SFA now supports the standards for foundation design for China
1. Isolated footings that have a sloping profile (also known as sloped footings or tapered footings)
2. Isolated footings that have a stepped profile (also known as stepped footings)
Both these features are available through the General mode of the program. In the Toolkit mode and PLANT mode, the Chinese code is currently not available for these or other types of foundations.
For both these footings, the column is considered to be symmetrically placed at the center of the footing. In other words, pedestals/columns located at an eccentric position with respect to the center of the footing are not supported in this version.
On a general basis, the analysis and design of these footings follows the principles described in the SFA Help manual for those foundations. The salient points of the procedure are:
Create the footing data using any of the following methods.
i. STAAD.Pro model
ii. an ISM file that has been exported from any software that performs the analysis of the superstructure
SFA uses the following axis system for recognizing the directions of loads and moments.
X and Z are the horizontal axis, and, Y is the vertical axis.
FX, FY and FZ are the forces, acting along the global X, Y and Z axes respectively, transferred into the foundation from the supports for the columns.
MX, MY and MZ are the moments, acting along the global X, Y and Z axes respectively, transferred into the foundation from the supports for the columns.
Note that in the event of data that is imported from a STAAD.Pro model or an ISM file, these forces and moments will have a sign that is opposite to the reactions in the STAAD.pro model or ISM file. For example, if the reaction in the STAAD.Pro model is FY = +47.5 kNs, then, the load on the foundation will be -47.5 kNs.
Thus, the sign convention for these terms is the same as that for JOINT LOADs in STAAD.Pro.
For data that brought from a superstructure analysis model which uses the “Z UP” system, the data is transformed into SFA’s axis system before the data is displayed in the SFA screens.
In practice, pedestals are generally present in the case of footings that support steel columns. The pedestal is the block of concrete that supports the base plate beneath the column, and hence has a slightly larger plan dimensions than the base plate.
If a pedestal is present, the engineer can specify that information through the Pedestal and Anchor Bolt tab of the Foundation Plan page as shown in the next figure. Note that the Anchor bolt data is not required to be specified, nor is the program able to use that information at this time.
If the “Yes” option is chosen, then, the forces from the column are assumed to act at the top of the pedestal. This is the point usually considered as a support for the superstructure. Thus, the support reactions for the superstructure analysis, which are the algebraic opposite of the loads transferred to the foundation from the columns, are assumed to act at the top of the pedestal.
If there are lateral forces FX and FZ acting at the top of the pedestal due to wind load, seismic load, or other reasons, those forces are assumed to produce additional moments at the at the base of the footing in the following manner:
MX at footing base = MX at column base (from the column reaction) + FZ * Lever Arm
MZ at footing base = MZ at column base (from the column reaction) - FX * Lever Arm
where Lever Arm = Pedestal height plus the total thickness of the foundation
If the engineer chooses “No” for pedestal, then, the forces from the column are assumed to act at the top of the footing. In other words, in the absence of a pedestal, the top of the footing is assumed as the point where the support is specified for the superstructure model.
Top of footing and specifying the height of soil
For tapered footings, one of the input terms that the engineer is required to provide is the Depth of Soil Above footing. The height of soil to be used in the calculations is determined by the program from this term, in the following manner.
The calculation requires the knowledge of which part of the footing is considered to be the datum position.
If the term Type of Depth is set to Fixed Top, the datum line is the top of footing.
If it is set to Fixed Bottom, the datum line is the bottom of footing.
The height of soil is calculated accordingly, as shown in the next figure.
For stepped footings, the term used is “Footing Embedment Depth”.
It has the same meaning as “Depth of Soil above footing” described for the tapered footing. The same rules described above are used for determining the height of soil for the stepped footing.
Designing the footing
Use one of the following types to design the footing:
If the Set Dimension option is chosen, Set the dimensions of the footing (length, width, thickness at bottom of slope and at top of slope for tapered footings, thickness of the various steps for stepped footings, etc.) in the footing geometry page, which instructs the program to check the suitability of the footing for those dimensions.
Instruct the program to Calculate the footing dimensions (initial values for length, width, thickness at top and bottom of the of slope for isolated footings, etc.) required to ensure the safety of the foundation.
If Set Dimension is used, the provided dimensions are used in checking the safety of the footing as per the details below. If Calculate Dimension is used, the program iteratively finds a footing size (using the plan dimension increment specified in the Footing Geometry page) which satisfies the checks described below.
The following checks are performed by the program:
Base pressures are first calculated using a rigid footing approach. In the Help manual, the section titled Calculating Isolated Footing Sizes provides the details of the approach used for such footings. A similar approach is used for combined footings too. The procedure used by the program to calculate soil pressures for axial load + biaxial bending caters for the possibility of partial uplift (partial loss of contact).
For any service load case, the footing is considered safe if all of the following conditions are satisfied.
For any strength (ultimate) load case, the footing is considered safe if the following conditions are satisfied.
If any of the above conditions are violated, the footing is deemed to have failed. For Calculate Dimension, the program will iterate until it can find a size (plan dimensions) that satisfies these checks provided that the required dimensions do not exceed the maximum permissible set by the user in the Footing Geometry page.
The above steps are referred to as the analysis.
In the event of a successful analysis:
For Calculate Dimension, the largest foundation size obtained from the checks described above from among all the service and ultimate load cases is considered the governing footing size, and the corresponding load case is considered to be the governing load case.
The program then proceeds to perform the concrete design. Calculations performed are to determine whether the depth provided is adequate (if Set Dimension used) or to find the required depth (if Calculate dimension is used) to resist the following:
For a successful design, a calculation sheet is displayed containing the details for the checks mentioned above, and the governing load case for each check. For a failed design, the cause of the failure has to be interpreted by reading the messages displayed in the Output Pane, which is the panel below the drawing area.
The report can be obtained in either Chinese or English. This choice is available from the “Report Customization” dialog box.
Drawings of the foundation plan and layout are also produced.
Modification to the parameters used in length and width increment for footing sizing
In past versions of SFA, for isolated footings, the Footing Geometry page had two options known as
as shown in this figure.
This has now been replaced with the following 2 options
which represent the amount by which X or Z dimension of the footing should be incremented by for each iteration (Dim stands for Dimension).
As a result of this change, two of the options available previously called
(if the design type was set to Calculate Dimension) are no longer available. Fixed Width can be simulated by setting the Dim Increment Along Global Z to 0.0, and, Fixed Length can be simulated by setting the Dim Increment Along Global X to 0.0. But note that both cannot be 0.0 simultaneously if the design type is Calculate Dimension.