CBM Engineers

Observations on Indian High Rise Construction

By

Dr. Joseph Colaco & Vimal Parikh

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Assessing the appropriateness of Indian Codes for Tall Building Design.

Detailing the use and abuse of ETABS.

Review of Indian Construction Practices

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Comparison of Indian and US Standards

Codes and Standards – Main Source of Information to Designers of Civil Engineering Structures.

Indian Standards - IS 875, IS 1893, IS 456

US Standard - IBC - 2003

Gravity Loads - Imposed / Live Load

Lateral Loads – Wind Load Seismic Loads

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LIVE LOADS

• IS 875 - 1987 – PART 2

• IBC 2003 – Table 16 A

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SR. NO Item

Live Loads as per

IS 875 (Part 2) 1987in KN/m^2

Live Loads as per

IBC 2003in KN/m^2

1 Office – Typical Floor 4.0 (With storage)2.5 (Without storage)

2.5-

Office – Corridors 4.0 5.0

2 Residential – Typical 2.0 2.0

Residential – Corridors 3.0 5.0

3 Garages 5.0 2.5

4 Car Bumper Loads No Specific Values 26.7 KN

Live loads

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WIND LOADS

• IS 875 - 1987 – PART 3

• IBC 2003 – SECTION 1609 (ASCE-7-02 - Section C6.0)

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SR. NO

Item IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

1 Drift Requirements H/500(Not clear whether this

limit is applicable for Design or Service

Wind Load)

Not specified Generally H/400

(Under Design Wind Load)2 Structural Properties Not Specified Cracked Properties

3 P-Delta Effects Not Specified Required to be Included

4 Torsion Not Specified ASCE-7-02 (pg.48)

5 GEF Method Old & hard to read from charts

ASCE-7-02 based on Latest research

Wind Sway Requirements

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IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

Wind Sway Drift Requirements

• As per IS 456 - 2000 Sec. 20.5, it shall not exceed H/500.

(Not clear whether this limit is applicable for Design or Service Wind Load)

• Not specified Generally H/400

(Under Design Wind Load)

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IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

Wind Sway Structural Properties Requirements

• Not specified • Members with Cracked Structural Properties as per Section 10.10.4.1 of ACI - 318 shall be used.

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IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

Wind Sway P-Delta Requirements :-

• Not specified • Required to be included

Wind Sway Torsion Requirements

IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

• Not specified • ASCE-7-02 (pg.48)

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IS 875 (Part 3) 1987 IBC 2003 (Section 1609)ASCE 7-02 (Section 6)

Gust Effect Factor Method (GEF Method)

• Old & hard to read from charts

• ASCE-7-02 based on latest research

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SEISMIC LOADS

• IS 1893 – 2002

• IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

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IS 1893 – 2002 IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

Fundamental Period of Structure

Very old - Leads to Large design forces for Low rise Structures and Smaller Forces for High-rise Structures

•More Realistic

* h

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IS 1893 – 2002 IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

Base Shear

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IS 1893 – 2002IBC 2003 – SECTION 1617

(ASCE -7-02 - Section C9.0)

Torsional Effects

• Accidental Torsion (Sec. 12.8.4.2) :-

Design Eccentricity, Edi = ± 0.05 * bi• Amplification of

Accidental Torsion Moment (Sec. 12.8.4.2) :-

With max. limit of Ax = 3.0

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IS 1893 – 2002IBC 2003 – SECTION 1617

(ASCE -7-02 - Section C9.0)

Vertical Irregularities – Weak Story

CBM EngineersVertical Irregularities – Weak Story

Examples of Weak Story

• Outrigger Floors

• MIVAN / TUNNEL FORM Systems Transferred above Ground Floor

• Major Transfer of Lateral Elements above Ground Floor

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IS 1893 – 2002

• Misuse of Flat slab as OMRF in Shear Wall + Frame system in high seismic Zone

IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

Misuse of Shear Wall + Slab Frame System

• Table 12.2-1 DESIGN COEFFICIENTS AND FACTORS FOR SEISMIC FORCE–RESISTING SYSTEMS

NL – No LimitNP – Not Permitted

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IS 1893 – 2002

• As per Section 7.12.2.2 it shall be designed and checked for Five times the design vertical coefficient. (= 3.33 * Ah)

For Zone III with R=5.0, Factor = 0.0533 W

IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

Seismic Forces on Cantilever Projections

• As per Section 9.5.2.6.4.3 it shall be designed and checked for 0.2 * SDS*W.

