Are you looking for a strip base design? Block foundations, combined foundations, strip foundations, inverted T foundations, strip foundations, etc. are more commonly used than shallow foundations. Depending on the nature of the soil, different types of shallow foundations are used to build structures.
Strip foundations are used in weak soil conditions as recommended by geotechnical engineers.
Laying a strip foundation significantly increases the supporting area of the foundation.
Therefore, these types of foundations can be used on soils with low bearing capacity.
Two methods can be used to analyze strip foundations.
- Rigid analysis method
- Flexible analysis method
Rigid analysis
The supporting pressure under the foundation is assumed to be constant throughout the length and width of the foundation.
Foundation area = (Total column load) / (Allowable bearing pressure)
The above equation is most often used to calculate foundation area.
Since we know the column loads and the pressure on the foundation, the bending and shear forces can be determined with a simple analysis. This can be done using software such as SAP2000, SAFF, ETAB or manual calculations.
Flexible analysis
The pressure on the ground beneath the foundation is assumed to vary along the entire length of the foundation.
Even in real-world conditions, pressure varies throughout the foundation, creating greater pressure on the ground beneath the columns. Using software such as SAP2000, SAFF or ETAB is the easiest way to carry out this type of analysis, as manual calculations are more demanding.
However, the foundation area is calculated using the equation shown above, which is used in strict analysis to keep the soil pressure under the foundation within allowable limits.
The main elements of this analysis are pillars, foundation and floor.
The column load can be applied to the foundation as a point load and the foundation can be modeled with shell elements while the floor is modeled with spring elements. In the software mentioned above, we can model the soil as spring elements, defining the underground reaction.
According to Bowel's basic book, in most cases we can define the background reaction using the following equation.
Underground reaction = (SF) x 40 x (permissible load capacity)
“SF” refers to the safety factor that was taken into consideration when determining the permissible load capacity. If the value of this factor is not available, a value in the range of 2-3 is generally assumed.
If the column loads, assumed foundation thickness, and subgrade reaction are known, the bending moments and shear forces required for foundation design can be determined.
