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Loads

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Surcharges

 

This surcharge is treated as additional soil weight – if the surcharge is 240 psf and the density is 120 pcf, then the program 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 passive pressure at the toe. Consider this if modeling a point load toe surcharge.

 

When a heel surcharge is defined, it is considered to be uniformly applied to the top surface of the soil over the heel. It may be entered whether or not the ground surface is sloped. This surcharge is always taken as a vertical force. This surcharge is divided by the soil density and multiplied by the Active Pressure coefficient to create a uniform lateral load applied to the wall. You can choose to use this surcharge to resist sliding and overturning by checking the option box adjacent to the load input field. Typical live load surcharges are 100 psf for light traffic and parking, and 250 psf for highway traffic.

 

Both the toe surcharge and the heel surcharge have associated checkboxes that can be used to dictate whether the respective surcharges should be considered as resisting sliding and overturning of the wall.

 

 

Axial Load Applied to Top of Stem

 

These loads are considered uniformly distributed along the length of the wall. They are applied to the top of the topmost stem section. 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. AVOID A HIGH AXIAL LOAD (say over 3 kips plf Total Load) SINCE IT COULD CAUSE A REVERSAL OF BENDING IN THE HEEL.

 

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.

 

Consider a point load (such as a beam reaction) applied to the top of a wall. The intensity of that point load will decrease at locations that are more distant from the point of application, because the lateral distribution width will increase as one moves away from the point of application.  For this reason, the intensity of the axial load felt at the base of the stem will be significantly less than the intensity immediately beneath the beam bearing. To account for this effect, the magnitude of the axial point load entered should be reduced proportionately (since the input actually represents a uniformly distributed load along the length of the wall). But the top of the wall may need to be checked for localized stress by appended calculations.

 

The input for axial load applied to the top of the stem allows the load magnitudes to be defined as either Dead Load or Live Load.  The load will be factored accordingly.  This type of load also allows the specification of an eccentricity value, where the eccentricity is defined 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  accepted in the Restrained Retaining Wall module, where tension is already expected on the toe side.  But negative eccentricities are not accepted in the Cantilevered Retaining Wall module.

 

 

Adjacent Footing Data

 

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 Reference Diagram for locations where input measurements should be taken.

 

Adjacent Footing Loads will be factored by the Live Load factor for strength design.

 

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 program uses equation (11-20a) in Bowles’ Foundation Analysis and Design, 5th Edition, McGraw-Hill, page 630.

 

When the Boussinesq analysis is used, the program may require additional computing time (hundreds of internal calculations are done after each entry), depending upon the speed of your computer. 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.

 

 

Footing Width: 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.

 

Footing Eccentricity: 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 is toward the toe, resulting in greater pressure at the side of the adjacent footing closest to the stem. (An eccentricity value of zero means that the adjacent footing load will be considered to act at the center of the adjacent footing.) The program 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.

 

Wall to Footing Centerline Distance: This is the distance from the center of the adjacent footing to the back face of the stem at the retained height. The nearest edge of the footing should be at least a foot away from the wall face – otherwise suggest using an equivalent heel surcharge instead. Do not use a horizontal distance greater than the vertical distance from the top of the footing to the bottom of the adjacent footing, since the effect on the wall will be insignificant.

 

Footing Type: This drop down menu selection allows you to enter either an isolated footing using the "Square Footing" selection, or a continuous footing using the "Line Load" selection.

 

Footing Base Above/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 elevation of the soil measured at the back of the wall. A positive entry would typically only be used when the soil is sloped and the footing resides "uphill" from the retained height elevation. 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 Boussenesq analysis may be used for the vertical load applied to the soil from the adjacent retaining wall footing, 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 program. Caution is urged for this condition. See discussion in the companion book: Basics of Retaining Wall Design. The designer should be advised that the program does not incorporate  any form of global stability analysis.

 

Poisson’s Ratio: Since the resulting pressures are sensitive to Poisson’s Ratio, there is an entry allowing you to specify 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.

 

 

Applied Lateral Load on Stem

 

This input allows you to specify an additional uniformly distributed lateral load applied to the stem. This is generally not the preferred method of applying seismic load.  Use the Seismic sub-tab instead.

 

This entry can be useful for a point load, such as due to an impact of a car or similar force.  When used in this way, it may be easiest to enter the load as a one-foot high increment, and specify the "Height to Bottom" and "Height to Top" to define a one-foot high strip of application.

 

This load will be factored by whatever value is specified in the adjacent Load factor input. To apply an additional factor (such as an impact factor), increase the applied load proportionately (e.g. an impact load of 1000 lbs requiring an impact factor of 2.0 would be entered as 2,000 lbs). You may need to do several designs to check multiple load combinations.

Use engineering judgment when applying a point lateral load.  The magnitude may be able to be reduced to account for the fact that the load distributes horizontally at levels below the point of application, so its intensity reduces at elevations below the point of application.

 

Height to Top: Defines the upper extent of the applied lateral load measured from the top of the footing. Do not enter a dimension higher than the top of the wall ("Retained Height" plus "Wall height above retained soil").

 

Height to Bottom: Defines the lower extent of the applied lateral load measured from the top of the footing.

 

Load FactorWill be applied to the lateral load when performing strength design checks.  It is not applied for service level load checks such as sliding, overturning, or  soil bearing pressure checks.

 

Wind on Stem above Soil: 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 generate sliding force, overturning moment, stem design moment and shear, and soil pressures.  

 

Wind Type: Note that recent building codes have started to determine wind forces at the strength level as opposed to at the traditional service level.  Consequently the program allows the user to indicate whether the specified wind pressure is at the Strength-Level or at the Service-Level.

 

 

When performing a design based on IBC 2012 / CBC 2013 or later:

 

The wind should be entered as a Strength-Level load.

When designing a masonry stem by strength design methodology or a concrete stem, the wind load factor (which should be 1.0) will be applied to the specified wind loads.  

When designing a masonry stem by ASD methodology, the wind load factor is not used, and the specified wind loads will be reduced to a service level by multiplying the specified pressure by 0.6.

Regardless of the stem construction, when determining service-level soil bearing pressure and when performing sliding and overturning checks, the specified wind loads will be reduced to a service level by multiplying the specified pressure by 0.6.

 

 

When performing a design based on codes earlier than IBC 2012 / CBC 2013:

 

The wind should be entered as a Service-Level load.

When designing a masonry stem by strength design methodology or a concrete stem, the wind load factor (which should be 1.6) will be applied to the specified wind loads.  

When designing a masonry stem by ASD methodology, the wind load factor is not used, and the specified wind loads will be used exactly as specified.

Regardless of the stem construction, when determining service-level soil bearing pressure and when performing sliding and overturning checks, the specified wind loads will be used exactly as specified.

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