THE CONCRETE BRIDGE MAGAZINE

SPRING 2018

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

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CONCRETE BRIDGE TECHNOLOGY Structural Design Using Stainless Steel Strands by Dr. Henry G. Russell, Henry G. Russell, Inc. This article highlights some of the articles in the American Association of State Highway and Transportation Officials’ AASHTO LRFD Bridge Design Specifications that need special consideration when stainless steel strands are used in the design of pretensioned concrete members. The article numbers and titles referenced here are those of the 8th edition of the specifications.1 The numbers in parentheses are article numbers (if different) from the 7th edition.2 5.4.4 Prestressing Steel Article 5.4.4.1 states that strand shall conform to AASHTO M 203. The AASHTO Standard Specification for Steel Strand, Uncoated Seven-Wire for Concrete (AASHTO M 203)3 provides the required properties for Grades 250 and 270 carbon steel prestressing strands. No similar standard currently exists for stainless steel. A comparison of the properties of carbon steel strand and 2205 stainless steel strand is provided in the following table. Currently, the availability of 2205 stainless steel strand is limited to Grade 250, which has a slightly smaller nominal cross-sectional area than Grade 270. The lower strength of the stainless steel requires more strands compared to a design using Grade 270 strands. 5.4.4.2 Modulus of Elasticity Article 5.4.4.2 states that the modulus of elasticity of prestressing steel may be taken as 28,500 ksi, if more precise data are not available. As shown in the table, 2205 stainless steel has a lower modulus of elasticity than conventional 1080 strand. Therefore, the lower modulus of elasticity shown in the table would be more appropriate to use when designing with 2205 stainless steel strand. However, it is most accurate to use the value recommended by the manufacturer. 5.5.4.2 Resistance Factors As shown in the table, 2205 stainless steel strand has considerably less elongation at rupture than conventional 1080 strand. Consequently, flexural members are likely to have less maximum ductility when stainless steel strand is used. To offset this lower maximum elongation, it may be appropriate to use a lower value for the strength resistance factor ϕ in tension-controlled sections when 2205 strand is used. However, specific values for ϕ have not been determined. 5.6 Design for Flexural and Axial Force Effects – B Regions Equations in Article 5.6.3.1.1— Components with Bonded Tendons are based on the assumption that the distribution of steel is such that it is reasonable to consider that all of the prestressing force is located at the centroid of the prestressing steel. In addition, an average stress in the strands may be used for calculation of nominal flexural resistance. However, because of the lower ductility of the 2205 stainless steel strand, it may be more appropriate to use a method based on the condition of equilibrium and strain compatibility, with the stress in the extreme row of strands

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