Pad base design workflow
The small concrete building model shown below will be used to demonstrate the pad base design process.
The model has already been designed prior to placing the bases.
Apply pad bases under supported columns
Pad bases can only be placed under, and be loaded by supported columns; strip bases can only be placed under and be loaded by supported walls.
If a ground beam is attached to the same support, loading from the beam will also be considered in the base design.
At this stage, as you are not aware of the individual base size and depth requirements; you can simply choose to place the bases where required, accepting the default size/depth offered.
Auto-size pad bases individually for loads carried
To obtain an idea of the range of potential sizes, bases should initially be designed individually for their respective loads, as follows:
- Access Design Groups page of the Design Settings dialog box to ensure that group design is turned off for Isolated Foundations.
- Select the bases to be auto-sized and in the Properties window and choose to auto-design both the size and depth; In this way the program
establishes suitable base dimensions for you.
Similarly, the reinforcement can be set to be auto-designed in the same manner.
- From the
Foundations tab click
Design Pad Bases.
Each base will be sized accordingly (any that are not in auto-design mode will simply be checked).
With the auto-design options cleared, you can then adjust individual base dimensions and re-check if required (by right-clicking the base that has been edited and choosing Check Member).
The site boundary may impose restrictions on the positioning of an isolated foundation relative to the column/wall it supports. This restriction may result in a requirement for an offset base, this can be achieved by specifying the eccentricity required in the base properties.
The overall auto-design procedure is summarized as follows:
- Bearing Design: - increase size
- Bending Design: - increase reinforcement - If max allowable reinforcement is reached then increase depth, set reinforcement back to start point, and go back to step 1.
- Shear Design - increase depth, set reinforcement back to start point, and go back to step 1
- Punching Shear Design - increase depth, set reinforcement back to start point, and go back to step 1
- Sliding Checks - increase depth, set reinforcement back to start point, and go back to step 1
- Uplift Checks - increase size, set reinforcement back to start point, and go back to step 1
At every stage, if the max allowable depth is reached the design fails.
Apply grouping to rationalize pad base sizes
Grouping can only be applied to pad bases - not to strip bases.
Once pad bases have been sized individually, the designs can be rationalized by activating grouping, in order to obtain one design per group sufficient for all bases within the group.
This is done as follows:
- Select a base that you want to be in a particular group.
- In the Properties window, ensure it is set to auto-design.
- Right-click the same base and from the context menu choose Create Property Set...
- Select all the other bases that you want to be in the same group.
In the Properties window, click Apply... to apply the property set you have just created to the selected bases.Note:
Ensure you click Apply... from the Properties window and not from the right-click menu, otherwise the properties will only be applied to the last base selected.
- From the Groups page of the Project Workspace, right-click Pad Bases (under the Design branch) and choose Regroup Members - this will put those bases that share similar properties into the same group.
- Open Design Settings dialog box, and from the Design Groups page select the option to design isolated foundations using groups.
Click Design Pad Bases - the results obtained will reflect the grouping that has been applied.
Review/optimize base design
In the Review View you can examine the design efficiency by switching from Foundations Status to Foundations Ratio. Note that the tool tip also indicates the base size and status as you hover over any base.
If the selections are unacceptable you may need to review the settings in Design Settings> Concrete> Foundations.
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
Create a model report that includes the concrete pad base design calculations that have been performed. (The default Building Design report includes these along with design calculations for other member types in the model).