Moment Redistribution Limits for Beams with High Strength Steel Reinforcement

CSA Standard A23.3:19 “Design of Concrete Structures” currently requires that the maximum yield strength, used in design calculations, be no larger than 500 MPa. The objective of the research, reported in this paper, is to determine whether the current limits for moment redistribution in continuous flexural members, Clause 9.2.4 of A23.3-19, are appropriate if High Strength Reinforcement is used. Sensitivity analyses were conducted, using SAP-2000, for two-span continuous beams with various quantities of ASTM A615/A615M Grade 100 (690 MPa) and ASTM A706 Grade 60 (420 MPa) steel reinforcement, and concrete strengths of 30 and 70 MPa. Loading cases where the live load was applied simultaneously on either both spans or on one span only were considered. Preliminary results suggest that if a full plastic collapse mechanism is present at failure, the moment redistribution percentage is independent of the grade or quantity of reinforcement. If the failure load is limited by insufficient inelastic rotation capacity at the first plastic hinge, the maximum permitted redistribution must reduce for beams with increasing mechanical reinforcement ratio, ω, values. Although the maximum permissible redistribution percentages are less for beams reinforced with Grade 100 steel than Grade 60 steel, the A23.3-19 provisions remain conservative in the cases investigated.