TABLE OF CONTENTS A number of important issues have been incorporated into recent editions of the Code. One significant issue is consideration of whether a structure is regular or irregular. In addition, specific overturning requirements for earthquake forces have been modified. It is important for the designer to understand the similarities and differences in the overturning requirements for wind and seismic forces.
This chapter provides an introduction to seismic irregularity considerations, gives a more detailed review of the overturning requirements for both wind and seismic forces, and takes a detailed look at the diaphragm flexibility issues introduced in Sec. 9.11.
The reader is reminded of the general convention in this book to use the term load to refer to a gravity effect such as dead load, live load, snow load, and so on. On the other hand, force generally refers to a lateral effect such as wind force and seismic force. Therefore, the phrase combination of loads and forces implies that both gravity and lateral effects are involved. Although there are not hard-and-fast rules regarding the use of these terms, the pattern of addressing vertical (gravity) loads and lateral (wind and seismic) forces seems to be emerging.
Experience has proven that regular structures perform much better in earthquakes than irregular structures. The subject of irregularity has added to the length and complexity of the seismic code. Recall how the SEAOC Blue Book and its commentary (Ref. 16.11) have...
