Here is a listing of design capabilities contained in this Cantilevered Retaining Wall module:
|•||Cantilevered stem wall can have up to five different stem sections, of either masonry or concrete, each with a different thickness and/or reinforcing size and spacing. You may also include a weightless fence on top of the wall for the purpose of collecting additional wind load.|
|•||Surcharges on either side of the wall.|
|•||Axial dead and live load applied to the top of the wall, with eccentricity.|
|•||Wind acting on a wall projection above grade.|
|•||Add lateral loads against the stem -- uniform or concentrated (impact) loads.|
|•||Effect of an adjacent footing behind the wall, line or point loading.|
|•||Option to use user-defined active and passive pressure or input angle of internal friction and module will compute pressures using the Rankine or Coulomb formulas.|
|•||Specify percent passive and frictional resistance to be used to prevent sliding.|
|•||Option to specify sliding resistance using cohesion, in lieu of friction.|
The retaining wall module divides the screen into a left and right portion. The left portion contains all of the input data (and in some cases intermediate calculated values). The right portion contains the calculated results and sketches.
GENERAL WALL INFORMATION
This tab allows you to enter the general information affecting the retaining wall. More specific data will be entered on other tabs dedicated specifically to the stem, footing and loads.
This is the height of retained earth measured from top of footing to the top of soil behind the stem (over the heel). When the backfill is sloped, the soil will slope away and upwards from this height. The actual retained height used for overturning and soil pressure calculations will be the retained height projected at the vertical plane of the back of the heel, but for stem moments, no such increase will be made.
Using the spin-buttons you can vary the retained height in 3-inch increments. You can also type in any number. After each entry, you can press [Tab] to advance to the next entry, or use your mouse to reposition the cursor.
Wall Height above Retained Soil
Use this entry to specify if the wall extends above the retained height. This entry is typically used to define a "screen wall" projection above the retained soil. This extension can be used as a weightless "Fence", or it can be defined as a concrete or masonry stem section without any soil retained behind it. You can enter wind load on this projection using the entry labeled "Load @ stem above soil" on the Loads tab. We'll handle the fence when we get to the Stem design screen. The total height of the wall (above the footing) will be equal to the Retained Height plus the Wall Height Above Retained Soil.
Height of Soil over Toe
Measured from top of footing to top of soil on the toe side, this may vary from a few inches to a few feet depending upon site conditions. (Note that it is specified in inches.) It is used to calculate passive soil resistance (but its effective depth can be modified by the "Soil over toe to Neglect" entry in the Sliding Resistance category on the Footing tab). This depth of soil is also used to calculate the resisting moment, and to reduce the net lateral sliding force. You can negate the latter effects on the Options screen if desired.
Water table height over heel
If a portion of the retained height is below a water table, the active pressure of the saturated soil will increase below that level. This additional pressure for the saturated soil is equal to the pressure of water, plus the submerged weight of the soil (its saturated weight - 62.4), plus the surcharge of the soil above the water table. The submerged weight of a soil can be approximated as 62% of its dry unit weight.
If you want to design for a water table condition, enter the maximum height from top of footing to water table level. The module will then compute the added pressures for saturated soil on the heel side of the footing, including buoyancy effect. It will also calculate increased moments and shears on the stem, and an increased overturning moment. Don’t enter a height more than the retained height, nor a liquid other than water. If the water table is near the top of the retained height, it may be advisable to enter the saturated soil density and specify the resulting active pressure for the full retained height.
You may enter any backfill slope behind the wall. Use the drop-down list box or type the slope ratio as Horiz/Vert. The soil must be level or slope upward. Negative backfill slopes (grade sloping downward, away from the wall) are not allowed.
The module will use this slope to:
|1.||include the weight of a triangular wedge of soil over the heel as vertical load, and|
|2.||compute overturning based upon an assumed vertical plane at the back face of the footing extending from the bottom of the footing to the ground surface – a steeper slope will result in a higher overturning moment.|
When the Rankine or Coulomb method is used, the final calculated pressures do include the effect of the slope on those Rankine or Coulomb equations.
The module will not accept a backfill slope steeper than the angle of internal friction.
