Mat foundation design workflow (metric units)

Tekla Structural Designer
2021
Tekla Structural Designer

Mat foundation design workflow (metric units)

The small concrete building model shown below will be used to demonstrate the process for modeling and designing a mat foundation.

For mat foundations the overall procedure is basically the same as that for flat slabs, with the exception that an extra check for mat bearing pressure is required.

Design the structure before supporting it on the mat

The model should already be designed and member sizing issues resolved prior to placing the mat foundation.

Determine the soil parameters

Unless you are going to define discreet piled supports, the mat will need to be supported on ground bearing springs. These are activated by selecting Use Ground Bearing Springs (located under the Slab parameters > Soil Parameters sub-heading in the mat properties).

When these are activated you have then to also specify:

  • Allowable bearing pressures
  • Ground stiffness type (Linear, or non linear spring)
    • Linear ground stiffness, or,
    • Non-linear ground stiffness (+/-) and tension/compression limits
  • Horizontal support

Allowable bearing pressures

When using ground bearing springs, a check is performed as part of the design process to ensure the allowable bearing pressure you enter is not exceeded.

Ground stiffness type

Both linear and non-linear ground springs can be defined, although in the majority of cases it is suggested that linear springs should suffice.

When linear spring are applied:

  • Allowable bearing pressures are checked
  • Uplift (tension) is checked
  • If no problems then linear springs are sufficient

When non-linear springs are applied:

  • You can have compression only
  • And also capped compression
  • Either way analysis takes longer
Note: For compression-only supports ( -ve stiffness set to zero ) a loadcase of lateral load only will never solve for Non-linear analysis. What happens in the solver is what would happen in reality if Gravity suddenly turned off! Structures stabilized by gravity load would flip over in the presence of lateral load - i.e. no static equilibrium position exists to be found.

Ground stiffness

You are required to enter an appropriate stiffness for the soil conditions on site.

The actual value entered is your responsibility, but as a guide the table below1 illustrates the potential range of values that might be considered.

Material Lower Limit (kN/m3) Upper Limit (kN/m3)

Loose Sand

4,800 16,000
Medium Dense Sand 9,600 80,000
Dense Sand 64,000 128,000
Clayey Medium Dense Sand 32,000 80,000
Silty Medium Dense Sand 24,000 48,000
Clayey Soil (qa<200kPa) 12,000 24,000
Clayey Soil (200<qa<800kPa) 24,000 48,000
Clayey Soil (qa>800kPa) 48,000 200,000
  1. Reference: "Foundation Analysis and Design", Joseph E. Bowles, 1995 (Table 9-1)

Horizontal support

The degree of horizontal support provided by the ground springs can be set as:

  • Fixed
  • Free
  • Spring (default 20% of vertical spring stiffness)

If set to "Free" a mechanism will result unless you provide additional discreet supports.

Determine the remaining mat properties

You are required to manually specify the Reduce live loads by (US head code), or Reduce imposed load by (other head codes) mat property. This could vary for individual mat panels, depending on the columns and walls attached to each panel, and the number of floors they are supporting.

If using an "area" method of mat creation you will also need to specify the amount of slab overhang.

The remaining properties are then similar to those used to define a typical concrete flat slab.

In this example the minimum area method is used to create a mat with the following properties:

  • Imposed/live loads reduced by 30%
  • Default overhang
  • Mat thickness 600mm
  • Ground bearing springs used
  • Default allowable bearing pressures
  • Default linear spring properties

Create the mat

To create the mat:

  1. Choose the method, (e.g. Minimum Area)
  2. Enter the mat properties, (see above)
  3. Click on those columns (or walls) that define its perimeter,
  4. Either press <Enter>, or re-click on one of the selected columns to complete the mat definition.

Note: The "Mesh 2-way slabs in 3D Analysis" option gets activated automatically for the level in which the mat is created, enabling the 3D analysis model to be supported on the ground springs.

Enable soil structure interaction

When not supported by a mat, columns and walls typically have supports at their bases.

When a mat is introduced these supports must be removed - as the mat now supports the whole building on ground bearing springs. Consequently adding a mat means re-analysis and hence re-design of the whole building.

Note: A simple technique for detecting and removing the supports is described in the next section "Model Validation".

Inherent in the re-design is the inclusion of "soil structure interaction" (or support settlement). In the past this has often been ignored, even though design codes suggest that it should be considered.

