Geotechnical Engineering Circular No. 9 Design, Analysis, and Testing of Laterally Loaded Deep Foundations that Support Transportation Facilities
Table 11-2: Minimum yield strengths for permanent steel casing
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- 11.3.5 Minimum and Maximum Amount of Longitudinal Steel Reinforcement
| Table 11- 2: Minimum yield strengths for permanent steel casing. Standard Minimum Yield Strength (f y ) ASTM A 36 ksi ASTM A 50 ksi for thickness ≤ 0.5 inch 46 ksi for 0.75 inch < thickness ≤ 1.5 inch 42 ksi for 1.5 inch < thickness ≤ 4 inches ASTM A252 Grade 2 36 ksi ASTM A252 Grade 3 45 ksi Note: The modulus of elasticity, E, for steel casings can be assumed to be 29,000 ksi. The thickness of casings should be shown in the contract documents as “minimum”. The minimum thickness of casings should be that required for reinforcement or for strength required during installation, whichever is greater. The latter is a function of both the site conditions and the method of installation. AASHTO Specifications (2007) require the contractor to furnish casings of greater than the design minimum thickness, if necessary, to accommodate the contractor’s choice of installation equipment. Casings used for structural support in permanent foundation applications also need to consider potential corrosion. In such cases, the casing needs to be provided with protective coating and/or additional thickness to maintain the required structural thickness of the casing for the defined design life of the structure. 11.3.5 Minimum and Maximum Amount of Longitudinal Steel Reinforcement AASHTO Section 5.7.4.2 (2014) specifies a range for the amount of steel reinforcement allowed in the cross-section of a concrete foundation element. The maximum allowable area of longitudinal reinforcing steel, As, is 8.0 percent of the gross cross-sectional area of the shaft Ag, or 𝐴𝐴 𝑠𝑠 𝐴𝐴 𝑔𝑔 ≤ 0.08 Equation 11-6) 149 In addition, AASHTO (2014) (Sections 5.10.11.3 and 5.10.11.4.1) limits the longitudinal reinforcement for Seismic Zones 2, 3 and 4 to not more than 6.0 percent. Typical amounts of reinforcement are between one and two percent but maybe greater than 3 percent in high seismic zones. Construction of concrete foundation elements with longitudinal reinforcement greater than 4 percent is difficult, and should be avoided if at all possible. Difficulties with construction of concrete foundation elements with the higher percentages of longitudinal reinforcement include the flow of concrete through the rebar cage to the outside faces of the element. See Brown et al. (2010) for additional discussion of the amount of longitudinal reinforcement for drilled shafts. The minimum amount of longitudinal reinforcement is affected by both the strength of steel and concrete. In the portions of the drilled shaft that behave as a column, defined as any portion of the shaft above the depth at which the shaft is laterally supported, the minimum longitudinal reinforcement amount is determined as 𝐴𝐴 𝑠𝑠 𝑓𝑓 𝑦𝑦 𝐴𝐴 𝑔𝑔 𝑓𝑓 𝑐𝑐 ′ ≥ 0.135 Equation 11-7) In which fy = yield strength of the longitudinal steel bars. Furthermore, the minimum longitudinal reinforcement area in the portions of the shaft that behave as a column should be not less than 1 percent of the gross concrete area of the shaft. Below the section where the drilled shaft behaves as a column (i.e., is laterally supported) the amount of reinforcement is typically governed by the moment demand along the length of the element. However, 0.5 percent of the gross concrete area of the pile is suggested as a practical minimum. The longitudinal reinforcing bars should be evenly distributed among not less than 6 bars in a circular arrangement. The minimum size of longitudinal bars is No. 5 (AASHTO Section 5.7.4.2). Per AASHTO Section 5.13.4.5.2, the clear distance between parallel longitudinal reinforcing bars shall be not less than 5 times the maximum aggregate size or 5.0 inches, whichever is greater. However, recent research has indicated that, for drilled shafts constructed using tremie concrete, the proper flow of concrete through the rebar cage cannot be assured unless the clear spacing is equal to or greater than 10 times the maximum aggregate size. When necessary, vertical reinforcing bars should be bundled in order to maximize the clear space between vertical reinforcement bars. For drilled shafts constructed by the dry method, a clear spacing of 5 times the maximum aggregate size, with a minimum of 5.0 inches, is sufficient. Download 6.03 Mb. Share with your friends:
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