Hello,
I am designing a combined footing in STAAD Foundation. After several designs using the software, I noticed that the distribution or transverse reinforcements are placed equally in spacing throughout the combined footing. In contrast, I found some combined footing designs in books/online references that have different spacing per section (closer spacing near the column, larger spacing in other areas). I attached below a visual comparison of the two. What is the design principle/justification as to why STAAD Foundation provide an equal spacing all throughout?
Textbooks that demonstrate the procedure for designing combned footings typically deal with columns transmitting only an axial force. Meaning, there is no bending moment transferred to the foundation by the columns. Thus, the design is done primarily for longitudinal bending moments. For transverse bending, the books suggest that we should assume a transverse beam beneath the columns having a width equal to the column width + 2*(0.5d) where d is the effective width.
However, SFA cannot follow this approach because most real life situations involve a biaxial bending moment at the base of the column. So, we calculate the soil pressures at the 4 corners of the footing and compute the bending moments for the 2 directions using those pressures. For the transverse direction, we assume that the footing behaves like a beam whose width is equal to the longutudinal dimension of the footing. Thus, the area of steel to resist that bending is spaced equally across that width.
If you want to obtain a design that takes the flexibility of the footing and the soil into consideration, you need to model it as a mat foundation. Zones closer to the column will then receive a higher amount of steel than regions farther from it.
Answer Verified By: Louis Flores
Hello Kris! Thank you for providing such a detailed answer.