Analysis & design of 36 to 45 storeyed towers in Noida and...

Analysis & design of 36 to 45 storeyed towers in Noida and Kolkata

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Abstract: As the height of building increases, its structural form, system and design are significantly affected by lateral forces due to wind and Earthquake action. Therefore, in functional planning, choice of structural form influences safety and economy.

Though Dead Loads and imposed loads due to functional usage can be assessed with certain reliability, there will be some uncertainty in the assessment of the magnitude of lateral loading due to wind and Earthquake actions, as they are dynamic and random in nature. Moreover, lateral deflection and oscillatory movements of flexible tall structures due to the wind can induce considerable discomfort in occupants unless properly accounted and controlled in the design.

This paper discusses the results based on Design and Analysis of a number of tall structures done by the author in the storey range of 36 to 45, which are constructed in Noida and Kolkata.

The paper presents interesting results that in some tall towers though wind loading primarily governs the design, in some cases Earthquake loading governs in one direction and wind loading in the other orthogonal direction. For skew frames, Earthquake forces are specified by I.S. 1893 Code, but for the wind, governing cases are obtained for skew frames only by wind tunnel tests which show how the wind components affect the design, in both directions similar to that due to Earthquake.

Air Building (G+40) at Uniworld City Kolkata

The project consists of the construction of AIR building, Ground + 40 floors in height on a large basement of car parking, which is fairly rectangular in size at Uniworld City Kolkata.

Structural System

The structural system consists of shear wall/ core interacting with beam- column frames.

Special Investigation

For the 41 storey building, it was necessary to investigate the dynamic effect of the wind along and across the direction of the wind. Therefore, design pressures and forces have been ascertained using a rigid (pressure) model of the structure in a wind tunnel. This test was also necessary to ascertain the venture- the effect of flow of high wind through the continuous central vertical opening provided as upper levels. The test was done at I.I.T. Roorkee.

Based on wind tunnel test, it was possible to assess the simultaneous actions of the wind on the structure, both along and across, during the motion of wind. Based on Wind Tunnel Test the design forces were given by I.I.T. Roorkee for wind motion from 0 to 1800 at 450 intervals with respect to the minor axis. Stand alone and interference conditions were also considered.

Distribution of wind loading along the height of the building as given in IS: 875 has been scaled to match the base shear assessed through the wind tunnel tests. Load combination has been modified to account for the simultaneous action of both along and across, during wind motion parallel to the incidence considered.

Alterations in the sizes of few members such as columns and walls were required based on the enhanced forces as per wind tunnel tests.

Seismic Loads

Seismic loading has been considered as per IS: 1893 (part-1) – 2002. Dynamic analysis has been carried out using response spectrum. The time period of the structure has also been worked out using STAAD Pro software. The building has been designed for base shear based on codal time period in accordance with i.e. 7.8.2 of IS : 1893 – 2002 using modification factor = ‘VB/ VB where ‘VB is calculated based codal time period and VB is calculated based on time period given by STAAD Pro software.

Wind Loads

Wind loads have been worked out based on the basic wind speed of 50m/s for a Return Period of 50 years. Based on the parameters listed above, average wind pressures including external pressure co-efficient are calculated at different heights.

This has been, scaled appropriately based on the special investigation report of Wind Tunnel Test.

Analysis, Model and Software Used

Super Structure: – The building has been analyzed as a 3- dimensional structure using STAAD Pro software. Rigid diaphragm action of the floor has been stimulated in distributing lateral forces due to Earthquake/wind.

Foundation: – 1000mm dia bored Cast-in-Situ piles 40m long with pile caps.