Allow Soil Bearing
The maximum allowable soil bearing pressure for static conditions. Using the spin buttons you can increment in 50 psf steps. Typical values vary from 1,000 psf to 4,000 psf or more.
Soil Density (heel side)
Enter the soil density for all earth (or water if applicable) above the heel of the footing. This weight is used to calculate overturning resistance forces and soil pressures using the weight of the soil block over the projecting heel of the footing. When surcharges are applied over the soil, the surcharges are transformed to equivalent uniform lateral loads acting on the wall by the ratio force = (Surcharge/ Density)*Lateral Load. Input this value in lbs. per cubic foot. Usual values are 110 pcf to 120 pcf. More if saturated soil. Water is usually assumed to be 64 pcf.
Soil Density (toe side)
Enter the soil density on the toe side, which may be different than the heel side. When surcharges are applied over the soil on the toe side, the surcharge is transformed to equivalent uniform lateral loads acting on the wall by the ratio force = (Surcharge/ Density)*Lateral Load. Input this value in lbs. per cubic foot. Typical values are 110 pcf to 120 pcf.
Lateral Pressure Method
Here you can choose between E.F.P. (Equivalent Fluid Pressure), Rankine formula or Coulomb formula. Based on your choice for Lateral Pressure Method, you will be offered the following input fields to fully define the lateral forces acting on the wall and footing.
When the EFP Method is selected:
Active Soil Pressure - Heel Side
Enter the equivalent fluid pressure (EFP) for the soil being retained that acts to overturn and slide the wall toward the toe side. This pressure acts on the stem for stem section calculations, and on the total footing+wall+slope height for overturning, sliding, and soil pressure calculations.
Commonly used values, assuming an angle of internal friction of 34°, are 30 pcf for a level backfill; 35 pcf for a 4:1 slope; 38 pcf for a 3:1 slope; 43 pcf for a 2:1 slope; and 55 pcf for a 1.5:1 slope. These values are usually provided by the geotechnical engineer.
When the retained soil is sloped, a vertical component of the lateral earth pressure over the heel can be applied vertically downward in the plane of the back of the footing. You can choose to apply this force for overturning resistance, sliding resistance, and/or for soil pressure calculations, by checking the boxes on the Options tab.
Active Soil Pressure - Toe Side
Enter the active pressure to be used on the toe side of the wall. This active pressure is used along with the "Soil Height over Toe" value (entered on the Sliding tab) to calculate a stabilizing soil force on the wall. This front side of the wall is assumed to be level. The active pressure from soil over the toe counteracts the heel-side active pressure to reduce net overturning and net sliding force.
This action is arguable, therefore the default is set to not use this counteracting force.
This is the resistance of the soil in front of the wall and footing to being pushed against to resist sliding. Its value is in psf per foot of depth (pcf). This value is usually obtained from the geotechnical engineer. Its value usually varies from 100 pcf to about 350 pcf.
When the Rankine or Coulomb Method is selected:
Soil Friction Angle
This value is entered in degrees and is the angle of internal friction of the soil. This value is usually provided by a geotechnical engineer from soils tests, but can also be found in reference books or building codes for various typical soil classifications. This value is used along with Soil Density within the standard Rankine and Coulomb equations to determine "Ka" and "Kp" multipliers of density to give active and passive soil pressure values.
Active Soil Pressure
This value will be computed using the Rankine or Coulomb formulas. This represents the lateral earth pressure acting to slide and overturn the wall toward the toe side. The result will be presented in units of psf/ft. This pressure acts on the stem for stem section calculations, and on the total footing+wall+slope height for overturning, sliding, and soil pressure calculations.
When the retained soil is sloped, a vertical component of the lateral earth pressure over the heel can be applied vertically downward in the plane of the back of the footing. You can choose to apply this force for overturning resistance, sliding resistance, and/or for soil pressure calculations, by checking the boxes on the Options tab.
Passive Soil Pressure
This value will also be computed using the Rankine or Coulomb formulas. This is the resistance of the soil in front of the wall to being pushed against to resist sliding. Its value is in psf per foot of depth (pcf). Common values usually vary from 100 pcf to about 350 pcf.
This tab allows you to enter all the loads that will be applied to your retaining wall in addition to lateral earth pressure.
