Structural Systems, Analysis And Design Of Precast Concrete Multi-storeyed Buildings – A Perspective

Harisankar S., Senior Project Officer, Department of Civil Engineering, Indian Institute of Technology Madras

Amlan K. Sengupta, Professor, Department of Civil Engineering, Indian Institute of Technology Madras

Meher Prasad, Professor, Department of Civil Engineering, Indian Institute of Technology Madras

 

Introduction

It is the need of the hour in the construction industry to have greener, faster and sustainable approaches towards construction, to meet the ever-increasing demand of housing. One such method is the precast concrete technology. Though this technology has been in use in India for several decades, it is not utilised substantially because of a few impediments. The Ministry of Housing and Urban Affairs has recently launched ‘Global Housing Technology Challenge – India’, in which precast concrete technology is considered to be one proven technology. The Indian Concrete Institute published the Handbook on Precast Concrete for Buildings for the benefit of professionals involved in construction of buildings using precast concrete. The Handbook on Precast Concrete for Buildings is a source of compiled information. For better understanding, it provides several illustrations and sketches. The references in each chapter can be accessed for additional material.

 

Fig. 1: Cover of the ‘Handbook on Precast Concrete for Buildings’

 

The benefits of precast concrete can be summarised under the following categories:

  1. A) Quality
  • - Better control in a factory environment
  • - Suitable during inclement weather
  • - Efficient quality management
  • - Accuracy in dimensions
  • - Possibility of textured finish

 

  1. B) Time
  • - Rapid construction with robust planning
  • - Use of mechanised ways, such as extrusion, battery and tilting moulds, etc.
  • - Suitable for modular and repetitive construction

 

  1. C) Cost
  • - Optimum use of materials
  • - Limited use of temporary supporting structures, such as scaffolding
  • - Multiple use of form work
  • - Availability of standard shapes
  • - Reduced maintenance leads to reduced life-cycle cost

 

However, there are challenges in adopting precast concrete technology. The challenges are as follows:

  • - High initial costs for setting up factories
  • - High transportation costs for delivery of precast components
  • - Erection of components
  • - Excise duty of precast products
  • - Appropriate design and detailing of connections and joints between the precast components

 

There are different types of precast concrete buildings and hence, it is necessary to classify them for better understanding of the structural system. The overall system can be classified as skeletal frame system, large panel system and cell system. The lateral load resisting system can be low-rise portals and frames, multi-storeyed frames, walls or dual system. The connections between the members can be emulative (wet) or mechanical (dry or jointed).

 

The present article highlights two projects of multi-storeyed buildings under construction, which have used precast concrete technology extensively. Aspects of structural system, method of analysis and certain design issues are briefly highlighted. Several relevant references are listed at the end.

 

Residential Buildings In Bangalore

Several residential tower blocks are being constructed in Bangalore South, by Star Worth Infrastructure & Construction Ltd. Each building consists of a basement and fifteen storeys. The basement and ground storey are used for car and two-wheeler parking. The construction up to the transfer floor at the first floor level is with cast-in-place concrete. Above this level, it is made of precast concrete. The building has two staircases and two lifts, and a central corridor in each floor which connects the eight apartments to the lift and staircase areas (Figure 2). The circumscribing floor area is approximately 38.5 m × 16 m. With a precast plant at site, each floor was completed in 7 to 8 days.

 

Structural System

The structural system of each building consists of composite slabs and precast large panels for the structural walls. A composite slab is made of two-way precast concrete plank on which, a reinforced cast-in-place structural concrete topping is placed. This type of slab system is commonly known as precast half slab system. The precast planks are designed to carry the dead and live loads. They also act as the permanent shuttering for the topping concrete, and are shored till the hardening of the concrete. The reinforcement in the topping concrete is designed for the diaphragm forces, generated during an earthquake. To ensure the composite action of a plank and the topping, lattice girders made of reinforcing bars are inserted in the precast planks to act as interface shear reinforcement, along with the roughening of the top surface.

 

  1. a) Elevation                        b) Aerial view                                    c) Typical Floor Plan

 

  1. a) Sectional Elevation of a Horizontal Connection

The slabs are supported on walls made of storey-height panels. The connections between the panels are emulative, i.e., the design aims to emulate or mimic the behaviour of cast-in-place construction. In a horizontal connection, dowel bars protrude upwards from the lower panel. The upper panel has ducts which accommodate the dowels bars while it is lowered on the supporting panel. The ducts are subsequently grouted with non-shrink grout (Figure 3). The vertical connections between panels are of the following types:

  1. U-bars from adjacent panels are overlapped with a vertical locking bar in between
  2. Looped wire ropes from adjacent panels are overlapped with a vertical locking bar in between
  • Shear keys at the interfaces of the panels, without any reinforcement through the joint.

