Wall Pier Regions

The design parameters for a wall pier design are defined in the properties area. An example layout is shown below.

To navigate through the options, hover the cursor over the area and use the mouse wheel to scroll up or down. Alternatively, click and hold the left mouse button with the cursor over right side scroll bar and move the scroll bar up or down.

The Wall Pier properties give inputs to control the geometry of the pier, including its position either locally or globally, parameters to control the design method including the subdivision of the pier to give more refined design, control over the reinforcement and parameters to control the effective length of the wall pier, including the ability to overwrite the effective length of the wall pier.

Each area of the design parameters is outlined below.

General

The general input area allows each wall pier zone to be individually named. This can be done by simply selecting the input area and overtyping the default pier name.

The Parent FE surface notes the FE surface the wall pier is associated with. A wall pier can be associated with any of the FE surfaces within the plane of the wall. That means that the wall pier need not be defined to lie wholly or partly within the parent FE surface, though in must lie within the same plane set of FE surfaces as the set parent FE surface. The setting out coordinates of the FE surface are defined relative to the selected parent FE surface – see the Geometry input area.

The parent FE surface can be changed by selecting the input line for the Parent FE surface, which will display an  icon.

Clicking on the icon, the parent FE surface can then be selected by moving the cursor over the required FE surface and selecting with the left mouse button. When a new parent FE surface is selected, the name of the Parent FE surface will update, as will the setting out coordinates in the Geometry properties area.

Geometry

The Geometry parameters area gives tools to assist in the definition of the wall pier setting and overall dimensions. The typical layout for a wall pier is shown below.

Horiz. Anchor to Node – defines whether or not the pier is defined by basic nodes

Start x(m) – local surface x coordinate for start of pier

End x(m) - local surface x coordinate for end of pier

Vertical placement – sets whether the height of the pier is set by local or global coords

Bottom y(m) – local surface y coordinate for bottom of pier

Top y(m) – local surface y coordinate for top of pier

If the Horiz. Anchor to Node option is set to Yes, the options change to give inputs for the start and end node numbers.

By anchoring to nodes, this means that in the event that the wall geometry is amended, the wall pier width will automatically update to remain associated with the nodes after the amendment of the model. Anchoring is not affected by node renumbering, as the node numbers mapped to the new numbering, which means the pier is anchored to node positions rather than the specific numbering. Only basic nodes can be used for anchoring; that is, the nodes which are part of the basic wireframe model and are present before the FE surfaces are meshed.

The vertical placement selection gives four options to define the vertical height of the wall pier. These can be selected by selecting the input line which then gives a drop-down list with four options.

The four options are:-

yCordLocal – positions the top and bottom of the pier relative to the bottom of the parent FE surface

yCordGlobal – defines the top and bottom of the wall pier using the global coordinate system

Level – defines the top and bottom of the wall in terms of the defined floor levels (set in the Loads>Floor and Roof panels>Default Loading and Construction Per Level menu area)

Node – similar to anchoring the horizontal nodes, allows the panel to be defined vertical using basic nodes. When anchoring to nodes, additional offset dimensions can be added to control the vertical positioning of the pier.

Design Method

The Design Method area provides inputs to control the design of the wall pier. These options include the ability to specify the subdivisions of the pier over its width, the limit on the averaging of the forces within each sub-pier, an option to control whether or not the vertical reinforcement is used in the compression zone and how the major axis shear is apportioned across the pier subdivisions. There is also an option to control whether the Auto Design will include the wall pier.

A typical layout for a wall pier is shown below.

The default setup gives the following options.

