RKALC Learning Centre

strut and tie modelling for reinforced concrete D-regions

A practical guide to identifying disturbed regions, developing rational load paths and designing concrete struts, reinforcement ties and nodal zones in accordance with AS 3600, demonstrated through a shear wall supported on columns using RKALC STM Wall.

Watch the video series Preview the engineering guide
Project overview

About this strut and tie tutorial

Follow the design from gravity loading and structural idealisation through nodal checks, tie reinforcement, strut verification and bursting reinforcement.

Introduction

This tutorial considers a reinforced-concrete shear wall that supports twenty storeys and transfers its gravity action to two columns below. The abrupt change in support conditions creates a disturbed region where conventional sectional beam theory does not adequately describe the internal stress flow.

The lower part of the wall is therefore represented using a strut and tie model. Concrete compression fields are idealised as struts, reinforcement carrying tension is idealised as a tie, and the intersections of these force paths are checked as nodal zones.

The objective is not simply to obtain a reinforcement area. The workflow makes the assumed load path visible and allows the engineer to review equilibrium, strut angles, nodal dimensions, concrete stresses, anchorage and bursting reinforcement.

What the series covers

  • Recognition of Bernoulli regions and disturbed regions.
  • Establishment of gravity actions and support reactions.
  • Development of a rational strut and tie idealisation.
  • Selection of strut angles and nodal dimensions.
  • Design of the principal reinforcement tie.
  • Checks of CCC and CCT nodal zones.
  • Verification of concrete compression struts.
  • Assessment of bursting forces and distributed reinforcement.
  • Review of anchorage and constructible reinforcement detailing.
  • Generation of a calculation report using RKALC STM Wall.
Strut and tie model of a reinforced concrete shear wall supported on two columns
Idealised load path through the wall transfer region, showing compression struts, the principal tie and supporting nodal zones.
Guide contents

Strut and tie workflow

Move directly to a specific part of the worked example.

01 · Sample structure

Shear wall supported on two columns

The example represents a heavily loaded wall whose support changes abruptly at the transfer level.

The wall supports a twenty-storey residential building and has an approximate tributary width of 8 metres. The plan arrangement gives a representative wall length of 10 metres and a tributary depth of approximately 12 metres.

At the transfer level, the wall is supported by two columns. The load must spread from the continuously loaded upper wall into the discrete supports, creating a deep disturbed region above the columns.

Supported levels20 storeys
Wall width10,000 mm
Wall height used5,700 mm
Wall thickness350 mm
Support width1,200 mm each
Structural behaviourConcrete D-region

Why the lower wall requires special treatment

Away from the supports, the stress field is comparatively regular. Near the transfer, the stresses become nonlinear as the load fans towards the columns. This region must be designed using a method that follows the actual force path rather than a simple linear stress distribution.

Plan and elevation of the twenty-storey shear wall supported on two columns
Sample building arrangement and transfer-wall geometry used in the worked example.
02 · Gravity actions

Establishing the wall design action

Slab actions and wall self-weight are combined to establish the ultimate load transferred through the disturbed region.

The representative slab loading includes a 220 mm slab, finishes and residential imposed action. Wall self-weight is added over the full twenty-storey height.

Representative actions

220 mm concrete slab
5.5 kPa
Finishes
1.5 kPa
Residential imposed action
1.5 kPa
Wall self-weight per level
187.5 kN
Ultimate slab line action
2,045 kN/m
Ultimate wall line action
450 kN/m
Adopted total line action
2,500 kN/m
Total wall action
25,000 kN

For the symmetrical demonstration model, the total ultimate action produces nominal support reactions of 12,500 kN at each column. Actual projects may attract unequal reactions where the loading or tributary areas are not symmetrical.

Gravity load calculation and reactions for the wall transfer example
Representative ultimate gravity action and nominal column reactions adopted for the model.
03 · Behaviour

Bernoulli regions and disturbed regions

Strut and tie modelling is used where the strain and stress fields are too disturbed for ordinary sectional design assumptions.

