In this document, after introducing the basic concepts related to the design of shallow foundations and the structural elements that compose a foundation system, an ultra-efficient design workflow is described to perform the complete design of shallow foundation systems using software, applicable to most industrial, commercial, or residential structures. The described workflow covers everything from data extraction from the superstructure model to the generation of calculation reports and construction plans. With this workflow, it is possible to perform complete designs of foundation systems in between 15 to 30 minutes.
1. Types of Shallow Foundations and Structural Foundation Elements
1.1 Isolated Foundations
A shallow foundation element designed to transmit the load of a single structural element, normally a column, to the soil. It generally consists of a reinforced concrete footing with a square, rectangular, or circular plan, dimensioned to distribute vertical loads and moments to the ground without exceeding the allowable bearing capacity or generating excessive settlements.
Related Article: Efficient Design and Optimization of Isolated Foundations
Video Tutorial: Creating an Isolated Foundation in Foundaxis
1.2 Strip Foundations
A type of continuous shallow foundation that extends longitudinally under bearing walls or an alignment of columns. Its function is to distribute the linear loads of the wall or several nearby supports toward the ground, maintaining uniform pressure and controlling differential settlements along the structure.
Related Article: Efficient Design and Optimization of Strip Foundations
Video Tutorial: Creating a Strip Foundation in Foundaxis
1.3 Mat Foundations
A type of continuous shallow foundation that extends longitudinally under bearing walls or an alignment of columns. Its function is to distribute the linear loads of the wall or several nearby supports toward the ground, maintaining uniform pressure and controlling differential settlements along the structure.
Related Article: Efficient Design and Optimization of Mat Foundations
Video Tutorial: Creating a Mat Foundation in Foundaxis
1.4 Seismic Tie Beams
A horizontal structural element whose function is to link foundations to each other to restrict horizontal displacements of the foundation system. These beams work primarily in tension, acting as a tie between footings or other foundation elements to prevent separations or relative horizontal movements, particularly against seismic actions, lateral thrusts, or tension effects at the base of the structure. They are not intended to transmit significant vertical loads to the soil or resist major moments, but rather to maintain the geometric integrity of the foundation system by restricting horizontal movements.
Related Article: Automatic Generation of Tie Beams for Foundations
Video Tutorial: Automatic Generation of Tie Beams for Foundation Systems in Foundaxis
1.5 Foundation Beams
A horizontal structural foundation element designed to work as a beam that connects two or more foundations, allowing them to act jointly against structural loads. Unlike tie beams, foundation beams are designed to resist bending, tension, and shear stresses, transmitting and redistributing loads between the connected foundations. They are used when it is necessary to harmonize deformations, balance loads, or improve the global behavior of the foundation system, making several foundations participate jointly in structural resistance.
Related Article: Foundation Beams
Video Tutorial: Automatic Generation of Foundation Beams in Foundaxis
1.4 Foundation Systems
An organized set of foundation elements that work in an integrated manner to transmit superstructure loads to the soil. A foundation system can be composed of different types of foundations, such as isolated footings, strip footings, mat foundations, piles, and connecting beams, configured according to soil characteristics, the magnitude and distribution of structural loads, and the performance requirements of the structure.
2. Design Parameters and Verifications
2.1 Design Loads
The loads required for foundation design are those occurring at the base of pillars and/or walls of the structure under the various load combinations for which it was designed (Dead Loads, Live Loads, Wind, Seismic, etc.). For a complete design, two types of load combinations will be required:
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Service Load Combinations: Correspond to unfactored load combinations (ASD or Allowable Stress Design) and are those that will determine the sizing of each foundation.
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Ultimate Load Combinations: Correspond to factored load combinations (LRFD or strength design) and are used for the design of steel reinforcement.
2.2 Design Parameters
The design parameters, i.e., the limit parameters that each foundation must meet for correct performance, usually come from a previously performed soil mechanics study or estimates made by the geotechnical engineer, and are as follows:
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Static Allowable Stress: The allowable stress to prevent ground collapse under static loads (Dead weights, Service loads, etc.); it normally incorporates a safety factor and is a value provided by the geotechnical study of the site where the structure will be built.
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Dynamic Allowable Stress: The allowable stress to prevent ground collapse under dynamic loads (Seismic, Vibrations, etc.); it normally incorporates a safety factor and is a value provided by the geotechnical study of the site where the structure will be built.
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Minimum Bearing Area Percentage: The minimum percentage of supported area allowed for the foundation under loads that cause overturning; it is a value usually provided in local structural regulations, typically around 80% (this means that the foundation will not be allowed to lift more than 20% of its area supported on the soil).
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Maximum Settlement: When soils are of low quality, soil mechanics will require that foundation design verifies settlements (how much the ground sinks upon installation of the structure and after some time), providing a limit value for this purpose.
