MasterSeries Blog

Masterseries 2023 now includes the Eurocode 7 ‘direct method design approach’ within the MasterSeries Concrete Design Module.

Eurocode 7 allows for different methods of design for spread foundations such as concrete pad foundations, with both the direct and prescriptive methods now able to be carried out within the software.

The Prescriptive Method

Previously, within the MasterSeries Concrete Design Module. Pad Foundation designs were limited to the Eurocode ‘Prescriptive Method’ which took user defined, presumed bearing resistances for the capacity of the soil in the serviceability limit states, for the controlling of the settlement of the foundation.

Prescriptive measures usually involve the application of charts, tables and procedures that have been established from comparable experience; they implicitly contain their own safety factor. Very often, the concept of ‘allowable stress on the soil’ is used in these charts or tables (typically from BS8004).

Safe Working Pressure values can be input directly into MasterSeries under the 'Global Basic Data and Design Defaults'

The Direct Method (MasterSeries 2023 Update)

The direct method is the most common method for Geotechnical Foundation Design in the UK, calculations are carried out for each limit state including ULS in the design methodology. At the ULS, the bearing resistance of the soil should be checked using partial factors on the soil properties as well as on the actions and at SLS the settlement of the foundations should be calculated and checked against permissible limits.

STR / GEO (Structural, Bearing and Sliding) Partial Factors

Vertical and Horizontal Load factors (labelled A1 and A2 'actions'), and Soil Parameter factors (M1 and M2) are considered within 'Design Approach 1' of the Eurocode 7 Direct Method.

Combination 1, which is equivalent to 'Eurocode 0 - Set B for STR / GEO', and Combination 2 - equivalent to 'Eurocode 0 - Set C for STR / GEO', include factored inputs for A1 & A2, and M1 & M2 as follows:

EQU (Overturning and Uplift) Partial Factors

The following 'Partial Factors on Actions' for checking Equilibrium cases, are used in combination with factored Soil Parameters - identical to Set 'M2' in the Combination 2 / EC0 Set C (above), for overturning and uplift checks.

Application of Load Combinations within MasterFrame

These ST / GEO / EQU partial factor values are found within MasterSeries Main Menu > Utilities > Preferences > Design Codes > "EC7 - Geotechnical Design"

Relevant Set B and new Set C Load Combinations with predefined 'A2' Partial Factors can be found within the Auto Load Case Generator in MasterFrame.

Bearing Pressure Calculations

Unlike the prescriptive method where Bearing Capacity is an input value, the direct method calculates the design bearing resistances for drained and undrained soil conditions using the following formulae:

Again, these soil parameters can be input directly into MasterSeries under the 'Global Basic Data and Design Defaults'

MasterSeries Concrete Design: Global Settings and Design Defaults

Design

One the frame has been analysed within MasterFrame, design of the Pad Foundation following the EC7 Direct Method is carried out within the Concrete Design Module.

Settlement Analysis and Factors of Safety

BSEN 1997-1 permits settlement calculations to be replaced by bearing capacity calculations with a high factor of safety so that ‘a sufficiently low fraction of the ground strength is mobilised’; however, advice on this high factor value is not given in the Code.

Traditionally, from BS 8004 a factor of safety of 3 for spread foundations was found from experience, by testing and from the back analysis of observations, to prevent ‘failure’ and to ensure that settlements under working loads remained acceptable. Therefore a safe design without the need for settlement analysis, can be used by taking a factor of safety of 3.

MasterSeries then checks the Critical load combinations for each of the Ultimate STR , GEO & EQU, and Serviceability load cases to ensure a satisfactory design.

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