Model your slab with
plate elements
The slab design module requires you to model your slab with plate elements.
Of course your model can also contain normal members for modelling
columns, beams, braces and the like that connect to the plate elements.
The plate elements in the slab should be meshed to a suitable size,
taking into consideration that a finer mesh may be required along
edges, around voids, at support points (such as columns), at stress
concentrations or any other areas of interest. Various mesh
tools are available for this purpose.
Drop panels or beams underneath the slab can be incorporated into the
slab design by the use of thicker plate elements that have been offset
downwards so that their top surfaces line up with the top of the slab.
The slab design would then consider them as a combined T or inverted
L cross section.
The local axes of all the plate elements in your slab should be aligned
so that any contour diagrams (and in particular reinforcement contours)
are based on the same direction throughout the slab. If your plate
axes aren't aligned then different parts of a contour diagram will
be for different directions. You must also ensure that the local z-axes
of all the elements in your slab are pointing in the same direction
so that the "top" and "bottom" faces are consistent
for all elements in the slab. When you use any of the mesh
tools they automatically align all the axes for the plate elements
that are generated, however the "Align
plate axes" tool can be used to do this later if required.
You can also use this tool to change the direction of all the plate
axes in your slab to match the desired direction of your reinforcement.
In order to choose an initial slab thickness that should result in
satisfactory deflections based on L/d ratios, you can use the "Span-to-depth calculator"
tool.
Footings
If you are using the concrete slab design module to design a raft footing
then correctly modelling the soil stiffness underneath the footing
is an important step. The soil stiffness can be modelled using vertical
node springs that resist downward loads but allow lift-off. Because
the meshing of the footing generally results in elements of various
shapes and sizes, calculating the tributary area of each node in order
to determine its spring stiffness is not straightforward, and so a
"Generate
plate spring supports" tool is available for that purpose.
The tool also attaches a short compression-only member to each spring
support so that lift-off can occur. For further information refer
to "Concrete
footing design".
Note that a wizard for generating standard raft footings is available
via the "Structure
wizard". The footing wizard generates the fully meshed model,
including compression-only spring restraints underneath and pattern
loading.
Apply the slab loads
You can use the normal graphical or datasheet tools to load your slab
with self
weight, pressure
loads, thermal
loads and node
loads, plus there is a pattern
loading tool that allows you to quickly create plate pressure
load cases comprised of various adjacent and alternate span pattern
loads. It is strongly recommended that you apply the plate loading
after the geometry and meshing of the slab is complete. If you change
the geometry or meshing after plate loads have been applied then some
of the loads may have been moved, deleted or become invalid, especially
if you have applied pattern loading. Note
that the "Mesh plates (advanced)"
and "Refine
plate mesh" tools replace the plate elements being re-meshed
(along with their nodes), and so any node loads, plate loads, restraints
or constraints previously applied to the affected nodes and plates
will be deleted. It is therefore recommended that you finalise the
use of these re-meshing tools before you apply your restraints, constraints
and loads in order to avoid having to re-apply them.
Decide between the
strip method and the finite element design methods
The strip method allows you to define
column and middle strips in the critical areas of your slab and then
design the reinforcement for each strip, taking into account flexure,
shear, torsion and deflection. Mesh, bars and stirrups are selected
to satisfy the required areas of steel. A comprehensive report can
be produced that includes a schedule of the reinforcement along each
strip and the various parameters that the design is based on.
The finite element method designs
the slab reinforcement on an element-by-element basis and presents
the results as contour diagrams of the top and bottom reinforcement.
This method requires minimal input from the user and gives a fast
overall reinforcement design, however no mesh or bars are selected
and so you must fit them manually based on the calculated areas of
steel. Because no bars are selected in the design, the cover you specify
in this method is to the centerline of the reinforcement, whereas
the cover in the strip method is to the edge of the reinforcement.
Both methods should give similar results, however the different treatment
of cover and ductility in the two methods could cause different results
in some circumstances. The differences are due to the fact that the
strip method selects bars and/or mesh with known properties and dimensions
from a reinforcement library, whereas the finite element method just
calculates areas of steel. Cover in the strip method is measured as
the clear distance from the edge of the outermost bars, mesh or stirrups
to the edge of the concrete, whereas cover in the finite element method
is measured to the centroid of the reinforcement. Ductility in the
strip method is known based on the bars or mesh selected, however
the finite element assumes normal ductility reinforcement with a capacity
reduction factor of 0.8. If you are using low ductility reinforcement
in the finite element method then you could manually increase the
calculated areas of steel in proportion to the ratio of the normal
and low ductility capacity reduction factors.
Define plate strips
(strip method only)
If you have chosen the strip method then you must first define plate
strips throughout the critical areas of your slab. It is common
to define a strip along each line of columns with a width equal to
25% of the distance to the next line of columns on each side (column
strips) plus a strip in-between each column strip with a width equal
to 50% of the distance between the lines of columns (middle strips).
You would normally define column and middle strips in both the longitudinal
and transverse directions so that they cross over each other. Strips
may also be required at other locations such as across re-entrant
corners. For further information refer to "Plate
strips".
