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Study on Statistical Characteristic of Strength of Steel Plate Used for Penstocks of Hydropower Stations

Jianguo Hou Shaojun Fu Xuwen An Yingming He
College of Civil & Architectural Engrg., Wuhan Univ. of Hydr.&Elec. Engrg.,Wuhan 430072 China
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Abstract:

1 Introduction

2 Collecting and preprocessing testing data

3 Counting and analysis

4 Determination of steel plates' strength in analysis of reliability of penstocks

5 Conclusions

    We can make the following conclusions in terms of the analysis results from more than 20,000 groups testing data of steel plates.
  1. After the verification of hypothesis, the yield strength of steel grades Q235 and Q345 are approximately satisfied to normal distribution, and the ultimate tensile strength of steel grades Q235 and Q345 are approximately satisfied to logarithmic normal distribution.
  2. The average and the standard deviation and the variation coefficient about yield strength and ultimate tensile strength of steel grades Q235 and Q345 are obtained from analyzing the collected more than 20,000 groups of testing data, referring to tables 3 to 5. These achievements can be used to analyze the reliability of penstocks when revising the code SD144-85 [1]. It also has the practical significance and the referential merit to the production of steel and the quality control in China.
  3. If s s and s b specified in the current national standard of steel plates are compared with the average of the measured values of s s and s b in terms of the collected testing data of steel plates produced by a few representative Chinese steel mills, it can be discovered that the values about s s and s b specified by current standards is basically equal to the average of the measured s s and s b minus 1.645 times of standard deviation. That shows that it is practicable to take the strength indexes s s specified in the current standards as a basis of the determination of the strength normal value of steel plates of steel grades Q235 and Q345, and it is basically satisfied to the requirements, which are that the dividing value is equal to 0.05 and the assurance probability is not less than 0.95 in GB 50199-94 [2]. This also indicates that the mechanical properties of steel plates produced by Chinese steel mills in recent years are stable and reliable.
  4. If tensile strength limit of steel plates is regarded as the basic parameter, the analysis of the reliability of penstocks according to the stipulation of the current code SD144-85ss =0.67s b whenss /s b > 0.67 shows the reliable index of tensile strength limit is usually higher than that of yield strength. Therefore, with relation to the high strength steel with higher ratio of yield strength to ultimate tensile strength (ss /sb), according to the analyzing results and referring to many home and overseas documents and some Chinese experts' suggestions, the restrictive ratio A(A=ss /sb) of yield strength to tensile strength limit of steel plates may be increased from 0.67 prescribed in old code to 0.75 in order to make full use of high strength steel and acquire a certain technical-economy benefits. It is preliminarily analyzed that steels can be saved some 10% or so in the structural designs if designers will adopt A=0.75, the structural reliability b will decrease a little, but it is still satisfied to the degree of acceptable value.

References

  1. Design Code of Penstocks in Hydropower Stations(SD 144-85).Beijing:China Water and Power Press,1985 (in Chinese)
  2. Unified Design Standard for Reliability of Hydraulic Engineering Structures (GB 50199-94).Beijing:China Planning Press,1994(in Chinese)
  3. Carbon Structural Steels(GB 700-88).Beijing:China Standards Press,1988(in Chinese)
  4. High Strength Low Alloy Structural Steels(GB/T 1591-94).Beijing:China Standards Press,1994(in Chinese)
  5. Crubbs F. E., Procedures for Detecting Outlying Observations in Samples.Technometrics, 1969,1~21
  6. Hou,J.G.,He,Y.M.,An,X.W.,and Fu.S.J.,Calibrating the Reliability and Determining Partial Factors for Penstocks of Hydraupower Stations.Wuhan Univ. of Hydr. & Engrg.,1999(in Chinese)

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