It should be noted that the preceding load combinations given by Eqs. (2.19) reflect the amendments to UBC-97 recommended by the seismology committee of the Structural

Engineers Association of California (SEAOC). The recommendations are published in SEAOC's 1999 edition of Recommended Lateral Force Requirements and Commentary, commonly referred as the Blue Book in California. As always, it is prudent to verify with the presiding building official that SEAOC-recommended revisions have been approved by the authority having jurisdiction over building permits.

The load factor of 1.0 on the earthquake load E reflects the fact that the specified forces in UBC-97 are at strength design levels, without further amplification. They are typically about 1.4 times as high as the allowable stress design levels given in previous provisions. The earthquake load E is a function of both horizontal and vertical components of ground motion (Eh and Ev). This is represented in the earthquake load equation.

The component Eh is due to the horizontal forces corresponding to the prescribed base shear V. The component Ev is due to an additional increment of dead load D and may be taken as zero for allowable stress design. In seismic zone 4, this component is at least equal to 0.2D, which, when added to 1.2D, results in parity with the 1.4D factored load in the previous provisions for strength design of reinforced concrete and masonry elements. In allowable stress design, the vertical component is indirectly included in the load combinations, which would need to be multiplied by about 1.4, compared to directly with strength design requirement. Alternate Basic Load Combinations Using ASD

One-Third Increase for Wind or Seismic Allowed

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