RAFT is a unique computer program for yield line analysis and ultimate strength design of stiffened raft and waffle raft slabs supported on moisture reactive soil, commonly applied in residential construction in Australia. The program was specifically developed for engineers wanting a practicable design methodology instead of selecting a prescription from Australian Standard AS 2870 – Residential slabs and footings (2011).

How RAFT works

Ultimate strength performance graph

The RAFT computer program is based on the limit state ‘Worst-case’ scenario of identifying and analyzing the critical failure mechanism of raft slabs on moisture reactive soil. RAFT deploys conventional ultimate strength design methodology, with appropriate factors of safety and design criteria to ensure adequate in-service performance.

The ultimate strength of slabs on reactive soil is a complex function of dynamic surface movement, static loading, floor plan size and shape, and soil stiffness. At right is a characteristic graph of ultimate bending moment (Mu value) versus surface movement (Ys value) at which the slab fails, covering both subsidence and heave. This  graph comprises a straight line for a short distance on either side of the vertical axis, followed by exponential curves to subsidence and heave ultimate strength asymptotes at infinite Ys values.

The first phase of the RAFT computer program generates the ultimate strength performance graph. Design ultimate bending moments for subsiding and heaving surface movements are determined by interpolating the ultimate strength performance graph at specific and appropriately factored Ys values applicable to the building site.

The final phase of the RAFT computer program calculates the slab dimensions, stiffening rib layout, steel reinforcement and other details in accordance with conventional structural design practice, including compliance with Australian Standards AS2870 and AS3600 and other relevant documents.