Client: Navi Mumbai Municipal Corporation
Architect: Hiten Sethi Associates
Structural Consultant: Shanghvi & Associates Consultants Pvt. Ltd. ( SACPL )
Civil contractor: Ashwini Infra
The recently completed office building for NMMC is an iconic building that stands apart from other structures in aesthetics, functionality and the structural design competencies associated with it. Primarily, the structure intends to be the main office building for the NMMC (Navi Mumbai Municipal Corporation) and it stands at sector 15A at CBD, Belapur. M/s Hiten Sethi Architects Pvt. Ltd were the Architects for this project and M/s SACPL – the Structural Design Consultants.
The idea of building the Navi Mumbai Municipal Corporation (NMMC) came around 2005. Prior to the present location, the site selected was in Vashi, where two existing buildings were present at the site and the NMMC wanted to transform these buildings into the NMMC headquarters. The idea was to bring all the departments under one roof.
By creating such a structure, the people of Navi Mumbai could avail better services all under one roof. However, later it was decided that a new plot at a prominent location should be selected to create the NMMC headquarters. CIDCO has always stated that Navi Mumbai is the city of the 21st century. Besides infrastructure, we required a landmark that will boast the city spirit that is evergreen. They finally settled for the current plot on which the NMMC headquarters now stands due to its strategic location.” Both geographical and topographic studies have been carried out for this iconic project.
The grand entrance pier is made of only five to seven columns and it is supported at the entrance with three-level high floors supported by a flat slab on top, hence there is no false ceiling involved. About 80 per cent of the slab is exposed and painted.” This project also comprises 20-m column-less office spaces on all floors. These columns are created with PT slabs. Apart from the triple height grand entrance foyer, the project features a 9-m-wide suspended glass canopy supported by only one column and has made use of a composite stone dry cladding system. Every detail has been executed with the vision that each element should have its own story to tell.
HITEN SETHI, CEO, HITEN SETHI & ASSOCIATES
The idea was clear; the NMMC headquarters had to be an iconic structure that becomes a landmark structure for the city of Navi Mumbai. The Architect knew what they needed and hence did not want to copy or replicate any structure. They realised that features such as a dome or shell should be part of the structure.
Owing to the iconic landmark that had to be created, designing the structure was a challenge. Everything had to be of a bigger and larger volume. Post-tensioned (PT) beams and slabs have been used to achieve large column-less spaces for the office areas. Apart from this, PT span beams of 45 m are rested on column corbels and float bearings by making use of a bridge like technology and methodology to ensure structural safety in case of an earthquake.
The structure is a circular building that is 100 m in diameter and comprises a 45-m diameter triple height atrium. The Architect did not want any columns in the atrium and hence carried out a span of 45 m to cover it at a height of 22 m above ground level. In addition, interesting detailing has been carried out at the entrance with the help of spider glazing systems. The NMMC headquarters comprises three glass-reinforced concrete (GRC) domes; the mega dome at the centre has a diameter of 40 m and is positioned over the Mahasabha Hall. It has been created from structural steel framework and has been given a GRC finish. In fact, it is the largest dome in India made of GRC.
It also includes two other domes that are 12 m in diameter designed in a similar manner. These domes are positioned above the mayor and commissioner’s chambers on the fourth floor. The grand entrance pier is made of only five to seven columns and it is supported at the entrance with three-level high floors supported by a flat slab on top, hence there is no false ceiling involved. About 80 per cent of the slab is exposed and painted. This project also comprises 20-m column-less office spaces on all floors. These columns are created with PT slabs. Apart from the triple height grand entrance foyer, the project features a 9-m-wide suspended glass canopy supported by only one column and has made use of a composite stone dry cladding system. Every detail has been executed with the vision that each element should have its own story to tell.
Geometrics of the Iconic
The total construction area of the project is 2.5 lakh sq. ft. The structure has 7 levels, above the ground floor. The first, second and third floors are having the central portion as void. So, a person standing at the ground floor lobby gets an astounding view of the circular shaped outer structure with circular columns and the high ceiling of about 25m.
The fourth floor and the fifth floor are having full floor plate including the centre circular area. The roof level is 12.7m above the fifth level and in between there are intermediate part levels. The dome projects above the roof level over the inner circular area. The maximum height of the structure is 56m above the ground level. There is one basement also for the vehicle parking. The extended basement area is having a maximum size of 160m in length and 150m in width.
The foundation of the structure is done with Independent footings. The foundations are supported on rocky strata which is 7.5m below the road level. The rock formations are having a bearing capacity of 80t/m2 as recommended by the Geotech consultant. The floors are either normal beam/slab system or Flat slab system. Flat slabs with peripheral beams were used extensively in the floors. If the slab panel size is more, they were given pre-compression using post tensioning.
Normal concrete of Grade up to 40 N/mm2 (M40) is used for the structure. Two-hours fire resistance is adopted as per the NBC guide lines and the analysis is done as per the relevant IS codes. 3D models of the structures were done using the latest finite element software ETAB. The foundation and the slabs were designed using FEM software SAFE.
There are various structural engineering aspects which makes this structure unique among other office buildings.