Using the strip method
In order to design a slab strip, you should select the plate strip
that you want to use, right-click and then select "Input/Edit
Concrete Slab Strip" from the popup menu that appears. This will
open the slab strip editor and attempt
to perform an initial design.
You should carefully set the ultimate, serviceability and sustained
load case lists in the editor before accepting any design results.
You can also change the other parameters in the slab strip editor
to suit your requirements.
You may have to specify different cover for strips that cross each
other to allow for the reinforcing crossing over at slightly different
levels.
If you want to prevent the sometimes annoying delay that happens due
to the design/check it does each time you change a setting then you
should untick the "Auto" option near the bottom of the editor
and just click the "Check" or "Design" button
when you're ready.
If you want to perform a check instead of a design then as soon as
you change any of the reinforcement details it will lock the design
and perform a check instead.
Once you are happy with the design or check, you should click the Ok
button to save the slab strip and exit from the editor. You can then
repeat the procedure for the next plate strip.
When your slab strips are defined, you can access all of them via the
"Concrete manager".
The concrete manager allows you to edit individual strips, run a design
or check on all of them or generate reports for all of them.
For further information refer to "Strip
method".
Using the finite
element method
The finite
element method requires you to set the reinforcement parameters,
display the reinforcement contour diagrams and select the initial
combination load case that the design is to be based on.
You can get to the reinforcement parameters by selecting "Reinforcement
Contour Options" from the "Show plate contours" button
as shown below. You are then able to select the design code and set
the cover, steel strength, minimum reinforcement ratios, Wood-Armer
moment adjustment option, etc. For further information refer to "Reinforcement
Contour Options".
You can then select the "Reinforcement Along Local X" or
"Reinforcement Along Local Y" contour diagram to generate
a reinforcement diagram for the entire slab.
You must also choose the load case or combination that the reinforcement
design is to be based on via the load case selection box in the top
toolbar. If you change the selected load case then the contour diagram
will be updated accordingly. If you have more than one load case selected
then the diagram will apply to the maximums from the selected load
cases.
For further information refer to "Finite
element method".
Check the punching
shear
The punching shear check is independent of the strip or finite element
methods. You can view the punching shear perimeters via the 
button on the
side toolbar. The perimeters and utilization ratios can also be labelled
on the slab for your visual checking. The punching shear calculations
are based on the load case or combination that is selected in the
top toolbar. If you have more than one load case selected then
the results will apply to the maximums from the selected load cases.
For further information refer to "Punching
shear".
Get the results
Strip method
If you have used the strip method, you can report the results by clicking
the "Report" button in the slab strip editor or you can
generate reports for multiple slab strips at once by opening the "Concrete manager",
setting the parameters in the "Report" part of the form
and then clicking the "Generate Reports" button.
You can also query the results for any strip by right-clicking on it
and then selecting "Concrete
slab design results" from the popup menu that appears. The
query displays a summary of the reinforcement, load cases used, areas
of steel and critical deflections. You can then click on any other
strip to have the query form updated with the results for that strip.
Finite element method
If you have used the finite element method, the results are displayed
graphically in the form of reinforcement contour diagrams that you
can view or print. A contour diagram is available for reinforcement
along the local X and Y plate axis directions for both the top and
bottom of the slab. In order to show the bottom reinforcement contour
you can either view the slab from underneath or you can select "Bottom
face only" from the contour button menu or set it in the "Display
side" field of the "Reinforcement
contour options" form. This will then show the bottom reinforcement
contour from a plan (top) viewpoint. Note
that the "top" face is defined as being on the side of the
positive local z-axis of a plate element. This means that if the local
z-axes for your plate elements are pointing downwards then the "top"
face will actually be at the bottom and the "bottom" face
will actually be on top.
You can also query the results for any plate element by right-clicking
on it and then selecting "Reinforcement
results" from the popup menu that appears. The query displays
the areas of steel for both directions in the top and bottom of the
element plus the key values used in the area of steel calculations.
You can then click on any other plate element to have the query form
updated with the reinforcement for that element.
Punching shear
The punching
shear results are displayed in the form of perimeters and punching
shear utilization ratios at each column support location. You can
also query the punching shear results by right-clicking on a support
column and then selecting "Punching
shear results" from the popup menu that appears. The query
displays the perimeter length, punching shear stress, utilization
ratio and other key values used in the calculation of the punching
shear. You can then click on any other support column to have the
query form updated with the punching shear for that column.
Re-design/check after
a change to the model
If you have made a structural or load change to your model after previous
slab designs or checks, you can redo the design or check for every
slab strip by opening the "Concrete
manager" and clicking the "Design/Check" button.
You can control which slab strips are re-designed/checked by listing
them in the "Slab strips" field above the "Design/Check"
button. Leaving the list blank will re-design/check them all.
If you have used the finite element method then you just need to re-display
the desired reinforcement contour diagrams.
Similarly, if you want to re-check the punching shear after a model
change then you should just re-display the punching shear perimeters
and stresses.