RATIONALE FOR LIMITS TO REINFORCEMENT OF TIED CONCRETE COLUMNS

Most design regulations for concrete columns contain rules that limit the amount of longitudinal reinforcement. Neither the lower bound nor the upper bound values are associated with a rational derivation. Longitudinal bar reinforcement must be assembled in a cage that employs transverse bars for which size, spacing, and arrangements are based largely on tradition and experience rather than specified performance needs.;Laboratory tests of eight rectangular reinforced concrete columns subjected to eccentric loading are described. Four of the columns contain full ties required by ACI Code 318-77. The other four contain only perimeter ties. All eight columns are tested to failure under axial load with an eccentricity of one tenth of the column thickness. The variable in this study is the effect of the presence of intermediate ties on the ultimate strength of the column for different concrete strength under short time loading. Three other research projects on influence of tie spacing on the behavior of reinforced concrete columns are also described. The results from all these tests are compared. The variables involved in these three projects are tie spacing and tie arrangement. All the tests described herein lead to a conclusion that the current ACI Code requirements on tie spacing and arrangement are more than adequate.;A finite element computer program is used to analyze the reinforced concrete column under concentric loading. Plane strain treatment is employed. The parameters in this analytic study are bar size, number of bars, column size, material strength and cover thickness.;The results show that only in extreme cases (high strength concrete with low strength steel and both materials of high strength) will the column need transverse reinforcement to stabilize the longitudinal steel in order to reach ultimate under short time loading. When time effect is considered, the column will need transverse steel in most cases in order to reach its ultimate strength. Transverse steel becomes essential in reinforced concrete column design. The amount of transverse steel needed is a function of the strength and size of longitudinal steel, size of column, strength of concrete and amount of longitudinal reinforcement.;The rational upper limit for longitudinal steel can be calculated based on steel congestion in the column; it also can be determined based on resistance to cracking of concrete. The lower bound can be determined as the replacement of the effective tension area of concrete to control cracking; also it can be derived based on shrinkage and creep to control steel stress under service load.