Dawn of New Era for Structural Engineers

Prof. (Dr.) Hardeep S Rai

Prof CE and Dean Consultancy

Guru Nanak Dev Engineering College, Ludhiana, India

Ensuring safety through BIS codes

National Building Code of India (NBC) 2016

  • incorporated changes as per latest technologies and users' requirements

  • incorporated changes as per latest technologies and users' requirements

IS1893(Part 1):2016

Criteria for Earthquake Resistant Design of Structures General provisions and buildings 6th Revision

  • Tremendous changes for structural design requirements.

Duplicate IS1893(Part 1):2016

Criteria for Earthquake Resistant Design of Structures General provisions and buildings 6th Revision

  • 1 Tremendous changes for structural design requirements.

  • 2 Tremendous changes for structural design requirements.

  • 3 Tremendous changes for structural design requirements.

IS13920:2016

Ductile Design and Detailing of Reinforced Concrete Structures Subjected to Seismic Forces

  • many clauses have far reaching repercussions

Draft code CED38(10639)

Criteria for Structural Safety of Tall Buildings

  • First code in India, related to tall buildings.

New era for

  • structural design requirements

  • architectural planning to achieve safety of structures during earthquakes

  • interaction between and Architects and Structural Engineers would become much more important.

Impact

of IS1893(Part 1):2016 and related codes on:

  • Design of Tall Buildings

  • including buildings under construction

  • Proof checking/peer review of structural designs

For faster construction

  • Different types of form work

  • New and innovative construction technologies were adopted

IS13920:2016

Minimum dimension of a column

  • 300 mm

  • 20 d_b

  • d_b mean db, dia of largest bar from beam coming to column.

  • Minmun dimension is 300 for dia < 16

  • 16 dia: Column size 320 mm

  • 20 dia: Column size 400 mm

  • 25 dia: Column size 500 mm

  • This will affect architectural planning.

IS13920:2016

Design shear force capacity of RC beams shall discard contributions of following:

  • bent up bars,

  • inclined links, and

  • concrete in the RC section.

IS1893(Part 1):2016

  • For normal structures.

  • Structures has to sustain damage during strong earthquake ground shaking.

  • To control the serious loss of life and property, base isolation or other advanced techniques may be adopted, by referring specialist literature for design, detail, installation and maintenance of such buildings.

IS1893(part 1):2002

  • No intend to lay down regulation to make all structures damage free, during earthquake of all magnitudes,

  • but to ensure that, structures are able to respond, without structural damage to shocks of moderate intensities and

  • without total collapse to shocks of heavy intensities.

Base isolation / energy absorbing devices

  • Only standard devices having detailed experimental data on the performance should be used.

  • Designer must demonstrate by detailed analysis that these devices provide sufficient protection to the buildings and equipment.

  • Performance of locally assembled isolation and energy absorbing devices should be evaluated experimentally.

  • Competent authority must review such design before it is used in practice.

revised code is 1893- criteria for earthquake

resistant design of structures (part-1 – general provisions for all

structures and specific provisions for buildings) importance factor s. no.

  1. structure (2) i (3) 1. important service and community buildings or

structures (for example, critical governance buildings, school), signature

structures, monument structures, lifeline and emergency structures (for

example, hospitals, telephone exchanges, television stations, radio

stations, bus stations, metro rail structure and metro rail stations,

railway stations, airports, water main lines and water tanks, food chain

structures, fuel stations, electricity stations, fire stations, and

bridges), and large community halls (for example, cinema halls, shopping

malls, assembly halls and subway stations) and power stations. 1.5 2.

residential or commercial buildings or structures, with occupancy more than

200 persons 1.2 3. all other buildings or structures 1.0 4. buildings with

mixed occupancies (different applicable for the respective occupancies) i

factor larger of the i values s. no. 2 & 4 would affect the new buildings

designs as per existing code is 1893 (part 1) : 2002 table 6 importance

factors, i (clause 6.4.2) s. no. structure (1) (2) i) important service

and community buildings, such as hospitals, schools monumental structures,

emergency buildings like telephone exchange, television stations, radio

stations, railway stations, fire station buildings, large community halls

like cinemas, assembly halls and subway stations, power stations ii) all

other buildings importan ce factor (3) 1.5 1.0 irregular buildings plan

irregularities (see figure) a building is said to be torsionally irregular,

when maximum horizontal displacement of any floor in the direction of the

lateral force at one end of the floor is more than 1.5 times its minimum

horizontal displacement at the far end of the same in that direction. in

torsionally irregular buildings, when the ratio of maximum horizontal

displacement at the other end is in the range (1)1.5 – 2.5, three-

dimensional dynamic analysis method shall be adopted; and (2)more than 2.5.

