How STAAD Foundation Design for Flexure

Question

Hello, 
 
 In designing combined footings, how does STAAD Foundation compute the &ldquo;Ultimate Moment (Mz)&rdquo; and &ldquo;Critical Moment for Distribution Steel (Mx)&rdquo;? I attached my calculation sheet for reference. 
 
 Thank you in advance! 
 PDF

Kris Sathia · Accepted Answer

Pressure at corners due to the load from the columns = 2.199273 kip/ft2 (as shown earlier) 
 
 Bending moment about the line AB : 
 A = 0.5 * (Width of foooting - Column dimension along Z direction) = 0.5 * (40-12) = 14 inches = 1.166666667 ft B = Length of footing = 22.91667 ft 
 Bending moment = (Pressure * B) * A * (A/2) = (2.199273 * 22.91667) * 1.166666667 * 0.583333333 = 34.3 kip-ft 
 SFA reports 34.3586 kip-ft 
 I will get back to you on the 8.9795 kip-ft moment

Anirban · Answer

This is the moment at top surface for Mx. Actually Mx would remain zero for full contact cases. As a minimum moment calculation, SFA is assuming that bottom soil vanishes and a hogging moment occures due to self weight soil weight and surcharge- 
 
 Here Mx Top = [{ footing length x ( Footing width - pedestal width)/2} x footing depth x concrete density + { footing length x ( Footing width - pedestal width)/2} x soil depth x soil density ] x Footing width - pedestal width)/4 
 
 here Mx Top = 275 x {(14 x24 x 0.159) + (14 x24 x 0.127)} x (14/2) x (1/12^3) = 107 kip.inch = 8.9 kip.ft

Anirban · Answer

Mx calculation in combined footing is a bit different than Mz calculation in combined footing. In Mz calculation, SFA takes the center line approach and calculate the actual Mz at different places. This approach is not safe for Mx calculation. As, for some cases, the L/W ratio of combined footing is very high , So, if we consider center line for Mx direction, it may under design to one end (Please ref the attached sketch). Therefore, Mx = Maximum corner pressure x Footing Length x (Foortng Width - column width)/2 x (Foortng Width - column width)/4

Kris Sathia · Answer

I presume your question is about the Combined footing example 1 in the verification manual for the ACI code. 
 The downward pressure due to selfweight and soil weight will be counteracted by an equal and opposite upward pressure from soil. Thus, the bending moments are caused solely by the loads from the columns. 
 Dowward pressure on soil due to column loads = 
 Length of footing = 16.25 + 3.3333 + 3.3333 = 22.91667 ft Width of footing = 3.3333 ft 
 Total factored downward load from the 2 columns for load case 103 = 2 * 60 * 1.4 = 168 kips 
 Uniform pressure at bottom due to the column load = 168 / (22.91667 * 3.3333) = 2.199273 kip/ft2 
 Sagging Moment at the outer face of the column = 2.199273 * 3.3333 * (3.333 - 0.5) * (3.333 - 0.5) / 2 = 29.42545 kip-ft 
 SFA reports 29.49 kip-ft, and the location as 2.83 ft

Hogging moment at the midspan (maximum moment is at mid-span due to symmetry = 22.91667/2 = 11.45833 ft from edge of footing) 
 2.199273 * 3.3333 * (11.45833) * (11.45833/2) - 60 * 1.4 * (11.45833 - 3.3333) = 481.25 - 682.5 = -201.25 kip-ft 
 SFA reports -201.54 kip-ft, and the location as 11.34 ft

How STAAD Foundation Design for Flexure

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