Comparison Of Raft Foundation And Strap Beam Foundation In RCC Tall Structures

Er. Bhupendra Singh Thakur (B.Singh),
Government of India, (Valuer) & Civil Engineering Consultant
Dr. S. V. Deodhar, Consultant, Indore

Due to increase in population construction of high rise buildings has become very common. However, 8 to 10 storyed residential apartment constructions are very common, as such buildings have a total height less than 30 metres and column-beam structure is sufficient to resist wind

Generic placeholder image Er. Bhupendra Singh Thakur (B.Singh)
Government of India, (Valuer)
& Civil Engineering Consultant
pressures and special provision of shear wall or other is not required. The cost of foundation, however, is always large due to construction of Raft Foundation in general. It is not essential to provide raft, but Indore being a belt of Black Cotton Soil, it is preferable to construct raft. In this paper, we have compared Raft Foundation with that of Strap Beam Foundation and have shown that Strap Beam Foundation is economical than that of Raft. We have also provided Strap foundation in one building and saved cement as well as reinforcing bars considerably thereby reducing construction cost considerably.

Case Study
The building under consideration is a G+8 storyed apartment building under construction at Pagnis Nagar, Indore. The plot size is 16,000 Sq.ft. with ground coverage of 30% i.e. 4800 Sq.ft. The total
Generic placeholder image Dr. S. V. Deodhar
Consultant, Indore
constructed area of all the floors is 43,200 Sq.ft. Four floors of this building are completed, and rest is in progress. The building foundation was designed for Raft as well as for Strap Beam foundation. The building was designed using STAAD PRO software and then checked as per Codal provisions. The typical floor plan of ground floor and other floors are shown in Figure 1 and foundation plan using Raft foundation and Strap Beam foundation is shown in Figure 2.
Design Considerations
The design considerations are as given below. The total height of building above ground level is 26.15m. The structure was designed using STAAD-PRO Software.
Wall load: 200 mm wall = 12.54 kN/m; 100 mm Wall = 6.27 kN/m; 100mm Parapet wall = 2.85 kN/m Slab Load: Dead Load = 6.53 kN/m2; Live Load = 2.5 kN/m2 Wind load: As per IS 875

CECR Fig.1: Typical Ground and Other Floor Plans


CECR Case1-Raft Foundation

CECR Case 2-Strap Beam Foundation
Fig.2: Foundation Plan of Building (Both Cases)

Assumed Stresses
Compressive Stress in Concrete Fck = 20 N/mm2 Yield Tensile Stress in Steel FY = 415 N/mm2 Ultimate bearing capacity of soil Q (Ultimate) = 225 kN/m2 Safe bearing capacity of soil Q (Safe) = 120 kN/m2 Concrete Mix = M 20 Grade Comparison
In comparison, the quantity of material required for the design of “Raft Foundation” and “Strap Beam Foundation” is shown in Table. It shows the quantities of Steel and Cement saved by using Strap Beam Foundation with that to Raft Foundation. Conclusion
It is found that in actual practice the conventional foundation (raft) consumed more steel as well as concrete as compared with the strap beam foundation (costonomical foundation). Hence it is proved that providing costonomical strap beam foundation is economical to make the structure economical and financially sustainable. This makes the structure cheaper. keeping the other considerations like Safety, Security and User friendliness. It is therefore recommended that for medium rise buildings Strap foundation be used in place of Raft foundation.