Piled mat foundation design workflow (US customary units)
The following example illustrates the typical process to model and design piles in a piled mat foundation.
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.
In order to retain the existing reinforcement design all members should be set to check mode. Alternatively you might choose to "check and increase" the reinforcement instead, (by leaving members in autodesign mode with the option to select bars starting from current.)
Create the mat
As piled mats are typically designed on the basis that they are supported on the piles alone, in most situations you would generally clear Use Ground Bearing Springs (under Soil Parameters in the mat properties.)
In this example the minimum area method is used to create a mat with:
- An overhang of 40in.
- Mat thickness 24in.
- The Use Ground Bearing Springs option cleared
Define the pile catalogue
The pile catalogue should be created prior to placing piles in the mat. You are required to define the pile properties in the catalogue manually.
In this example it is assumed this structure is stable horizontally so that the pile type horizontal restraint is left as Fixed.
Add piles to the mat
Although piles can be added individually, it is often more efficient to add multiple piles at regular spacings in the form of a pile array.
These can either be placed vertically or on an incline if required.
A preview of the array is displayed making it easier to adjust the array properties before placing the layout.
After piles have been placed as an array, you are then free to edit individual pile positions, or delete them as required.
Remove existing column and wall supports
When not supported by a mat, columns and walls typically have supports at their bases.
When a piled mat is introduced these supports must be removed - as the mat now supports the whole building on pile springs. Consequently adding a piled mat means re-analysis and hence re-design of the whole building.
Inherent in the re-design is the inclusion of support settlement. In the past this has often been ignored, even though design codes suggest that it should be considered.
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.
Once the piles have been placed it is worth running a validation check from the Model ribbon to check for potential conflicting supports.
Before the mat was placed the structure would have required supports underneath columns and walls for it to be designed. As support is now being provided by the piles the member supports are no longer required and should therefore be removed.
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 axial forces in the piles and the moments to be used for the mat design.
Gravity, lateral and seismic combinations can be analyzed by running Analyze All (Static), and any seismic RSA combinations by running 1st or 2nd Order RSA Seismic.
In each of the 3D, FE chasedown, and grillage chasedown analyses that get performed mats are modeled as meshed 2-way slabs, either supported on ground bearing springs, or on discreet piled supports, (or a combination of both).
In the FE chasedown and grillage chasedown models the mat and first level above the mat are always combined in a single sub-model.
Perform the pile design
The piles are checked (and the mat is designed) by running Design Mats from the Foundations ribbon.
Review the pile design status and ratios
You can display the Pile Status and Pile Ratios from the Review View in order to determine if any remodeling of piles is required.
In this example a number of piles are not heavily loaded so you could consider deleting some of them, or switching the pile type that has been applied.
If you make any changes, to see their effect simply re-run Analyze All followed by Design Mats once more.
At any stage if you reapply a pile array to a mat any previously existing piles in the mat are removed.
Design results can be examined in more detail by selecting Tabular Data from the ribbon and setting the View Type to Design Summary. By selecting Piles as the Characteristic (and the appropriate material) the full results for each pile can be examined.
Add and run pile 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.
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
Punching shear resistance is assumed to be provided by the concrete alone - there is no option to add specific punching shear reinforcement in the form of studs and rails (as there is for column punching checks, including those supported by mats).
The check considers 3D Building Analysis, FE Chase-Down and Grillage Chase-Down results for all active gravity, wind, seismic and RSA load combinations.
The check considers only a single pile - not a pair - and only vertical shear not moment (as piles are modeled a pinned spring supports without moment fixity).
Just as for punching checks of columns supported by piled mats, all loading and reactions (from ground bearing springs) within the punching perimeter are considered.
There is an additional pile-specific Design setting to use the pile capacity in the punching check in Design Settings > Concrete > Cast-in-place > Foundations > Mat Foundations > Piles (default Off).
Perform the mat design
The Design of the mat itself is described in the Mat foundation design workflow (US customary units) topic.