Wind Load on exposed stem above soil
This wind force will be applied to that part of the stem projecting above the retained height defined by the entry "Wall height above retained soil." It is used to calculate overturning moment and sliding, stem design moment and shear, and soil pressures. Only positive values of wind load should be specified. This will ensure that the wind load acts in the direction of the active soil pressure, increasing the overturning moment, the sliding force, the soil bearing pressure, and shear and moment in the stem.
Vertical Surcharge (Surcharges will be factored as Earth Load, H for LRFD designs.)
Surcharge over Toe
This surcharge is treated as additional soil weight – if the surcharge is 240 psf and the density is 120 pcf, then the module uses two feet of additional soil. Similarly, if 50 psf is added for the weight of a slab over the footing, this will be equivalent to 0.41 feet of soil (50 / 120). This surcharge will affect sliding resistance and active toe pressure. Keep this in mind if modeling a point load toe surcharge.
Use TOE Surcharge to resist sliding & overturning
Checking this box will include the weight of soil overburden on the toe to resist overturning and add to its weight for frictional resistance.
Surcharge over Heel
This surcharge is considered uniformly applied to the top surface of the soil over the heel. It may be entered whether or not the ground surface is sloped, but it is unlikely a surcharge could apply to a sloped backfill. This surcharge is always taken as a vertical force. This surcharge is divided by the soil density and multiplied by the Active Pressure to create a uniform lateral load applied to the wall. You can choose to use this surcharge to resist sliding and overturning by clicking the box on the Options tab. Typical live load surcharges are 100 psf for light traffic and parking, and 250 psf for highway traffic.
Use HEEL Surcharge to resist sliding & overturning
Checking this box will include heel surcharge. If the surcharge includes live load, then using it to resist sliding and overturning could be non-conservative. In this situation, it might be advisable to deselect this checkbox.
Vertical Load Applied to Top of Stem
These loads are considered uniform load along the length of the wall. They are applied to the top of the topmost stem section and affect the design of masonry stems only. The dead and live loads are used to calculate stem design values and factored soil reaction pressures used for footing design. Only the dead load is used to resist overturning and sliding of the retaining wall.
If a wall is subjected to a high axial load (say more than 3 kips/ft) it could cause a reversal of the bending moment in the heel. Under these conditions, it might be advisable to investigate the design with and without the high axial load, to be sure that an acceptable design is found for all conditions.
Since slenderness ratios (h/t) for retaining walls are generally small, usually less than 10, and axial stresses are low, slenderness effects are checked but usually have a small effect.
If a point load is applied to the top of a wall, such as a beam reaction, the point load is typically assumed to distribute itself laterally at a rate that is based on engineering judgment for the materials under consideration. As a result of this distribution, the point load will result in a uniformly distributed load of some relatively low magnitude by the time it reaches the base of the stem. This module does not have an explicit input field for point loads, so they must be represented as uniformly distributed loads. To properly account for the lateral distribution that is characteristic of an axial point load applied to a wall, the magnitude that is entered to represent the point load should consider this distribution effect. The top of the wall may also need to be checked by appended calculations for the localized effects of the full magnitude of the concentrated load.
Axial Load Eccentricity
This is the eccentricity of the axial load with respect to the centerline of the uppermost stem section. Positive values of eccentricity move the load toward the toe, causing bending moments that are additive to those caused by the lateral soil pressure over the heel. Negative eccentricities are not accepted.
Vertical Adjacent Footing Load
This entry gives you the option of placing a footing (line or square) adjacent and parallel to the back face of the wall, and have its effect on the wall included in both the vertical and horizontal forces on the wall and footing. Refer to the General Reference Diagram for locations where input measurements should be taken.
For "Line (Strip) Load" the entry is the total load per ft. parallel to the wall (not psf). If the adjacent footing is specified as "Square Footing" (not line load), the load entered should be the adjacent footing load divided by its dimension parallel to the wall, giving a pounds per lineal foot value, as for a continuous (line) footing.
