Combined Footing Design Excel Sheet

Combined Footing Design

A combined footing is a simple pad foundation which accommodates two columns or load point instead of one. Combined footings are commonly used in cases where columns are very closely spaced, or where the loads are so large that individual foundations for each column would overlap. Combined footings can also be used in some cases where columns are close to the edge of a boundary and a combined footing can reduce the size of the foundation in one direction.

Strap footings are a special type of combined footing where two isolated pads are connected by a concrete beam. This reduces the size of the two isolated pads and is often used when a column is close to the site boundary. This is also sometimes called a balanced foundation. As the pads are connected by a narrow concrete beam, this type of foundation can be used when the two columns are further apart. Strap footings can be designed using our Strap Footing Design Excel Sheet.

Combined footings work in the same way as any pad foundation. The concrete footing spreads the load over a larger area, allowing the pressures to be reduced to within the bearing capacity of the underlying soils. The bearing pressure calculations are made much more complex by the interaction between the two columns connected by the single foundation. The bearing pressure calculations are too complex and time consuming to complete by hand. A spreadsheet or similar is the only practical solution for combined footing.

CivilWeb Combined Footing Design Excel Sheet - Inputs

First the designer must input the loading conditions for both columns and the geometry of the columns and proposed foundations. The loadings and position of the columns are usually set as part of the superstructure design and in most cases cannot be adjusted by the foundation designer. The load inputs can include axial, horizontal and moment loads in both directions.

The designer must also input a proposed foundation size. This initial trial size can be taken from experience of similar footings or simply guessed. The rectangular combined footing design xls creates a useful dynamic drawing of the combined footing being analysed. This shows the designer exactly what is being analysed and prevents any confusion over the meaning of the inputs.

After the loading and geometry inputs the designer must input the soil bearing capacity. This is taken as a simple maximum allowable bearing capacity for the underlying soils. This simplified analysis is suitable for combined footings in most cases. The bearing capacity of the soil can be taken from typical values or from experience for preliminary design. For final design a detailed analysis including site testing must be undertaken. An easy to use spreadsheet such as the CivilWeb Soil Bearing Capacity Spreadsheet can be used.

In some cases the structure may be particularly vulnerable to settlement. Some clay soils can continue to settle for many years, even when the bearing capacity of the soil is not exceeded. This consolidation settlement occurs due to the increase in pore water pressure caused by the foundation. In these cases a separate settlement analysis can be undertaken using a simple design spreadsheet such as the CivilWeb Foundation Settlement Calculator Spreadsheet.

Once the loading, geometrical and soil design conditions have been entered, the CivilWeb Combined Footing Excel Sheet now completes the bearing pressure calculations. The spreadsheet can complete the calculations in accordance with either BS EN 1997 or BS 8004. These complex calculations are completed instantly and presented to the designer using unique analysis charts showing the designer exactly where the maximum bearing pressures occur. The spreadsheet also calculates the sliding resistance of the foundation in both directions.

After the dimensions of the foundation have been finalised, next the reinforcement can be designed. The spreadsheet calculates the maximum bending moments and shear forces acting on the foundation. These are then used to design the reinforcement. The CivilWeb Combined Footing Excel Sheet includes a unique reinforcement design chart which shows the designer exactly which combinations of reinforcement bar size and spacing are best suited to the footing.

The CivilWeb Combined Footing Design Spreadsheet then presents the designer with a summary of the completed combined footing design. This summary highlights for the designer any areas where the combined footing design is not valid. The spreadsheet also creates a diagram of the completed design which the designer can use in the construction phase.

CivilWeb Combined Footing Design Excel Sheet

The CivilWeb Combined Footing Design Excel Sheet spreadsheet can be used to complete a combined footing design in accordance with BS EN 1997 or BS 8004. The spreadsheet includes unique design and analysis tools which allow the designer to complete a fully optimised and fully compliant combined footing design in minutes.

The CivilWeb Combined Footing Design Excel Sheet can be purchased here for only £20.