2.3 Required Verifications
2.3.1 Service Limit States (To be verified with Service loads)
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Bearing Capacity Verification: It is verified that static and dynamic allowable stresses are not exceeded in any of the corresponding load combinations.
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Overturning Verification: It is verified that no uplift occurs greater than allowed according to the "Minimum Bearing Area Percentage" design parameter for each service load combination.
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Sliding Verification: The lateral displacement of the foundation is verified; this verification is somewhat obsolete, as local design codes normally require foundations to have tie beams between them and because foundations are generally laterally restricted by the ground. It must be performed in the event that a foundation does not meet at least one of the previously mentioned conditions.
2.3.2 Ultimate Limit States (To be verified with Ultimate loads)
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Punching Shear Verification: Verifies that no failures occur as a result of concentrated loads from pedestals on the foundation.
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Structural Concrete Failure Verification: It must be ensured that the foundation resists bending due to shear and tensile forces for all Ultimate load combinations; for this, a steel reinforcement design is performed to ensure this condition.
2.3.3 Geotechnical Verifications
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Subsurface Pressure Verification (Pressure Bulbs): In some cases, it is necessary to verify that stresses below the ground are met, for example, when tunnels or ducts exist under the foundations. For this, the combined pressure bulbs occurring under the foundation system can be calculated.
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Settlement Verification: If requested by the geotechnical engineer, it must be reviewed that total settlements (immediate settlements plus settlements over time) do not exceed allowable settlements. If significant settlements are detected, differential settlements between different areas of the foundation system must also be verified to avoid damage to the superstructure.
3. Changes in Traditional Foundation Design
3.1 Traditional Design Workflow
Designing the dimensions of a foundation under a single set of loads with approximate equations can be simple with a basic Excel sheet or a series of manual calculations. Even so, iterations are required to reach an optimal dimension and then design the steel reinforcement from there. However, buildings usually have many foundations subjected to many load combinations, and the resulting design loads often have horizontal force components in both directions at once. Even if an approximate analysis is performed, in one direction first and then the other, using an extensive Excel sheet, multiple situations arise that multiply the time invested in performing iterations looking for optimal dimensions: the type of foundations to use must be chosen, and this choice often varies during the search for dimensions.
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Approximate Estimates
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Trial and Error Iterations
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Fragmented Tools
4. Efficient and Precise Design Workflow for a Shallow Foundation System
In this chapter, we will explore an ultra-efficient design workflow for the optimal structural design of a complete system of shallow foundations for any structure previously designed in any finite element software, such as SAP2000, ETABS, STAAD, or others. With a little practice of this scheme, it is possible to design complete, optimized, and safe foundation systems in times between 15 and 30 minutes, covering the entire process: from pre-design to the complete generation of associated documentation. Below is a diagram of the optimal design workflow, and the following subchapters detail each stage.
4.1 Efficient Data Extraction from Structural Model (2-5 minutes)
The extraction of the data required for the design of the foundation system is fast and effective if the following conditions are previously met in the structure's model:
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The design of the superstructure must be finished; that is, the members and components of the structure that will be supported by the foundation system are definitive.
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The load combinations to be used for foundation design (usually Service Combinations for sizing and Ultimate Combinations for reinforcement design) must have been included in the model calculation.
The sequence to follow upon meeting these conditions is as follows:
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Select all constrained nodes of the model (nodes that will connect to the foundations) and the structural elements that reach them.
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Export the model definition and reactions at the constrained nodes to a file readable by the foundation design software (for example, in SAP2000 .s2k).
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Open the file in the foundation design software.
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Select the Service and Ultimate load combinations; it is relevant to select only the critical combinations and not consider combinations that are obviously known not to determine the design; it is usually sufficient for all structural codes to consider up to 10 service combinations and 10 ultimate combinations.
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Define basic design parameters for the foundations from soil mechanics (static and dynamic allowable stresses and minimum bearing area).
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Define a foundation depth and thickness¹.
Once these data are entered, an initial basic system is obtained where the following are automatically assigned: an isolated foundation per constrained node or strip foundations in the case of walls detected in the structural model.
¹ In general, the soil mechanic recommends an ideal depth for the foundation bearing level; local design codes and structural criteria based on the general dimension of the superstructure are key to establishing the thickness to be used; in general, thicknesses greater than 30 cm are used, and depending on the loads involved, it can reach 1, 1.5, or 2 m in thickness.
Video Tutorial: Loading Data from SAP2000 to FOUNDAXIS
Video Tutorial: Loading Data from ETABS to FOUNDAXIS
Video Tutorial: Loading Data from STAAD to FOUNDAXIS
Video Tutorial: Loading Data from EXCEL to FOUNDAXIS
4.2 AI-Based Foundation Type Selection (1-3 minutes)
Press the AI pre-design tool; based on the loads involved and the locations of pillars and walls, the AI will provide a set of possible structuring options that consider all load combinations. The different options play with:
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Foundation types in the layout.