A Boussinesq analysis is used to calculate the vertical and lateral pressures acting on the stem and footing. The module uses equation (11-20a) in Bowles’ Foundation Analysis and Design, 5th Edition, McGraw-Hill, pages 630. When the Boussinesq analysis is used, the module may require additional computing time, depending upon the speed of your computer (hundreds of internal calculations are done after each entry). To avoid this delay (which occurs any time any entry is changed) we suggest you use a vertical load of zero until your data entry is nearly finalized. Then enter the actual footing load and modify your final values.
For adjacent truck or highway loading, it may be preferable to use a heel surcharge (uniform) of 250 psf (or more), instead of treating it as an adjacent footing.
It is generally not necessary to use this feature if the adjacent footing load is farther from the stem than the retained height, less the depth of the adjacent footing below the retained height, since at this distance it will not have significant effect on the wall.
Wall to Footing Centerline Distance
This is the horizontal distance from the center of the adjacent footing to the back face of the stem (measured at the top of retaining wall footing). The nearest edge of the footing should be at least a foot away from the wall face – otherwise it is suggested to use an equivalent heel surcharge instead.
Note: If the horizontal distance from the center of the adjacent footing to the back face of the stem is greater than the vertical distance from the top of the retaining wall footing to the bottom of the adjacent footing, then the effect on the retaining wall will be insignificant.
Width of the adjacent footing measured perpendicular to the wall. This is necessary to create a one-foot long by Width wide area over which the load is applied.
Height of Base Above (+) or Below (-) Retained Height
Use this entry to locate the bottom of the adjacent footing with respect to the Retained Height. Entering a negative number places the footing below the Retained Height. A positive entry would typically only be used when the soil is sloped and the adjacent footing resides uphill. To insert a negative number, first type the number, then press the "-" (minus) sign.
Note: If the Adjacent Footing is another retaining wall at a higher elevation, the Boussinesq analysis may be used for the vertical load applied to the soil from the wall, however the design must also consider the lateral (sliding) loads from that adjacent wall. This load could be applied as Added Lateral Load, however this is at the discretion of the designer and is not within the scope of the module. Caution is urged for this condition. See discussion in the companion book: Basics of Retaining Wall Design. For questionable soil or site conditions a global stability analysis is advised.
This entry is provided in case the soil pressure under the adjacent footing is not uniform. Enter the eccentricity of the resultant force under the adjacent footing from the centerline of the adjacent footing. Positive eccentricity shifts the load toward the toe, resulting in greater pressure at the side of the adjacent footing closest to the stem of the retaining wall. The module will use the vertical load and eccentricity and create a trapezoidal pressure distribution under the adjacent footing for use with the Boussinesq analysis of vertical and lateral pressures.
This drop-down list box allows you to enter either an isolated footing using the "Square Footing" selection, or a continuous footing using the "Line Load" selection.
Since the resulting pressures are sensitive to Poisson’s Ratio, there is an entry allowing you to select a ratio from 0.30 to 0.55. This value should be provided by the geotechnical engineer. A value of 0.50 is often assumed.
Lateral Load on Stem
This input allows you to specify an additional uniformly distributed lateral load applied to the stem.
This is for an impact point load, such as due to an impact of a car or similar force. Enter the load as a one-foot high increment, separating the "Height to Bottom" and "Height to Top" by one-half foot (or meter).
Note: This load is not factored. To apply a load factor (such as for an impact load), increase the applied load proportionately (e.g. an impact load of 1000 lbs requiring a load factor of 2.0 would be entered as 2,000 lbs). You may need to do several designs to check load factor combinations.
Keep in mind that when considering a concentrated lateral load, it may be possible to reduce the magnitude to account for the fact that the load distributes horizontally at levels below the point of application.
Height to Top
This dimension defines the upper extent of the added lateral load measured from the top of the footing. Do not enter a dimension that exceeds "retained height" plus "Wall height above retained soil".
Height to Bottom
This dimension defines the lower extent (or bottom) of the added lateral load measured from the top of the footing.
STEM DESIGN TAB
Use the buttons to select Masonry, Concrete, or Fence. Fence is only allowed on top of the wall, higher than the Retained Height, and is considered weightless.
Use the drop-down list box to input the wall thickness. If masonry is chosen, you will be given standard masonry thickness (e.g. 6", 8", 12"). If concrete is chosen, you can increment in one-inch steps. If Fence is chosen, this entry is unavailable since the fence is assumed to be weightless.
