Slab on beams design workflow
A simple slab on beam model as shown below is used in order to demonstrate the techniques involved in the slab design process.
Note that not all the slab panels are rectangular.
Set up pattern loading
By default, only beam loads, and slab loads that have been decomposed on to beams are patterned. Loads applied to slabs should be manually patterned using engineering judgement; this is achieved on a per panel basis for each pattern load by toggling the loading status via Update Load Patterns on the Load ribbon.
Establish slab design moments
Analysis is required to establish the moments to be used in the slab design - this analysis is automatically performed when either Analyze All, Design Concrete or Design All are run.
Typically these moments are taken from the FE chasedown model results - as each floor is analyzed individually this method mimics the traditional design approach.
If you have elected to mesh 2-way slabs in the analysis, the 3D Analysis model results will also be considered.
It is NOT suggested that it should be standard practice to mesh 2-way slabs, for slabs that are not needed to participate in the lateral load analysis it is better not to mesh in 3D, however: - you may choose to mesh them to cater for the possibility of un-braced flat slab design. - more likely, you may do so to deal with significant transfer slabs - e.g. a shear wall supported by a slab, (there is no need to create a system of fictitious beams).
Select a level, or sub-model
In models with distinct floor levels you should use 2D Views to work on the floor 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.
When working in a 2D View use the right-click menu to design or check slabs and patches: this saves time as only the slabs and patches in the current level are considered. Running Design Slabs or Design Patches from the Design ribbon will take longer as it considers all the slabs and patches in the model.
Add beam and wall top patches
You may optionally want to add beam and wall "top surface" patches to cover the full extent of non-zero top bending moments. By doing so you can then set the top reinforcement in the slab panels to none and the panel design should still pass.
This is an interactive process - requiring a certain amount of engineering judgement.
Typically you should expect to work on this one floor at a time, (making use of multiple views when creating beam patches as discussed below).
It is suggested that you add patches in the Results View while looking at relevant moment contours - For example you might (after selecting the FE chasedown result type) have Mdx top moments on in one view on the left and Mdy top moments in a second view on the right, as below
By doing this, it is possible to see how patches extend over the moment contours.
It is suggested that 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 should be reviewed/resolved at the patch design optimization stage.
To design multiple slab panels, either:
- From the Design ribbon run Design Slabs in order to design or check all the panels in the model - by default newly created panels will all be in "auto-design" mode - so reinforcement is selected automatically.
- In the 2D View of the floor which you want to design right-click and choose either Design Slabs or Check Slabs. Working in this way restricts the design or checking to the slab panels in the current view. This will be a much faster option on large buildings and avoids time being wasted designing slab panels at levels where patches have not yet been set up.
- Design Slabs will re-design slabs (potentially picking new reinforcement) regardless of the current autodesign setting.
- Check Slabs will check the current reinforcement in slabs regardless of the current autodesign setting.
Review/optimize panel design
It is suggested that you use split Review Views to examine the results as indicated below.
The view on left shows Slab Design Status, (with slab patches turned off in Scene Content to assist clarity), the view on right shows 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 slab, 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).
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.
- 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.
- If the span-effective depth check is failing, review the panel properties to confirm that edge categories are set correctly, (and if the panel shape is irregular consider whether it is being suitably idealized as a rectangular panel). See: Slab on beam idealized panels
Design beam and wall patches
Having established and rationalized the slab panel reinforcement which sets the background levels of reinforcement, it is now logical to design any beam or wall patches that you may have defined.
To do this, either:
- From the Design ribbon run Design Patches in order to design or check all the patches in the model - by default newly created patches will all be in "auto-design" mode - so reinforcement is selected automatically.
- In the 2D View of the floor 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.
- 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:
- Beam patches - can the width be adjusted (minimized)
- Wall patches - can the width be adjusted (minimized)
- 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.
This chapter includes two sub-chapters:
- Slab/Mat Design Summary - by default this chapter summarizes all the slab
panel and patch designs, and punching shear checks in the model. It can be
filtered as required.
- Slab/Mat Design per Plane - this chapter contains sub-chapters for the
individual slab panel and patch designs, and punching shear checks. Each of
these have a member report style assigned which controls the level of output
produced.The style can be selected for each sub-chapter from the right click context menu as shown below.Note: You can either choose a named style, or you can choose "Active Style" (in which case when you come to display the model report it will adopt whichever member report style is active at that time).Each named style can be configured to include more or less detail, which can be done in a separate operation as follows:
- Close the above dialog box, and open the Member Reports dialog box.
- Choose the member types in turn (e.g. Punching Check, Slab/Mat Panel, Slab/Mat Patch) and select an existing style, (or add an new one if required).
- Drag chapters and options into the report structure as required.Note: This dialog box is also used to review which member report style has been nominated as the "Active Style". You can switch this to the currently selected style by clicking the >> Active button.
- Configure the level of calculation detail, by right clicking on the Design
Calculations chapter and selecting Settings from the context
- Choose the level of detail that the calculations are to be expanded to
in the report, (critical, with no levels expanded being the minimum),
then click OK to close the dialog box.
- Choose the level of detail that the calculations are to be expanded to in the report, (critical, with no levels expanded being the minimum), then click OK to close the dialog box.
- Click OK to close the Member Reports dialog box.
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
- slab/mat layout - created from the Draw tab of the ribbon by selecting
Slab/Mat Detailing. This variant has pre-configured layer configurations
available to limit the reinforcement displayed:
- Slab/Mat Detail (All) - this configuration displays top and bottom
reinforcement, and also displays any punching perimeters
- Slab/Mat Detail (Top Reinforcement) - this configuration displays top
- Slab/Mat Detail (Bottom Reinforcement) - this configuration displays
bottom reinforcement only
- Slab/Mat Detail (Punching) - there were no punching checks specified
in this example, but if there were they could be displayed on the layout
using this configuration. If any stud rail reinforcement had been designed
this would also be included to the side of the layout, as shown below.
- Slab/Mat Detail (All) - this configuration displays top and bottom reinforcement, and also displays any punching perimeters
- punching check detail - if punching checks had been specified in this
example, this variant could be used to display the details of any stud rail
reinforcement by right clicking on an individual punching check item in a scene
view and selecting Generate Drawing from the context menu.