Review of Indian Seismic Codes

Importance of Seismic Design Codes Ground vibrations during earthquakes cause forces and deformations in structures. Structures need to be designed to withstand such forces and deformations. Seismic codes help to Improve the behaviour of structures so that they may withstand the earthquake effects without significant loss of life and property. Countries around the world have procedures outlined in seismic Codes to help design engineers in the planning, designing, detailing and constructing of structures. An earthquake-resistant building has four virtues in it, namely:

Good Structural Configuration: Its size, shape and structural system carrying loads are such that they ensure a direct and smooth flow of inertia forces to the ground. (b) Lateral Strength: The maximum lateral (horizontal) force that it can resist is such that the damage induced in it does not result in collapse. (c) Adequate Stiffness: Its lateral load resisting system is such that the earthquake-induced deformations in it do not damage its (d) Good Ductility: Its capacity to undergo large deformations under severe earthquake shaking even after yielding, is improved by favourable design and detailing strategies. Seismic codes cover all these aspects.contents under low-to-moderate shaking.

Indian Seismic Codes Seismic codes are unique to a particular region or country. They take into account the local seismology, accepted level of seismic risk, building typologies, and materials and methods used in construction. Further, they are indicative of the level of progress a country has made in the field of earthquake engineering.

The first formal seismic code in India, namely IS 1893, was published in 1962. Today, the Bureau of Indian Standards (BIS) has the following seismic codes: IS 1893 (Part I), 2002, Indian Standard Criteria for Earthquake Resistant Design of Structures (5th Revision) IS 4326, 1993, Indian Standard Code of Practice for Earthquake Resistant Design and Construction of Buildings (2nd Revision) IS 13827, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Earthen Buildings IS 13828, 1993, Indian Standard Guidelines for Improving Earthquake Resistance of Low Strength Masonry Buildings IS 13920, 1993, Indian Standard Code of Practice for Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces IS 13935, 1993, Indian Standard Guidelines for Repair and Seismic Strengthening of Buildings

The regulations in these standards do not ensure that structures suffer no damage during earthquake of all magnitudes. But, to the extent possible, they ensure that structures are able to respond to earthquake shakings of moderate intensities without structural damage and of heavy intensities without total collapse.

The revised 2002 edition, Part 1 of IS1893, contains provisions that are general in nature and those applicable for buildings.

The other four parts of IS 1893 will cover: Liquid-Retaining Tanks, both elevated and ground supported (Part 2); Bridges and Retaining Walls (Part 3); Industrial Structures including Stack- Like Structures (Part 4); and Dams and Embankments (Part 5). These four documents are under preparation. In contrast, the 1984 edition of IS1893 had provisions for all the above structures in a single document.

What is the Seismic Design Philosophy for Buildings? Earthquake Design Philosophy The earthquake design philosophy may be summarized as follows (Figure 1): (a) Under minor but frequent shaking, the main members of the building that carry vertical and horizontal forces should not be damaged; however building parts that do not carry load may sustain repairable damage. (b) Under moderate but occasional shaking, the main members may sustain repairable damage, while the other parts of the building may be damaged such that they may even have to be replaced after the earthquake; and (c) Under strong but rare shaking, the main membermay sustain severe (even irreparable) damage, but the building should not collapse.

Calculation of Design Seismic Force by Static Analysis method Problem Statement: Consider a four-storey reinforced concrete office building shown in Fig. The building is located in Shillong (seismic zone V). The soil conditions are medium stiff and the entire building is supported on a raft foundation. The R. C. frames are infilled with brick-masonry. The lumped weight due to dead loads is 12 kN/m2 on floors and 10 kN/m2 on the roof. The floors are to cater for a live load of 4 kN/m2 on floors and 1.5 kN/m2 on the roof. Determine design seismic load on the structure as per new code.

Solution Design Parameters: For seismic zone V, the zone factor Z is 0.36 (Table 2 of IS: 1893). Being an office building, the importance factor, I, is 1.0 (Table 6 of IS: 1893). Building is required to be provided with moment resisting frames detailed as per IS: 13920-1993. Hence, the response reduction factor, R, is 5. (Table 7 of IS: 1893 Part 1)

Seismic Weights The floor area is 15×20=300 sq. m. Since the live load class is 4kN/sq.m, only 50% of the live load is lumped at the floors. At roof, no live load is to be lumped. Hence, the total seismic weight on the floors and the roof is: Floors: W1=W2 =W3 =300×(12+0.5×4) = 4,200 kN Roof: W4 = 300×10 = 3,000 kN (clause7.3.1, Table 8 of IS: 1893 Part 1) Total Seismic weight of the structure, W = ΣWi = 3×4,200 + 3,000 = 15,600 kN

Fundamental Period: Lateral load resistance is provided by moment resisting frames infilled with brick masonry panels. Hence, approximate fundamental natural period: (Clause 7.6.2. of IS: 1893 Part 1) EL in X-Direction: T = 0.09h / d = 0.09(13.8) / 20 = 0.28 sec

The building is located on Type II (medium soil). From Fig. 2 of IS: 1893, for T=0.28 sec, g Sa = 2.5 h A R ZI 2 = g Sa 2.5 2 5 0.36 1.0 × × × = = 0.09 (Clause 6.4.2 of IS: 1893 Part 1)

Design base shear B V A W h = = 0.09×15,600 = 1,440 kN (Clause 7.5.3 of IS: 1893 Part 1)