Continuous Strip Footing With Stem Wall Stem Wall

Wall Footings - Modeling

Here we will talk about the steps in modeling a wall footing and get into advanced wall footing modeling considerations. We will also talk about which type of wall footing would be appropriate for your application.

See the Wall Footing Definitions topic for more information on defining wall parameters.
See the Wall Footings - Design topic for more information on calculation details.
See the Wall Footing Results topic for more information on the output.

Wall Footing Types

There are three types of wall footings possible in RISAFoundation: a retaining wall, a strip footing and generated slab/stem wall combination. See the Draw Wall Footings section for how you select each option.

Retaining Wall

This is the traditional retaining wall. This type of wall will perform all stability checks (sliding, overturning and soil bearing) due to hydrostatic pressures and any superimposed load on the wall. This wall type is a single entity in that the wall and footing are considered together and defined in the Wall Footing Definitions spreadsheet.

Strip Footing (Gravity)

This is a simplified version of the retaining wall and would be used in cases where you have a simple strip footing layout. Hydrostatic pressures on the wall are not a concern here. This type of wall footing pares down the input and output in the program, removing a lot of the complexity necessary for retaining walls, but not required for a simple strip footing. This wall type would generally be used for simple gravity loads. This wall type is also a single entity in that the wall and footing are considered together and defined in the Wall Footing Definitions spreadsheet.

Strip Footing (Lateral/Custom)

This is a wall type that is really a generation utility that will create a slab, slab design strip(s), and a stem wall to represent your wall footing. This type of footing would not be controlled by the Wall Footing Definitions, but rather by each individual entity (slab, slab design strip and stem wall) separately. This option generates these elements one time and then allows you to edit/modify these elements as necessary.

The main purpose of this type of wall is to allow for any customization and to allow the program to perform a finite element analysis of the footing itself. The other two wall types do not perform an FEA on the footing itself.

Draw Wall Footings

Once you've decided which wall type you want (Wall Footing Types) you can select the appropriate option here. To draw a wall footing click the Draw or Modify Wall Footings button and select the Draw Wall Footings tab. Choose the wall type, including the Definition if applicable.

If you are modeling a Retaining Wall or Strip Footing (Gravity Only), click OK to immediately start drawing wall footings by clicking the start and end location of the wall footing in the graphic view.

If you are modeling a Strip Footing (Lateral/Custom), you have the option to make the wall footing continuous by checking Keep Wall Continuous, then click OK and you will be brought to a Strip Footing Generator dialog where you can input the properties of your composite slab, slab design strip and stem wall entity. This dialog is similar to the Wall Footing Definition Editor.

After saying OK in this dialog you can then start drawing wall footings by clicking the start and end location of the wall footing in the graphic view.

Modify Wall Footings

How you modify Retaining Walls and Strip Footings (Gravity Only) can be done from the information below. Modifying Strip Footings (Lateral/Custom) requires you to edit the individual entities (slab, slab design strip(s) and stem wall) that make up that wall footing.

For non-custom wall footings you may view and edit the wall footing data in the Wall Footings Spreadsheet or you can use the Modify Wall Footings tool to graphically modify multiple wall footings at once.

The graphical Modify Wall Footings tool discussed here lets you modify the properties of wall footings that already exist in your model. To use this, specify the properties you want to change, then select the wall footings you want to modify.

The Use? check boxes next to the data fields indicate whether the particular parameter will be used or not when the modification is applied. If the box next to a field is checked, that parameter will be applied to any selected retaining walls. If the box is NOT checked, the parameter will NOT be applied, even if a value is entered in the field. This lets you easily change one or two properties on wall footings without affecting the rest of the properties.

To modify a wall footing click the Draw or Modify Wall Footings button and select the Modify Properties tab. Then set the parameters that you want to change. Check the Use? box for the items to apply.

If you have drawn your wall footing so the soil is on the wrong face of the wall you can use the Out of Plane Flip option. This will flip the start and end nodes of the wall so that the wall footing is flipped to the correct orientation.

Wall Footings Spreadsheet

To access this spreadsheet click the Wall Footings button on the Data Entry toolbar or choose Spreadsheets - Wall Footings from the drop-down menu.

Label

This gives a unique label to each wall footing in the model.

Start Point/End Point

This defines the start and end location points for the wall footing and can be modified here.

Definition

This is the Wall Footing Definition to be used. This setting gives all of the other information required for design of the wall footing.

Wall Footing Modeling Tips

Here are some tips to think about when modeling wall footings.

Wall Footing Orientation

When drawing wall footings, click on two points to define the length of the wall footing. The wall (with footing) will be drawn so that the centerline of the wall lines up along the two nodes defining the wall. These elements, though drawn in a 3D rendering, are really just line elements.

