5. Nonparallel system

Remedial measures

3. Columns or other elements supporting discontinuous walls or frames shall have the design strength to resist special seismic load combination: 1.2D + f1 L + 1.0Em and 0.9D ± 1.0Em where Em = Q.oEh, the estimated maximum earthquake force that can be developed in the structure.

5. Multiply the sum of actual and accidental torsion by torsional amplification factor Ax.

6. Increase forces determined by static procedure by 25% for connection of diaphragms to vertical elements and to collectors, and for connection of collectors to the vertical elements.

7. Design for orthogonal seismic effects. Analyze for 100% of seismic forces in one direction plus 30% of forces in the perpendicular direction. Alternatively, combine orthogonal effects by using square root of the sum of squares (SRSS) procedure.

Nominal Loads for Allowable Stress Design (ASD) One-Third Increase Not Permitted.

4. D + H + F + 0.75(L + T) + 0.75(Lr or 5 or R) (2.26)

6. D + H + F + 0.75(W or 0.7E) + 0.75L + 0.75(Lr or 5 or R)

Notations used in the load combinations are as follows:

D = dead load E = earthquake load

F = load due to fluids with well-defined pressures and maximum heights H = load due to lateral earth pressure, ground water pressure, or pressure of bulk materials L = live load Lr = roof live load R = rain load 5 = snow load T = self-straining force W = wind load

The designer is referred to AISC 7-02 for load combinations that include ice, flood, and wind on ice.

Special Seismic Load Combinations. In seismic design, certain elements such as those supporting discontinuous systems, collectors, and transfer diaphragms require special consideration. ASCE-7 requires the design of these elements for a maximum seismic load effect given by


E = estimated maximum earthquake force that can be developed in the structure. Q,o = system overstrength factor Qe = horizontal seismic force due to base shear V 5ds = short-period spectral acceleration for design earthquake = 2/3 5MS D = dead load

Special load combinations apply to both ASD and LRFD. Redundancy Factor p

This factor recognizes multiple paths of resistance that may be present in a lateral-force-resisting system. It takes into account: 1) plan area of the building; 2) number of lateral-force-resisting elements; and 3) magnitude of shear force resisted by the lateral-force-resisting elements. p is given by the relation

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