For Zone III with R=5.0 Factor = 0.0373 W

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IS 1893 – 2002

• As per Section 7.11.1 it shall not exceed 0.004 * Story Height

Drift Limitations are Close

IBC 2003 – SECTION 1617 (ASCE -7-02 - Section C9.0)

Drift Limitations

• As per Section 1630.9.2 of UBC 97, The Maximum Inelastic Response Displacement, ΔM,

ΔM = 0.7 * R * ΔS

Where,ΔS = Storey drift based on

Analysis of the structure incl. P-Delta Effects

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ANALYSIS OF SAMPLE BUILDINGS

FOR BOTH IS & US

STANDARDS

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• A Sample building 65-Story in Mumbai is analyzed for both Indian Standards and IBC- 2006.

Building Data • H = 235m• Building dimensions - 24.8m x 35.0m• Structural System - Ductile Shear wall + OMRF• Soil Type - Hard Soil/Rock

Comparison for a Sample Building

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• Total building Weight = 1066314 KN• Zone Factor = 0.16• Importance Factor = 1.0• Soil Type = I (Hard Rock)• Response reduction Factor = 4.0• Base Dimension, Dx = 24.8m Dy = 35.0m

• Code Specified Time Period, Tx = 4.247 sec Ty = 3.575 sec• Sa/g,x = 0.235 Sa/g,y = 0.280

Design Data - (As per IS 1893 – 2002)

CBM EngineersSR. NO.

ITEM AS PER INDIAN

STANDARDS

AS PER US STANDARDS

1 Seismic Base shear, Vx Vy

5332 KN (0.7 times)5972 KN

(0.78 times)

7617 KN

7617 KN

2 Wind Shear, Wx Wy

17133 KN11179 KN

13673 KN8680 KN

3 Code Specified Time Periods, Tx Ty

4.253 sec3.575 sec

2.926 sec2.926 sec

4 Displacements @ top, Δx EQ

Δy EQ

0.185m

0.115m

0.32m

0.19m

5 Displacements @ top, Δx WIND

Δy WIND

0.368m

0.140m

0.288m

0.107m

SUMMARY OF RESULTS

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• Consider the same building 20-Stories tall now - in Mumbai. Analyzed for both Indian Standards and IBC- 2006.

Building Data • H = 73m• Building dimensions - 24.8m x 35.0m• Structural System - Ductile Shear wall + OMRF• Soil Type - Hard Soil/Rock

Comparison for a Sample Building

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• Total building Weight = 290076 KN• Zone Factor = 0.16• Importance Factor = 1.0• Soil Type = I (Hard Rock)• Response reduction Factor = 4.0• Base Dimension, Dx = 24.8m Dy = 35.0m

• Code Specified Time Period, Tx = 1.32 sec Ty = 1.11 sec• Sa/g,x = 0.76 Sa/g,y = 0.90

Design Data - (As per IS 1893 – 2002)

CBM EngineersSR. NO.

ITEM AS PER INDIAN

STANDARDS

AS PER US STANDARDS

1 Seismic Base shear, Vx Vy

4409 KN(2.13 times)

5221 KN (2.52 times)

2071KN

2071 KN

2 Wind Shear, Wx Wy

3186 KN2046 KN

2919 KN1860 KN

3 Code Specified Time Periods, Tx Ty

1.32 sec1.11 sec

2.926 sec2.926 sec

4 Displacements @ top, Δx EQ

Δy EQ

0.026m

0.017m

0.011m

0.0077m

5 Displacements @ top, Δx WIND

Δy WIND

0.01m

0.0042m

0.0097m

0.0039m

SUMMARY OF RESULTS

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DESIGN ISSUES

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IS 875 – Part 5, IS 456

1.5 D + 1.5 L

1.5 D +1.5 (W or E)

1.2 D +1.2 L + 1.2(W or E)

0.9D ± 1.5(W or E)

IBC 2003 – SECTION

1.2 D + 1.6 L

1.2 D + (1.3 W or 1.0 E)

1.2 D + 0.5 L+ (1.3 W or 1.0E)

0.9D ± (1.3W or 1.0E)