This displayed value is based upon the wall data entered earlier. For concrete stems, the unit weight of concrete can be specified on the Stem tab. For masonry stems, 140 pcf grout is assumed, and the unit weight of the completed stem is a function of the specified thickness, CMU Type, and the status of the Solid Grouting checkbox, all of which are located on the Stem tab. A multiplier is also available on the Options tab to modify the tabular weights for masonry walls.
The industry standard masonry unit weight values used by this module may be modified by clicking Databases > Concrete Masonry Unit Data from the main menu and then clicking the [Change] button.
When a masonry stem section is chosen, this allows a choice of ASD or LRFD methods. When the latter is selected the input notations change (e.g. fs to fy) and all calculations are based upon LRFD.
Make your selection from the pull-down menu for bar sizes #3 to #10. “Soft Metric” sizes will be displayed in parentheses alongside.
Chose between Center or Edge. If Center is chosen, the rebar d distance will be 1/2 the actual wall thickness. If Edge is chosen it will be located at the heel side of the stem.
For masonry design, the module contains a table of the appropriate "d" values to use for various block sizes and center/edge locations, as shown in the table below.
Rebar Position Depth for Masonry, Default Values.
Rebar Depth (in)
For concrete, the edge rebar depth is always stem thickness less 1.5" for #5 and smaller bars (or 2" for #6 or larger), less one-half the bar diameter.
Specify Position Box
Click this box to change the default "d" value.
Enter the allowable steel stress, based on working stress design, which should be used for design of the masonry stem section. The spin button changes this value in increments, and is not visible when a concrete wall has been specified.
Short Term Increase
This factor is applied to masonry ASD design and allowable soil bearing as permitted by IBC 2009, section 1806.1, and ACI 530-08, section 126.96.36.199. This is applicable only when wind and/or seismic is applied.
This applies to masonry only. If this box is checked, the weight of the wall will be based upon industry standard solid-grout weight for either lightweight, medium weight, or normal weight block regardless of the specified spacing of reinforcing. If this box is not checked, the module will calculate the weight based on the assumption that only cells containing reinforcing are grouted.
This also affects equivalent solid thickness for stem shear calculations, and area for axial stress calculations (combined with moment for masonry stems).
Modular "n", Ratio
This is the multiplier used to calculate the modulus of elasticity of masonry. The ACI 530-05 and ACI 530-08 both specify Em = 900*f’m which is the default value. The multiplier can be modified on the Options tab.
Equivalent Solid Thickness
If partially grouted (not solid grout), this value is generated from an internal database that is accessible by clicking Database > Concrete Masonry Unit Data.
Stem Design Heights
IMPORTANT! The term “Stem Design Height” used in this module is the height above the top of the footing (i.e. above the base of the stem). It is the height above the bottom of the stem where you want the module to compute moments and shears.
You can divide the stem into up to five sections (increments of height). Each section represents either a different material (concrete, masonry, or fence), a change in thickness, or a change in reinforcing size or spacing.
For most walls, only two or three changes in stem sections are used. For example, it would be logical to create a change of section at the top of the dowels projecting into the stem from the footing and perhaps another change in section further up the wall where a more economical design is desired.
You must start the stem design here, at the base (height above footing = 0.00), where the stem moment and shear are maximum. As you manipulate the bar sizes, spacing, and position (you first, of course, will have selected a wall material and trial thickness) until the Summary box shows you an acceptable stress ratio (the higher and closer to 1.0, the more efficient).
To check the wall at a higher Design Height, such as at least the LAP REQ’D IF ABOVE distance, where reinforcing or thickness can be reduced, click the [Insert Stem] button and enter the next higher section. Advance the spin button to the desired height above the top of the footing or enter it by typing. This will move (and dim) the Bottom Section and you can now design this new section.
Continue this way, clicking [Insert Stem] button after each stem section design is completed, up to a maximum of five heights. A new Design Height should only be entered when you want to change the material, thickness, or reinforcing, and should never be less than about two foot intervals.