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Grouping of isolated foundations for homogenization.
Choose the layout that makes the most sense according to your engineering judgment.
Related Article: Foundation System Design and Optimization with AI
Video Tutorial: AI Pre-design of Foundation Systems and Customization in FOUNDAXIS
4.3 Customization of the Foundation System Layout (1 to 3 minutes)
Using the rapid foundation layout customization tool, make the adjustments you deem necessary: vary thicknesses, join/separate foundations as strip or mat, transform foundation types.
Related Article: Foundation System Design and Optimization with AI
Video Tutorial: AI Pre-design of Foundation Systems and Customization in FOUNDAXIS
4.4 Automatic Dimension Optimization (2-5 minutes)
Once satisfied with the foundation system layout, simply press optimize, and all foundation types will adjust to the optimal horizontal dimensions to comply with the specified working stresses and minimum bearing area percentage.
Once the foundations are optimized, it is prudent to verify compliance with the design parameters in the general view, where foundations that meet the design parameters will be marked with a green checkmark. If required, you can verify the detailed performance of a particular foundation in the detailed results view.
Related Article: Foundation System Design and Optimization with AI
Video Tutorial: AI Pre-design of Foundation Systems and Customization in FOUNDAXIS
4.5 Automatic Generation of Seismic Tie Beams (1-3 minutes)
Activate the automatic generation of seismic tie beams; if necessary, add additional beams to those proposed, remove excess beams, or modify their location.
Related Article: Tie Beams and Foundation Beams, Foundation Engineering
Video Tutorial: Automatic Generation of Tie Beams in FOUNDAXIS
4.6 Steel Reinforcement Design (2 to 5 minutes)
Select the steel reinforcement design module, verify:
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The steel bar diameters available in your locality are selected.
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Covers match your local code.
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The minimum required reinforcement area matches your local code.
The default values in the software correspond to those specified in ACI-318. Once these parameters are verified, press DESIGN, and the automatic design of the required reinforcement in foundations and pedestals will be performed.
If you wish to inspect demand moments, interaction diagrams, or other design parameters, click on a particular foundation.
Related Article: Automatic Steel Reinforcement Design for Shallow Foundations
Video Tutorial: Automated Steel Reinforcement Design of Foundations in FOUNDAXIS
4.7 Geotechnical Analysis (4 to 8 minutes)
If you need to perform an analysis to determine combined pressure bulbs, immediate, consolidation, and differential settlements, the first step is to configure the ground layers. The ground layers can have irregular geometry and are defined through boreholes, specifying the thickness of each layer at each borehole point. Predefined soil types in the software or user-defined soil types can be used; it is possible to create your own soil type library. Do not forget to correctly enter the water table depth.
Once the soil strata are defined, enter the GEOTECHNICAL ANALYSIS module and press RUN; after a couple of minutes of calculation, the following will be available:
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Natural Pressures: pressures due to the ground's own weight.
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Direct Pressures: pressures resulting from the foundation system (Pressure Bulbs).
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Total Effective Pressures: Pressures combining the weight of the ground considering the water table and those exerted by the foundation system.
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Immediate Settlements: Elastic settlement at the time of installing the foundations and the structure.
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Primary Consolidation Settlements: Settlements to occur in the medium term.
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Total Settlements: Immediate settlements plus consolidation settlements.
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Differential Settlements: Analysis of differential settlements based on the evaluation parameters entered after pressing RUN.
Related Article: Fast and Precise Calculation of Pressure Bulbs and Settlements in Foundation Systems Video Tutorial: Generation of Stratified Soils in FOUNDAXIS
4.9 Documentation Generation (2 to 3 minutes)
It is possible to generate 3 types of project documents:
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Complete calculation report: you can download it in editable Word format or PDF.
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Construction plans: available in PDF and DXF CAD.
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Cost Estimation: based on the entered material prices, you will obtain an estimate of the foundation system costs.
Related Article: Foundation Systems: Automated Design Plans and Calculation Reports Video Tutorial: Generation of Cost Estimation, Design Plans, and Calculation Report in Foundaxis
4.10 BIM Model (1 minute)
Additionally, if you wish to export the complete design (Foundations + Reinforcement) to integrate it into a BIM model, just export in .ifc format.
5. Practical Example of Optimized Workflow
In the links below, you will find step-by-step practical examples applying the design workflow just described.
Related Article: Automated Foundation Design for a 6-Story Building from SAP2000
Related Article: Automated Foundation Design for a 6-Story Building from ETABS
Video Tutorial: Automated Foundation Design from SAP2000 with Foundaxis
Video Tutorial: Automated Foundation Design from ETABS with Foundaxis