Slab Deflection and Cracking Analysis

Note: Before carrying out a long term deflection and cracking analysis for a slab, it is necessary to ensure that a full FE Slab design has been carried out and that all ultimate limit state design checks are satisfactory.

Basic Controls - Adding, Copying and Deleting Basic Regions

The basic controls for adding, copying, deleting and selecting basic regions are the icons in the Basic Reinforcement Sets area. These icons are:

  - Add a new Basic Region

 - Copy a region (automatically adds a new region with the same settings as the copied Basic Rebar)

 - Delete a region

 - Select all regions

 - Invert current selection

 - List visible items only (lists only those basic regions currently displayed in the graphics window

Creating a Slab Deflection and Cracking design brief

A design brief consists of the design parameters and loading events that are to be applied as part of the long term deflection and cracking design to a slab. For any slab in a model, to carry out a deflection and cracking check on that slab, a design brief must be associated with that particular slab so each slab to be checked must have at least one design brief that applies to it. It is possible, however, to apply multiple design briefs to a slab, to give design checks for multiple scenarios. This can be useful for considering the effects of pattern loading on a slab, for instance, or for comparing the results with modified parameters such as striking times or differing construction loads due to different construction processes.

On entering the Concrete Slab Deflection and Cracking module for the first time on a model, the initial screen layout will be as shown below, with no deflection brief.  The floor will be shown in the graphics area.

A deflection design brief is added by selecting the  icon. A deflection brief will now be indicated in the right hand pane. The slab deflection brief then needs to be assigned to a floor. This is done by selecting the relevant slab in the graphics area - move the mouse pointer to the boundary of the slab until the slab highlights. Once highlighted, left-click on the slab. The selected slab name will be added to the Slab Surface input line.

If a slab is made up of multiple FE surfaces, it is only necessary to select one of the FE surfaces as the software will automatically include all connected FE surfaces. FE surfaces will be treated as separate surfaces if the slabs have a full edge release condition defined at their common boundary, or if the slabs are physically disconnected.

Deflection brief analysis inputs

Once a design brief has been created and applied to a slab, it is necessary to define the various material and analysis parameters. These parameters are input in the right hand panel. The load events and humidity inputs are defined separately. Once the inputs have been made, the slab analysis can be run by selecting the  icon. Where there are multiple design briefs, the analysis for all design briefs can be run by selecting the  icon.

Each design brief can be selected by selecting the brief name in the right hand panel.

General

The general inputs area is as shown below. This area allows a user defined name to be added to a design brief, as wall as defining the long term time period for the analysis.

Name - displays the current name for the selected design brief.

Long term (days) - defines the time period for long term deflections. Default input is 50 years

Slab surface - notes the name of the FE slab the brief is appended to

Concrete

The concrete inputs area defines the deflection parameters for the cement type and restraint as well as allowing the user to define the crack width limits.

Cement Type - displays the currently selected cement type. The default setting is Type R

Shrinkage restraint - displays the current set value for shrinkage restraint. The recommended minimum value is 50% which is the default value

Crack width Top(mm)- displays the current crack width limit for the top surface of the slab. The default value is 0.3mm

Crack width Bottom (mm) - displays the current crack width limit for the bottom surface of the slab. The default value is 0.3mm

For each input the values are amended by selecting the relevant row and then either using the drop down option or over-typing the required values.

Defining Slab Load Events

A key component of the long term deflection analysis is defining the time at which the load on the slab changes, either by the application of load or, equally important, the removal of a load. Loading events and their associated time periods are defined in the Slab Load Events for Deflection table. This is accessed by clicking on the  icon to the right of the Edit the slab load events line. This will open the following input area.

On first opening, a default load event has been created for the load in Load Group D1. This load event starts on Day 7 and ends on day 18250, which is the defined load term time period. The Event Name label has been left blank.

A new slab load event is added by selecting the  icon. This will add a new line to the table. The added line will be set up as the default line shown above. If a row is added and is not required, it can be deleted by clicking on the icon.

Loading events are controlled by the Load Group drop downs and Start and End date inputs. The Event Name is simply a label used to aid identification of a loading event. The Load Groups refer to the loads defined in Masterframe. Because of the time dependent nature of the loading it will be necessary to refine the load definition in Masterframe such that each component of loading is in a separate load group.

An example of input loading events is shown below.

The Quasi-permanent load factor and Beta factor values are amended for each load event by highlighting the relevant cell of the table and amending as required.

The Frequent Load Case can be included by checking the tick box. When this is done, the option to select the Frequent Load Group becomes active. The Table will now also include a column to input the Frequent Load Factor. The Frequent Load Factor only applies to variable imposed loads.

To auto fill the load factors, select the  icon. This will apply a set of default load factors to the input loads, based on the Load Group Letter.

