*** . WARNING: “SINGLY OVER-REINFORCED SECTION BEING BRITTLE SHOULD ALWAYS BE AVOIDED IN EARTHQUAKE RESISTANT DESIGN. MODIFY SECTION DIMENSION..”- What is the meaning for this?
In case of Earthquake resisting structures, you are supposed to use ductile members. So, if you are using IS: 13920 ductile detailing, you have to first verify if the member is ductile or not. Suppose, you have designed a section and provided the reinforcement accordingly. Now, you have to check the limiting moment resistance capacity of the section (Mu,lim). If the moment of resistance of the section after providing the required reinforcement becomes higher that the limiting value, the section becomes over reinforced. In such a section, the concrete reaches its ultimate capacity before steel yields. This results in sudden failure of the section. It behaves like a brittle member. So such a section should not be used. In STAAD, the warning is provided. You need to change the section size.
In STAAD, an error message is reported if the program finds a section to be over reinforced (in IS: 13920 beam design). This error is reported from the engineering sense as we don’t desire to have a brittle section present in IS: 13920 ductile design of beams. In this case, the program reports the area of steel required only. It doesn’t provide detailed output containing the shear reinforcement