Set Dimension

In this method, you are required to specify a value for length (dimension along X), width (dimension along Z) and thickness (dimension along Y) of the footing. This input is denoted by the terms Minimum Length (Fl), Minimum Width (Fw), and Minimum Thickness (Ft) in the Footing Geometry page. The program checks whether this size is sufficient to satisfy all the service load cases and ultimate load cases.

For service load cases, the size is deemed sufficient if the following conditions are satisfied for each load case.

1. The footing must be large enough that the eccentricity, e = M/P, for both global axes (X and Z) is smaller than half the footing dimension for the respective direction.

   ex = Mz/P < Fl/2

   ez = Mx/P < Fw/2

   where

   M and P

   =

   the total moment and total vertical load, respectively, at the base of the footing for the load case under consideration. This includes contributions from selfweight of the footing, weight of pedestal (if any), weight of soil on top of the footing, and weight of surcharge.

   
2. The maximum soil pressure is within the allowable soil pressure specified. For any given load case, the allowable soil pressure is obtained as the product of the Soil bearing capacity (specified in the Cover and Soil page) and the Pile/Soil bearing Capacity Factor Multiplier specified through the Soil Properties page.
3. The sliding and overturning factors do not exceed the allowable values specified for those terms in the Sliding & Overturning page. The computation of the factors for sliding and overturning is described in Factors of Safety in Sliding and Factors of Safety in Overturning.
4. The area of the footing in contact with the soil does not fall below the permissible value specified through the Minimum % of Contact Area for Service loads in the Cover and Soil page times the footing plan area (footing length × footing width).

Calculate Dimensions

The program then uses an iterative procedure to find the smallest required size that satisfies the conditions similar to those described above for service load cases, with the following differences.

1. For (a), the total weight may include or omit the selfweight of the footing, weight of soil on top of the footing, and weight of surcharge, depending on the choice you make in the Global settings dialog on the Rigid Foundation Settings tab. The two options are:
   • Neglect Footing Self Weight for Concrete Checks – if selected, then the weights of the footing, soil, and surcharge are not considered.
   • Consider Net Pressure for Concrete Checks – if selected, then the weights of the footing, soil, and surcharge are considered.

   If the moments on the footing are so high as to cause partial uplift, it is advisable to choose the Consider Net Pressure for Concrete Checks option, since it can lead to a smaller footing size.

   From one iteration to the next, the size increases are achieved using the values specified for the Plan Dimension Increment and Thickness increment in the Footing Geometry page. If the conditions cannot be satisfied using the maximum values of these dimensions, the footing design is reported as failed.

2. For (b), the permissible soil pressure is obtained as the product of the value computed in (b) above times the term Multiplier on Soil Bearing Capacity for Ultimate Loads on the Cover and Soil page.
3. Sliding and overturning are not computed for ultimate load cases.
4. For (d), the minimum contact area required is calculated as the permissible value specified through the Minimum % of Contact Area for Ultimate Loads in the Cover and Soil page times the footing plan area (footing length × footing width).

Obtaining an Optimum Value of the Footing Dimensions

Since there are multiple conditions to satisfy as described above (bearing pressure, contact area, sliding and overturning), and the thickness must ensure that the footing is safe from a concrete design standpoint, it may not be possible to obtain the optimum footing size on the basis of the minimum volume of concrete. The failure to calculate the optimum size may manifest itself in the following manner.

For example, you select Calculate Dimension as the Design Type and specify the minimum (starting) values as 1.0m × 1.0m × 0.1m. Then the program recommends a size 4m × 4m × 0.35m. You may then find that if you use Set Dimension and specify the fixed values 4m × 4m × 0.30 m, (that is, the same length and width but a lower thickness than that recommended by the program), it may pass. In such situations, we recommend that you use the following procedure using Calculate Dimension to arrive at an estimate of the size that would be as close as possible to the optimum.

Specify the minimum thickness as a value that is quite close to what you think is reasonable from an engineering standpoint, say 6 inches, 250mm, etc. If the loads are large, you may want to specify a higher minimum thickness. Use caution as to not specify too shallow of a minimum thickness (for example, 4 inches or 100 mm).

Specify the minimum length and minimum width as small numbers, say 1.0m × 1.0m , or, 3ft × 3ft.

The program will recommend a value for Length, Width and Thickness. If the suggested thickness is higher than the minimum thickness you specified, increase the minimum value and try again. If it is smaller than the minimum thickness you specified, reduce the minimum value and try again. Repeat this until a convergence is attained between the value you started out with and the value reported by the program. The length and width reported at that point be the most optimum for that thickness.

Maximum efficiency in the footing sizing is achieved when the Plan Dimension Inc. and Thickness Increment on the Footing Geometry page are set to as small values as possible. But the smaller those values, the more the time required to calculate the suitable footing size.