A B-region is a part of the member where the usual beam assumptions remain reasonably valid and the stress distribution can be related to sectional behaviour. A D-region occurs near discontinuities such as concentrated loads, supports, openings, corbels, abrupt changes in geometry and transfer zones.

For the wall supported on columns, the lower zone above the supports is a D-region. The uploaded example adopts an effective disturbed height of approximately 0.8 times the clear support span as an initial engineering estimate.

Elastic stress trajectories through a deep wall supported at two points
Elastic stress trajectories help reveal the principal compression and tension paths before the STM is idealised.
Finite element horizontal and vertical stress contours for the wall transfer region
Finite-element stress contours can support understanding of the disturbed region, but do not replace rational detailing.

General modelling principles

Principle 1

Follow the load path

Align the idealised truss with the principal flow of compression and tension.

Principle 2

Maintain equilibrium

Every strut, tie and node must form a complete and balanced force system.

Principle 3

Respect geometry

Use practical strut angles and nodal dimensions compatible with the structure.

Principle 4

Detail the reinforcement

Provide sufficient anchorage, distribution and bursting reinforcement.

04 · STM idealisation

Turning the stress field into a truss model

The wall is idealised using two diagonal concrete struts, a lower reinforcement tie and nodal zones at the load and support regions.

Compression is carried from the loaded upper wall towards the two supports through inclined concrete struts. The horizontal components of these strut forces are balanced by a reinforcement tie near the bottom of the wall.

The loaded upper region is represented by compression-only CCC nodes, while the lower support regions are CCT nodes because two compression actions and one tension tie meet at each support.

Upper nodesCCC
Support nodesCCT
Concrete membersCompression struts
Steel memberHorizontal tie
Trial strut angleApproximately 70°
Model basisLower-bound equilibrium

A good model is also buildable

More than one equilibrium model may be possible. The preferred model should produce a credible force path and reinforcement arrangement that can be anchored and placed within the available wall thickness and nodal depths.

Strut and tie idealisation showing CCC and CCT nodes, diagonal struts and a horizontal tie
Lower-bound idealisation of the transfer region used for the worked example.
05 · Design workflow

Iterate geometry, stresses and reinforcement

Strut and tie design is an iterative process rather than a single isolated calculation.

  1. Build a model that follows a credible force path.
  2. Review the resulting strut angles and adjust the effective D-region if required.
  3. Apply the ultimate and service actions.
  4. Adopt an initial wall thickness and nodal dimensions.
  5. Check stresses at the upper and lower nodes.
  6. Increase thickness or revise node depths where stresses exceed the permitted limits.
  7. Design the principal tie reinforcement and its anchorage.
  8. Check the concrete struts and their effective compression widths.
  9. Provide horizontal and vertical bursting reinforcement where required.
  10. Review the complete detailing arrangement and issue the calculation report.
Flowchart for the strut and tie design workflow
Typical iterative workflow from model construction through nodal, tie and strut checks.
06 · Nodal checks

Checking the load and support zones

The nodal zones are checked using their geometry, the intersecting force system and the applicable concrete stress limits.

The upper loaded zone is divided into left and right parts according to the share of load attracted by each support. The lower support nodes receive the column reaction, the inclined strut force and the horizontal tie force.

Upper node

CCC condition

Compression acts through all principal faces of the upper nodal zone.

Lower node

CCT condition

The support reaction and strut compression are balanced by the reinforcement tie.

Stress state

Combined faces

Base, side, inclined and shear stresses are reviewed rather than one bearing value alone.

Iteration

Revise dimensions

Wall thickness and nodal depths are adjusted until the stress state is acceptable.

RKALC STM Wall nodal stress checks for the upper and lower nodes
Representative nodal stress results from RKALC STM Wall.
Mohr circle display for a non-hydrostatic concrete nodal stress state
Mohr-circle review of the combined stress state at a non-hydrostatic node.
07 · Tie reinforcement

Designing and anchoring the principal tie

The tie reinforcement carries the horizontal tension required to balance the inclined compression struts.