Defining Slab Humidity and Temperature Conditions

Both the ambient temperature and relative humidity have an effect on the long term deflections of the structure, with lower deflections resulting from higher humidity while lower temperatures result in slower hydration and lower strength gain with time. During the construction process, the slab will potentially be constructed in external conditions, while over time as the building envelope is constructed, the slab will eventually transition to internal conditions. The relative humidity and ambient temperature are defined in the Slab Humidity and Temperature Conditions for Deflection input area. This is accessed by click on the  icon to the right of the Edit the slab humidity and temperature conditions line. This will open the following input area.

The default inputs are a relative humidity of 50% and an ambient temperature of 20 degrees from the start of the analysis until time 18250 days (50 years).

Additional input conditions can be created by adding additional lines to the table. This is done by clicking on the  icon. The input temperature and humidity conditions are then displayed in the calculation area.

Running the analysis

Once the analysis input parameters have been set, the analysis needs to be run. This can be done by clicking on either the  or the  icons.

 - runs the deflection and cracking analysis for all slab deflection briefs

 - runs the deflection and cracking analysis for only the currently selected slab deflection brief

Displaying the results

Once a slab deflection analysis has been run, the analysis results can be viewed by selecting either the Deflection Check Lines or Crack Widths options in the Results line. The results to be displayed are then selected from the options given in the Deflection Results and Check Lines area.

When the Crack Widths outputs are selected, the left hand pane will display the calculations for the calculated slab crack widths. But initially, when the Deflection Check Lines option is selected, no calculations will appear. This is because the deflection calculations are based on deflection check lines and if no check lines have been created, then there are no calculations to display. Once a check line has been created, then the relevant calculations will appear. The Deflection Check Lines option will display selected contour outputs in the graphical area without the need to create a deflection check line.

Creating a check line

Check lines are required for displaying deflection check calculations. Multiple check lines can be added to any particular slab and deflection check criteria can be created for check lines.

A check line is created by selecting the  icon when the Deflection Check Lines option is selected. This will activate the Draw mode and a check line can be created by selecting the appropriate start and end points on the selected FE surface. Further check lines are added by repeating this process.

When check lines are created, they can then be selected using the drop down. The currently selected check line will highlight in red on the graphics screen. Alternatively, a check line can be selected by clicking on the check line itself in the graphics window.

Once check lines are created, the deflection check criteria for the check line need to be set. This is done by clicking on the  icon. This will open the Slab Deflection Check Line Criteria window, as shown below.

The default check line criteria is set for the final deflection, which looks at the deflection at the input time defined as long term.

New deflection criteria are added by clicking on . This will add a new line to the table. A user defined Name can be added to assist when selecting the required check line criteria for a check line. The following inputs are required

Differential deflection - set the deflection check to calculate the deflection between two times (requires an End Time to be input)

Start time - select the time the deflection is to be calculated at, or the start time for a differential deflection check

End Time - final time for a differential deflection check

Relative to ends - checks the deflection on a check line relative to the end points. This takes out the deflections that would be due to elastic shortening of columns, cores walls etc.

Span/limit - set the required deflection limit

The Cantilever Deflection check applies to specifically selected check lines. A check line is selected and then the Check Cantilever Deflection check is set. The Check Cantilever Deflection is then a check-line by check-line setting, whereas the deflection check criteria can be used for multiple check lines.

Sets of deflection check criteria can be set created by adding new sets (click on the  icon and add the required deflection check criteria). A new set of deflection check criteria can be created on the current set by selecting the  icon. It is also possible to import and export deflection criteria.

Once deflection criteria have been set, then each check line can be selected and the calculation for the selected deflection check criteria are displayed in the calculation area. A specific criteria can also be shown graphically in the graphics window by selecting a specific deflection check criteria and selecting the appropriate magnification.

Contour Outputs

Contour outputs can be displayed for both the crack width and deflection calculations for a selected slab. This is done by first selecting either the Deflection Check Lines or Crack Widths options and then selecting the required contour output from the drop down options.

The available contour outputs are as follows:

Crack Widths

Crack Widths Set 1 - the calculated crack widths in direction 1

Crack Widths Set 2 - the calculated crack widths in direction 2

Crack Unity Set 1 - the crack width ratio (crack width/crack limit) in direction 1

Crack Unity Set 2 - the crack width ratio (crack width/crack limit) in direction 2

Deflections

The deflection options will be based on the input times created in the slab load events inputs, with a deflection and creep factor output available for each time step. Therefore, the number of available options in the drop downs will depend on the number of slab load events created.

The contour outputs for deflections are:

Deflection @ xx days - the absolute slab deflections at day xx

Creep factor @ xx days - the creep factor  at day xx

Max Crack Ratio Set 1 - the maximum crack factor α in direction 1

Max Crack Ratio Set 2 - the maximum crack factor α in direction 2

First Crack Time Set 1 - the time to first cracking in a finite element in direction 1

First Crack Time Set 2 - the time to first cracking in a finite element in direction 2