the building plan shall have to be revised thus interaction between

architect structural engineer would be required. and as per existing is

1893 (part – 1) – 2002 torsional irregularity to be considered to exist

when the maximum storey drift, computed with design eccentricity, at one end

of the structures transverse to an axis is more than 1.2 times the average

of the storey drifts at the two ends of the structure at any storey, the

minimum width of floor slab along any section after deduction of openings

shall not be less than 5 m. and, the minimum width of the slab beyond an

opening to edge of slab shall not be less than 2 m. further, the cumulative

width of the slab at any location shall not be less than 50% of the floor

width floor slabs having excessive cut-out and openings a0 table 6 response

reduction factor r for building systems (clause 7.2.1) s. no. (1) lateral

load resisting system (2) r (3) moment frame systems 1. rc buildings with

ordinary moment resisting frame (omrf)1 3.0 2. rc buildings with special

moment – resisting frame (smrf) 5.0 3. steel buildings with ordinary

moment resisting frame (omrf)1 3.0 4. steel buildings with special moment

resisting frame (smrf) 5.0 braced frame systems 5. buildings with ordinary

braced frame having concentric braces 4.0 6. buildings with special braced

frame having concentric braces 4.5 7. buildings with special braced frame

having eccentric braces 5.0 s. no. (1) lateral load resisting system (2) r

  1. structural wall systems 8. 9. load bearing masonry buildings (a)

unreinforced masonry (designed as per is 1905) without horizontal rc seismic

bands 1.5 (a) unreinforced masonry (designed as per is 1905) with horizontal

rc seismic bands 2.0 (a) unreinforced masonry (designed as per is 1905) with

horizontal rc seismic bands and vertical reinforcing bars at corners of

rooms and jambs of openings (with reinforcement as per is 4326) 2.5 (a)

reinforced masonry [refer sp 7 (part 6) section 4] 3.0 (a) confined masonry

3.0 buildings with ordinary rc structural walls1 3.0 10. buildings with

ductile rc structural walls 4.0 s. no. (1) lateral load resisting system

  1. r (3) dual systems 11. buildings with ordinary rc structural walls and

rc omrfs1 3.0 12. buildings with ordinary rc structural walls and rc smrfs1

4.0 13. buildings with ductile rc structural walls with rc omrfs1 4.0 14.

buildings with ductile rc structural walls with rc smrfs 5.0 flat slab –

structural wall systems 15. rc building with (a) ductile rc structural

walls (which are designed to resist 100% of the design lateral force), (b)

perimeter rc smrfs (which are designed to independency resist 25% of the

design lateral force), and (c) preferable an outrigger and belt truss system

connecting the core ductile rc structural walls and the perimeter rc smrfs1

  1. punching shear shall be taken care and drift at the roof be limited to

0.1% 3.0 as per existing code is 1893 (part 1) : 2002 as per revised code is

1893 (part 1) : 2016 sa g design acceleration coefficient corresponding to 5

percent damping for different soil = as per existning is 1893 (part 1) :

2002 the value of damping for buildings may be taken as 2 and 5 percent of

the critical, for the purposes of dynamic analysis of steel and reinforced

concrete buildings, respectively. however in the revised code damping is 5%

for both steel and rcc. as per existing code is 1893 (part 1) : 2002 table

3 multiplying factors for obtaining values for other damping (clause 6.4.2)

damping percent factors 0 2 5 7 3.20 1.40 1.00 0.90 10 15 20 25 30 0.80 0.70

0.60 0.55 0.50 in the revised code the table has been deleted. floating or

stub columns such columns are likely to concentrated damage in the

structure. cause as per existing code is 1893 (part 1) : 2002 a) stiffness

irregularity — soft storey a soft storey is one in which the lateral

stiffness is less than 70 percent of that in the storey above or less than

80 percent of the average lateral stiffness of the three storeys above b)

stiffness irregularity —extreme soft storey a extreme soft storey is one

in which the lateral stiffness is less than 60 percent of that in the storey

above or less than 70 percent of the average stiffness of the three storeys

above. for example, buildings on stilts will fall under this category. as

per revised code is 1893 (part 1) : 2016 soft storey – is one in which the

lateral stiffness is less than that in the storey above. the storey lateral

stiffness is the total stiffness of all seismic force resisting elements

resisting lateral earthquake shaking effects in the considered direction.

as per existing code is 1893 (part 1) : 2002 discontinuity in capacity —

weak storey a weak storey is one in which the storey lateral strength is

less than 80 percent of that in the storey above, the storey lateral

strength is the total strength of all seismic force resisting elements

sharing the storey shear in the considered direction. as per new code is

1893 (part 1) : 2016 weak storey – is one in which the storey lateral

strength (cumulative design shear strength of all structural members other

than that of urm infills) is less than that in the storey above. the storey

lateral strength is the total strength of all seismic force resisting

elements sharing the lateral storey shear in the considered direction.

