STAAD FOUNDTION FOOTING SIZE CALCULATION PROCESS

Question

Kindly help me to know how SFA is calculating the dimension of footing. 
 
 Input Data 
 1. SBC - 120 KN/m2 
 2. Net Bearing capacity Input Selected. 
 3. Load Case Table 
 
 4. Multiplier Factor - 2 
 Question - How staad is calculating min area required is 1.58 m2 ? Please explain calculation to be followed here ?

5. How Final area of footing is coming 2.25 m2 ? 
 
 As per my calculation, area should be = Total Load /SBC 
 Total Load - 213.80+33.75+73.96 = 321.51 
 Therefore, Area = 321.51 / 120 = 2.67m2 
 
 6. How pressure value are being calculated in STAAD. I am not able to find the calculation done here in verification manual ?

Kris Sathia · Answer

Results as per the latest version - SFA Build 9.6.1.74 
 Total vertical load = 383.069 + 54.15 + 120.473 = 557.692 kN 
 MZ at base of footing = (-1.0 * -1.389 * 0.6) + 0.254 = 1.0874 kN-m 
 MX at base of footing = (-10.523 * 0.6) -4.297 = -10.6108 kN-m 
 Plan area of footing = 1.9m * 1.9m = 3.61 sq.m Section Modulus = (1/6) * 1.9 * (1.9*1.9) = 1.143 m3 
 Maximum "Gross" corner pressure = (557.692/3.61) + (1.0874/1.143) + (10.6108/1.143) = 164.7185 kN/m2 
 Minimum "Gross" corner pressure = (557.692/3.61) - (1.0874/1.143) - (10.6108/1.143) = 144.2522 kN/m2 which is greater than zero, hence no uplift 
 Gross Bearing capacity = Net bearing capacity + Gamma*H = 120 + (18.0 kN/m3 * 2.5m) = 165 kN/m2 
 164.7185 < 165 --- Hence safe.. 
 For reporting purposes: 
 Maximum Net Bearing pressure = 164.7185 - 45 = 119.7185 kN/m2 Minimum Net Bearing pressure = 144.2522 - 45 = 99.2522 kN/m2

Kris Sathia · Answer

In the current version, 
 Reactions from the superstructure columns are assumed to act at the top of the footing. The lateral forces FX and FZ produce a moment at the bottom of the footing. 
 MZ due to FX = -1.0 * FX * D MX due to FZ = +1.0 * FZ * D 
 Total Moment for each direction = moment due to the lateral force + moment from the reactions at the base of the column. 
 D is the thickness of the footing = 0.6 m 
 The multiplier 2 is used to obtain the allowable bearing pressure for utimate load cases. That is why the term is called Multiplier on Soil Bearing Capacity for Ultimate Load cases. 
 allowable bearing capacity for ultimate load cases = Multiplier on Soil Bearing Capacity for Ultimate Load cases * Base value of Soil bearing capacity 
 Allowable Gross bearing capacity for service load cases = 120 + 45 = 165 Allowable Gross bearing capacity for Ultimate load cases = 2*120 + 45 = 285 
 Load case 3 is declared as a Primary type, which means it serves as a Service case and an Ultimate case. The check being done here is for the service case, hence we use 165 and not 285.

Kris Sathia · Answer

Ans to Q1: 
 Your statement that a service load is not a factored load is not true. Codes like ASCE have load factors for service conditions that are not necessarily just 1.0. Here is an example of some of the service level combinations in ASCE 7 
 D + 0.75L + 0.75(Lr or S or R) D + 0.6W D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R) 7. 0.6D + 0.6W 
 A load case that is set to the Service type is used for the following checks: 
 1) Comparison of the maximum corner pressure for that load case (demand) with allowable soil pressure for that case (capacity) 2) Stability checks - safety of the footing in sliding and overturning 3) percentage area of the footing in contact with the soil (demand) and comparison with the minimum required percentage for service cases (capacity). 
 Load cases set to the Ultimate type are used for the following checks: 
 1) Comparison of the maximum corner pressure for that load case (demand) with allowable soil pressure for that case (capacity) 3) percentage area of the footing in contact with the soil (demand) and comparison with the minimum required percentage for ultimate cases (capacity). 3) Concrete design - Punching shear, Oneway shear, Flexure, development length ... 
 The program knows what checks to do for a service type case, and what checks to do for an ultimate type case. 
 No special instruction is required from your side. You just need to assign the type to the individual load cases as service, or ultimate. A third option is "Primary" which then gives that load case both types. So, a load case set to Primary goes thru the Service level checks as well as the Ultimate level checks. 
 Ans for Q2: 
 For load cases tagged as Primary, the soil pressure checks are done twice. In both instances, the demand (the pressure calculated from the loads) is the same. In the first instance, the allowable (capacity) is the service level value. In the second instance, it is the ultimate level value. The service level allowable is almost always smaller than the ultimate level allowable, so, it will produce a higher ratio of demand/capacity. In the calculations I provided, only the first instance was shown. But internally, the program automatically does the second one too. 
 Ans for Q3: 
 Please get hold of the latest version - 9.6.1.74. The calculations I provided are based on this version. 
 Ans for Q4: 
 Exactly the same approach as in the case of service level. Calculate 
 P = Total ultimate level loads = column reactions for the ultimate load case + factored selfweight + factored soil weight + buoyancy. Factors for selfweight and soil weight are taken from the selfweight factor table. MX = Total ultimate Moment about the X axis from the column reactions for that case (contribution from FZ and MX that form the column reactions) MZ = Total ultimate Moment about the Z axis from the column reactions for that case (contribution from FX and MZ that form the column reactions) 
 Apply the formula (P/A) +/- (MX/SX) +/- (MZ/SZ) where 
 A = Plan area of the footing SX = Section modulus for the X axis SZ = Section modulus for the Z axis 
 If a negative value is obtained at any of the corners, use an iterative approach to calculating the actual area in contact and apply the same formula as above using the properties (A, SX an SZ) of the final area in contact for that load case.

Kris Sathia · Answer

We are following just what code committees and companies around the world do. Hundreds of companies have accepted the procedure implemented in SFA. 
 If you provide a hand calculation showing the procedure that you wish to follow for an isolated footing design, I can have a look and let you know if SFA is capable of following that approach, and if so, how the input should be specified. 
 Also, you can model the foundation in STAAD.Pro using plates, solids, PLATE MAT etc., and analyse it using the finite element method. You will get the base pressures, contact area, moments, shears, etc. using which you can do your own design without having to rely on SFA.

Kris Sathia · Answer

Your calculation in step 5 is not correct. You are dividing a force which is in the gross frame of reference by an allowable soil pressure which is in the net frame of reference. You need to use the gross bearing capacity in the denominator. (net bearing capacity needs to be converted to the gross bearing capacity as explained in an earlier discussion ). 
 Also, you need to add the pressure contribution from the 2 moments at the base of the footing. The Canadian example 6 in the verification manual shows you how to calculate the pressures due to a vertical force plus two moments. If you attach your SFA file, we can show you how the values in your step 6 are obtained.

STAAD FOUNDTION FOOTING SIZE CALCULATION PROCESS

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