Floor Slab at the 4th Level:
The size of the central circular floor area is 43.2m diameter. To support such a huge span, various farming methods using reinforced concrete and others like structural steel framing and pre-stressed concrete were prepared and compared. Finally, pre-stressed beams with RCC slabs were selected. PT beams with sizes of 600mm width and 1600mm depth are used to effectively transfer the huge floor loads to the supports. These were pre-stressed with bonded post tensioning methodology.
It now holds a record for the maximum span for a pre-stressed concrete beam and entered in to the Limca book of world records. Specially designed shuttering systems were used for this floor as it stands at a height of 18.25m from the ground floor level.
The Circular Dome:
The dome at the roof level is having a diameter of 43.2m and the apex point is 15 m high. Such a large size dome was conceived with structural steel framing with GRC sheeting. This Dome has been recognized as the biggest dome by Limca Book of World records. Apart from the Central dome, there are two smaller domes as well at the fifth floor. But the sizes of those are only 15m in diameter.
Major Corbels for the Floor Supports:
The central circular areas are supported on the corbels taken from the inner ring columns at the 4th and upper levels. These are 1m wide and have a depth of 1.2m- 2.2m. These allow for the natural expansion joint between the outer portion of the building and the inner circular area.
Finite element models were done to capture the stress flow patterns and it clearly shows the desired load path as expected from these elements.
Suitable bearing pads are designed in such a way to allow uniform transfer of load from beams to these corbels. They permit beam rotation at the bearing point due to deflection and also allow lateral movement. These also insulate the sound and vibration transfer between the two parts.
Seismic Design of the Structure:
Stringent norms were considered in the lateral load design of the structure, especially for seismic forces. The code prescribed values (like seismic zone) were enhanced further, considering the importance of the structure. The detailing is also done to ensure that proper ductility is available for all the main lateral load carrying elements.
As the central portion is structurally detached from the main structure, it can move separately during seismic condition. The main structure is checked and designed for the associated forces from this phenomenon.
Construction sequence analysis is done to capture the non-linear behavior of the structure when it is being casted in different stages. It is important for this structure as the loads coming on to the columns are quite large, though the no. of stages is not much.
Free Standing Columns at the Ground Level
There are some columns which are free- standing (without tie at intermediate levels) up to the fourth level slab, i.e. about 25m height from the fixed base. The sizes of the columns were fixed by considering the slenderness moments and the buckling aspect.
Structural Health Monitoring System (SHM)
Due to the large span of the central floor area, proper monitoring of the structural responses is to be observed for the service life of the structure. Keeping this as objective, strain gauges and penetrometer were planted in the concrete at these levels to check whether any excessive deflection is happening at these levels.
Further, it is planned to have much more sophisticated tools to measure the accelerations and vibrations also. These facilities will provide the building management team, the real-time information on the building movements and characteristics to effectively service the structure during its life time.
Construction Methodology and Quality Control
High standard construction methodologies were used throughout the full construction phase of the structure. Stringent quality control norms were also followed. At the super structure, special shuttering methods were adopted for the fourth floor. The erection of the central dome also demanded precision formworks. Pour sequence for the casting of this structure also is an important aspect because of the parameters like large spans, pre-stressing, corbels, expansion joint etc.
When we were assigned the structural design of this project, which is only about 2.5L sq. ft., we never imagined that this one to be turned out as a landmark project in the Navi Mumbai area with such an iconic proportion. The entire project duration of about 4 years was mixed with many challenges, in the structural designs as well as in the site execution. The complexities of the structure are obvious in the geometry itself and the hardships which we had gone through in materializing the vision of the Architectural intend has finally become fruitful in all aspects, which in fact is the true reward for any professional.
JITESH PATEL, Principal Engineer, SACPL
In this project, composite marble with a six-side sealed stone has been utilized for exterior dry cladding.
The support system for dry cladding has also been used with the help of aluminium-based technology. All decorative elements in column cladding, cornices and domes, have been created by GRC. For the triple height atrium, we have used a metal ceiling.
Composite marble with higher aesthetic value has also been used in the entrance foyer and upper floors. For all glazed areas, DGU is used and lustre paint has been applied on walls. Acrylic emulsions are applied to ceilings instead of plastic emulsions for the interiors.
Epitome of efficiency
The building has been registered with LEED-IGBC for certification and aims to receive the Gold rating for this project. The project also features a rainwater harvesting system with 13 groundwater recharge pits for harvesting surface rainwater and collection tank for terrace rainwater. A biogas plant has also been constructed to generate cooking gas for the project. In addition, a sewage treatment plant of 0.150 mld along with a water purification system has been installed at the premises.
The treated water is used for cooling towers, flushing and gardening. Sustainable materials in the form of GRC and manufactured stone have been used for dry cladding. This ensures a ventilated facade and keeps interiors cooler. The glass does not overpower the boldness of the structure; and a minimal percentage of glass in and around the structure has been kept. Apart from this, water-cooled chillers have also been utilized in the project. The cost of the project is about Rs 180 crore and the payback period is seven to eight years. The final result: a green success!