 

After placing the panels, the gap in-between is grouted.

 .       .                      
a) Sectional Elevation                b) Sectional Plan of a Corner                  c) Sectional Plan of a 
of a Horizontal Connection       Vertical Connection with U-bars               Vertical T-connection
                                                                                                                      with Looped Wire Ropes

                                     Fig. 3: Connections between Wall Panels

 

The walls constitute the lateral load resisting system. They are designed as shear walls for adequate lateral load resistance in each of the x- and y- directions. In the first floor, the panels are supported on cast-in-place walls or beams. The beams act as transfer girders and are supported on walls or columns.

Structural Analysis

The structural system of the building was analysed for the loads, using the finite element software ETABS (Figure 4). Frame elements were used to model the columns and beams. Membrane elements were used to model a slab of regular geometry for computational simplicity. Since a membrane element cannot support out-of-plane vertical loading, the loads transferred from the slab were assigned on the supporting members by the software. A non-regular slab was modelled using shell elements, where the load transfer was accounted for, based on the assigned distributed super-imposed dead and live loads. A diaphragm constraint was assigned at each floor level. The nodes of the floor slab elements and the nodes of the wall elements at that floor were connected to the diaphragm. The diaphragm was analysed to check the chord stresses at the edges of the slab.

 


a) Isometric View                 b) Plan at a Level

 Fig. 4 Computational Model for the Building in Bangalore

 

 

The walls were modelled using shell elements to consider both in-plane and out-of-plane resistance to lateral loads. An emulative horizontal connection (referred to as joint for the computational model) transfers vertical stress due to gravity loads and horizontal stress due to lateral loads. Such a joint was considered to be rigid. This can be justified even for lateral load analysis, as long as there is compression across the joint due to gravity loads. The nodes of the top and bottom wall elements were directly connected as in monolithic construction.

 

The vertical connections were modelled as three types:

  1. Shear transfer joint: Connections with U-bars were considered to transfer in-plane shear without deformation. This was modelled as rigid joint.
  2. Partial shear transfer joint: Connections with looped wire ropes were considered to be semi-rigid to transfer in-plane shear, especially after cracking of the grout. This was modelled with separate shell elements with shear stiffness modifiers for the joint region equal to 10% of the in-plane shear stiffness of the wall panels.
  • Gap joint: Connections with shear keys and without any reinforcement were considered to slide after cracking of the grout. This was simulated by modelling a small horizontal gap (10 mm) between the wall panels. The wall panels were connected only at the floor levels through the slab elements.

 

In presence of the raft foundation, the boundary condition of the vertical members of the bottom storey was considered to be fixed. The analyses for wind and earthquake loads were as per IS 875 (Part 3) and IS 1893 (Part 1), respectively. The equivalent static and response spectrum methods of analysis were done for earthquake loads. The applications of these methods are briefly described in the Handbook.

 

The foundation was analysed separately using software SAFE. The raft was modelled using plate elements. To consider soil-structure interaction, springs were assigned below the plate elements, based on the subgrade modulus from the geotechnical report.

 

 Commercial Building In Hyderabad

The project consists of a podium and tower type commercial building with 4 basements for the podium, and ground plus 24 storeys for the tower. The building is designed by Melior Structural Consultants and constructed by Aurobindo Realty & Infrastructure Pvt. Ltd. The total height of the building is 99.5 m above the ground level, with a typical storey height of 4.1 m (Figure 5). The tower structure above the podium was initially planned as a fully precast structure. However, due to delay in setting up the precast plant, the vertical members up to 6th floor of the tower were constructed as cast-in-place. The rest of the tower, including the helipad, is made of precast concrete. The slabs in all the floors are composite made of precast pre-tensioned one-way hollow-core slabs with cast-in-place structural concrete topping. The 197 m long tower is divided in the middle with an expansion joint. Each part of the tower contains 2 staircases and multiple lifts for passenger and freight. The central portion of each part containing the lifts, staircases and service areas is constructed with structural walls. The subsystem acts as a core. The rest of the building is made of beam–column frames. Belt walls along the periphery in certain floors and outrigger beams connected to the core are added to increase the lateral stiffness. A precast plant was set up for this and future projects in the city. It is to be noted that with the precast plant in full operation, the entire structural system was completed in less than 6 months.

