Criteria for earthquake-resistant design of structures
1984 Edition

IS 1893:1984 categorizes India into five seismic zones based on Modified Mercalli Intensity, each with defined basic horizontal seismic coefficients and zone factors as follows:

Key considerations include multiplying the basic coefficient by an importance factor depending on the structure's criticality, special provisions for underground structures at depths beyond 30 meters, and the emphasis on ductility in design. This zoning assists in estimating seismic forces for earthquake-resilient designs.

According to IS 1893 Clause 4.2.1, earthquake forces for tall buildings are determined by considering the floor diaphragm action and the building height alongside seismic zoning:

1. Floor diaphragms are assumed rigid to distribute seismic shear to lateral resisting elements. In buildings with shear walls and frames, frames must carry at least 25% of seismic shear.

2. The analysis method depends on height and zone:

• Buildings over 90 m in zones I and II require modal response spectrum analysis.
• Structures between 40 m and 90 m in all zones use modal analysis, with the seismic coefficient method permitted in zones I to III.
• Buildings under 40 m can use the seismic coefficient method in all zones.
• For structures exceeding 40 m in zones III to V, advanced dynamic analyses are recommended.

3. Storey shear force is computed by superimposing modal responses with height-dependent weighting factors to capture dynamic effects accurately.

IS 1893 recommends different analysis techniques for dams and retaining structures:

• For earth and embankment dams, a pseudo-static approach is employed, applying additional horizontal and vertical seismic loads on soil masses within potential failure surfaces. Dynamic analysis is preferred for critical dams to predict deformations.

• Concrete and masonry dams up to 100 meters use a seismic coefficient method, where horizontal coefficients reduce linearly from 1.5 times the top seismic coefficient at the crest to zero at the base, and vertical coefficients are 0.75 times the horizontal. For dams exceeding 100 meters, response spectrum methods are adopted. Dynamic analysis is encouraged for final design stages.

These approaches ensure preliminary and detailed seismic evaluations suited to dam types and sizes.

IS 1893:1984 primarily centers on horizontal seismic forces, considering them the dominant ground motion component during earthquakes. It does not explicitly define vertical seismic acceleration coefficients or detail vertical force provisions within the design clauses.

While vertical ground motions are acknowledged, their influence is less emphasized, and typical practice involves taking vertical seismic forces as a fraction (commonly 0.5 to 0.7) of horizontal coefficients, though this is not formally specified in this edition. Modern IS 1893 revisions provide clearer guidance on vertical accelerations.

Consequently, for IS 1893:1984, design focus remains on horizontal seismic forces, with vertical effects considered implicitly or via advanced analyses when critical.

IS 1893:1984 stipulates that all bridge structures, including concrete, masonry, and earth dams, must be designed to withstand lateral earthquake forces acting on the entire structure and its components.

Key points include:

• Horizontal forces are assumed and applied at the center of mass of each structural element from any direction.
• Bridges shorter than 60 meters in length or with spans under 15 meters are exempt from seismic design except when located in seismic zones IV and V.
• Detailed dynamic analysis is not mandatory for all bridges; simplified methods or bracing may be sufficient for smaller or less critical bridges.
• Soil and foundation types influence seismic coefficients and must be incorporated.
• Use of light, well-braced construction materials is encouraged to enhance seismic performance.

The design lateral force is calculated using the product of the horizontal seismic coefficient and the effective seismic weight of the bridge element.