The ACI 318 Building Code contains special seismic requirements for design that apply only for areas where the probability of earthquakes capable of causing major damage to structures is high, and where ductility reduction factors for lateral seismic loads are utilized (ACI 319-99, Chap. 21). The general requirements of ACI 318- 99 for reinforced concrete provide sufficient seismic resistance for seismic zones (or seismic performance categories) where only minor seismic damage is probable and no reduction factor for ductility is applied to seismic forces. Designation of seismic zones (or seismic performance categories) is prescribed in general building codes, as are lateral force loads for design. (See also Art. 5.18.7.)
Special ductile-frame design is prescribed to resist lateral movements sufficiently to create plastic hinges and permit reversal of direction several times. These hinges must form in the beams at the beam-column connections of the ductile.
Shear walls used alone or in combination with ductile beam-column frames must also be designed against brittle (shear) failures under the reversing loads (Commentary on ACI 318-99).
Ductility is developed in reinforced concrete by:
Conservative limits on the net flexural tension-steel ratio p <= 0.025, to ensure underreinforced behavior. At least two continuous bars must be provided at both top and bottom of flexural members.
Heavy confining reinforcement extending at joints through the region of maximum moment in both columns and beams, to include points where hinges may form. This confining reinforcement may consist of spirals or heavy, closely spaced, well-anchored, closed ties (hoops) with hooked ends engaging the vertical bars or the tie at the far face.
(ACI Detailing Manual, SP-66, American Concrete Institute.)