This is the width of the Toe of the footing, and is measured from the front edge of the footing to the front face of the stem. Can be set to 0.00 for a property line condition. All overturning and resisting moments are taken about the bottom-front edge of the toe.
Distance from front face of stem to back of heel projection. If a dimension is entered that is less than the stem width at the base, the module will automatically reset the heel dimension to at least the stem width. For a property line at the rear face of the stem, this dimension would be the stem width.
The calculated width of the footing, Toe Width + Heel Width.
Total footing thickness, NOT including the key depth (if used). For bending and shear design of the footing, the rebar depth "d" is taken as Footing Depth - Rebar Cover - ½" (to account for the rebar radius). If footing thickness is inadequate for shear capacity a red warning indicator will appear.
The footing thickness must be sufficient to allow for rebar development (for hooked dowels) plus rebar cover (adjacent to the soil). If you enter a dimension less than required for stem bar development, a red message will appear at the top of the screen. If the thickness is inadequate, increase the footing thickness, or change the stem dowels, until this message disappears.
Enter concrete compressive stress for footing.
Allowable rebar yield stress to be used for design of footing bending reinforcement.
This option is necessary since, if there is any buoyancy effect, it will reduce the effective weight of the footing concrete.
Min "As" Ratio
Enter the absolute minimum steel percentage to be used to calculate rebar spacing requirements (commonly 0.0018 Ag for Fy=60,000 psi, but code applicability for footings is arguable). If the % steel required by stress analysis is less that 200/Fy, the minimum of (200/Fy or 1.333 * bending percentage required) is calculated and compared with the Minimum As% entered here, and the greater of the two is used to calculate rebar spacing requirements.
Depth of the key below the bottom of footing. The bottom of the key is used as the lower horizontal plane for determining the size of the passive pressure block from the soil in front of the footing. Adjust this depth so the sliding safety factor is acceptable (typically a value of 1.5 is used).
Width of the key, measured along the same direction as the footing width. This is usually 12"-14", but generally not less than one-half the key depth so flexural stress in the key is usually minimal.
Enter the distance from the front edge of the toe to the beginning of the key. Do not enter a distance greater than the footing width minus key width.
Sliding Resistance Method
Enter whether sliding resistance will be by friction and passive pressure or by cohesion and passive pressure.
Soil over toe to neglect
Since the soil over the toe of the footing may be loose and uncompacted, it may have little or no passive resistance. This entry gives you the option of neglecting any or all of the Height of Soil Over Toe that you entered in the Criteria tab. You can neglect the soil over toe plus the footing thickness, if desired.
Ftg/Soil Friction Ratio
Enter the friction factor here. It is generally provided by the geotechnical engineer and usually varies from 0.25 to 0.45.
% FRICTION Usable for Sliding Resistance
This may be a stated restriction in the geotechnical report. Enter a value from zero to 100%.
% PASSIVE Usable for Sliding Resistance
This may be a stated restriction in the geotechnical report. Enter a value from zero to 100%
Lateral Forces at Base of Footing
This is the total lateral force against the stem and footing which causes the wall to slide and which must be resisted. It is the total active pressure on the heel side less the active pressure on the toe side.
less Passive Pressure Force
This uses the allowable passive pressure in pcf times the available depth (footing thickness plus soil above toe minus height to neglect) and multiplied by the percent usable that you indicated, to compute the total passive resistance. Weight due to toe surcharge, if applicable, will also be added. If a key is used, the available passive pressure depth will be to the bottom of the key.
less Friction Force
This is the total vertical reaction multiplied by the friction factor and multiplied by the percent usable that you indicated.
Added resisting force required
If this is 0.0 lbs., the forces balance, but there may be no safety factor. Watch the Sliding Factor of Safety for an adequate value (usually 1.5). Consider adding a key or revising the footing dimensions if required.
Additional force required for a 1.5 Factor of Safety
This is the additional resisting force that would be required in order to achieve a 1.5 safety factor. If this value indicates zero, then the sliding factor of safety is already greater than or equal to 1.5.
