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General Footing by FEM

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The General Footing by FEM module is useful for footings of a variety of shapes and supporting up to five pedestals.  

 

GFxFEM

 

The module gathers geometric info, soil properties, and loading data, and then generates a finite element model that is analyzed behind the scenes in ENERCALC 3D.  The results are then passed back to the module for review and refinement.

 

This process allows for considerably more flexibility than some of the other footing modules.  For example, this module supports many common shapes of footings.  It also allows square, rectangular and round pedestals to be modeled, and the square and rectangular pedestals can even be rotated.  A full biaxial analysis is performed using compression-only springs for each load combination, including service-level combinations for soil pressure evaluation and stability checks, and strength-level combinations for concrete design.

 

The module allows the user to control the mesh density, and to review the resulting mesh that is automatically created prior to running the analysis.  It also offers two options for the stiffness of the footing.  One option is to use the calculated stiffness based on material properties, thickness, and specified cracking factors.  A second option is to consider the footing as a "Rigid Body", which evaluates the footing as if it was an infinitely stiff plate.

 

 

General Tab

 

Concrete & Rebar Properties

 

Enter material properties and design parameters.

 

 

Stability Settings

 

Set minimum allowable factors of safety for sliding and overturning checks.

 

 

Soil Design Values

 

Specify allowable soil bearing pressure, subgrade modulus, soil density, coefficient of friction and passive pressure.  

 

 

Footing Tab

 

Footing Shape

 

Select the desired footing shape and specify the data necessary to define the plan dimensions of the footing.

 

 

Suggested Footing Target Element Size

 

The module suggests a value to be used as the Footing Target Element Size.  The value generally represents one-fiftieth of the longest dimension of the footing.  

 

 

Footing Target Element Size

 

This field is used to specify the desired target size of the finite elements that will be used to model the footing.  Note that this input is specified in units of inches, and that this is only a target size.  The meshing procedure will attempt to achieve this size for as many elements as possible, but it is not a rule or a limit.  The smaller the target element size, the larger the model (roughly a squared function).

 

 

Footing Thickness

 

Specify the thickness of the footing in units of inches.

 

 

Footing Stiffness

 

This section offers two options.  The first is to treat the footing as a rigid body.  The second is to consider the actual stiffness of the footing based on thickness, material properties, and the specified cracking factor.

 

 

Pedestals & Applied Loads Tab

 

Pedestal Selection

 

Click the desired button to enter geometric and loading data for a particular pedestal.  Up to five pedestals can be specified per footing.

 

 

Description

 

The description field is available for a text description of each individual pedestal.

 

 

Shape

 

The shape dropdown list box offers Unused, Square, Rectangular and Round.  Dimension fields are displayed based on the selected shape.  Note that pedestal dimensions are always collected in units of inches, even if the "pedestal" is long, like a wall.  Enter the Pedestal Height (in inches) above the top of the footing.

 

 

Pedestal Centroid to Footing Datum

 

This area collects dimensions to locate the centroid of the pedestal.  Note that dimensions are in units of feet, and that the pedestal outline is visible on the Mesh Plot for reference.

 

 

Punching Shear Perimeter Adjustment Factor

 

This factor acts as a multiplier on the calculated punching perimeter.  The punching perimeter is calculated as follows:

 

Square Pedestal:  4 * (d/2 + Pedestal dim + d/2)

 

Rectangular Pedestal:   2 * [(d/2 + Pedestal X dim + d/2) + (d/2 + Pedestal Z dim + d/2)]

 

Round Pedestal:   pi * (d/2 + diameter + d/2)

 

So by applying the Punching Shear Perimeter Adjustment Factor to the calculated punching perimeter, it is possible to account for situations where the punching perimeter falls partially outside the footing, such as in corner and edge conditions.  Note that this factor is separate and distinct from the Punching Alpha s Factor described in the next section.  For example, if it is determined that only three quarters of the calculated punching perimeter actually falls within the footprint of the footing for a particular corner or edge column, then the Punching Shear Perimeter Adjustment Factor can be specified as 0.75 for that column.

 

 

Punching Alpha s Factor

 

Based on the position of a pedestal (interior, edge, or corner), ACI requires the use of a specific value for the Alpha s factor: 40, 30 or 20 respectively.