If you want to include design for situation without support settlement then you need to think about the workflow sequence or use runs with differing ground stiffness assumptions.

Note however that soil structure interaction only affects the 3D analysis model results, and the chasedown results in the lowest sub-model. Members in all other sub-models are thus already being designed both with and without the affects of support settlement.

Model validation

Once the mat has been placed it is worth running a validation check from the Model ribbon at this stage - specifically to check for potential conflicting supports.

A "Supports exist within area of Mat Foundation" warning is issued if member supports conflict with ground springs. (This can be remedied by right-clicking on the warning and choosing Delete Items).

Perform the model analysis

Analysis is required to establish the bearing pressures and the moments to be used for the mat design.

Gravity, lateral and seismic combinations can be analyzed from the Design ribbon by running Design All (Static) and any seismic RSA combinations by running Design All (RSA). These processes will also recheck all the member designs taking account of the effects of soil structure interaction.

In each of the 3D, FE chasedown, and grillage chasedown analyses that get performed mats are modeled as meshed 2-way slabs supported on ground bearing springs.

In the FE chasedown and grillage chasedown models the mat and first level above the mat are always combined in a single sub-model.

Note: In a large model, if you anticipate several analysis might be required to determine the size of mat required, you might find it more efficient to just run the analyses that are required from the Analyze ribbon then re-run the member design at a later stage.
Note: For compression-only supports ( -ve stiffness set to zero ) a loadcase of lateral load only will never solve for Non-linear analysis. What happens in the solver is what would happen in reality if Gravity suddenly turned off! Structures stabilized by gravity load would flip over in the presence of lateral load - i.e. no static equilibrium position exists to be found.

Check foundation bearing pressure and deformations

You can check the mat bearing pressure and 2D deflection contours from the Results View for each of the analysis types that have been performed before commencing the detailed design.

By viewing the 2D deflection results for combinations based on "service" rather than "strength" factors the stiffness adjustments that you apply (via Analysis Options> Modification Factors> Concrete) do not need to account for load factors.

The default stiffness adjustments are dependent on the design code. For design to EC2 the default adjustment factor applied is 0.2. For design to ACI the default adjustment factor applied is 0.25.

Re-perform member design

Depending on the steps that have been taken to establish the mat size, at this point the design condition reported in the Status Bar for the members will either be "Valid" or "Out of Date".

Unless the status is "Valid" you should recheck the member designs (taking into account the effects of soil structure interaction) by running Design All (Static) from the Design ribbon.

Note: Similarly if an RSA design has previously been performed, but is now out of date Design All (RSA) should be re-run.

If any concrete members now fail, you can switch them back into Autodesign mode, (ensuring Select bars starting from is set to Current rather than Minima) and then design again. This ensures that the bars that were satisfactory for the original design are only ever increased not reduced.

Open an appropriate view in which to design the mat

In models with mats at distinct floor levels you should use 2D Views to work on the mat design one level at a time. Occasionally the "3D geometry" of a model may make it less easy to distinguish between individual floors, in which case it may be easier to design the floors one sub-model at a time.

Note: When working in a 2D View use the right-click menu to design or check mats and patches: this saves time as only the mats and patches in the current level are considered. Running Design Mats or Design Patches from the Foundation ribbon may take longer as it considers all the mats and patches in the model.

Add patches

This is an interactive process - requiring a certain amount of engineering judgment.

When placing patches under walls you can choose to place a single patch along the wall, an internal patch under the middle, or end patches at the wall ends (as shown above).

Typically at the initial patch creation stage you should make the patches a reasonable size and not be concerned if they are a bit too big - as this will be reviewed/resolved at the patch design optimization stage.

Note: Whilst the command to add Patches appears on both the Foundations and Design ribbons, the former defaults to creating patches on the bottom surface, the latter to the top surface. For mats the Foundations ribbon default will generally be correct.

Design mats

Note: Mat design is dependent on the areas of patches (patch areas which are excluded from mat design) - hence patches should be added before mats are designed.

