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9 of the concrete cover, tieback reinforcement is required. Design Steps (General Where stacked ducts are used in curved girders, the moment Sectional Model) resistance of the concrete cover, acting in flexure, shall be investigated, but no specific methodology or stress require- The following outlines the basic steps of designing the ments are provided. For curved girders, the global flexural exterior web of a box section for the combined actions of flex- effects of out-of-plane forces shall be investigated. ural shear, torsion, and moment. It is based on the provisions Article, "Diaphragms," requires the use of di- of Article 5.8.3 and, therefore, does not cover the steps for a aphragms at abutments, piers, and hinge joints. Interme- segmental post-tensioned concrete box-girder bridge. diate diaphragms may be used in beams in curved systems or where necessary to provide torsional resistance. Inter- Step 1 Determine the Controlling mediate diaphragms shall be used in curved box-girders Load Cases with an inside radius of less than 800 feet. Diaphragms may be omitted where tests or structural analysis show them to Determine the controlling load cases for the applicable be unnecessary. strength limit states. Consider the concurrent actions on the Article 14.4.1, "General," specifies the movement require- section. As a minimum, consider the following two cases: ments for joints and bearings. It includes the requirement to consider the effects of curvature, skew, rotations, and 1. Maximum flexural shear and concurrent actions support restraint. The commentary includes additional 2. Maximum torsion and concurrent actions discussion pertinent to curved bridges. Perform Steps 3 through 7 separately for each the above cases and any additional cases that may potentially govern the design. With respect to torsion design, a detailed review of the specifications was performed. The following briefly describes the design methods, outlines the basic steps of designing a Step 2 Determine the Cross-Section box section for the combined actions of flexural shear, torsion, Parameters and moment, and includes a discussion (in italics) where fur- Acp total area enclosed by outside perimeter of concrete ther guidance is required in interpreting or applying the cross section (Article LRFD specification. pc the length of the outside perimeter of concrete cross section (Article Design Methods 2 / Pc 2A b (Equation A cp o v Two basic design methods, specified in Articles 5.8.3 and 5.8.6, depend on construction method and structure type. (Comment LRFD Article does not address the case A sectional model using the modified compression field theory when the thickness of the flange of a non-segmental box sec- with a variable angle truss model is the basis of Article 5.8.3 and tion is less than the effective web width. This is addressed in applies in most cases. Article 5.8.6 contains the flexural shear Article 12.2.10 of the Segmental Guide Specification (Reference 2) and torsion provisions specific to segmental post-tensioned and LRFD Article for segmental bridges.) concrete box-girder bridges. A conservative expression for the concrete contribution and a 45 truss model are assumed. Ao area enclosed by the shear flow path (in.2) (Article ds the length of the torsional shear flow path on the exte- General Comment There is a minor conflict between Arti- rior web (in.) (Commentary cles 5.8.3 and 5.8.6. Article 5.8.3 states that Article 5.8.6 may be ph perimeter of the centerline of the closed transverse tor- used for segmental post-tensioned concrete box-girder bridges sion reinforcement (in.) (Article while Article 5.8.6 states that Article 5.8.6 shall be used for seg- bv effective web width (in.) (Article mental post-tensioned concrete box-girder bridges in lieu of dv effective shear depth (in.) (Article Article 5.8.3. It needs to be clarified whether Article 5.8.6 is a permissible or mandatory procedure for segmental bridges. Additionally, Commentary Article C5.8.6.1 states, "For types of Step 3 Check the Web Width construction other than segmental box-girders, the provisions Verify that the effective web width is adequate to prevent of Article 5.8.3 may be applied in lieu of the provisions of Arti- web crushing: cle 5.8.6." It appears that the word may should be replaced by shall, unless the intent is to permit Article 5.8.6 as an alternative Vu Vn design method for bridge types other than segmental. Vn 0.25fc b v d v +Vp (Equation