Reinforced concrete (RC) frame buildings are very common in the world. In such types of structure for functional requirements of parking space under the buildings no masonry in fill are provided resulting a construction with stilts.
Open ground storey building is inherently poor systems with sudden drop in stiffness and strength in the ground storey. In the current design practice, stiff masonry walls are neglected and only bare frames are considered in design calculations. Thus, the inverted pendulum effect is not captured in design.
Safeguard Against Failure
The failure can be avoided following two considerations in structural proportioning:
a) To avoid soft storey
b) When soft storey cannot be avoided, providing special design provision in designing such structure.
How to Avoid Soft Storey
Architects and structural designers can use the following conceptual design strategies to avoid undesirable performance of open ground storey buildings in earthquake:
Ø Provide some shear walls at the open ground story level : this should be possible even when the open ground story is being provided to offer car parking
Ø Select an alternative structural system (e.g., RC shear walls) to provide earthquake resistance: when the number of panels in the ground storey level that can be filled with masonry walls is insufficient to offer adequate lateral stiffness and resistance in the ground storey level, a ductile frame in not an adequate choice. In such cases an alternative system, like a RC shear wall, is required to provide earthquake resistance.Some remedial measures to counter the bad performance are shown in fig:
Special Design Provision
To safeguard the soft first storey from damage and collapse code provides two alternative design approaches:
1) The dynamic analysis of the building is to be carried out which should include the strength and stiffness effects of infills as well as the inelastic deformations under the design earthquake force disregarding the reduction factor R.
2) The building is analyzed as a bare frame neglecting the effect of infills and, the dynamic forces so determined in columns and beams of the soft (stilt) storey are to be designed for 2.5 times the storey shear and moments: or the shear walls are introduced in the stilt storey in both directions of the building which should be designed for 1.5 times the calculated storey shear forces.