Our program calculates soil bearing pressures on the basis of service or unfactored loads and not ultimate or factored loads. The Bearing Capacity page of the Soil Analysis output shows the results of a soil-supported foundation. It consists of the actual gross bearing pressure on the soil, allowable gross bearing pressure, actual stability ratios in both directions, allowable stability ratio as defined by the user, load eccentricities at the bottom of the footing in both directions, unfactored moments at the bottom of the footing in both directions and factor of safety against sliding in the Z and X directions. The governing or controlling values are indicated in bold fonts.
For footing design, if soil overburden needs to be neglected so that only allowable net bearing is reported and compared to the actual soil bearing, a percent of soil overburden can be neglected on the Soil Parameters Tab. For more details, refer to Soil Parameters.
Bearing pressures under the foundation base are calculated by two different methods based on the user selected option in the Soil Tab of the Options dialog.
Linear Soil Pressure Method
The linear soil pressure method assumes the soil pressure distribution under the footing base to be linear in nature.
In general, the soil bearing pressure for square and rectangular footings is calculated as follows:
Condition I:
Eccentricity e < Footing length or width L/6
Max soil bearing = P/A*(1 + 6*e/L)
Min soil bearing = P/A*(1 – 6*e/L)
Condition II:
Eccentricity e > Footing length or width L/6
Max soil bearing = 2*P/(3*B*(L/2 – e))
Min soil bearing = 0
P = Total axial load on the footing
A = Area of the footing
L = Footing dimension in the direction of soil pressure consideration or overturning moment
B = Footing dimension perpendicular to the direction of soil pressure consideration or overturning moment.
For biaxial bending conditions, a trial and error solution is used in obtaining the maximum bearing pressure under the foundation base by considering various positions of the neutral axis resulting in different compression lengths along each orthogonal direction.
Soil bearing pressure under an octagonal footing is calculated on the basis of the table listed in the reference “HANDBOOK OF CONCRETE ENGINEERING”.
Reduced Effective Area Method
Bearing pressure using this method is based on Meyerhof's theory of bearing capacity on footings with eccentric or inclined loadings. According to Meyerhof's theory, effective footing dimensions obtained as
L' = L - 2ex B' = B - 2ey
should be used in bearing capacity analyses to obtain an effective footing area defined as Af = L'B' with the center of pressure using a rectangular pressure distribution of q' is the center of area Af.
For an octagonal foundation, the effective area is based on the effective area of the inscribed circle within the octagon.
For more details on this method, reference may be made to "Foundation Analysis and Design" by Joseph E. Bowles (Fifth Edition).
Note: Reduced Effective Area method only computes the bearing pressure based on the effective area described above. To complete the concrete design of the footing, pressures obtained using Reduced Effective Area method are not used. The pressures are determined using Linear Soil Pressure method considering uniaxial bending. However if concrete design is to be completed using reduced effective area method, refer to the topic Ultimate Soil Analysis.
When the actual bearing pressure exceeds the allowable gross bearing pressure for one or more unfactored or allowable load combinations, Foundation3D displays a message stating that the bearing pressure has exceeded. However, it will continue to display the results in the "Bearing Pressure" dialog. Clicking OK at this dialog will display the message one more time and prompts if you would like to continue with the design. If Yes is selected, Foundation3D will continue with the footing design. The safe bearing capacity includes the soil overburden calculated using a load factor specified in the load combinations.
The allowable soil bearing pressure or Safe bearing capacity is calculated as follows:
For all concrete design codes except AS 3600,
Gross allowable soil bearing pressure = Net allowable bearing pressure specified in the Options dialog * Allowable increase in bearing pressure due to short term loads (if applicable) + soil overburden using a load factor of the load combination under consideration
When AS 3600 is used,
Safe bearing capacity = Net bearing pressure specified in the Options dialog * Soil bearing reduction factor * Allowable increase in bearing pressure due to short term loads (if applicable) + soil overburden using a load factor of the load combination under consideration
Percent Compression Under Base
Foundation3D and Mat3D also allow input of what percent of the entire base be in compression in the Soil tab of the Workspace settings. The percent base in compression value is used in the Design mode to determine the foundation dimensions that would provide the required percent of the total base in compression for each unfactored load combination. To view the percent base in compression for each load combination, click the "Calculation Report" under the Reports menu after the analysis/design is completed.
In the Analysis mode, if one or more unfactored load combinations do not satisfy the percent base in compression, that row is displayed in orange color. In the Design mode, the program tries to find a solution that will meet this requirement.
Allowable Bearing Pressure
Allowable bearing pressure is calculated based on several different inputs. For details, refer to the topic Tabular Allowable Bearing Pressure
Overturning, Resisting Moments; Sliding Resistance
When the button "Show Intermediate Values" is clicked, a dialog Stability, Sliding dialog displays that lists overturning moment, resisting moment, sliding resistance and applied lateral load in each direction for each load combination.
Sample Bearing Pressure Calculation
A more detailed sample bearing pressure calculation for the load combination that generates maximum bearing pressure is listed on the Calculation Report.
Bearing Pressure Diagram
The "Bearing Capacity" dialog displays a button called "Show Bearing Diagram". Clicking on this button displays a Bearing Pressure Diagram window. This window lists the following:
Allowable Load Combinations
A diagram showing the mat or the footing in blue highlighted line and the load elements (piers, base plates etc.) in dark gray color
Hatched area on the footing indicating the footing area in contact with soil
Bearing pressures at each corner of the footing with the maximum bearing pressure highlighted in red
Load eccentricity location at the base of the footing indicated by a yellow circle along with its X and Z coordinates from the center of the mat
When the footing contact area is less than 100%, a black line inside the footing demarcates the contact area. This black line represents the neutral axis. The coordinates in red are listed at the two ends of the neutral axis where it crosses the footing perimeter
Percent footing area in compression or contact with soil
When the load combination is selected with a click, the bearing pressure diagram for that load combination displays.
Note: For an irregular footing, the bearing pressure diagram may show the neutral axis line past the footing shape to indicate the computed coordinates.
Ratio of Maximum bearing to allowable bearing:
One of the columns on the Soil Analysis page lists the maximum bearing to allowable bearing ratio. This ratio provides an indication of how understressed or overstressed the footing is with respect to soil bearing pressure. If the ratio exceeds the value listed on the Soil tab of Workspace Settings for any load combination, that load combination row is highlighted in red.
Strap Footings:
The bearing pressure is resisted only by the two footings and therefore Bearing Capacity tab displays only under the two footings and not under the strap beam sub tab denoted with S-{Footing1Name}-{Footing2Name}. The Bearing Pressure diagram displays the uniform pressure under each of the two footings with a compression percent of 100% meaning that both footings are completely in contact with soil under normal loading conditions. This is because the footings are assumed to only resist axial loads.
If the applied loads create a negative bearing pressure on the footings, a message will display that one or more load combinations result in the footing not in contact with soil and to revise the footing geometry or the loads. When this message is displayed, the footing geometry or loads must be revised to get the correct results.