When looking at a new building project, construction firms need to find the most appropriate design and construction methods for that particular build, in order to come up with a cost-effective solution that can deliver on the project requirements and handle site-specific conditions. When it comes to foundations, traditional trench footings aren’t always the most suitable route, which is why every construction professional should have a firm understanding of raft foundations.
What are raft foundations?
Also known as mat foundations or raft footings, raft foundations are basically slabs of concrete that sit on a firm, level bed of soil and extend across the largest part or even the entire footprint of the build. Whilst these are shallow foundations, they can provide much better stress distribution than conventional trench footings. Because the weight of the building is spread across a much larger surface area than with traditional trench footings or pile footings, the stress acting on the soil below is greatly reduced.
When to use raft foundations
In the past, raft foundations have been widely used in the construction of commercial buildings such as warehouses or supermarkets. However, over the last few decades they are increasing in popularity as a simple and inexpensive solution for domestic construction projects, such as extensions and conservatories.
Raft foundations are suitable where a building’s footprint is reasonably small, and the structural load requirements are not onerous. They are also suitable for basem*nt constructions where the foundation slab can receive directlive loadsdepending on the use of the building. On sites where the soil conditions are poor or access for large excavation plant is limited, raft foundations can again prove to be an excellent solution.
There are three types of loads which must be considered when designing concrete slabs:
- Dead load of the slab:
(Dead Load: Self weight of the slab= Mass / Weight of the slab i.e. Density *Area.) - Live load of the slab:
(Weight of Humans (Living Beings), Furniture etc. = Refer IS 875 for load condition in various types of buildings.) - Floor finish load:
(Load acting due to plaster, painting, tiles etc.)
