Design Of Raft Foundation Spreadsheet

The CivilWeb Design of Raft Foundation Spreadsheet is an easy to use spreadsheet for the design of simple raft foundations. The spreadsheet completes all the calculations required for a raft foundation design in accordance with BS EN 1997 and BS EN 1992. The spreadsheet completes the calculations instantly and includes unique design and analysis tools allowing the designer to complete a fully compliant and fully optimised raft foundation design in minutes. This will save many hours of design work on every raft foundation design.

The CivilWeb Design of Raft Foundation Spreadsheet can be purchased at the bottom of this page for only £20. Or why not purchase our Mat & Raft Design Bundle including our raft and mat foundation design spreadsheets together for only £25.

Alternatively the Foundation Design Suite includes all 12 of our foundation design spreadsheets allowing the design of any common foundation type. This is our best value bundle and can be purchased at the bottom of this page for only £50 (a discount of 80%).

Design of Raft Foundations

Raft foundations are foundations which are continuous under the entire footprint of the building. Instead of isolated pads or strips beneath the columns or walls, a raft foundation presents a continuous concrete slab beneath the entire footprint of the building. Raft foundations are common in areas with very poor soils as they spread the load from the building over an area as large as possible.

The raft foundation is also capable of spanning small areas of poor soils which prevents differential settlements. If a building is built on a raft foundation it cannot experience differential settlement which is the most common cause of cracking and damage to small buildings.

The CivilWeb Design of Raft Foundation Excel Spreadsheet models the raft foundation as a single slab subject to a universally distributed load. The spreadsheet checks that the slab is sufficiently strong to span over any small areas of poor soil. The raft foundation can also accommodate a linear load such as from a wall at the edge and the corners of the raft. This covers the most common types of raft foundation.

Large single foundations which accommodate many column or point loads cannot be modelled by this spreadsheet. These mat foundations can be designer using our Mat Foundation Design Spreadsheet.

CivilWeb Design of Raft Foundation Spreadsheet - Inputs

The first step in the design of raft foundations is for the designer to input the loading conditions. These are a universally distributed load over the entire area of the raft foundation and linear loads at the edges and corners of the raft. These are the critical loads which will be used to determine whether the raft foundation is adequate or not.

Next the design must input a trial thickness of the slab and the underlying layer of hardcore stone. A thicker slab will be more capable of resisting large linear loads at the edges and corners of the raft. However thicker slabs will increase the bearing pressure acting on the underlying ground. For this reason it can be difficult for the designer to balance the thickness of the slab.

The raft foundation design xls makes this easy using as unique analysis graph which shows the designer exactly what thickness of slab is required for the loadings and exactly which thickness will start to overload the subgrade. This can save the designer a great deal of time optimising the thickness of the slab. This particularly helps the designer when the design is not possible as the unique analysis tools show the designer this instantly without the need to go through multiple iterative design attempts.

The CivilWeb Design of Raft Foundation Spreadsheet analyses the suitability of the slab using a simple maximum allowable bearing capacity for the underlying soils. This simplified approach is suitable for simple raft foundations such as these. The bearing capacity of the soil can b estimated from typical values or from experience for preliminary design. However it is recommended that this is verified with onsite testing before the design is finalised. An easy to use spreadsheet such as the CivilWeb Soil Bearing Capacity Spreadsheet can be used for this detailed analysis prior to final design.

Raft foundations protect structures from differential settlements even on poor soils. However the raft foundation can still be susceptible to total settlements. In most cases this will not exceed 25mm when the maximum bearing capacity of the soil is not exceeded. However in some clay soils consolidation settlement can continue to occur for many years after the structure is built, even when the maximum bearing capacity of the soil has not been exceeded. In these cases an additional settlement analysis of the raft foundation may be prudent. A simple design spreadsheet such as the CivilWeb Foundation Settlement Calculator Spreadsheet can be used for this analysis.

The CivilWeb Design of Raft Foundation Spreadsheet completes the geotechnical analysis of the foundation in accordance with either BS EN 1997 or BS 8004. Then the excel spreadsheet for raft foundation design calculates the maximum bending moments and shear forces produced in the slab. The design critical forces are produced in the centre of the raft when spanning over an area of poor support, and at the edge and corner of the raft foundation when subjected to linear loads from walls. The CivilWeb Raft Foundation Design excel sheet includes unique reinforcement picker tools which show the designer the optimum reinforcement combinations to use for this slab.

CivilWeb Design of Raft Foundation Spreadsheet

The CivilWeb Design of Raft Foundation Spreadsheet can complete all the calculations required for the design of raft foundations in accordance with either BS EN 1997 or BS 8004. The spreadsheet includes unique analysis tools which show the designer the optimum thickness for the slab at a glance. Using these tools the designer can complete a fully optimised and fully compliant design in minutes.