To design multiple mats, either:

  • From the Foundations ribbon run Design Mats in order to design or check all the mats in the model (each according to their own autodesign setting), or,
  • If you have created mats at multiple levels you may prefer to work on one level at a time. To do this, open a 2D View of the level to be designed then right-click and choose either Design Slabs or Check Slabs.
    Note: When accessed from the right-click menu, these commands operate as follows:
    • Design Slabs will re-design slabs and mats (potentially picking new reinforcement) regardless of the current autodesign setting.
    • Check Slabs will check the current reinforcement in slabs and mats regardless of the current autodesign setting.

Review/optimize mat design

It is suggested that you use split Review Views to examine the results. You could arrange one view to show Mat Design Status, and then a second view to show Slab Reinforcement in the panels. Note that the tool tip indicates all panel/patch reinforcement as you hover over any panel.

If the selections are unacceptable you may need to review the design settings. e.g - if 8mm dia bars are selected and you would just never use anything less than a 10mm bar in a mat, then set that as a minimum.

Once the selections are reasonable it is advisable to select all the panels and swap them out of auto-design mode (and after this point be careful not to use the right-click option to design panels unless you really want to.)

Note: Auto-design will select the smallest allowable bars at the minimum centers necessary to provide sufficient reinforcement area. Larger bars are only tried when the minimum permissible spacing is reached. Design options allow you to control the minimum acceptable bar sizes and spacings. It is important to use these settings so that the reinforcement established in the slabs is sufficiently spaced to allow introduction of additional reinforcement in the patches. For example you might set a "minimum spacing (slab auto design)" = 150mm.

After using the Review View update mode to standardize reinforcement you can then run Check Slabs from the right-click menu to check the revised reinforcement.

Remember:

  • Consider swapping between Status and Ratio views - if utilizations all < 1 but some panels failing then problems are to do with limit checks. The Ratio view is better for helping you focus on the critical panels.
  • During this process it will also make sense to be adding panel patches in which the reinforcement is set to none and strip is set to average. The purpose of this is to smooth out local peaks at the most critical locations which would otherwise dictate the background reinforcement level needed to get a pass status.

Design patches

Having established and rationalized the slab panel reinforcement which sets the background levels of reinforcement, it is now logical to design the patches which will top up the reinforcement as necessary within the strips of each patch.

To do this, either:

  • From the Foundations ribbon run Design Patches in order to design or check all the foundation patches in the model - by default newly created patches will all be in auto-design mode - so reinforcement is selected automatically, or,
  • In the 2D View of the level which you want to design right-click and choose either Design Patches or Check Patches. Working in this way restricts the design or checking to the patches in the current view.
    Note: When accessed from the right-click menu, these commands operate as follows:
    • Design Patches will re-design the patches (potentially picking new reinforcement) regardless of the current autodesign setting.
    • Check Patches will check the current reinforcement in the patches regardless of the current autodesign setting.

Review/optimize patch design

At this stage the patch sizes can be reviewed:

  • Wall patches - can the width be adjusted (minimized)?
  • Column patches - Is the size reasonable? - this relates to code guidance about averaging in columns strips - it is not reasonable to average over too wide a width. If in doubt safe thing to do is err on the side of caution and make them a bit smaller.
  • Having got the sizes sorted out and the patches re-designed, swap them out of auto-design mode.
  • Now click Slab Reinforcement in the Review View to review and standardize the patch reinforcement. For instance in column patches this might include forcing the spacing of the mat reinforcement to be matched (if the mat has H12-200, then in the patch don't add H12-125, swap to H16-200 - then there will be alternate bars at 100 crs in this strip of the patch.

Add and run punching checks

Punching checks require slab reinforcement to be defined/known in order to determine punching capacities.

Punching checks can be added individually, or by windowing over an area. See: Create punching checks

You can then select any check and review the properties assigned to it.

Once added click Design Punching Shear. See: Design and check punching shear

punching_status.png

The checks are done and status is shown as:

  • Pass - if no shear reinforcement is needed
  • Warning - if shear reinforcement is needed
  • Fail - if it is impossible to achieve required capacity by adding share reinforcement
  • Unknown - if check not run yet
  • Beyond scope or error - if for example the centroid of the column/wall lies outside the mat

Create drawings and quantity estimations

Drawings that convey the structural intent are easy to create. It should be borne in mind that these are NOT the final detail drawings, their purpose is to eliminate the need for manual mark-up drawings as a means of communication between the engineer and the detailer

Print calculations

Create a model report that includes the panel, patch, and punching check calculations that have been performed. (The default Building Design report includes these along with design calculations for other member types in the model).

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