Design Load Factors and Combinations

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IS 875 – Part 5, IS 456

Design Load For a Sample Residential Building

SR. NO

ITEM INDIAN STANDARD

(KN/m2)

US STANDARD(KN/m2)

1 Self wt. of Slab (200mm thk.)

5.0 5.0

2 Floor Finish 1.5 1.0

3 Ceiling & Mechanical 0.5 0.5

4 Partition Walls 2.5 1.0

5 Sunk Areas 2.0 --

6 Live Load 2.0 2.0

Total DLTotal LL

11.52.0

7.52.0

Ultimate LC 1.5 DL + 1.5 LL 1.2 DL + 1.6 LL

TOTAL (Ultimate) 20.25 12.2

(1.66 times)

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IS 456 – Table 20

ACI - 318 – 05, Section 11.7.5-------0.2* fc’ or 5.52 N/mm^2(max.)

Maximum Allowable Shear Stress

Concrete Grade M15 M20 M25 M30 M35 M40

τ N/mm^2 3.0 4.0 5.0 5.52 5.52 5.52

The values given above include a ф factor of 0.75

CBM EngineersDesign of Post Tensioned Concrete

CBM EngineersDesign of Post Tensioned Concrete

CBM EngineersDesign of Post Tensioned Concrete

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IS Codes

No Provisions available

Design of Composite Columns

AISC Manual – Chapter I

Complete Procedure for Design of Composite Members

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Outrigger Floors

- Differential Axial Shortening

Fundamental Periods

- Closeness or Combination of Torsional and Lateral Modes

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Detailing the Use and Abuse

of ETABS Analysis

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• Over Estimation of Dead & Live Loads due to Common/Overlapping areas of Beams & Columns with Slabs.

PROGRAM LIMITATIONS

CBM EngineersPROGRAM LIMITATIONS

CBM EngineersCOMPARISION OF LOADS - 3 STORY BUILDING

• A 3-Story RCC building of 6.0m x 5.0m modeled in ETABS

Story Height = 3.0m

Beam Size = 230mm x 300mm

Column Size = 300mm x 300mm

Slab Thickness = 120mm

SDL = 1.5 KN/m^2

LL = 2.0 KN/m^2

Unit Weight of Concrete = 25 KN/m^3

CBM EngineersCOMPARISION OF 3 STORY BUILDING

CBM EngineersCOMPARISION OF 3 STORY BUILDING

Item ETABS results (KN) MANUAL results (KN)

Self Weight of Slab 1080.00 990.82

Self Weight of Columns 182.25 182.25

Self Weight of Beams 341.55 322.92

Total Dead Load 1603.8 1495.99

• Dead Load is over estimated by 7 %

• If Partition Walls are present, Uniform SDL & LL will also be Over Estimated by the Program.

•Dead Loads and Live Loads may be Over Estimated up to 20 % Depending Upon the Geometry of the Building.

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BRACED FRAME STRUCTURE WITH RIGID DIAPHRAGM UNDER LATERAL LOAD

• ZERO AXIAL LOADS in Beams since there is no relative displacement of end nodes of beams.

PROGRAM LIMITATIONS

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BRACED FRAME STRUCTURE WITH RIGID DIAPHRAGM UNDER LATERAL LOAD

SOLUTION –

• Release one node of the beam from the Rigid Diaphragm.

• Provide Semi-rigid Diaphragm - Parametric study with diaphragm flexibility required to obtain correct amount of axial force.

PROGRAM LIMITATIONS

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CRACKED PROPERTIES OF COUPLED SHEARWALL

• CAN NOT BE MODELED ACCURATELY due to inherent

PROGRAM LIMITATIONS

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CRACKED PROPERTIES OF COUPLED SHEARWALL

• ACI 318-05 Provisions

PROGRAM LIMITATIONS

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CRACKED PROPERTIES OF COUPLED SHEARWALL

• CORRECT WAY TO MODEL

• Reduce both Axial Area and Moment of Inertia.

• Add a Frame Element with only with missing axial area and Zero Moment of Inertia ( Like a Column).

