Strap Footing Design Excel Sheet

Strap Foundation Design

A strap foundation is a special kind of combined foundation where two individual footings are joined by a structural concrete beam. This type of footing can be economical when a building includes columns which are close together. In some cases pairs of foundations can be so close together that the individual foundations would almost overlap. In these cases the individual footings can be combined using a concrete beam which strengthens both footings.

A strap footing can also be used where an individual footing is close to the site boundary. In these cases an individual footing centred on the column might be too large to fit within the site boundary. A strap footing can use the stability of a nearby footing to reduce the size of the pad required. This can allow the foundation to fit within the site boundary. Note standard combined footings can be designed using our CivilWeb Combined Footing Design Spreadsheet.

CivilWeb Strap Footing Design Excel Sheet - Inputs

First the designer inputs the loading conditions acting on the two footings. These loads are usually determined during the superstructure design. The spreadsheet can be used to model any combination of axial or moment loads on both of the columns.

The designer must then input trial dimensions for the proposed strap footing design. Trial dimensions can be determined from experience of similar strap footings or simply guessed. In most cases with strap footings the critical dimension is the space available beyond the first column. For this reason the spreadsheet allows the designer to input this dimension and size the rest of the foundation dimensions around this parameter.

The CivilWeb Strap Footing Design Excel Sheet includes a handy dynamic drawing of the proposed strap footing. This ensures that the designer can see exactly what strap footing dimensions being analysed.

Next the designer must specify the soil design parameters. The CivilWeb Strap Footing Design Spreadsheet uses a simple bearing capacity analysis to check whether the proposed footing design is valid. For preliminary design the maximum allowable bearing capacity of the soil can be taken from experience or from typical values. This must be checked with onsite testing of the soils before the final design is completed. A simple spreadsheet such as the CivilWeb Soil Bearing Capacity Spreadsheet can be used to complete the detailed soil bearing capacity analysis for use with the final design.

In some cases an additional settlement analysis may be required. This is likely for structures which are sensitive to settlement or for strap footings where the two foundations are situated on different types of soils. A simple design spreadsheet such as the CivilWeb Foundation Settlement Calculator Spreadsheet can be used to complete this analysis if required.

When the designer has input all the required information, the spreadsheet completes the stability calculations to determine whether the proposed foundation is suitable. The spreadsheet can complete these calculations in accordance with BS EN 1997 or BS 8004. The spreadsheet calculates the minimum width for each of the pads. This can be used to quickly optimise the design whether the length of the pads is critical or whether a square foundation is desired.

When the designer is happy with the geometry of the strap foundation, next the CivilWeb Strap Footing Design XLS can be used to design the reinforcement. The spreadsheet calculates the maximum bending moments and shear forces in both the pads and in the connecting concrete beam. The spreadsheet presents a useful moment and shear force diagram allowing the designer to see exactly where the foundation is most stressed.

These bending moments and shear forces are then used to design the required reinforcement. The spreadsheet includes unique reinforcement picker tools which show the designer exactly what reinforcement is best suited to the foundations. Then the spreadsheet checks whether shear reinforcement is required in either of the pads or in the connecting beam.

When the reinforcement has been designed the CivilWeb Strap Beam Footing Design XLS presents a summary of the main design checks, highlighting any areas where the strap footing design is not valid.

CivilWeb Strap Footing Design Excel Sheet

This powerful design spreadsheet can be used to design strap footings in accordance with either BS EN 1997 or BS 8004. The spreadsheet completes all the required calculations instantly allowing the designer to complete a fully compliant and fully optimised design in minutes, saving many hours of detailed design work on every strap foundation design.

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