Rebar Cover in Heel & Toe
These input fields allow you to specify the clear cover that will be used at the heel and at the toe. When specifying these values, keep in mind that the toe rebar is placed closest to the bottom of the footing, and the heel rebar is placed closest to the top of the footing. When calculating the "d" dimension for bending and shear strength calculations, this module will consider the footing thickness and then deduct the specified clear cover and an additional 1/2" to account for the radius of the rebar.
Toe Reinforcing Options
This list gives you choices for reinforcing sizes and spacing for the bottom toe bars. Typically the toe bars are extensions of the stem dowels, which are bent out toward the toe. Therefore, it may be most efficient to simply verify that the bar size and spacing used for the stem dowels is within the range of the selections offered for toe reinforcing options.
NOTE: If “No reinf’ req’d” message appears, it means the flexural capacity of the footing (modulus of rupture times the section modulus, with 2” deducted from the thickness for crack allowance per code) is adequate to resist the applied moment. However, the designer in some cases may consider it prudent to add reinforcing regardless of the theoretical flexural capacity. For plain concrete per ACI 22.5.1, Fr = phi(5)(f’c)1/2, where phi = 0.55.
Heel Reinforcing Options
This list gives you choices for acceptable sizes and spacing of top heel bars. It is desirable to select a spacing that is modular with the stem dowel bars for ease of construction. Note: The module does not calculate the heel bar development length inward from the back face of the stem (where the moment is maximum).
NOTE: If “No reinf’ req’d” message appears, it means the flexural capacity of the footing (modulus of rupture times the section modulus, with 2” deducted from the thickness for crack allowance per code) is adequate to resist the applied moment. However, the designer in some cases may consider it prudent to add reinforcing regardless of the theoretical flexural capacity.
ALSO NOTE: The heel design moment may be influenced by the setting used for the item labeled "Neglect Upward Pressure at Heel for Ftg M & V" on the Options tab. See the section on the Options tab for additional information.
Key Reinforcing Options
If flexural tension is insufficient to resist bending in key, a message will appear indicating reinforcing required. You can vary the width of the key until the message disappears. If reinforcing is required options will be shown on the Footing tab.
Toe Active Pressure Used
This drop-down list box provides the option to specify whether the module should or should not apply the toe side horizontal active pressure to reduce the overturning moment and sliding force to be resisted. Typically this is NEVER used. It was added to assist in cases where the footing was buried very deeply in soil.
Slab is Present to Resist all Sliding Forces
Check this box when a slab is in front of the wall to resist lateral sliding. When this box is checked, sliding is not a design issue – passive and friction resistance are ignored -- but the lateral sliding force is displayed for checking the resistance offered by the slab.
The slab is assumed to be at the top of the footing – not higher, so selecting this checkbox will not reduce the design shear or moment in the stem.
Neglect Upward Pressure at Heel for Ftg M & V
When the user DESELECTS the option to “Neglect Upward Pressure at Heel for Footing Moment and Shear”:
|•||The program is considering the upward pressure at heel.|
|•||The upward pressure tends to reduce the moment caused by the weight of the soil and the self-weight of the heel itself.|
|•||The program is reporting the actual net moment found to exist in the heel.|
When the user SELECTS the option to “Neglect Upward Pressure at Heel for Footing Moment and Shear”:
|•||The program is NOT considering the upward pressure at heel.|
|•||The program determines the heel moment caused by the weight of the soil and the self-weight of the heel itself.|
|•||The program then applies the following logic:|
- The calculated heel moment is conservative when the upward pressure at heel is neglected.
- The calculated heel moment would act in the same direction as the toe moment.
- The sum of the heel moment and the toe moment can’t be any greater than the stem design moment, which is delivering the moment to that joint in the first place.
- So the program conservatively assumes that the heel design moment is the calculated heel moment but not to exceed the stem design moment.
Use 2010 CBC Section 1807.2.1
Section 1807.2.1 of 2010 CBC and IBC 2009 requires the designer to consider in the sliding analysis the effects of the active pressure extending all the way to the bottom of a keyway when one is used. Selecting this option will ensure that the analysis properly considers the full extent of the active pressure on a keyway.
Choices for Use of Vertical Component of Active Pressure
The vertical component of the lateral pressure is applied at a vertical plane at the back of the heel. You can choose whether or not to use this force to resist overturning, to resist sliding, and to reduce soil bearing pressure.
