Reinforcement Design Methodology

Basic Regions

The reinforcement design of a basic region is based on the maximum bending moments occurring within the region contained within the boundary of the basic region under consideration. The basic region reinforcement identifies the maximum bending moment for the four directions, Top Reinforcement Direction 1, Top Reinforcement Direction 2, Bottom Reinforcement Direction 1 and Bottom Reinforcement Direction 2. Areas of the slab defined within the peak and strip zones are excluded and their design is dealt with in the peak or strip zone.

The bending design is based on the Wood and Armer moments in the x- and y-axis directions of the FE surface, transformed to account for the orientation of the direction 1 bars. If the direction 2 bars are skewed relative to Direction 1, then the reinforcement areas are transformed in the orthogonal directions. The design of the reinforcement also accounts for the axial force in the slab.

Peak smoothing can be applied to the results for a basic region. When active, the software will identify peak regions within the results and will apply peak smoothing over a user definable radius.

The critical location is determined taking account of both the bending moments and the axial forces in the slab. The critical location is determined by examining the results over the full basic region and calculations the required reinforcement, with the largest area of reinforcement being determined and thus identifying the critical location for each direction. Once the critical location is identified, a more exact analysis, based on an iterative approach to determine the moment capacity of the cross section, is used to give a refined moment capacity at each critical location. This iterative design check is more computationally expensive than determining the required reinforcement and so it not suited to carrying out a check on the entire basic region.

Peak Zones

Peak Zones are intended to for the design of the reinforcement in localised areas of high hogging or sagging moments, such as occur at support positions or under transfer columns. Each peak zone designs the reinforcement in two orthogonal directions in either the Top or Bottom direction. That is, a peak zone will design either the Top Direction 1 and 2 reinforcement or the Bottom Direction 1 and 2 reinforcement. Should the design of both the top and bottom reinforcement be required at the same location, this will require that two peak zones be applied to the same location, one designing the top reinforcement, the other set to design the bottom reinforcement. However, providing peak regions at the same location would not be the recommended usage of peak zones.

To deal with the high peak moments which occur with the FE analysis at support positions or the location of point loads, it is standard practice to employ averaging over a width to determine the design forces. The peak zones provide four options to control the width of the design force averaging. The width of the peak zone can be taken as a single zone i.e. the full width of the peak zone, or refined to split the peak zone into a middle 1/2 strip and two outer 1/4 strips. This allows refinement of the reinforcement design within the peak zone in each direction by allowing control over the width for design force averaging but also allows for different designs for the outer strips for which reinforcement based on the middle half would be conservative.

The Design Force Averaging options are:

Average over full width: uses the full width of the peak zone for where a single zone is used, or uses the full width of the middle strips and outer strips, in effect doing a full peak zone design on each strip. The software will give a warning where the peak zone width is greater than 8d.

Average - 0.5 full width (one zone only): for use where the peak zone is design in based on one zone. This option limits the average width to half the zone, but provides the same reinforcement over the full width. Limiting the design force averaging to half the strip produce a design for the inner column strip and so provides a limit on the amount of averaging provided on the peak results. The software will give a warning where the peak zone width is greater than 8d.

Average - limit width (m): provides an option to specify the width to be used for force averaging. The upper limit is defined by the peak zone itself. Therefore you cannot average over a width greater than the peak zone.

Average - limit effective depth multiple: provides the option to specify the limit on the average width as a multiple of the slab effective depth. The upper limit is then the lesser of the effective depth multiple or the peak zone width.

For peak zones which are divided into two zones, the average over full width, limit width and effective depth multiple options, the reinforcement design is carried out on both the inner and outer strips, designing each of the inner and outer strips.

Strips

Strip regions are intended to provide reinforcement in a single direction in either the top or the bottom of the slab in areas of localised high sagging or hogging. An example would be providing strip zones in column strips, to provide enhanced sagging reinforcement, or in an area underneath a wall supported on the slab. Since strips provide only reinforcement in a singe direction, if reinforcement was required in two orthogonal directions then multiple strips would be needed.

