FE Slab Design - Overview

The FE Slab Design module provides tools to enable to the design process to be automated for reinforced concrete slabs modelled using FEA. This is done by dealing with the resulting forces the slabs in 4 sets of calculations. These calculations are intended to cover the full set of slab results, while allowing the tailoring of the design in specific regions. These calculation sets are:

1. Basic region

2. Peak zones

3. Strips

4. Punching shear

Basic Region

The Basic Region reinforcement defines the basic reinforcement which is to apply generally to an FE surface or part of an FE surface. This represents a basic mat of top and bottom reinforcement that can be added to, or replaced, in regions of higher bending moments. The Basic Region reinforcement allows for separate reinforcement to be defined in the T1, T2, B! and B2 layers. The Basic Region reinforcement orientation can be defined by the user by inputting an angle relative to the FE surface local x-axis. It is also possible to input a skew angle for the T2 and B2 reinforcement relative to the T1 and B1 layers, up to a skew angle of 45 degrees.

Peak Zones

Peak zones are intended to provide enhanced reinforcement in zones where peak bending moments occur, such as at support points or at locations of high localised loading to the slab. Peak zones provide options to define reinforcement in two orthogonal directions in either the top of bottom of the slab to deal with the peak bending moments. The reinforcement in a peak one can be defined to be either in addition to the basic reinforcement, or as replacement for the basic rebar. The reinforcement in the peak zone can be designed as a single zone, or split over the middle half and outer quarter widths. There are also options to control the design force, with options to define how the force averaging is carried out. The default shape of peak zones is rectilinear but it is possible to amend the peak zone shape to be a more general quadrilateral. Reinforcement in a peak zone is aligned on orthogonal axes, orientated relative to Direction 1 or 2.

Strips

Strips are intended to provide enhanced reinforcement in strips where higher bending moments occur along lines, for example, within column strips in slabs. The strip rebar provides options to define reinforcement in either the top or bottom of the slab and also whether the reinforcement is orientated parallel or perpendicular to the strip length. Each strip is intended to represent reinforcement in a single direction, unlike the Basic and Peak zones which can define reinforcement in multiple directions. As for peak zones, the reinforcement in a strip can be set to be in addition to the basic rebar, or to replace the basic reinforcement. Similarly to the peak zones, there are options to control the design force averaging. The default shape of peak zones is rectilinear but it is possible to amend the peak zone shape to be a more general quadrilateral. The reinforcement in a strip is aligned on orthogonal axes, orientated relative to centreline of the strip.

Punching shear

The punching shear design checks are intended to carry out the required punching shear design checks at support points or points of localised high loads, such as over the head of columns or support points, at the ends of shear walls, or under transfer columns. The punching shear design is based on the concrete slabs effective depth and the defined reinforcement applicable to each punching shear check applied to the slab. The punching shear check involves a check on the perimeter of the column and then check on subsequent punching shear perimeters. The design shear force can be derived from the Code methods or directly from the FE analysis results. The punching shear check allows the user to define both the arrangement of shear reinforcement using a drop down of pre-set arrangements and to also indicate a shear bar reinforcement angle should if inclined reinforcement is to be used. The punching shear check provides inputs to define the column cross section (which defines the punching perimeter) for cases where a nodal support is used rather than modelling the column. Finally, the punching shear check provides inputs that allow the user to specify a drop in a slab to provide enhanced punching shear resistance.

Rebar Zone interactions

There are a number of interactions between the defined rebar zones that need to be taken account of when designing a slab. These include zones which do have an interaction, but also cases where zones do not interact.

The basic rebar region carries out design checks on all those areas defined under it's extent which are not covered by peak and strip zones. That is, the design checks for the basic region specifically exclude those areas which are design as a peak or strip region.

Peak zones design the reinforcement within the region defined by as a  peak region and can design either provide the reinforcement in full replacement of the basic rebar or design the reinforcement that would be required in addition to the basic rebar. Therefore, where the rebar is provided in addition to the basic region reinforcement, the peak zone will design will be partly dependent on the rebar define within the applicable basic region, so there is an interaction between the peak zone and basic region. Thus changes in the basic region reinforcement can lead to design changes in peak zones.

Strip zones design the reinforcement for bars running in a single direction within the boundary of the slab. The reinforcement within any strip can be provided in addition to the basic rebar, or set to fully replace the basic reinforcement. Where the reinforcement within a strip is provided in addition to the basic region reinforcement, the design of the reinforcement within the strip is then partly dependant upon the basic region reinforcement, and so changes in the basic reinforcement can lead to changes being required within the strip reinforcement.

Punching shear design is based on the tension reinforcement on the basic control section. This is calculated on the basis of either the peak reinforcement, or if no peak has been applied, the basic reinforcement. If a strip region overlaps with a peak, the reinforcement in the strip is not considered.

Peak Zone and Strip zones do not interact. Both of these are wholly independent from each other. This means that is a peak zone and a strip region overlap, the design is not based on a combination of the reinforcement from the peak and strip zone. The strip zone design will exclude forces from those areas which are dealt with in a peak zone, but the reinforcement required to satisfy the design of the strip will be provided over the full length (or width, depending on reinforcement orientation relative to the strip). It is recommended that strip zones and peak zones are not overlapped.

Design Workflow

When first designing the reinforcement for slab or region of a slab, the first step is to define the basic reinforcement. However, with no peak and strip zones determined, when first applying a basic region, the design checks will be carried out on the peak values of the design forces found within the defined basic region. In some structures it may be the case that the basic mat is intended as the reinforcements, so an auto design or manual adjustment of the reinforcement can be done at this stage to satisfy the design checks for the maximum design forces in the basic region. However, where it is the intention to use both peak regions and strips in addition to the basic reinforcement, then the initial design of the basic region will be carried out after the peak and strip regions are defined. In this case, the design process will be

1. Basic region - defined the desired basic mat of reinforcement to apply to non-peak areas

2. Peak regions - assign peak regions to the areas intended to have a refined peak reinforcement design

3. Strips - assign strips to regions intended to have refined reinforcement design applied in a strip

4. Basic region review - review and refine the reinforcement in the basic region now that the peak and strip zones are not being included in the basic region

5. Peak and strip region review - changes to the basic region reinforcement may affect the reinforcement in the peak and strip regions if the reinforcement in these zones adds to the basic rebar of the basic region

The flexibility of the slab design software allows the user to customise the slab design at stages 4 and 5 to suit their deigns intention. For instance, at stage 4, the basic region rebar can be increased to satisfy the design requirements for all areas not included in the peak and strip zones, or, alternatively, stage 4 can be used to identify areas not currently in a peak or strip zone where the basic region reinforcement is not satisfactory and so either the existing peak and strip regions can be extended to cover more areas of the slab or additional peak and strips can be created.