Pier subdivision – sets the method by which the wall pier is subdivided

Equal pier widths fixed – sets the fixed pier width for the Equal strips subdivision method

Out of plane averaging limit – sets a maximum width over which force averaging occurs. The width is a multiple of the wall thickness

Round design widths for bar spacing – choose whether to have pier divisions to happen midway between vertical reinforcing bars

Use compression bars – sets whether the reinforcement in the compression zone is considered for the section bending capacity

Total Major axis shear proportioned – sets whether or not the total major shear force is equally split between the pier subdivisions or forces in each pier are determined by the FEA results for each subdivision

Auto design – sets whether the wall pier will be included in the auto design or not

The Pier Subdivision gives five options for considering how the forces in a wall pier are determined for the wall design. These are accessed by selecting the input line and clicking on the  icon to expand the list. This gives:-

The options are:-

Equal strips – fixed width: starting from the left and right, the pier is subdivided into strips of the input fixed width, rounded to subdivide midway between vertical bars. A central ‘make-up’ subpier is used in the central zone of the pier

Equal no. of strips: subdivides the pier into an equal number of sub-piers. The widths are based on the wall pier overall length divided by the number of strips, but rounded to the nearest midpoint between reinforcement if that option is enabled. A central ‘make-up’ strip width is used

User defined strip widths: pier widths can be manually assigned, starting from left to right by inputting strip widths, separated by semi-colons. The round design widths is not active with this option

One zone:  the wall pier is not subdivided. The design forces are determined over the full width of the single pier, but averaged over a width controlled by the out of plane averaging limit value.

Left and right zone – axial and bending: the in-plane bending forces is resisted by two reinforced zones at the ends of the walls

The reinforcement arrangement for the above options also depends on the ‘Same bar all strips’ option which is found in the Bars area. With this option set to ‘On’, the vertical strips within a wall pier zone will use the same reinforcement across the whole of the wall pier. Since the use of the same bars across the whole wall would not be desired when using the Left and Right zones to resist bending, the option to use the same bars in not available when the Left and Right zone option is selected.

Out of plane averaging limit

The purpose of the out of plane averaging limit is to prevent the wall forces being averaged over extreme widths such that the design forces are no longer representative of the forces in the wall. As an example, consider a wall with equal tension and compression vertical forces along the wall, varying linearly with a zero value at the middle of the wall – taking the average force over the full wall would return a vertical force of zero. To prevent this, the software uses a maximum width for averaging, based on a multiple of the wall thickness. The default setting is a multiple of 5 times the wall thickness.

Use compression rebar

When analysing the cross section in bending, the effect of the reinforcement in the compression zone can be included by setting the ‘use compression rebar’ to ‘Yes’. With compression reinforcement included, this will generally reduce the compression zone, affecting the position of the section neutral axis and also the lever arm for the tension reinforcement. However, when reinforcement is used in compression, additional detailing rules must be complied with, these detailing rules are not considered in the FE wall design module and so the user must satisfy themselves that the detailing of the reinforcement of the wall satisfies the full requirements of the design code.

Total Major Axis Shear Proportioned

Finite element results can frequently show high stress concentrations in specific areas. While often representative of actual stress distributions, the results can also show high stresses in areas where forces area applied to specific nodes in the FE surface, such as locations where line elements, such as beam, connect to the FE surface, such as would occur where beams are modelled connecting to core walls. Designing for these localised high forces may not be appropriate in all cases, particularly for lateral forces acting on the major axis of the wall and their resulting shear force.

Where designing for local high shear stresses in the major axis is not representative of the actual forces, the total shear force can be equally divided along the length of the wall, so each sub-pier takes a proportion of the shear force based on the ratio of the strip width over the total wall pier width.

The option to proportion the shear force is set to on by default.

Bars

The Bars area provides options to control the vertical and horizontal reinforcement bars within each wall pier. This includes options to set the reinforcement cover if it is different from the default values, define whether the sub-pier uses the same reinforcement and inputs to define the horizontal and vertical reinforcement manually.

A typical layout is shown below.

If the ‘Same bars all strips’ option is set to ‘Off’ the vertical bars line will change to display the number of all sub piers, and on expanding this area, each subpier can then be selected and the vertical reinforcement defined, as shown in the following example

The horizontal reinforcement for a wall pier applies to all strips/subpiers.

Edge Restraints

The edge restraint input area allows the out of plane restraint conditions for the wall pier to be defined. This controls the effective height of the wall pier which then affects the wall pier slenderness and nominal eccentricities.

