Simplified Frame Analysis - Simplified Design of Reinforced Concrete Buildings - page 35

2.4 FRAME ANALYSIS BY ANALYTICAL METHODS

For continuous beams and one-way slabs not meeting the limitations of ACI 8.3.3 for analysis by coefficients,
an elastic frame analysis must be used. Approximate methods of frame analysis are permitted by ACI 8.3.2 for
“usual” types of buildings. Simplifying assumptions on member stiffnesses, span lengths, and arrangement of
live load are given in ACI 8.7 through 8.11.

2.4.1 Stiffness

The relative stiffnesses of frame members must be established regardless of the analytical method used. Any
reasonable consistent procedure for determining stiffnesses of columns, walls, beams, and slabs is permitted by
ACI 8.7.

The selection of stiffness factors will be considerably simplified by the use of Tables 2-8 and 2-9. The stiffness
factors are based on gross section properties (neglecting any reinforcement) and should yield satisfactory results for
buildings within the size and height range addressed in this topic. In most cases where an analytical procedure is
required, stiffness of T-beam sections will be needed. The relative stiffness values K given in Table 2-8 allow for
the effect of the flange by doubling the moment of inertia of the web section (b w h). For values of h f /h between
0.2 and 0.4, the multiplier of 2 corresponds closely to a flange width equal to six times the web width. This is
considered a reasonable allowance for most T-beams. 2.2 For rectangular beam sections; the tabulated values should
be divided by 2. Table 2-9 gives relative stiffness values K for column sections. It should be noted that column
stiffness is quite sensitive to changes in column size. The initial judicious selection of column size and uniformity
from floor to floor is, therefore, critical in minimizing the need for successive analyses.

As is customary for ordinary building frames, torsional stiffness of transverse beams is not considered in the
analysis. For those unusual cases where equilibrium torsion is involved, a more exact procedure may be necessary.