Introduction:
In the design of a raft foundation, the weight of the raft itself is not considered as a significant factor. Instead, the design focuses on the load-carrying capacity of the subsoil and the distribution of loads to the supporting columns. This approach is based on the assumption that the raft does not remain in direct contact with the soil, but rather transfers the load to the columns.

Explanation:
The weight of the raft is not considered in the structural design of a raft foundation due to the following reasons:

1. Weight is carried by subsoil:
The primary purpose of a raft foundation is to distribute the loads from the superstructure to a larger area of the subsoil. The weight of the structure is transferred to the subsoil through the raft, which acts as a load-spreading mechanism. The subsoil is responsible for carrying the weight of the raft and transmitting it to the underlying strata. Therefore, the weight of the raft is indirectly accounted for through the load-carrying capacity of the subsoil.

2. Raft does not remain in contact with soil:
A raft foundation is typically designed to be a floating foundation, meaning that it does not remain in direct contact with the soil. Instead, it floats on a layer of granular material or a cushion of concrete. This floating action allows the raft to distribute the loads evenly over a larger area, minimizing differential settlements. As the raft does not remain in contact with the soil, it does not contribute significantly to the overall weight-bearing capacity of the foundation.

3. The weight is transferred to columns:
The primary load-bearing elements in a raft foundation are the columns or piers that support the superstructure. The weight of the structure is transferred to these columns through the raft. The columns are designed to carry the vertical loads and transmit them to the ground. The raft acts as a horizontal load distributor, ensuring that the loads from the superstructure are evenly distributed to the supporting columns.

Conclusion:
In summary, the weight of the raft is not considered in the structural design of a raft foundation because it is assumed that the weight is carried by the subsoil, the raft does not remain in direct contact with the soil, and the weight is ultimately transferred to the supporting columns. By focusing on these factors, the design of a raft foundation ensures the safe and efficient distribution of loads to the underlying soil, minimizing the risk of settlement and structural failure.