The worked model produces a representative tie force of 4,579 kN and a required reinforcement area of approximately 10,775 mm². One possible arrangement is eighteen N28 bars distributed over six layers of three bars.

The bars should be distributed through an appropriate portion of the lower nodal depth rather than concentrated into a single narrow layer. Their centroid should align as closely as practical with the assumed tie force path.

Detailing requirements

  • Anchor the tie beyond the support nodal zones.
  • Provide sufficient development length or mechanical anchorage.
  • Maintain practical spacing and concrete cover.
  • Coordinate the tie with vertical and bursting reinforcement.
  • Confirm congestion can be constructed and concreted properly.
  • Review the actual tie centroid against the analytical model.
Principal tie reinforcement distributed through the lower wall node and anchored beyond the supports
Representative arrangement of the principal tie reinforcement through the lower nodal zone.
08 · Strut verification

Compression capacity and bursting reinforcement

The diagonal concrete struts are checked for compressive stress and transverse splitting or bursting effects.

Each diagonal strut is defined by the geometry of its adjoining nodal zones. The effective width changes along the strut, and the average compression width is used with the strut force to assess the applied stress.

Representative strut force13,312 kN
Strut angle69.88°
Average compression width3,168 mm
Efficiency factor0.92
Applied compression stress29.26 MPa
Representative permitted stress34.93 MPa

The worked example also calculates transverse bursting actions. Horizontal and vertical reinforcement is distributed through the wall faces to resist splitting and control cracking around the spreading compression field.

RKALC STM Wall strut checks including compression stress and bursting reinforcement
Representative left and right strut checks and distributed reinforcement requirements.
Where STM applies

Typical reinforced-concrete D-regions

The same force-path principles can be applied to many concrete details where ordinary sectional theory becomes unreliable.

Application

Transfer walls

Walls supported by columns, transfer beams or discontinuous foundations.

Application

Deep beams

Members where the load reaches the support through a direct compression field.

Application

Corbels

Short cantilevers transferring concentrated reactions into columns or walls.

Application

Pile caps

Foundation elements distributing column actions into discrete piles.

Application

Wall openings

Regions where force paths must divert around doors, windows or service penetrations.

Application

Beam-column joints

Congested nodal regions where several compression and tension actions intersect.

Application

Anchorage zones

Post-tensioning or concentrated-load zones affected by local bursting forces.

Application

Diaphragms

Bridge and building diaphragms transferring concentrated actions across deep regions.

Video series

Learn Strut-and-Tie Modelling with RKALC

Follow practical engineering examples that explain the theory behind Strut-and-Tie Modelling before demonstrating complete design workflows using RKALC.

Episode 1

Introduction to Strut-and-Tie Modelling

Learn the fundamentals of Strut-and-Tie Modelling, understand where it applies within reinforced concrete design, and see how RKALC simplifies the development and verification of engineering models.

  • Introduction to STM concepts.
  • Identifying B-regions and D-regions.
  • Load paths using struts, ties and nodes.
  • Overview of the RKALC STM workflow.
10 · Engineering guide

Preview and download the complete worksheet

Keep the full marked-up worked example beside the video series as a reference for the complete design process.

PDF engineering guide

Shear wall on columns — strut and tie model

The guide contains the sample structure, gravity calculations, STM theory, design workflow, RKALC STM Wall results, reinforcement detailing and the generated calculation report.

  • Twenty-storey transfer-wall example.
  • Ultimate and service gravity actions.
  • B-region and D-region discussion.
  • CCC and CCT node checks.
  • Principal tie reinforcement.
  • Strut stress and bursting reinforcement.
  • Calculation-report output.
Preview PDF Download guide
Cover preview of the shear wall on columns strut and tie engineering guide
Marked-up educational guide and calculation report.

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Engineering perspective
Strut and tie modelling is not about drawing triangles. It is about making the real load path through concrete visible.

A good model satisfies equilibrium, respects the available geometry and leads to reinforcement that can be placed, anchored and clearly communicated. RKALC STM Wall is designed to keep those assumptions, forces and governing checks visible to the engineer.

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