Isolated Footing Design per Canadian A23.3 2019
Salient Features
The footing(s) can be designed to service and ultimate load cases/combinations which are created within the SFA environment, as well as to those imported from a STAAD.Pro superstructure model.
From the standpoint of the size of the footing, two types of design are available.
- Set Dimension where the user specifies the dimensions and the program determines if that size is sufficient to carry the loads.
- Calculate Dimension is the other design method where the program starts from a user specified minimum size and increments it iteratively till a satisfactory size is obtained.
- The column can be located eccentrically with respect to the center of the footing. The soil pressure calculation will account for the moments caused by the eccentricity.
- The lateral loads from the column reactions (FX and FZ) are assumed to act at the top of the footing in the absence of a pedestal, and at the top of the pedestal if a pedestal is present. These forces are multiplied by the thickness of the footing (or footing + pedestal) and added to the moments from the column reactions for calculating the soil pressures.
Load combinations can be generated to the 2005 edition of the NBCC code within the SFA environment provided that the column reaction loads for primary load cases, categorized as Dead, Live, Wind, Seismic, etc., have been specified or have been imported from the STAAD.Pro model. Alternatively, the combinations can be specified in the STAAD.Pro superstructure model and, after the analysis of that model, the support reactions for those combination cases can be imported into SFA for the service and ultimate checks.
In both modes – General and Toolkit – from the standpoint of load cases and/or combinations, the minimum that needs to be present is either A or B, where,
- One primary load case
- One service load case/combination, and, one ultimate load case/combination
Checks Performed for Service Load Cases/Combinations
- Soil pressures are calculated for each service load combination that is included
in the isolated footing job. The forces acting at the founding level are
computed from the following sources:
- forces and moments on the footing from the column in that combination case
- selfweight of the footing
- soil weight
- surcharge load
Items b, c and d are multiplied with the factors specified in the "Selfweight and Deadweight Factor table" correspondng to the load case being designed.
- For each service load that is solved, the maximum soil pressure is compared with an allowable bearing pressure. Under Global Settings, the engineer can choose to define the allowable soil pressure input value as being of the type "Gross" or "Net". The resulting "Gross" allowable pressure at the founding level is calculated and further factored by the value in the "Pile/Soil Bearing Capacity Factors" table to arrive at the maximum permissible soil pressure for the load case being evaluated. The footing is considered to have "Passed" this check if the maximum soil pressure for the load case being solved is less than the permissible soil pressure.
- For column reaction loads that cause uplift, the footing size is increased (in the case of Calculate Dimension) till the weight of footing and soil after multiplication by the appropriate factors is large enough to counter the uplift force. For Set Dimension, a net upward force implies a footing that is not in contact with the soil and the footing is deemed to have failed.
- Stability checks – For each combination case, the factor of safety against forces that cause sliding and moments that cause overturning are computed. The footing is considered to have Passed this check if the computed factor of safety exceeds the user-specified minimum required factor of safety.
- The area of the footing in contact with the soil is calculated for each combination case. The contact area for cases where P (the force) and MX and MZ (the moments) are such as to cause partial uplift is calculated using an iterative method. The check is deemed to have Passed if the computed contact area exceeds the user-specified minimum required value.
- If the water table level is above the founding level, buoyancy effect is considered as contributing to the de-stabilizing effects. It is also considered for calculating the maximum soil pressures under partial uplift conditions.
- For each of the aforementioned checks, if the safety criteria described above is not met (meaning, the footing does not pass that check), the footing is considered to have Failed for the Set Dimension type of design, and, leads to another iteration (increase in dimensions followed by another round of the above calculations) for the Calculate Dimension type of design.
Checks Performed for Ultimate Load Cases/Combinations
After the service level checks described above are computed and the footing is found to be safe for all those checks, the soil pressures are calculated for the ultimate load cases/combinations. Following this, the program calculates the bending moments, oneway and twoway shear forces which are then used in the following checks.
- Design for flexure along both principal directions. Facilities are available in the program’s UI for specifying the "specified compressive strength of concrete", "specified yield strength of steel", minimum and maximum permissible bar sizes, etc. The reinforcement bar library for Canada is built into the program and used for flexure design.
- Check for oneway shear along the 2 global vertical planes. This check is performed based on the assumption that the entire shear has to be resisted by concrete alone, meaning, no shear reinforcement is provided.
