General Guidelines for Construction of Buildings in seismic zones.
REFERENCE DOCUMENT -
1) Government of Maharashtra, Buildings and Communications Department, Letter No. BDG-3870/87584-K dated 6-4-1971 Sachivalaya, Bombay-32 BR.
2) Recommendations for designing C.D. Works, minor bridges and major bridges for seismic affected have been issued under Government letter No. RRM-5070/445050 dated 21-9-1970.
3) 1) Government of Maharashtra, Buildings and Communications Department, Letter No. BDG-3870/87584-K dated 14-01-1972 Sachivalaya, Bombay-32 BR.
Similar question so far as building works are concerned was under examination and it that since the Koyna Earthquake there is a general tendency since discourage load bearing wall building in seismic areas, with the result that even small buildings are being proposed in R.C.C, thereby increasing the cost and in raising the period to complete the preliminaries such as preparing R.C.C. designs etc. which also results in delays during construction. But it is to be considered that we should not totally discourage load bearing construction in seismic areas but allow the same with proper precautionary measures against seismic affect. Accordingly, the following recommendations are made:
I. R.C.C. Frame Structure.
1. Buildings in Zone I and II
Structures in these zones need not be designed for seismic effect.
2. Buildings in Zone III and IV
Structures in these zones need to be designed for seismic effect.
II. Load bearing wall structures.
Generally, foundations of buildings rest on murum with a safe bearing capacity of 3 to 4 tons per Sq.ft. This can be classified as soil type II. Since the horizontal seismic coefficients for soil type II are comparatively small for zone I and II, the same can be safely neglected since increase in the stress of foundation level is permissible to the extent of 30% if seismic effects are considered in the design (IS:1893), Thus, for seismic zone I and II the buildings can be constructed as usual without making any special provisions for seismic effects. However, for load bearings wall structure, the following precautions should be taken as a matter of safety.
1. Buildings in Zone I and II
a. Foundations shall rest on soils with a minimum bearing capacity of 20 T/m^2, the building shall rest on a soil with uniform bearing capacity throughout the area.
b. The height of buildings shall be restricted to 8 M. or 2 Storeys for brick construction and to 15 m. or 4 Storeys for store construction.
c. Bricks shall have a minimum crushing strength of 35 Kg/Cm2 (soaked condition).
d. Cement mortar of at least 1:6 proportions be used. Lime mortar should be avoided.
e. Continuous band shall be provided at lintel level as per the following details (for buildings in Zone II only).
Width of band :- Full width of wall
Height of band :- 10 Cms
Concrete grade :- M -20 (minimum)
Reinforcement:- 4 bars 10 mm, links 6 mm @25 Cm.
2. Buildings in Zone III
a. Foundations shall rest on soils with a minimum bearing capacity of 30 T/m2. Attempts should be made to rest foundations on these strata, even by providing deepest foundations if there is possibility of obtaining such strata within a reasonable depth. In cases where such strata are not available at reasonable depth, the cases shall be studied and solved on individual merits.
b. The height of buildings shall be restricted to 5 m. or 2 Storeys for brick construction and to 15 m. or 4 Storeys for stone construction.
c. Brick shall have a minimum crushing strength of 35 kg/Cm2 (soaked condition).
d. Cement mortar of at least 1:6 proportion be used. Lime mortar should be avoided.
e. Projecting Cantilevers parts such as parapets, balconies, canopies, chajjas etc. should be avoided as far as possible, structural members supporting the cantilever elements should have proper anchorage at the supports of the member in Load Bearing structures as per sketch attached below. The anchor Bars of 2 Nos 12 mm diameter provided throughout at top of beams should be properly anchored in supported columns or continued at top in the adjacent beams in R.C.C. structure.
f. Ceiling plaster shall not be greater than 6 mm. thick, suspended ceilings should be avoided as far as possible.
g. Top of openings in a wall should be at one level. Total width of all openings shall not be greater than 1/2 the clear length of walls between adjacent cross walls. any openings shall be away from the corners of the wall by a distance equal to at least 1/8 the height of the openings.
Horizontal distance between two openings shall not be less than 1/4th the height of the higher openings.
Clear vertical distance between any two openings within the same floor shall not be less than 60 Cms.
h. Use of arched over the openings shall be avoided unless steel ties are provided.
Properly designed R.C.C. lintels shall be provided over all openings.
i. A continuous concrete band shall be provided at the sill level with the following details:
Width.:- Full width of the wall.
Height :- 7.5 Cm.
Concrete:- M-20 grade.
.Steel :- 2 bars 10 mm longitudinally
6 mm bars 25 ms. laterally.
J. A continuous band shall be provided at the lintel level with the following details.
Width.:- Full width of the wall.
Height :- 15 Cm.
Concrete:- M-20 grade.
.Steel :- 4 bars 10 mm longitudinally
6 mm bars 15 Cms. laterally.
k. The roof slab shall extend over the exterior load bearing walls for the full width.
l. In case of pitched roof construction, adequate breaking shall be provided in plan at the tie level.
m. At all corners and junctions of walls one M.S. bar of 12 mm. Ø shall be provided vertically.
n. A concrete casting shall be provided around all windows opening as per details below:
Width.:- Full width of the wall.
Height :- 7.5 Cm.
Concrete:- M-20 grade.
.Steel :- 1 bar of 88 mm for every half brick thickness of wall.
o. For pitched roofs C.G.I. or asbestos sheet shall be used in preference to Mangalore, tiles. All roofing material shall be properly tied to the supporting members. Heavy roofing materials should generally be avoided.
p. Plinth beams shall invariably be provided.
q. The columns should be connected to each other by R.C.C. beams in both directions as far as possible. The height of column should not exceed 4 m.
3. Buildings in Zone Iv.
For single storied structure in Zone IV load bearing construction should be adopted with all the precautions detailed for buildings in Zone III.
For buildings having more than one Storey in Zone IV, R.C.C. framed structure should be adopted.
4. Buildings with ground floor of ground + 1 upper Storey structures.
a) The precautions listed For single storied structure in Zone III should be followed and the RCC designs may be prepared without consideration of seismic forces in the design.
5. Buildings with ground + 2 upper Storeys.
The precautions listed For single storied structure in Zone III should be followed and the RCC. designs may be prepared without consideration of seismic forces in the design by observing the additional precautions listed below.
a) The reinforcement in columns should be equal to the maximum reinforcement required at the terrace level, 2nd floor, 1st floor or plinth level. The sections of the columns may be changed from floor to floor, but reinforcement should be the same throughout.
b) All RCC, beams shall be provided with minimum top and bottom steel as shown below which should be well anchored at the supports or adequately continued in the adjacent beams if any. The section of the beam may change depending upon the span. But the minimum reinforcement shall be as under: -
i) Beams with spans more than 4.572 m (15 feet) 2 Nos. 20 mm.
ii) Beams with spans 4.572 m. (15 feet) or less 2 Nos. 16 mm.
6. Buildings with ground + 3 or more upper Storeys.
a) The designs shall be got prepared from the designs Circle.
Note: The above instructions do not apply to structures located within a radius of 50 Kms. from Koyna.
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