Yield-Line Analysis and Design of Grids

Increasing attention has been given in recent years to design methods which recognize and account for the reserve strength which often exists in a structure beyond the occurrence of first fiber yielding. These methods, variously named, use the structures collapse behavior as the basis for proportioning. Service load checks must then be made on alternative failure modes, such as deflection. Most applications of collapse design have been confined to structures composed of "line elements," e.g., beams and frames. Reinforced concrete designers have been given guarded approval to extend collapse analysis to slabs, if such analysis is accompanied by verifying load tests. Thus the so-called "yield-line" approach affords a powerful slab preliminary design tool which, adequately supported by experimental verification, can be used to determine final proportioning of slab and reinforcing steel. A yield-line type of analysis can also be employed to appraise the collapse strength of steel grid systems. This paper presents yield-line analysis and design procedures for orthogonal grid systems. Confirming laboratory tests are also presented and discussed.