PROGRAM LIMITATIONS

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INCORRECT DESIGN OF COLUMNS WITH SMALL AXIAL LOAD

• ETABS Column Design module for ACI-318 does not check it correctly when -

Ultimate axial load (phi * Pn) < (0.10 * fc‘ * Ag)

where:phi = Strength Reduction FactorPn = Nominal Axial Load Strengthfc' = Compressive Strength of ConcreteAg = Gross Area of Section

• The design shall be done like a flexural member (like a beam)

• (Reference: ACI-318, Appendix B: B.10.3.3)

PROGRAM LIMITATIONS

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MODELING AND

ANALYSIS IN

ETABS

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MEMBRANE• Use only when In-plane stiffness properties of member are

desired.

PLATE• Use only when Out-of-plane bending stiffness properties of

member are desired.

SHELL• Use when both In-plane and Out-of-plane stiffness properties

of member are desired.

TYPES OF ELEMENTS

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SLAB• Simple RC Solid Slab.• By default modeled as 2-way slab.• Can also be modeled as 1-way slab.DECK• Used as 1-way load Transfer.• Metallic Composite Slab.• Filled Deck, Unfilled Deck & Solid Slab Deck.PLANK• By default use 1-way load transfer mechanism.• Generally used to model pre-cast slabs.• Can also be a simple RC solid slab.

LOAD TRANSFER FOR FLOOR AND RAMP

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• Walls can be modeled with membrane or shell elements depending on the desired type of behavior.

• Shell type of elements are generally recommended.

ELEMENT USED FOR WALLS

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AT BASE• For Typical RCC building, it is FIXED - All translational

and rotational degrees of freedom are restrained.

AT GROUND FLOOR

• Restrained in both Horizontal directions to account for the lateral restraint provided by Basement walls.

SUPPORT CONDITIONS

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Not Providing the restraint at the ground level will result in –

• A Fictitious Structure that is more Flexible.

• Over design of Foundation Structure.

• May Result in Under Design of Basement Walls.

SUPPORT CONDITIONS

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• Appropriate Modeling Technique shall be used for Outrigger Floors.

• A Separate Sequential Analysis Required for Axial Shortening and Transfer of Forces.

• Appropriate Cracking Coefficient shall be used.

OUTRIGGER FLOORS

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• Appropriate Modeling Technique shall be used to Account for Arching Action and Flow of Forces.

• A Separate Sequential Analysis required for Gravity Loads.

• Appropriate Cracking Coefficient shall be used.

MODELING OF MAJOR TRANSFER ELEMENTS

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• P-Delta Analysis For Lateral and Torsional Deflections.

• Temperature and Creep/Shrinkage Analysis.

• Construction Sequence Analysis for Correct Force transfer and Design of Structural Elements.

OTHER ANALYSES

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• Column/Wall Axial Shortening Analysis and Column/Wall Height Adjustments for Floor Levelness – Especially For Tall structures.

• “Performance Based Design” and “Non-Linear Analysis”.

OTHER ANALYSES

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REVIEW OF INDIAN CONSTRUCTION

PRACTICES

CBM EngineersReview of Indian Construction Practices

Overall Dead Load of Structure

Structures in India ~ 25 KN/m^2 Structures in the US ~ 11 KN/m^2

• Heavy Partition Load – Brick Partitions.

• Screed of 50-100 mm is commonly used.

• Heavy Water Proofing Load.

•Impact on Seismic Loads, Structure and Foundations.

CBM EngineersReview of Indian Construction Practices

Sunk Slabs at Toilets, Decks

• Structurally Challenging.

• Does not Allow the use of Certain Types of Framing Systems such as Post-Tensioning, Flat Slabs.

• Difficult for Construction – Complicated Formwork.

• More Cost and Time for Construction.

CBM EngineersReview of Indian Construction Practices

CBM EngineersReview of Indian Construction Practices

Suggestions/Recommendations

• Try to Reduce Overall Weight of Structure.

• Use of Light Weight Partitions – Reduction in Wall Weight up to 50% with Siporex, AAC blocks, Gypsum Walls.

• Elimination of Screed – Especially in garages.

•Elimination of Sunk Slabs.

CBM EngineersReview of Indian Construction Practices

Suggestions/Recommendations

• Lower Weight results in

- Lower Design Seismic Loads. - Lighter Structure. - Reduction in Foundation Sizes and Cost.

• Elimination of Sunk Areas

- Easier Construction. - Easy Formwork System. - Flat Slabs and Use of PT Systems Possible. - Saves Cost and Time.

CBM Engineers

THANK YOU