Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 31
31 CHAPTER 3 PROPOSED CHANGES TO LRFD BRIDGE DESIGN SPECIFICATIONS Sections 3.1 and 3.2 present the two change proposals. The After considering the provisions in numerous codes of first proposal is the Proposed Simplified Provisions (in a practice and of other suggested shear design approaches, the form similar to the AASHTO standard provisions) and the research team proposes to adopt a method that shares the second is a modification to the current LRFD Sectional approach taken in the current AASHTO Standard Specifica- Design Model. Section 3.3 presents an overview of design tions and in ACI 318-02 where, for shear design, the struc- examples using these two methods. Section 3.4 presents a ture is considered to be divided into regions of web-shear and justification for the proposals using experimental test data flexure-shear cracking. The ability to estimate a lower bound and Section 3.5 presents a justification through a comparison to the diagonal cracking load for the purpose of service eval- of required strengths of shear reinforcement for many design uations was considered important to include in the AASHTO cases. Section 3.6 examines how change proposals, if imple- LRFD Specifications, particularly because the AASHTO mented, may affect design, and then in Section 3.7 their Standard Specifications will be discontinued in time. The anticipated effect on safety and economy is presented. This proposed simplified Specifications differ from the current effect is evaluated for users of both the AASHTO Standard AASHTO Standard Specifications in the expression for Vcw, Specifications and LRFD Sectional Design Model who the assumed angle for , the maximum shear stress permit- choose to use either of the two proposed shear design meth- ted for design, the minimum required amount of shear rein- ods. Section 3.8 presents how design database was incorpo- forcement, and requirements for the amount of longitudinal rated in NCHRP Process 12-50. tension reinforcement that must be developed at the face of Two changes to the LRFD specifications are presented. the support. Furthermore, the values for Vcw are selected so Change Proposal 1 is the addition of alternative (or simpli- that they are consistent with the contribution of the concrete fied) shear design provisions that reintroduce the idea of bas- to the ultimate shear capacity of a beam in accordance with ing Vc on the lower of the calculated web-shear (Vcw) and the crack model with friction concept. The expressions for flexure-shear (Vci) strength but where a new and more Vcw are developed so that they can be applied easily to beams conservative relationship is used for Vcw and where a variable with deformed bar reinforcement only, with prestressed rein- angle truss model is introduced for evaluating the forcement only, and all combinations of those reinforce- contribution of the shear reinforcement based on the angle of ments. A need for seamlessness between reinforced and diagonal cracking. This report refers to this alternative as prestressed concrete provisions for shear was not recognized either the "proposed simplified provisions" or the "Modified when the AASHTO Standard Specifications for shear in Standard Approach." prestressed beams were developed because, at that time, Change Proposal 2 is that the current tables for determin- prestressed and reinforced concrete were seen as separate ing and , as well as the equation for evaluating x in the materials. Sectional Design Model (S5.8.3), be replaced by the rela- The basis for the proposed simplified provisions is sum- tionships for , , and x that have already been incorporated marized below, followed by the specific proposed relation- in the Canadian Concrete Design Code for Concrete Struc- ships for the simplified (alternative) LRFD shear design tures (CSA A23.3-04). Therefore this change proposal is specifications. The detailed explanation of the basis for referred to as the CSA Method. the equations of the proposed simplified provision is given in Appendix F, which is included in NCHRP Web-Only Document 78. 3.1 CHANGE PROPOSAL 1: PROPOSED SIMPLIFIED APPROACH (MODIFIED VCW AND VCI OR MODIFIED STANDARD) 3.1.1 Basis of Proposed Simplified Provisions An alternative (or simplified) shear design method is pro- Web-Shear Cracking Strength, Vcw posed to overcome the limitations of the modified LRFD Sectional Design Model as presented in change proposal 2 The estimate of the web-shear cracking force follows (CSA Method). directly from Mohr's Circle of stress.