If you choose the option to use this force to resist overturning, then for a level backfill, the module will back-solve the EFP method to find the equivalent internal friction angle, and then apply this vertical component equal to tanß. If either the Rankine or Coulomb method had been chosen, this vertical component would be tangent of .
If you choose the option to use this force to resist sliding, then the sliding calculation will incorporate the additional frictional force that can be generated as a result of the additional vertical force.
If you choose the option named "Soil Bearing Pressure", then the the effect of this vertical component at the back of the heel will be considered in the calculation of the soil bearing pressures.
Factor Applied to Masonry f'm for Calculation of Em
The modulus of elasticity for masonry is 900 times f'm per ACI 530-05. This field allows you to enter a multiplier of other than 900 if necessary.
Multiplier Applied to CMU Weight from Tables
This entry allows you to increase or decrease the internal default values of CMU weights, as displayed on the Stem tab.
For each type of load (DL, LL, etc) the default factor will be displayed. You can change them and set new defaults, but remember to review them for a new design since they may have been changed.
These are displayed for both overturning and sliding.
Soil Pressure @ Toe and Heel
This is the resulting unfactored soil pressure for both the toe and heel. If the eccentricity is outside the middle third, the heel pressure will show 0.00. (Note: when the resultant is outside the middle-third, the module calculates the toe pressure assuming no tension at heel).
Allowable Soil Pressure
This is for your reference as input on the Criteria tab.
Total Bearing Load
This is the sum of all vertical forces.
Distance from center of footing to resultant soil pressure.
Eccentricity Within/Outside Middle Third
The resultant is outside the middle third of the footing width if the eccentricity is greater than one-sixth the footing width. (If outside the middle third, the module computes the toe soil pressure assuming no tension at heel.)
ACI Factored Soil Pressure @ Toe and Heel
Load factors are applied to all dead and live loads to determine total vertical load for soil pressure used in calculating footing moments and shears. This load is then applied at the same eccentricity calculated for service load soil pressures to yield the actual factored soil pressures for footing design using ultimate strength design principles. Note that since only factored vertical loads are applied at the non-factored resultant eccentricity, a true 1.6 load factor applied to lateral earth pressure is not used for footing design. If resultant vertical load eccentricity were to be calculated using factored loads, the distance would not truly represent a correct state of stress in the soil. ACI load factors are intended to give conservative results for stress. Calculation of a factored load eccentricity would give soil pressure diagrams that would not always represent the actual soil pressure distribution under the footing, and yield unreasonable results. Factored lateral earth pressure, however, is always used for concrete stem design.
Mu Design @ Toe/Heel
These are the factored (by 1.2) moments at face of stem for toe and heel moments. Since neither can be greater than the stem base moment (factored if concrete stem), the latter may govern. These moments will be reduced if you choose to neglect the upward soil pressure on the Options tab.
A message will indicate which controls.
Shear @ Toe and Heel
The actual shear is calculated from the one-way action in the footing at a distance "d" (footing thickness - rebar cover) from the toe side of the bottom stem section, and at the face of the stem on the heel side. If "d" is greater than the projecting toe or heel length, then the one-way shear is zero.
Allowable Footing Shear
The allowable unit shear equals (0.75 * 2 * f'c½).
RSM - RESISTING MOMENTS TAB
This screen presents in tabular form each component contributing to resisting moment, giving weights and lever arms from the front edge of the toe to the centroid of the weight.
For calculating the vertical component, if checked on the OPTIONS screen, and if the EFP method was chosen, the module will back-solve using the Rankine formula to obtain an equivalent internal friction angle.
The force and moment displayed at the bottom accounts for deduction of effect of vertical component, if box on Options tab has been checked.
OTM - OVERTURNING MOMENTS TAB
This screen presents in tabular form each component acting horizontally to overturn the wall/footing system. The centroid of each force is multiplied by its distance up from the bottom of the footing. The Heel Active Pressure includes the effect of surcharges and water table, if applicable, and its Distance is to the centroid of the total lateral force.
The total overturning moment is displayed, along with the Resisting/Overturning ratio. The overturning moment is reduced by the toe side active pressure, if this option is selected on the Options tab.