The design of the reinforcement within a strip region is based on the maximum design forces determined by applying design force averaging. The method of design force averaging can be defined by the user. The design forces are averaged in a direction perpendicular to the reinforcement direction.

The Design Force Averaging options are:

Use max value (no averaging): uses the maximum moment value found within the strip applicable to the reinforcement layer and direction specified.

Average - limit width (m): averaging is carried out perpendicular to the reinforcement directions specified, up to a maximum width specified by the user.

Average - limit effective depth multiple: provides the option to specify the limit on the average width as a multiple of the slab effective depth. The upper limit is then the lesser of the effective depth multiple or the strip zone width for the direction perpendicular to the reinforcement direction

Average over full width: uses the full width of the strip zone perpendicular to the reinforcement to average the design forces.

The Design Force Averaging options provide a single design force for the design of the selected reinforcement. Therefore, caution is required when creating design strips. In large slabs, if a strip is created that runs the full length of the slab, then a single design value is applied to the full strip. For example, if a single strip was applied to a column strip over the full length of the slab, to deal with sagging moments, then the moment for the end bays would likely be the critical value and this would be applied to the full strip. In this instance, it would be recommended to create separate strip zones. Where strips potentially interface with peak zones, it is recommended that the strips are not carried through the same region as the peak zone.

Punching

Punch shear regions are intended to check slabs for punching shear at locations of high shear concentrations, such as at the head of column/slab interface for columns supporting the slab or at the base of columns supported on the slab.

The punching shear check uses the size of the column to check the shear force at the face of the support. The punching shear is then checked on a control perimeter u₁ and if shear reinforcement is required, then the shear bars are designed based on the selected user inputs which define the shear reinforcement pattern, bars diameter and angle. The punching shear is then checked on subsequent perimeters to determine the zone where punching shear reinforcement is no longer required.

The design forces used for the design of the punching shear reinforcement can be determined either using the methods outlined in the relevant design code, or directly from the FE analysis results. In both cases the effect of the bending moments transferred to the column will be taken into account when determining the shear force and shear force distribution. When using the FE analysis results, the shear on the punching perimeter under consideration is calculated directly from the FE results and the effect of the bending in the slab and column is automatically accounted for.

Where changes in slab thickness occur within the perimeter of punching shear check, these are taken into account in the punching perimeter design. The punching shear is then designed according to the following conditions at the boundary of the slabs

a) Full moment and shear continuity - the punching perimeters are taken to be continuous and the punching shear design is based on the average effective depths of the slabs encountered.

b) Moment and/or directional releases - punching perimeters are assumed to not pass the through the slab intersections and so the design is based on that part of the punching shear perimeter within the slab, bounded by the slab junction, with the effective depth being based on the slab within which the punching perimeter is being considered. To accommodate this, the software will allow multiple punching shear design zones to be assigned to a single column, with inputs to control with FE surface a punching shear design zone is associated with.

For punching perimeters are slab edges, slab corners or adjacent joints in slabs (where FE surface edge releases have been defined) the shape of the punching perimeter is determined based on the position of the centroid of the column relative to the slab boundary and the shape of the column. The punching shear checks are designed to apply to square, rectangular or circular columns.

The punching shear perimeter will also be amended to account for the presence of slab openings. The process by which this is done takes cast an arc from the centroid of the column to extent of the opening and excluded that part of the punching shear perimeter that falls within this arc.

Since the length of the punching shear perimeter for columns located near slab boundaries is determined to a degree by the position of the column centroid, there may be cases where modelling the column centroid on the slab edge, while often desirable from an overall modelling perspective since it reduces the mesh complexity, may lead to an under calculation of the shear perimeter. In these cases, it may be necessary to amend the position of the column centroid to better reflect the position of the column as it will be constructed relative to the slab edge or joint position. Similarly, with joints in slabs, while for a general FE analysis the joint position may often be located on the line of columns, it may be necessary to adjust the joint position to line with the edge of the column as it will be constructed to better model the punching shear perimeters as well as the shear force in the slabs.