to be posted for legal loads.