Design Related Advantages for adapting Precast Framing System for the Superstructure

Er. Kapilesh Bhate, Precast India Infrastructures Pvt.Ltd


In the wake of some technical revisions in the IS codes, following codes are mandatory to be followed

• IS 1893 (2016)
• IS 13920 (2016)

The total and exact effects of these changes is a subject of long discussions and probably, some debate due to interpretation differences, however, to summarise in short, for the benefit of the esteemed readers, following are the implied restrictions laid by the code on Flat Slab Post Tensioned structures, which used to be the most common system of construction, especially for commercial large span structures:

1. Flat Slab system cannot be considered as a part of earthquake (lateral load) resisting system, i.e. the columns shall not be allowed to contribute to resisting any earthquake forces, and should avoid punching failure anyway
2. The shear walls should be provided with ductile detailing to resist 100% of design earthquake forces
3. 25% of the design earthquake forces to be resisted by peripheral moment resisting frame
4. The Flat Slab system structures henceforth, will have to be designed with R=3 and not R=5, i.e. the earthquake forces shall increase on the current structure by 66%
5. Importance Factor for the buildings shall be 1.2 minimum as opposed to 1, in the previous code (this is mostly for all structures though, not only flat slab)
6. The top storey drift shall be restricted to 0.1%, which is a new and a very stringent constraint laid by the code, which will lead to increase in shear walls in following aspects:
   1. Locations
   2. Length
   3. Thickness
   4. Reinforcement Steel
   5. As per experience of numerous design experts, and outcomes of various technical seminars, it is believed that just the shear walls in the core area may not be sufficient to fit in these stringent criteria and we may require shear walls in other, more evenly spread out locations, to increase the stiffness of the building, which may bring obstructions in the clear floor plates of the IT office spaces, which may go against the very basis of selection of PT Flat Slabs for ‘flexible’ IT Office Spaces (Or introduction of grid beams, usually wider in nature – upto 2 to 2.5m wide and 400 to 500 deep, to act as a frame to transfer the lateral forces to the shear walls as efficiently as possible)
7. All shear walls must start from footing levels right till the top of the building (terrace), so, the shear walls mentioned in point 6 may obstruct parking drive ways in the lower floors (parking) as well
8. Above mentioned points are only a few important changes from the old to the new code, applicable to the CIS PT Flat Slab structure (our current structure type)
9. These clauses clearly can be translated to the following challenges
   1. increase in concrete
   2. increase in steel/ strands
   3. increase in challenges to frame the structure and pass in design critrea
   4. increase in shear wall locations, inconveniencing Office and Parking Floors
10. To summarise, increase in cost and work, hence adds cost and time

Design Related Advantages of Precast Framing Plan

1. Given the above heavy restrictions on the PT Flat Slab, an efficient solution opens out as a Frame Structure with Shear Wall System, and since in Precast System, we shall be providing emulative monolithic ductile moment resisting connections, we can provide for the above mentioned challenges, especially in case of
   1. Long Spans
   2. Creation of columnless Spaces
   3. Lower beam depths than conventional CIS frames
   4. Heavy loadings
2. Of the above mentioned clauses, only the Importance Factor Clause shall be applicable to the Precast System, so our earthquake systems will not double like in Flat Slab PT System, but only increase 1.2 times.
3. These increased forces can be tried best to be contained in the existing sizes and lengths of shear walls, as per architect’s requirement, due to the contribution of the moment resisting frames of the columns.

Other Non-Design Related Clear Advantages of Precast System

1. Quality of construction
2. Speed of construction
3. Less labour on site
4. Safe Working Procedures
5. Due to increase in quantities and cost of CIS PT Options due to the above mentioned factors, cost would almost not be a deciding factor any more
6. Due to elimination of excise, duty, and application of GST for both, conventional and Precast system, the cost comparison also remains apple to apple now
7. Less Noise
8. Less Dust
9. Pre-Engineering possible in the period upto hitting ground hence client budget can be frozen early enough
10. Advance Production of Typical elements to win race against time
11. Parallel progress of serially dependent different activities, not possible in CIS
12. 3D BIM for systematic design, planning providing coordinated results.