STAAD Foundation Discrepancy in Computed Corner Pressures

Hello,

I am verifying the computed corner pressures of a combined footing with 3 columns subjected to different magnitudes of axial loads and moments. However, I am getting a large discrepancy between the hand calculation and software results.

Loading Combination 04 - Net Pressure at Corners

Length = 12m

Width = 2.5m

Soil Unit Weight = 17.6 kN/m3

Depth to Footing Base = 2.5m

Footing Self Weight = 344.99kN

Top Soil Weight = 1029.56kN

Column 1:

Fy = 150kN

Mx = 60kN-m

Mz = 60kN-m

Column 2:

Fy = 90kN

Mx = -25kN-m

Mz = 40kN-m

Column 3:

Fy = 90kN

Mx = 30kN-m

Mz = 70kN-m

Determine Total Net Axial Pressure:

Gross Pressure = [(150+90+90)+(344.99+1029.56)/(12*2.5)] = 56.818 kPa

Net Pressure = 56.818 kPa – (17.6*2.5) = 12.818kPa

Determine Total Moments:

Summation of Mx = 60 – 25 + 30 = 65kN-m

Summation of Mz = 60 + 40 + 70 = 170kN-m

Calculate Z:

Zx = 12* (2.5)^2 / 6 = 12.5 cu. m

Zz = 2.5* (12)^2 / 6 = 60 cu. m

Pressures at the corner:

12.818kPa + (65/12.5) + (170/60) = 20.851 kPa

12.818kPa + (65/12.5) - (170/60) = 15.185 kPa

12.818kPa - (65/12.5) + (170/60) = 10.451 kPa

12.818kPa - (65/12.5) - (170/60) = 4.785 kPa

delte.sfa