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General Footing |
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General Footing |
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This module provides analysis of a rectangular footing with applied axial load, overburden, moment and shear loads. The module allows you to move the axial load application position off-center of the footing, and provides automatic calculation of allowable soil bearing pressure increases based on footing dimensions and/or depth below surface.
The module checks service load soil pressure, overturning stability, sliding stability, flexure and shear at each of the four column corners and punching shear around the column perimeter.
General
f'c
28-day compressive strength of the concrete.
fy
Yield point stress of reinforcing.
Ec
Modulus of elasticity of concrete.
Concrete Density
The density of the concrete is used to calculate the self weight of the pedestal and footing when that option is selected.
Phi Values
Enter the capacity reduction values to be applied to Vn and Mn.
Calculate footing weight & include in soil bearing
Click [Yes] to have the module calculate the self weight of the footing and apply it as a downward load when determining soil bearing pressures. The self weight will be multiplied by the dead load factor in each load combination.
Calculate footing weight & use to resist sliding, resist overturning and counteract moments & shears
Click [Yes] to have the module calculate the self weight of the footing and consider it when determining sliding and overturning factors of safety and when calculating moments and shears. The self weight will be multiplied by the dead load factor in each load combination.
Min Steel Ratio - Bending
Enter the minimum ratio for bending steel, calculated using the "d" distance from compression edge to rebar centroid. This will trigger a warning message if the section is under-reinforced.
Min Steel Ratio - Temperature/Shrinkage
Enter the minimum ratio for temperature/shrinkage steel, calculated using the footing thickness. This will also trigger a warning message if the section is under-reinforced.
Minimum overturning safety factor
Enter the minimum allowable ratio of resisting moment to overturning moment. If the actual ratio is less than the specified minimum ratio, it will trigger a message that overturning stability is not satisfied.
Minimum sliding safety factor
Enter the minimum allowable ratio of resisting force to sliding force. If the actual ratio is less than the specified minimum ratio, it will trigger a message that sliding stability is not satisfied.
Use ACI 10.5 as minimum reinforcing
Select this checkbox if the module should check ACI 318-05 equation 10-3 and trigger a warning if that minimum reinforcing is not supplied.
Soil Allowable Values
Allowable Soil Bearing
Enter the allowable soil bearing pressure that the soil can resist. This is a service load resistance and will be compared to calculated service load soil pressures (loads not factored as in strength design).
Increase bearing by footing weight
Click [Yes] to tell the module to calculate the weight of one square foot (plan view) of footing weight and add it to the allowable soil bearing value. This has the effect of not penalizing the soil for the self weight of the footing, and is useful for situations where the geotechnical engineering report provides allowable net bearing pressures.
Soil passive sliding resistance
Enter the value of passive soil pressure resistance to sliding. This value will be used to determine a component of sliding resistance that is generated by the passive pressure of the soil. The sliding resistance due to passive pressure is then added to the sliding resistance due to friction to determine the total resistance to sliding for each load combination.
Coefficient of Soil/Concrete Friction
Enter the coefficient of friction between soil and footing to use in sliding resistance calculations.
Soil Bearing Increase
This section allows you to specify some dimensions that, when exceeded, will automatically increase the allowable soil bearing pressure.
Footing base depth below soil surface: The distance from the bottom of the footing to the top of the soil. This value is used to determine allowable soil bearing pressure increases and soil passive sliding resistance, but it is not used in any other calculations in this module.
Increases based on footing depth: Provides a method to automatically apply increases to the basic allowable soil bearing pressure based on footing depth below some reference depth. Collects the following parameters:
Allowable pressure increase per foot: Specifies the amount that the basic allowable soil bearing pressure can be increased for each foot of depth below some reference depth.
When base of footing is below: Specifies the required depth in order to start realizing incremental increases in the allowable soil bearing pressure on the basis of footing depth.
Example: Assume the following: Basic Allowable Soil Bearing Pressure = 3 ksf. Footing base is 6'-0" below soil surface. The Geotechnical report specifies that a 0.15 ksf increase in bearing pressure is allowed for each foot of depth when the base is deeper than 4' below top of soil. Since you've indicated that the footing is 6' below the soil surface, the module will automatically calculate the adjusted allowable soil bearing pressure to be 3 ksf + (6' - 4') * 0.15 ksf = 3.30 ksf.