 

 

CW Rotation

 

This field allows square and rectangular pedestals to be rotated by a specified number of degrees.  Positive = CW.  Negative = CCW.

 

 

Loads

 

Loads can be specified for each of the primary load types.  Positive values of Py act downward.  Positive Mxx and Mzz tend to create higher soil pressure on the positive X and positive Z sides of the footing, respectively.  Positive Vx and positive Vz act in the positive X and positive Z directions, respectively.  Moment and shear can be defined with respect to the footing axis system or the pedestal axis system.  Overburden DL can be specified over the entire footing.  It can also be omitted at the footprint of pedestals.

 

Loading is facilitated by the fact that the module allows up to four independent wind load cases and four independent seismic load cases for each model.  This means that one calculation can include cases such as:

 

1.Net wind to the East,

2.Net wind to the West,

3.Net wind to the North, and

4.Net wind to the South.

 

It could also be set up to run:

 

1.Net wind in the East-West direction with positive internal pressure,

2.Net wind in the East-West direction with negative internal pressure,

3.Net wind in the North-South direction with positive internal pressure, and

4.Net wind in the North-South direction with negative internal pressure.

 

Likewise, this increased loading capacity means that a single calculation could include seismic load cases such as:

 

1.Seismic in the East-West direction with positive eccentricity,

2.Seismic in the East-West direction with negative eccentricity,

3.Seismic in the North-South direction with positive eccentricity, and

4.Seismic in the North-South direction with negative eccentricity.

 

It could also be set up to run:

 

1.100% Seismic in the East-West direction with a concurrent 30% seismic to the North,

2.100% Seismic in the East-West direction with a concurrent 30% seismic to the South,

3.100% Seismic in the North-South direction with a concurrent 30% seismic to the East, and

4.100% Seismic in the North-South direction with a concurrent 30% seismic to the West.

 

 

Suggested Target Mesh Element Size

 

The module suggests a value to be used as the Target Mesh Element Size at the pedestal footprint.  

 

 

Footing Target Element Size

 

This field is used to specify the desired target mesh element size of the finite elements that will be used to model the footing within the footprint of the selected pedestal.  Note that this input is specified in units of inches, and that this is only a target size.  The meshing procedure will attempt to achieve this size for as many elements as possible, but it is not a rule or a limit.  The smaller the target element size, the larger the model (roughly a squared function).

 

 

Load Combinations Tab

 

Select the desired load combination set with the Select button.  Specify Sds and Rho values to be incorporated into the generated load combinations if desired.  Combination sets are automatically generated for strength-level (concrete design), and service-level (soil pressure, sliding, overturning).  Each load combination in each set can be individually modified and/or turned on or off as desired.

 

 

Calculations Tab

 

Results

 

Displays a summary of failure modes, factors of safety or utilization ratios, design values, and governing load combinations.

 

 

Soil Pressure

 

Reports the maximum soil pressure and associated load combination, as well as soil pressures for each shell for each load combination.

 

 

Settlement

 

Reports the maximum settlement and associated load combination, as well as settlement for each node for each load combination.

 

 

Actual Moments

 

Reports the maximum Mxx and Myy and associated load combinations, as well as values of Mxx and Myy for each shell for each load combination.

 

 

Wood-Armer Results

 

Reports envelope results of Wood-Armer moments and required areas of steel for the top surface and the bottom surface at each edge of each shell.

 

 

Stability

 

Reports the overall overturning stability ratio and the associated load combination and orientation of the overturning axis.  Also reports the controlling overturning ratio for each load combination, including the overturning moment, the resisting moment, and the orientation of the overturning axis.

 

 

Sliding

 

Reports the lowest sliding ratio and the associated load combination.  Also reports the sliding ratio for each load combination, including the vertical load, friction coefficient, sliding resistance, and applied force.

 

 

Punching

 

Reports the punching shear and resistance.

 

 

Shell Info

 

Reports the area and bounding node info for each shell in the model.

 

 

3D Tab

 

Displays a 3D rendering of the footing.  Display options allow control over the individual elements to be shown.

 

 

Mesh Plot Tab

 

Displays a graphical depiction of the underlying finite element model mesh.  Click the Regenerate button to refresh the mesh display.

 

 

Stress Plots Tab

 

Displays plots that graphically depict soil pressure, pure flexural moments, Wood-Armer design moments, and area of steel required to satisfy Wood-Armer moments.

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