The default layout of the Edge Restraints area is shown below.

The edge conditions input gives 5 options that define the restraint conditions on the edges of the wall pier. The Top and Bottom fixed define whether or not the top and bottom edges are being considered as pinned or fixed edge restraints. The wall height override option allows the user to input the wall pier effective height which overrides any values determined by the wall pier dimensions and restraint conditions.

Edge conditions

The edge conditions drop down provides 5 restraint conditions to determine the wall pier effective height. These options are:

Cantilever

Top and Bottom restrained

Top, Bottom & one vertical edge restrained

All four edge restrained

User defined beta value

The available options in the Edge Restraints input area will change depending on the option selected.

Cantilever – assumes the wall pier is fixed at its base both positionally and rotationally, but is free at the top edge, with the wall pier edges also free. The beta factor is taken to be 2

Top and Bottom restrained – the wall pier is assumed to be positionally fixed along the top and bottom edges and free on the left and right edges. If the Top and Bottom Fixed option is set to ‘Yes’ then the top and bottom edges are taken to also be rotationally restrained. The beta factor is defined by Table 12.1 of BS EN 1992-1-1:2004.

Top, Bottom & one vertical edge restrained - the wall pier is assumed to be positionally fixed along the top and bottom edges and also on one of the left  or right edges. The other vertical edge is assumed to be free. If the Top and Bottom Fixed option is set to ‘Yes’ then the top and bottom edges are taken to also be rotationally restrained. The beta factor is defined by Table 12.1 of BS EN 1992-1-1:2004.

All four edges restrained – the wall pier is assumed to be laterally restrained on all four edges. The top and bottom edges can also be set as fixed using the Top and Bottom Fixed option. The beta factor is taken from Table 12.1, as in the above. However, for a wall restrained on all four sides, the wall effective height is based on the ratio of the wall width over the wall height, with the lesser of the two being the critical dimension. This is due to the fact that for the wall to buckling under vertical axial forces, the wall must bend in both the vertical and horizontal directions.

User defined beta value – for combinations of restraints that are not covered by the above options, the user defined beta factor allows the user to input an effective length factor manually. The effective length in this case is based on the wall pier height only.

Wall height override (m)

The basic wall height taken by default is based on the height of the defined wall pier zone as defined by the user. The effective height for slenderness and buckling is then based on the wall pier height modified by the defined edge conditions. However, there may be cases where it is convenient or desirable to define the wall pier over heights that are not necessarily related to the buckling length. For example, to simplify design and detailing, wall piers could be created over multiple stories, or in areas of more specific design interest, multiple piers could be created within a storey height to give a refined design. Where the wall pier height is not representative of the basic wall buckling height, the Wall Height override allows for an effective wall height to be input manually. The edge conditions inputs then apply the effective height factor to the user defined wall height input.

Place Current Pier Zone

The Place Current Pier Zone area provides options for creating the wall pier zones. The placement methods can also be used to redraw an existing pier to allow the pier to be moved or resized.

When a new wall pier zone is added and the relevant FE surface has been selected, the Place Mode is automatically activated, which is indicated by the  icon. The placement method can be selected by selecting either the Draw x1, x2, y1, y2 option, or by selecting the Draw Rect option. These options control how the wall pier is defined:

Draw x1, x2, y1, y2 – the rectangular wall pier is defined by selecting the horizontal start and finish points (x1 and x2) and then the vertical start and finish points (y1 and y2).

Draw Rect – the wall pier is defined by selecting the location of two opposite corners.

Once a wall pier has been placed, the draw mode is deactivated by selecting the  again. When deactivated, the icon will change colour to a solid blue, shown as .

To redraw a wall pier, select the required pier either graphically in the graphics display, or by selecting the wall pier in the Rebar Set Name list. The pier will highlight in the graphics window and in the Wall Pier Zone list. Next, select the  icon and the select the required Place Mode option and then re-draw the wall pier in the required position.