Implications of Revised Seismic Code is 1893

Implications of Revised Seismic Standards on Architectural Aspects of Buildings. Brief Description of revised clauses of IS 1893-2016 & IS 13920-2016

GMR CONSULTING ENGINEERS MUMBAI

Introduction: As Bureau of Indian Standards has published the revised Codes IS 1893-2016 & IS 13920-2016 for Seismic Design & Ductile Detailing of Structures. From the study of this latest codes and Clauses incorporated in them, it has come to our notice that it will have implications on the architectural Planning & design of Buildings. Also, the Local Authorities responsible for the approval of Buildings Like BMC are insisting on the compliance with the latest Design Codes. With this document, we would like to highlight the Clauses which will be directly affecting the architecture of Buildings and the Compliance to the Code requirement can only be made possible by incorporation of these guidelines right from the beginning of architectural planning. As being counterparts to Architects & Developer we as your Structural Consultant would like to bring this to your view. We hope this document will be useful for understanding the perspective & guidelines of latest codes which are meant to be fulfilled.

IS 1893-2016

S.no

Clauses

Implications

1.

Applicability:

Every temporary structure need to be designed for earthquake forces.

2.

Addition & Alteration:

Any additions in floors to the previously made structures left with future provisions need to be reexamined and checked as per new code. Stability of such buildings as whole needs to comply the provisions of IS 18932016.

3

Change in Occupancy:

If change of occupancy takes place in project, higher importance factor is to be used in such cases.

4

Design for Vertical Earthquake:

Along with the earthquake in two orthogonal directions, Seismic forces in vertical directional is also introduced. This will be inducing higher forces on all major cantilever & overhangs projecting outwards resulting higher member sizes.

All Vertical & Horizonal projections for 5 times Seismic Force as mentioned above.

5.

6.

Vertical Irregularities (a) Mass Irregularity:

(b) InPlane Discontinuity in Vertical Elements Resisting Lateral Force:

The probability of having a mass irregularity will be more at transfer girder level, as the depth of girders are high as compared to the beam size on above typical floors resulting in the variation of mass in adjacent levels. Along with the sizes the loading values also changes in parking Area and above placed residential area which may lead to mass irregularity. This clause says that the flaring of columns which generally happens in Multistorey building system is restricted to 20% ie for example: Column with plan dimension 600mmx1000mm can only flared upto 600mmx1200mm in respective direction in above floor. If its more than 20% it shall not be considered in in lateral load resisting system. Above this specified limits In- plane Discontinuity in vertical elements resisting lateral force will prevail which is not permitted in Zone 3,4&5.

7.

Flat slab system:

8.

Property Modifiers:

(a)Flat slab System need to have RCC structural walls (Shear walls) properly placed in structural system so as to take 100% of the lateral Forces. (b) RCC frame to be provided on the periphery capable of resisting 25% Earthquake forces. (c) lateral Drift at the roof level under lateral force shall not exceed 0.1%, which will result in higher column sizes as this clause is more stringent as compared to RC frame system. For analysis now we should apply property modifiers assuming cracked section which will increase the lateral deformation of structure and member sizes and percentage of steel in members.

9.

Floating Columns:

The clause with the most prominent implications on architectural design of Multi-storey & highrise buildings. It states that the Floating column which we have in our structural system starting from transfer girder can’t be the part of lateral load resisting system. Those columns will be designed as gravity columns. For compliance with these columns architectural designs need to be done in such a way that we can minimise stub & Floating columns.

10.

Soft Storey & Weak Storey: (a)Stiffness Irregularity (Soft Storey):

(b) Strength Irregularity:

(i)Open storey condition: As per the mentioned clause the stiffness of unreinforced masonry is to be accounted. It will happen in building with stilt/open ground storey used for parking with no infill walls. Creating soft storey. (ii)Secondly it can occur in case if fire check floor where the generally the storey height is less as compared to the storey heights of the adjacent levels, which leads to the increase in the stiffness and resulting into soft storey below & above the fire check floor. Provision: To get rid of this problem the simplest way is to make the storey height of fire check floor same as adjacent floors. (b) Strength Irregularity: This clause will have implications on sudden reduction in sizes of Columns & Shear walls at different storey levels. As sudden reduction in sizes may create a weak storey.

11.

Structural Wall Plan Intensity:

Buildings with the Stiffness Irregularity mainly with open ground story shear wall system need to have Structural wall plan density of At least 2% in each principal direction. That Implies we need to have thicker walls on open ground story.

IS 13920-2016 S no

1.

2.

Clause Beam Size

Size of columns:

Implications

Minimum size if Column will be greater of two criteria 20 times the Diameter of bar used in the beam passing through that Column or 300 mm. So now to finalise the column sizes the whole design should be freeze. Because any change at later stage leading to higher diameter bar in beam will

3.

Coupled Shear Wall:

consequently change the size of column. And due to above mentioned reason we will not be in position to calculate the quantities of steel correctly unless structural system has been finalised. For Eg: Bar of 20 dia is been used in beam then minimum dimension of the column shall be 20x20(Db)=400mm. Minimum thickness for coupled Walls shall be 300 mm. Normal Shear wall shall have minimum thickness of 160mm taking fire resistance into consideration.