 

  1. a) Elevation                          b) Aerial View

  1. c) Typical Floor Plan

   Fig. 5: Photos and Floor Plan of a Building in Hyderabad           

 

Structural System

The structural system of the building consists of composite slabs, precast frames and shear walls. A hollow-core slab rests on the ledges of inverted T-beams, with bearing of width at least 50 mm (Figure 6). The bearings are used to support the slab during erection. After the casting of the continuous structural topping, the slab–beam connection becomes emulative.

 

.       .         
   a) Connections of Precast Slabs                b) Connections of Pre.              c) Connection of Precast Columns

        and Beam                                                   Beams and Column      

 

                                                                Fig. 6: Connections in Frames

 

A precast beam rests on the corbels provided in the supporting columns. After casting of in-situ concrete with continuity reinforcement, the beam–column connection also becomes emulative. The moment resisting frames constitute part of the lateral load resisting system. The presence of the core walls makes it a dual system. The walls and frames are designed to carry 75% and 25% of the design base shear, respectively. The connection between the precast columns in two storeys is obtained through grouted splice sleeve couplers for the bars. This provides full force transfer between the columns making them act like a continuous column.

 

As mentioned for the previous project, the vertical connections between the wall panels are either with overlapped looped wire ropes or U-bars, along with locking bars. The gap in-between the panels is filled with non-shrink grout (Figure 7).

 

  1. a) Using Looped Wire Ropes               b) Using U-bars

Fig. 7: Sectional Plans of T-Connections between Wall Panels

 

Structural Analysis

Since the building is symmetric about the expansion joint, only one half of the building was analysed using ETABS (Figure 8). As before, the structure was modelled using frame, membrane and shell elements. A beam in the transfer floor supporting a wall above was considered to act integrally with the wall. The overturning moment from the wall was transferred to the supporting columns. The coupling beams between the walls were provided with a minimum width of 300 mm, and detailed with diagonal reinforcement, as per the provisions of IS 13920: 2016.

 

Conclusion

From the case studies discussed, it can be seen that the use of precast concrete leads to better quality, faster and economical construction of buildings. A further development of this industry will provide pre-fabricated pre-finished volumetric construction, as is being adopted in certain countries. The structural analysis and design of precast concrete buildings follow the same principles as used in conventional construction. However, attention is required to model the structure appropriately, considering the behaviour of the connections of the members. The modelling should be based on the adopted type of detailing, with appropriate assumptions.

 

The key features of analysis and design of precast concrete multi-storeyed buildings are highlighted.

  1. The analysis and design of tall buildings should follow the provisions of IS 15916: 2010, IS 16700: 2017, NBC 2016 and other relevant codes.
  2. The floors have to analysed and designed as diaphragms for adequate lateral load resistance (IS 10297: 1982 and IS 10505: 1983).
  3. The lateral load resisting systems have to be adequately designed. Appropriate representation of the behaviour of the connections is necessary in the analysis. If a horizontal connection between wall panels is considered to be rigid, then adequate number of dowel bars need to be provided based on the axial force and shear force transferred. If the connections in a moment resisting frame are considered to be emulative, then attention should be given to the detailing of the joints between the members; else, tests can be conducted to characterise a designed connection.

 

  1. a) Isometric View                              b) Plan at a Level

                       Fig. 8: Computational Model for the Building in Hyderabad

 

  1. Tie reinforcement is to be provided to avoid progressive collapse (IS 11447:1985).
  2. Advanced methods of analysis for earthquake loads are possible, such as pushover analysis, considering the non-linear behaviour of the joints and the members of a building structure.

 

References

  1. Handbook on Precast Concrete for Buildings (2018), ICI Bulletin 02, Indian Concrete Institute.
  2. IS 875: 1987, Code of Practice for Design Loads, Parts 1 to 5,
  3. IS 1893: 2016, Criteria for Earthquake Resistant Design of Structures, Part 1
  4. IS 10297: 1982, Code of Practice for Design and Construction of Floors and Roofs using Precast Reinforced/Prestressed Concrete Ribbed or Cored Slab Units
  5. IS 10505: 1983, Code of Practice for Construction of Floors and Roofs using Precast Concrete Waffle Units
  6. IS 11447: 1985, Code of Practice for Construction with Large Panel Prefabricates
  7. IS 13920: 2016, Ductile Design and Detailing of RC Structures Subjected to Seismic Forces
  8. IS 15916: 2010, Building Design and Erection using Prefabricated Concrete – Code of Practice
  9. IS 15917: 2010, Building Design and Erection using Mixed/Composite Construction – Code of Practice
  10. IS 16700: 2017, Criteria for Structural Safety of Tall Concrete Buildings
  11. National Building Code of India (NBC) 2016