For two-way slab systems, the ACI 318 Building Code permits a three-dimensional (space-frame) analysis in which the equivalent frame combines the flexibility (reciprocal of stiffness) of the real column and the torsional flexibility of the slabs or beams attached to the column at right angles to the direction of the bending moment under consideration. This method, applicable for all ratios of successive spans and of dead to live load, is an elastic (exact) analysis called the equivalent frame method.
An approximate procedure, the direct design method, is also permitted (within limits of load and span). This method constitutes the direct solution of a one-cycle moment distribution. (See also Art. 9.59.)
(E. S. Hoffman, et al., Structural Design Guide to the ACI Building Code, 4th ed., Kluwer Academic Publishers, Boston/Dordrecht/London.)
