Summary
Highlights
Story drift is the lateral displacement of a structure due to lateral loads. The maximum inelastic response displacement (Delta sub m) and design level response displacement (Delta sub s) are introduced. Drift limits are established to prevent excessive displacement. For structures with a period less than 0.7 seconds, Delta sub m should not exceed 2.5% of the total story height (hn), while for structures with a period greater than or equal to 0.7 seconds, Delta sub m should not exceed 2% of hn. Excessive drift not only causes structural damage but also affects the comfort of building occupants.
P-Delta effects describe the secondary moments and additional story drift that develop in a lateral force resisting system due to the axial load (P) in a column being moved laterally (Delta). These effects should be considered when the stability coefficient for a story exceeds 10%. The stability coefficient is the ratio of the secondary moment (P sub X * Delta sub S X) to the primary moment (V sub X * H sub X).
Structures must be designed to resist overturning effects caused by earthquake forces. Overturning moments are determined using seismic forces acting above the level under consideration. Incremental changes in the design overturning moment are distributed to various resisting elements, and these effects must be carried down to the foundation.
Building separations are necessary to prevent adjacent structures from impacting each other during an earthquake, a phenomenon known as pounding. Expansion joints are often used to divide large or irregular buildings into separate parts. The code requires these separated structures to maintain a specific distance, Delta sub NT, which is calculated using the maximum inelastic displacements of the two adjacent structures (Delta sub M1 and Delta sub M2).
The earthquake load for strength design involves combinations of dead load, live load, and earthquake load. The total earthquake load (E) is a combination of horizontal (E sub H) and vertical (E sub V) forces. E sub H results from the distribution of base shear, while E sub V is calculated as 0.5 * CA * I * dead load, where CA is the seismic response coefficient and I is the importance factor. It's crucial to include both horizontal and vertical earthquake forces in the computation. The horizontal earthquake force (E sub H) also needs to be multiplied by a redundancy factor, which is always greater than or equal to 1, typically up to 1.25.