- Design for twoway (punching) shear. The column is treated as Interior, Edge or Corner on the basis of some empirical rules. As in the case of oneway shear, for this calculation too, design is performed based on the assumption that the entire shear has to be resisted by concrete alone, meaning, no shear reinforcement is provided.
- Development length checks. If the required development length exceeds the available, this condition is not treated as a failure. Instead, a warning is displayed for Set Dimension, as well as for Calculate Dimension if no more iterations are possible. The reason is that the necessary length can be mobilized through a combination of bends and/or hooks.
- Check for bearing from column reaction.
- The punching and bearing checks are performed for only those load cases where the column exerts a downward force on the footing. Meaning, column reaction loads that cause uplift are ignored for these checks.
The above-mentioned checks are generally similar to the ones performed for other codes such as ACI 318. The following table shows the list of the equations and sections of the Canadian code that are used in the concrete design checks for the ultimate load cases.
| Description | Section of the A23.3-2019 code |
|---|---|
| Minimum thickness of the footing | 13.2.1, 13.2.3, 15.7 |
| Effective depth for oneway shear | 3.2 (Symbols) |
| Effective depth for twoway shear | 13.3.1.2 |
| Neutral Axis factor β1 | Equation 10.2, section 10.1.7 (c) |
| Concrete Strength Factor α1 | Equation 10.1, section 10.1.7 (c) |
| Concrete Density Factor λ | 8.6.5.a (Normal density concrete assumed) |
| Maximum Concrete Strain | 10.1.3 |
| Concrete Stress Distribution | 10.1.7 |
| Neutral Axis Depth & Ductility clause | 10.5.2 |
| Resistance Factor for concrete ϕc | 8.4.2 |
| Resistance Factor for reinforcement ϕs | 8.4.3 |
| Compressive & Tensile strengths of concrete | 8.4.2 |
| Maximum yield strength of reinforcement | 8.5.1 |
| Stress-strain curve for concrete & steel | 8.5.3.2 |
| Modulus of elasticity of steel | 8.5.4 |
| Lower & Upper limits for concrete strength | 8.6.1.1 |
| Minimum reinforcement for flexure | Smaller of (7.8.1, 10.5.1.1) |
| Cracking Moment & Modulus of rupture | 8.6.4 |
| Factored Shear Resistance of Concrete | Equation 11.6, section 11.3.4 |
| Maximum Factored Shear Resistance of concrete | Equation 11.5, section 11.3.3 |
| Effective web width for shear capacity calc | 11.2.10.1 |
| Shear capacity factor β | 11.3.6.3(b), 11.3.6.3(c) |
| Maximum size of coarse aggregate for β calculation | 20 mm (assumed), 11.3.6.3(b) |
| Critical location for oneway shear | 11.3.2 |
| Critical location for twoway shear | 13.3.3.1 |
| Factored punching shear stress resistance | Equations 13.5, 13.6, 13.7, section 13.3.4.1 |
| Reduction in effective depth for punching | 13.3.4.3 |
| Unbalanced moment (UBM) effects | 13.3.5.3, 13.3.5.4 and 13.3.5.5 |
| "J" and other terms in UBM effects | Section 8.4.4.2.3 – ACI 318-2014 |
| Concrete Bearing Check | 10.8.1 |
| Development Length calculation | 12.2.3, 12.11.3 |
Output from the Program
The following results are available for viewing through the program’s calculation reports.
- Maximum soil pressure at each corner of the footing and the associated load case.
- Load case found to be responsible for the final footing plan size if the design type is Calculate Dimension, and the corresponding Contact Area percentage.
- Details of the sliding and overturning checks for each service load case, along with highlighting of the smallest factor of safety for the two criteria.
- Soil pressures and contact area for each ultimate load case.
- Details of flexure design for the longitudinal and transverse directions. Maximum moment for which design is performed and the associated load case, moment capacity, bar size and spacing, and other related information.
- Details of check for oneway shear for the two principal planes. Maximum shear force for which design is performed, shear capacity of concrete, and other related information.
- Details of check for twoway shear. Maximum shear force for which design is performed, shear capacity of concrete, and other related information.
- Details of the check for column bearing on the footing.
- Development check details – required and available development lengths, and associated values.