Figure 1: Plan View of Wall Footing

When drawing a wall footing the orientation of the local axes for the wall will always have the local x-direction parallel to the line that defines the two nodes drawn.

Figure 2: Isometric View of Wall Footing

Here we can also see the orientation of the positive z-axis and that the soil side of the wall footing is oriented on the positive z-axis. So you must keep this in mind when drawing your walls.

If you are drawing basement walls for a structure then make sure to draw in a counter-clockwise fashion. This will ensure that the soil is on the exterior side of the walls. Note that this is only applicable for Retaining Walls.

Figure 3: Basement Walls Drawn Counter-Clockwise

If you have drawn your wall panel so the soil is on the wrong face of the wall you can use the Out of Plane Flip option on the Modify Wall Footings dialog. This will flip the start and end nodes of the wall so that the retaining wall is flipped to the correct orientation.

Load Categories

Once you have defined all of the required properties in the Wall Footing Definitions spreadsheet, the program calculates the weights, hydrostatic loads, soil pressures, etc, and applies them to the wall for analysis and design purposes. The program also places these loads in load categories for you automatically.

Dead Loads (DL): weight of concrete and weight of soil

Live Loads (LL): surcharge loads (applicable only to retaining walls)

Hydrostatic Loads (HL): Any lateral pressure/hydrostatic loads (applicable only to retaining walls).

Note:

Any loads applied to the top of the wall footing will be considered as the load category they are applied as.
If you have a sloped backfill on your retaining wall, then a portion of the hydrostatic loads will be applied in the vertical direction of your wall. The cosine of these forces will be applied horizontally as a pressure. The sine of these forces will be applied as a vertical force that is conservatively lumped at the top of the wall.
Water pressure (if there is a water table present) will always be assumed to act horizontally, even in the presence of a sloping backfill.
It is possible for the horizontal component of hydrostatic force to act ABOVE the height of the wall. All loading that occurs above this height will be lumped as a point load right at the top of the wall.

Figure 4: Hydrostatic Loads with Sloped Backfill

Loading of Wall Footings with Externally Applied Loads

Wall footings will accept externally applied loads to them, but only loads applied to the top of the wall. To apply loads to a wall footing you must apply the loads to the centerline of the wall at the base of the wall.

Figures 5A & 5B: Wall Footing with External Loads

In the figure shown above, we can see (from the red lines) how the loads were applied and how they ended up on the wall footing. The distributed load applied along the centerline of the wall ended up applied to the top of the wall and will be analyzed/designed as such. The other load did not line up with the centerline of the wall, so it will not be considered at all in the wall footing design.

Partial length distributed loads (even trapezoidal) and point loads work in the same fashion.

Figure 6: Wall Footings with Trapezoidal Dist. Loads and Point Loads

Looking at the model in a Plan View while drawing can make modeling these loads much easier.

Figure 7: Wall Footing Loading shown in a Plan View

Note:

If you wish to apply loads adjacent to the wall to work as surcharge loading, apply the loads in the Wall Footing Definitions - Soil tab in the Heel Surcharge entry.

If you have two wall footings adjacent (defined by the same node), then applying the load to model will always place that load on the wall footing that comes first in the Wall Footings spreadsheet.

Figure 8: Two Adjacent Wall Footings with a Point Load

Connectivity with Other Elements in the Model

Non-custom Wall Footings in RISAFoundation have no connectivity to any other elements in the model, not even adjacent retaining walls. So while it is possible to connect beams and place slabs adjacent to the wall footing, no forces will be transferred between the two. Here are some examples where wall footings and other elements may seem to come into contact with each other.

Beams and Wall Footings: Here it appears that the wall footing and a beam are in the same space. However, if any load was applied along the line where the beam and the wall footing overlap, the load would only be taken by the beam and none of the load would be seen by the wall footing.

Figure 9: A Beam Overlapping with a Wall Footing

Beams and Slabs: Slabs can be drawn so that they are adjacent with wall footings. Any load that is defined on the area of overlap of a slab and a wall footing will only be taken by the slab. The wall (and footing) will have no interaction with the slab.

Figures 10A & 10B: A Slab Overlapping with a Wall Footing

Other Items that Overlap: Many other overlapping situations in the program are not handled and an error will occur at solution.

Figure 11: A Slab Drawn Overlapping the Wall Portion

Figure 12: Two Wall Footings Where the Walls Cross

Figure 13: A Pile, Pile Cap, or Footing, that Intersects the Wall Footing Centerline

taylortose1960.blogspot.com

Source: https://risa.com/risahelp/risafoundation/Content/Foundation_Only_Topics/Wall%20Footings%20-%20Modeling.htm