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 ?
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.
PFA, SFA analysis. Please check for footing no 103. Also in the attached model for footing no 103, tota load is 557.69 KN ( 383.07 +54.15 + 120.47) and GBC is 330 KN /m2. Area is coming 557.69 /330 = 1.68 m2.However staad reports 3.61 m2. Please elaborate this also.
Assembly Hall Project 4 with meshing and FL and LC_foundation.sfa
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 * DMX 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 = 165Allowable 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.
1. When we talk about "SERVICE LOAD", it is not factored load as per procedure followed in books also it includes the (Load coming to the column from superstructure + weight of footing only), however in STAAD, "SERVICE LOAD" represents (Factored value of load which is summation of Load coming to the column from superstructure + weight of footing + Weight of soil). Hence it seems there is no difference in "SERVICE LOAD" AND "ULTIMATE LOAD" in SFA.
(How to avail the facility in SFA to design footing dimension under SERVICE LOAD case and Pressure check to be under "ULTIMATE LOAD" case).
2.
Kris Sathia said: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
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
Kris Sathia said:Gross Bearing capacity = Net bearing capacity + Gamma*H = 120 + (18.0 kN/m3 * 2.5m) = 165 kN/m2 164.7185 < 165 --- Hence safe
Gross Bearing capacity = Net bearing capacity + Gamma*H = 120 + (18.0 kN/m3 * 2.5m) = 165 kN/m2
164.7185 < 165 --- Hence safe
Here, we are applying pressure check with "Gross Bearing pressure for service condition" ( VALUE = 165) and we are using Load Case ( Primary = Service = Ultimate ) ( 557.692 IS THE SERVICE AS WELL ULTIMATE LOAD CASE) .Then, why we are not using "GBC for ultimate condition" for pressure check here ?
Kris Sathia said:Allowable Gross bearing capacity for Ultimate load cases = 2*120 + 45 = 285
3. STAAD is showing different approach to calculate GBC , 2*(120 +18*2.5)) = 330 KN /M2 . Please suggest whether 285 is correct or 330 is correct ?
4. How to do pressure check calculation for "ULTIMATE PRESSURE" ?
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 + 0.6WD + 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 overturning3) 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 footingSX = Section modulus for the X axisSZ = 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 said: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 + 0.6WD + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)7. 0.6D + 0.6W
In any case, Service Level Load can't be less than the actual load coming to the structure.
For Ex (Assume) - DL - 100 KN
LL - 5 KN
Lr - 5 KN
Actual Load coming to the structure - 100 + 10 = 110 KN
If I follow your approach mentioned above, Total Load (P) = D+ 0.75L + 0.75Lr
P = 100 +0.75*5 +0.75*5 = 107.5 KN
Hence 107.5 KN < Actual Wt of the structure (110 KN) ( Which is incorrect Design approach)
Coming Back to what ASCE 7- 05 says,
It follows two approach of design namely (ASD and LRFD)
ASD -Follows Serviceability Approach to Design
LRFD - Factored Approach to Design
www.bgstructuralengineering.com/.../BGASCE700202.htm
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.