design vertical earthquake effects effects due to vertical earthquake

shaking shall be considered when any of the following conditions apply: 1.

structure is located in seismic zone iv or v; 2. structure has vertical or

plan irregularities; 3. structure is rested on soft soil; 4. bridges; 5.

structure has long spans; or 6. structure has large horizontal overhangs of

structural members or sub- systems. table 8: minimum design earthquake

horizontal lateral force buildings clause 7.5.1) seismic zone (1) ρ (%) (2)

ii 0.7 iii 1.1 iv 1.6 v 2.4 (a) bare mrf buildings (without any masonry or

any other infills) ta = 0.075h0.75 0.080h0.75 for rc mrf building for rc –

steel composite mre building for steel mrf building 0.085h0.75 (b) buildings

with rc structural walls ta = 0.075h0.75 √aw in which, h awi lwi nw =

height of building (in meters) as defined in 7.6.2(a), = effective cross-

sectional area (m2) of wall i in first storey of building; and = length (m)

of structural wall i in first storey in the considered direction of lateral

forces, and = number of walls in the considered direction of earthquake

shaking. the value of lwi /h to be used in this equation shall not exceed

0.9 but not less than 7.6.3 (c) as per revised code is 1893 (part 1) : 2016

  1. all other buildings - where h = height of building, in meters, as

defined in 7.6.2 (a); and d = base dimension (in m) of the building at the

plinth level along the considered direction of earthquake shaking. as per

draft ced 38 – criteria for structural safety of tall buildings:- tall

building - it is a building of height greater than 45m, but less than 250m,

normally intended to be used as residential, office and other commercial

buildings. super tall building ― it is a building of height greater than

250m. height limit for structural systems the maximum building height (in

  1. shall not exceed values given in table 1 for buildings systems. with

different structural table 1 maximum values of height h above top of base

level of buildings with different structural systems structural system

seismi c zone structural moment moment structu structu wall system frame

frame + ral ral system structura wall wall + flat slab l wall system + tube

floor system system frame with perimeter system moment frame (1) (2) (3) (4)

    1. v not allowed not allowed 100 m 100 m 150 m iv not allowed not

allowed 100 m 100 m 150 m iii 70 m 60 m 160 m 160 m 220 m ii 100 m 80 m 180

m 180 m 250 m slenderness ratio the maximum values of the ratio of height h

to minimum base width shall not exceed values given in table 2. structural

system table 2 maximum slenderness ratio (b / h) seismic structural wall

moment moment structural zone system wall frame frame + system + flat slab

floor system structural system with wall perimeter moment system frame (1)

        1. v not not 8 8 allowed allowed iv not not 8 8 allowed allowed

iii 5 4 8 8 ii 6 5 9 9 structural wall + tube frame system (6) 9 9 10 10

plan aspect ratio the maximum plan aspect ratio (l/b) of the overall

building shall not exceed 5.0 lateral acceleration from serviceability

considerations, (human comfort) under standard wind loads with return period

structural of peak 10 years, the combined maximum lateral acceleration amax

in the building for along and across wind actions at any floor level shall

not exceed values given in table 4, without or with the use of wind dampers

in the building. table 4 permissible peak combined acceleration building

use residential maximum peak combined acceleration amax (m/s ) 0.15 office /

commercial 0.25 2 floor systems material all floor slabs shall be

cast-in-situ. precast floor systems without a minimum screed of 75 mm

concrete shall not be used in seismic zones iii, iv and v, but can be used

in seismic zone ii. structure modelling  rigid end offsets of linear

members in the joint region, when centerline modeling is adopted; 

cracked cross sectional area properties as per table 6; and  p-δ effects

table 6 cracked rc section properties un-factored loads structural element

area (1) slabs factored loads (2) 1.0 ag moment of inertia (3) 0.36 ig area

  1. 1.00 ag moment of inertia (5) 0.25 ig beams 1.0 ag 0.7 ig 1.00 ag 0.35

ig columns 1.0 ag 0.9 ig 1.00 ag 0.70 ig walls 1.0 ag 0.9 ig 1.00 ag 0.70 ig

in the final version of the draft tall building code there may be some

changes but the pholosophy of the new code has to be appreciated.

concluding remarks thus new earthquake resistant design of structures codes

have specified new guidelines for safety both for architects and structural

consultants. overall it would be a good code to follow. however it would

result in increase of earthquake forces in large number of cases. the main

problem would be that the existing buildings, recently completed buildings

as well as buildings under construction designed based on is 1893 (part - 1)

  • 2002 would be unsafe as per new is 1893 code. this would result in

anxiety in the minds of owners, structural designers and the people who

would use it / live in them. further, at the time of issue of compilation

certificate of the building, the authorities require structural safety

certificate for structural design as per codes published by bureau of indian

standards latest revisions and amendments. including thank you

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