Seismic Design Philosophy
Modern seismic design philosophy for buildings is based on the concept of ductile failure modes that allow a structure to survive strong earthquakes without collapse, even when structural elements are damaged. The design approach recognizes that it is not economically feasible to design buildings to remain elastic during the largest expected earthquake. Instead, buildings are designed with ductile detailing that allows controlled inelastic deformations at specific locations, dissipating seismic energy and preventing brittle failure modes. The strong column-weak beam concept ensures that plastic hinges form in beams rather than columns, maintaining the overall stability of the frame. Columns must be stronger than the beams they support so that yielding occurs in the beams where it can be distributed across multiple floors.
The building code defines three seismic design categories from A for low seismic risk through F for the highest risk. The SDC determines the permitted structural systems, height limits, and detailing requirements. Buildings in SDC A and B have minimal seismic requirements beyond basic lateral force resistance. Buildings in SDC C require special detailing for seismic resistance. Buildings in SDC D, E, and F require the most stringent detailing including special moment frames, special concentrically braced frames, or shear walls with boundary elements. The design process begins with the determination of the seismic design category based on the site soil conditions and the design spectral accelerations.
The response modification coefficient converts the elastic seismic demand into the design seismic force by accounting for the ductility and overstrength of the structural system. Special moment-resisting steel frames have an R value of 8, meaning the design force is one-eighth of the elastic demand. Ordinary moment-resisting steel frames have an R value of 4.5 because of their limited ductility. The deflection amplification factor converts the elastic deflection into the expected inelastic deflection for drift calculations. The calculated drift must not exceed the allowable drift limits, which range from 2 percent of the story height for buildings in SDC B to 1.5 percent for buildings in SDC C or higher.
Seismic Analysis Methods
The equivalent lateral force method is the simplest seismic analysis procedure and is permitted for regular buildings with limited height. The method distributes the total seismic base shear to each floor level based on the mass distribution and height of each floor. The base shear is calculated from the building weight, the design spectral acceleration, and the response modification coefficient. The lateral forces are applied at each floor level, and the resulting member forces and drifts are calculated using linear elastic analysis. sulfate attack on concrete in aggressive soil environments. cathodic protection systems for reinforced concrete corrosion prevention. penetrating sealers for concrete surface protection. The method assumes that the building responds primarily in its fundamental mode of vibration and is suitable for buildings up to 240 feet in height depending on the seismic design category.
The response spectrum analysis method uses the dynamic properties of the building to calculate the seismic response. The natural periods and mode shapes of the building are determined from an eigenvalue analysis of the structural model. The spectral acceleration for each mode is determined from the design response spectrum. The modal responses are combined using the square root of the sum of the squares method or the complete quadratic combination method for closely spaced modes. The response spectrum analysis provides a more accurate representation of the seismic response than the equivalent lateral force method, particularly for irregular buildings or buildings with significant higher mode effects.
Nonlinear analysis methods including pushover analysis and nonlinear response history analysis provide the most accurate assessment of building seismic performance. Pushover analysis applies incrementally increasing lateral loads to the structural model until a target displacement is reached, tracking the formation of plastic hinges and the degradation of structural stiffness. The results identify the weak points in the structure and the sequence of damage development. Nonlinear response history analysis applies actual or simulated earthquake ground motion records to the structural model and calculates the response at each time step. This method accounts for the cyclic degradation of strength and stiffness that occurs during an earthquake.
Seismic Detailing Requirements
Special moment frames in high seismic zones require detailing that ensures ductile behavior under cyclic loading. The beam-column joint region must be reinforced with closely spaced transverse reinforcement to confine the concrete and prevent shear failure. The beam reinforcing bars must be continuous through the joint with hooks developed within the confined joint core. Column transverse reinforcement in the form of hoops or spirals must be spaced at maximum 4 inches in the plastic hinge zone at the column ends. The volumetric ratio of transverse reinforcement in the plastic hinge zone must be at least 1 percent for columns with axial loads exceeding 10 percent of the gross section capacity.
Shear walls in seismic zones require boundary elements at the wall ends and around openings where compressive strains are highest. The boundary element is a region of closely spaced transverse reinforcement that confines the concrete and prevents buckling of the vertical reinforcement. The need for boundary elements is determined by the compressive strain calculated at the wall edge under the design seismic displacement. Special structural walls with two layers of reinforcement in both directions provide the highest ductility and energy dissipation capacity. The reinforcement ratio in each direction must be at least 0.25 percent of the gross concrete area, with maximum spacing of 18 inches. Coupling beams between wall piers must be designed with diagonal reinforcement that crosses the beam in both directions to resist the high shear forces that develop during seismic response.