Design & Detailing of RCC Structure

Some of the various load combinations considered are as follows:-

1a 1.5 (D.L. + Reduced L.L) as per fig. 1 of IS:875 – 1987 (Part-2) (For design of vertical members only)
1b. 1.5 D.L + L.L (For Beams Only)
2. 1.5 (D.L. ± E.Q. in X- Direction)
3. 1.5 (D.L. ± E.Q. in Z- Direction)
4. 1.2 (D.L. + K1L.L ± E.Q. in X- Direction)
5. 1.2 (D.L. + K1L.L ± E.Q. in Z- Direction)
6. 0.9 (D.L.) +1.5 E.Q. in X- Direction
7. 0.9 (D.L.) +1.5 E.Q. in Z- Direction
8. 1.5 (D.L. ± E.Q. in X- Direction ± 0.3 E.Q. in Z Direction)
9. 1.5 (D.L. ± E.Q. in Z- Direction ± 0.3 E.Q. in X Direction)
10. 0.9 (D.L.) + 1.5 (E.Q. in X- Direction + 0.3 E.Q. in Z Direction)
11. 0.9 (D.L.) + 1.5 (E.Q. in Z- Direction + 0.3 E.Q. in X Direction)
12. 1.2 (D.L. + K1 L.L ± E.Q. in X- Direction ± 0.3 E.Q.
in Z Direction)
13. 1.2 (D.L. + K1 L.L ± E.Q. in Z- Direction ± 0.3 E.Q.
in X Direction)
14. 1.5 (D.L. + W.L in X- Direction ± K2W.L in Z Direction)
15. 1.5 (D.L. ± W.L. in Z- Direction ± K3W.L in X Direction)
16. 1.2 (D.L. ± L.L. ± W.L in X Direction ± K2 W.L in Z Direction)
17. 1.2 (D.L. ± L.L. ± W.L in Z Direction ± K3 W.L in X Direction)
18. 0.9 (D.L.) 1.5 (W.L in X Direction ± K2 W.L in Z Direction)
19. 0.9 (D.L.) 1.5 (W.L in Z Direction ± K3W.L in X Direction)

Note: Load combinations from (14) to (19) are based on wind tunnel test results using factors K2 & K3 as per Wind Tunnel test results.

D.L = Dead Loads
L.L = Live Loads
E.Q. = Earthquake Loads
W.L = Wind Loads
K1 = 0.25 For live load class upto 300 Kg/m2

Detailing of R.C.C. beams and columns conforms with IS: 456-2000, IS: 4326 (Earthquake resistant design and construction of buildings), IS: 13920 (Ductile detailing of reinforced concrete structures subjected to seismic forces).

Material of Construction

All RCC work for columns, beam and slabs are based on design mix concrete.

For columns and core walls M50 for lower storeys, M40 for middle-level storeys, and M35 for remaining storeys are being used.

Unitech Grande Phase-II (Burgundy Towers)

 

Model of Air Building at Kolkata, Construction Stage of Air Building at Kolkata
Model of Air Building at Kolkata, Construction Stage of Air Building at Kolkata
Typical Structural Arrangement of 41 Storeyed Air Building at Kolkata
Typical Structural Arrangement of 41 Storeyed Air Building at Kolkata

 

Project Brief:- The Development consists construction of High rise residential towers (Tower-1 to Tower-6) which comprises of building with 36 to 45 Storeys in Sector 96, 97, 98 Noida (U.P.)

Structural System

Conventional beam slab system with large nos. of shear walls including core and corner long walls have been adopted for resisting gravity and lateral loads.

Foundation System

The foundation for the above project consists of 750mm dia. of the bored cast-in-situ pile with piled raft foundation along with pedestal at column location which has been provided to transmit loads of superstructure to the foundation.

Concrete Grade

The concrete grade for all RCC elements is as per table below:-

Seismic Loads

Seismic loading has been considered as per IS: 1893 (part-1)-2002.

 

table-1

The fundamental natural time period is taken as Ta = 0.009h/vd as per clause 7.6.2 of IS: 1893-2002.

Spectral acceleration coefficient Sa/g is taken as 1.36//T as per clause 6.4.5 of IS: 1893-2002 (for medium soil & time period in range of 0.55 to 4.0 sec.)

