Seismic Loads

Earthquakes have occurred in many states. Figures 6.2 and 6.3 show contour maps of the United States that reflect the severity of seismic accelerations, as indicated in Minimum Design Loads for Buildings and Other Structures (ASCE 7-95), American Society of Civil Engineers.
The engineering approach to seismic design differs from that for other load types. For live, wind, or snow loads, the intent of a structural design is to preclude structural damage.
However, to achieve an economical seismic design, codes and standards permit local yielding of a structure during a major earthquake. Local yielding absorbs energy but results in permanent deformations of structures. Thus seismic design incorporates not only application of anticipated seismic forces but also use of structural details that ensure adequate ductility to absorb the seismic forces without compromising the stability of structures. Provisions for this are included in the AISC specifications for structural steel for buildings.
The forces transmitted by an earthquake to a structure result from vibratory excitation of the ground. The vibration has both vertical and horizontal components. However, it is customary for building design to neglect the vertical component because most structures have reserve strength in the vertical direction due to gravity-load design requirements.
Seismic requirements in building codes and standards attempt to translate the complicated dynamic phenomenon of earthquake force into a simplified equivalent static force to be applied to a structure for design purposes. For example, ASCE 7-95 stipulates that the total lateral force, or base shear, V (kips) acting in the direction of each of the principal axes of the main structural system should be computed from

V = Cs W
C=s  seismic response coefficient
W  = total dead load and applicable portions of other loads.
Applicable portions of other loads are considered to be as follows:

1. In areas for storage, a minimum of 25% of the floor live load is applicable. The 50 psf floor live load for passenger cars in parking garages need not be considered.
2. Where an allowance for partition load is included in the floor load design, the actual partition weight or a minimum weight of 10 psf of floor area, whichever is greater, is applicable.
3. Total operating weight of permanent equipment.
4. Where the flat roof snow load exceeds 30 psf, the design snow load should be included in W. Where the authority having jurisdiction approves, the amount of snow load included in W may be reduced to no less than 20% of the design snow load.
The seismic coefficient, Cs , is determined by the following equation:

Coefficients CV and Ca are based on the soil profile and are determined as follows. From the descriptions in Table 6.13, determine the soil profile type for the site under consideration. From Fig. 6.2, determine the effective peak acceleration, Aa . Enter Tables 6.14 and 6.15 with Aa and the soil type to find coefficients CV and Ca . For the cases noted in Table 6.14, CV depends upon the effective peak velocity-related acceleration, AV , Fig. 6.3. The response modification factor, R, depends upon the structural bracing system used as detailed in Table 6.15. The higher the factor, the more energy the system can absorb and hence the lower the design force. For example, ordinary moment frames are assigned a factor of 3 and special moment frames a factor of 8 (see Art. 9.7.1). Note that the forces resulting from the application of these R factors are intended to be used in LRFD design, not at an allowable stress level (see Art. 6.12).
A rigorous evaluation of the fundamental elastic period, T, requires consideration of the intensity of loading and the response of the structure to the loading. To expedite design computations, T may be determined by the following:

where wx and wi = height from the base to level x or i
k = 1 for building having period of 0.5 sec. or less
 2 for building having period of 2.5 sec. or more
 use linear interpolation for building periods between 0.5 and 2.5 sec.

where Fi  the portion of the seismic base shear induced at level i. The seismic design story shear is to be distributed to the various elements of the force-resisting system in a story based on the relative lateral stiffness of the vertical resisting elements and the diaphragm.
Provision also should be made in design of structural framing for horizontal torsion, overturning effects, and the building drift.

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