Increases based on footing plan dimension: Provides a method to automatically apply increases to the basic allowable soil bearing pressure based on footing dimensions greater than some reference dimension. Collects the following parameters:
Allowable pressure increase per foot: Specifies the amount that the basic allowable soil bearing pressure can be increased for each foot of length or width greater than some reference dimension.
When maximum length or width is greater than: Specifies the required dimension in order to start realizing incremental increases in the allowable soil bearing pressure on the basis of footing dimension.
Example: Assume the following: Basic Allowable Soil Bearing Pressure = 3 ksf. Footing measures 12'-0" x 6'-0". The Geotechnical report specifies that a 0.15 ksf increase in soil bearing pressure is allowed for each foot when the largest plan dimension of the footing is greater than 4'. The module will automatically calculate the adjusted allowable soil bearing pressure to be 3 ksf + (12' - 4') * 0.15 ksf = 4.2 ksf.
Note: Increases based on footing depth and plan dimensions are cumulative.
Footing Dimensions
This tab is where you enter the footing and pedestal dimensions.
Width, Length & Thickness: defines the overall dimensions of the footing
Load Location: defines the offset from the center of the footing where the axial load is applied. Only one direction can be used.
Pedestal dimensions: If a concrete pedestal bears on the footing, its dimensions can be specified here. The px and pz dimensions are used to define the locations on all four sides where one-way shear, two-way shear and bending moment are calculated. If you enter a nonzero height, then you can choose to have the weight of that prism calculated and added as dead load. Note: Any applied overburden loads will be omitted from the area defined as the pedestal dimension along the xx and yy axis, regardless of the specified height of the prism.
Footing Reinforcing
This tab allows you to specify the reinforcing in each direction of the footing.
Applied VERTICAL Loads
This tab allows you to specify the axial load applied to the pedestal location and an overburden load applied to the entire plan dimension of the footing (except the area designated as the pedestal).
Enter loads with a positive sign for downward direction.
Note! This module will not allow a net uplift on the footing. If the result of the factored axial loads (dead, live, wind, etc) produces a negative load sign, the module will not recalculate and will notify you of which load combination resulted in net uplift.
Applied BENDING Loads
This tab allows you to enter applied moments.
Applied SHEAR Loads
This tab allows you to enter applied shear forces. These loads are applied at the location of the pedestal. If the pedestal is specified to have a height, the shear will be applied at that height and will create a moment on the footing equal to Shear Load * (Footing Thickness + Pedestal Height).
Load Combinations - Service
This is the typical load combination tab used throughout the Structural Engineering Library. The Service Combinations tab is used to calculate soil pressures to be compared to the allowable soil bearing pressure. The "Soil Increase" is a factor that can be specified individually for each load combination and is applied to the allowable soil bearing pressure.
Load Combinations - Factored
This is the typical load combination tab used throughout the Structural Engineering Library for strength design. These load combinations are used to calculate moments and shears in the footing for use in determining stresses and required reinforcing.
Note: The General Footing module is applying the factored loads to the footing and determining a different eccentricity than was determined using the service loads for the soil bearing pressure check.
Results Tab
This tab provides a summary of all calculated values. The stress ratios, applied & allowable values and load combination for those governing values are reported.
Soil Pressures Tab
This tab summarizes the calculated service load soil bearing pressure for moments & shears applied about the specified axis, for each load combination.
Overturning Stability Tab
This tab provides the calculations for overturning and resisting moment stability of the footing about each axis and for each load combination. Please note that the load combinations used here are internally generated and are NOT from the Service Load Combinations you have entered.
Sliding Stability Tab
This tab provides the calculations for applied and resisting sliding stability of the footing in each axis direction and for each load combination. Please note the load combinations used here are internally generated and are NOT from the Service Load Combinations you have entered.
Footing Bending Tab
This tab provides a summary of the calculated factored load moment at all four edges of the pedestal perimeter for each load combination.
Footing Shear Tab
This tab provides a summary of the calculated factored load shear at all four edges of the pedestal perimeter for each load combination. Two-way or punching shear is also calculated.
Sketch Tab