Zone factor Z has been taken from table 2 of IS: 1893-2002 as 0.24 for seismic zone IV for Noida (Delhi NCR region).

Response spectrum reduction factor is taken as 4.0 from table 7 of IS: 1893-2002 for the ductile shear wall.

The building has been designed for base shear based on codal time period in accordance with i.e. clause 7.8.2 of IS: 1893-2002 using modification factor = VB/vBwhere VB is calculated based on codal time period and VB is calculated by ETAB software.

Wind Loads

Wind loads have been worked out based on the basic wind speed of 47m/s for a Return Period of 50 years. Average wind pressures including external pressure co-efficient are calculated at different heights. As per Clause 7 of IS: 875 (Part 3) – 1987

Quote

Dynamic Effects

Flexible slender structures and structural elements shall be investigated to ascertain the importance of wind induced oscillations or excitations along and across the direction of the wind.

In general, the following guidelines may be used for examining the problems of wind induced oscillations:

a) Buildings and closed structures with a height to minimum lateral dimension ratio of more than about 5.0, and
b) Building and closed structures whose natural frequency in the first mode is less than 1.0 Hz.

Any building or structure which does not satisfy either of the above two criteria shall be examined for dynamic effects of wind.”

Unquote

Thus wind Tunnel test was also carried out to study the Structural Wind Loading on one of the towers to provide loading information for the overall structural design and to determine the wind- induced accelerations.

The overall wind- induced overturning moments, shear forces and torsional moments for the tower, acting at its base have been predicted for the 50 years design return period by the Wind Tunnel Test. Overall moments and shears are provided on a floor- by – floor basis.

It was seen that the predicted torsional velocities were acceptable for human comfort for the tower as recommended by the Council on Tall Buildings and Urban Habitat (CTBUH).

The overall wind- induced overturning moments, shear forces and torsional moments for the tower, acting at its base have been predicted for the 50 years design return period by the Wind Tunnel Test.

Overall moments and shears are provided on a floor- by – floor basis.

It was seen that the predicted torsional velocities were acceptable for human comfort for the tower as recommended by the Council on Tall Buildings and Urban Habitat (CTBUH).

After analysing for various load combinations given in Indian Standard Codes, for skew structures, it was seen that in short direction wind was governing and longer direction Earthquake was governing.

 

table-2

Typical Architectural PlanUnitech Grande Phase-II (Burgundy Tower), Noida
Typical Architectural PlanUnitech Grande Phase-II (Burgundy Tower), Noida
Model of Unitech Grande Phase-II (Burgundy Tower), Noida, Construction Stage of Unitech Grande Phase-II (Burgundy Tower), Noida
Model of Unitech Grande Phase-II (Burgundy Tower), Noida, Construction Stage of Unitech Grande Phase-II (Burgundy Tower), Noida

 

Conclusion

Wind Tunnel Test should be carried for Tall Slender structures. In the case of skew structures,these tests also indicate an effect of wind components in both directions similar to that due to Earthquake.

Another important aspect which should be considered for tall structures is the Human comfort. Council on Tall Buildings and Urban Habitat (CTBUH) recommends for 10 years return period acceleration as well as peak torsional velocity for residential as well as office buildings. ISO Standard (ISO 10 137: 2007 [E] – Annex- D) also recommends the acceleration criteria. These recommendations should be also be considered.w

Author’s Bio

 

Subhash Mehrotra   Managing Director Mehro Consultants New Delhi
Subhash Mehrotra
Managing Director Mehro Consultants New Delhi

Subhash Mehrotra is the Managing Director – Mehro Consultant. He is a M. Tech. (Structural Engineering) from I.I.T. Delhi. The author was the Former President – Indian Association of Structural Engineers as well as Consulting Engineers Association of India. He is a Former Executive Member Committee Member of International Federation of Consulting Engineers (FIDIC).

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