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Square Footing |
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Square Footing |
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This program provides design analysis for square footings subjected to axial load only. If you wish to design a rectangular footing subjected to axial loads and overturning moments about one or both axis, the "General Footing" program is required.
Each footing may have a dead, live, and short term axial load applied, have soil overburden weight included, and you may choose to omit live load during short term load combinations.
Allowable soil pressure and a short-term allowable increase factor is used to check soil pressures, and the program also allows you to specify allowable soil pressure increases based on footing size and placement depth.
The program calculates actual soil pressure for static and short-term loadings, one and two-way shears, and calculates required reinforcement using the factored soil pressures.
Basic Usage
| • | Dead, Live, and Short-Term loads are entered as loads applied to the footing (no horizontal shear or overturning moments are allowed with this program). Entering the Seismic Zone will instruct the program how to factor the short-term loads. Entering 0" will signal that short-term loads are wind loads, and 1" through 4" indicates zones. Overburden Weight will be applied as a dead load (for example assuming soil, concrete, or other static material), and you can specify to Include Live Load with short-term loads (typically combined for wind but ignored for seismic). |
| • | Footing Width, Thickness, and Material Strengths will be used to evaluate footing weight, pressures and shears. If Rebar Size and Quantity are entered, the program will calculate the moment capacity of the footing, otherwise it will provide design analysis and reinforcing requirements. |
| • | Soil Data is entered to define the allowable soil bearing pressure for static (DL+LL) and short-term (DL+LL+ST) load combinations. By entering the footing depth and Allowable Increases, you can simply modify footing dimensions and have the allowable pressures automatically modified. |
Unique Features
| • | Up to six footings can be designed at one time on one program calcsheet, allowing fast design of dozens of footings. |
| • | The program can both analyze and design steel reinforcing |
| • | Allowable soil pressure increases based on depth of embedment and width can be automatically calculated whenever footing dimensions change. |
| • | Program checks both beam and punching shear. |
Assumptions & Limitations
| • | Only a square footing can be designed using this program. |
| • | One and two-way shear and moments are calculated using the equivalent column dimension. |
Example
The data entry for this example is shown in the screen captures that accompany the Data Entry Tabs and Results & Graphics Tabs sections to follow.
Data Entry Tabs
This set of tabs provides entries for all input in this calculation. While you are entering data and switching between these tabs you can view the desired resulting information on the tabs on the right-hand side of the screen (calculated values, sketches, diagrams, etc.). A recalculation is performed after any entry data is changed. After each data entry you can view the results on the right-hand set of tabs.
General Tab
Footing Square Dimension
Since only square footings can be designed, the length and width must be identical.
Thickness
Total footing thickness.
# Bars
You can enter the size and number of bars in your footing for analysis of a footing, or allow the program to calculate this for you (see Reinforcing). If you specify the reinforcing, the program will automatically determine the bending capacity.
Bar Size
Bar Size #3 thru #18 bars are allowed.
Rebar Cover
Distance from the bottom of the footing to the centerline of the reinforcing.
Column Dimension
Enter an equivalent column square dimension to be used for calculation of moments and shears.
Allowable Soil Bearing
Enter the allowable soil pressure for static loadings.
f'c
Concrete compressive strength.
Fy
Yield stress of reinforcing steel.
Concrete Weight
By entering a non-zero number here, the weight of the footing will be included in soil bearing pressure calculations. Enter this number in PCF.
Load Duration Factor
This factor will be applied to the Basic Allowable Soil Pressure to calculate allowable pressure under short-term loads.
Seismic Zone
This entry is used to control overall ACI load factoring. When wind loads generate the short-term forces, enter a 0" here. When seismic loads generate the short-term loads, enter 1" through 4" here to indicate the UBC seismic zone. When zone 3 or 4 is used, the special load factoring provisions of the UBC are applied.
Min Allow % Reinf.
Loadings Tab
Loads
Each footing can support a dead, live, and short-term load. The load is applied to the center of the footing (no eccentricities are allowed).
Overburden Weight
This represents an applied uniform dead load over the footing. It can be from soil, concrete slab, or storage loads.
Combine LL & ST?
This entry instructs the program when to include live load with short-term loads. Typically, wind load analysis will include live load, while short-term loads due to seismic forces are not combined with live loads.
Depth & Width Increases Tab
Footing Base Depth Below Surface
Enter the depth of the bottom of the footing below the adjacent soil. This item will only need be entered if you wish to use the Depth Increase capability.
Increase per foot of Depth
These two values define how the soil pressure will be increased when increases based on depth are used. When Below defines the minimum depth of the footing below the soil for the increase to take effect. Per Foot Depth is the allowable pressure increase per foot of additional embedment greater than the minimum value.
Increase per foot of width
These values define how the soil pressure will be increased when increases based on footing width are used. When Wider defines the minimum footing width of the footing for the increase to take effect. Per Foot Width is the allowable pressure increase per foot of additional width beyond the minimum value.
ACI Factors
This tab specifies the load factors to be used by the program when calculation the factored dead, live, and short term loads to be used in the internal load combinations for determining Mu & Vu.
Results & Graphics Tabs
This set of tabs provides the calculated values resulting from your input on the "Data Entry Tabs". Because a recalculation is performed with each data entry, the information on these tabs always reflects the accurate and current results, problem sketch, or stress/deflection diagram.
Summary Tab
Max. Static Soil Press
This is the soil pressure due to dead, live, and overburden loads, and should not exceed the Allowable Static pressure.
Allow Static Press
This soil pressure is equal to the Basic Allowable soil pressure when depth and width increases are not used. When you have specified depth or width increases, and the footing is either wider or deeper enough for them to be used, this value will reflect those additions.
Max. Short Press.
This is the soil pressure due to dead, short term, overburden loads, and optional live loads. This pressure should not exceed the Allowable Short pressure.
Allow Short Press.
This is equal to Allowable Static Pressure described above multiplied by the Short Term Factor.
One Way Shear
This is the maximum one-way shear due to factored loads for both static and short-term conditions. The shear is calculated at a distance d + Col. Dim/2" away from the footing centerline.
Two Way Shear
This is the maximum two-way (punching) shear due to factored loads for both static and short-term conditions. The shear is calculated at a distance d/2 + Col. Dim/2" away from the footing centerline.
Mn
This is the allowable bending capacity of the footing using the reinforcement optionally specified earlier. If the reinforcing was not entered, you can review the reinforcement calculations below for minimum requirements.
Mu/Phi
This is the maximum actual moment in the footing due to static or short-term load combinations.
Reinforcing
The required steel percentage is calculated by first finding the required steel area due to bending moments: % Req'd = (1/m) * (1-[1-(2*m*Ru)/Fy)½ ]½
If this area is greater than 200/Fy then it is compared to your Min AS % value and the maximum used. If it is less than 200/Fy, it is multiplied by 1.33 and again compared to 200/Fy. The minimum of those two values is then compared to your Min AS % value and the larger used
Total As Required
This is equal to the As% value calculated previously multiplied by the width and d distances of the footing.
Rebar Choices
This section lists the reinforcing choices that will supply the required area.
Sketch Tab
This tab provides a sketch of the beam with loads and resulting values shown. Using the [Print Sketch] button will print the sketch in large scale
on a single sheet of paper.
Sample Printout
