CIV5136 Chap.9 Strut and Tie Model Foundations
Strut and Tie Model Foundations
Strut-and-tie modelling represents a disturbed concrete stress field as an equilibrium truss. Compression fields become struts, reinforcement becomes tension ties, and finite nodal zones transfer their forces.
The model begins with D-region boundary actions and must reproduce those resultants while remaining geometrically credible inside the concrete member.
Disturbed regions occur near concentrated loads, reactions, openings and sudden geometry changes, where ordinary beam stress assumptions no longer describe the local flow. Establish the axial force, shear and moment acting on the D-region boundary.
Place nodes where those external resultants enter or leave, then select direct compression paths through the concrete and tension paths that practical reinforcement can follow.
At every node, draw component arrows and enforce horizontal and vertical equilibrium. The compression struts should align with plausible concrete stress trajectories, while ties close the tension resultants and require anchorage.
The model is not unique. A useful choice minimises unnecessary tie length, avoids impossible crossing or overlap, stays inside the concrete outline and leaves space for nodal zones and reinforcement.
Nodal zones are finite volumes rather than dimensionless pins. Bearing plates, support widths and developed bars determine where force can actually transfer.
After solving the ideal truss, overlay strut and tie centrelines on the member, confirm that the internal reactions reproduce all boundary resultants, and identify where later component sizing may force a change in node position or load path.
What this chapter covers
- 01
Bernoulli regions and disturbed regions
- 02
Static and geometric discontinuities
- 03
D-region boundary resultants
- 04
Compression struts and tension ties
- 05
Nodal zones and external force entry
- 06
Equilibrium truss selection
- 07
Direct and buildable force paths
- 08
Global and nodal equilibrium checks
Worked example · free
Resolve a symmetric deep-beam truss
- +1Symmetry gives 300 kN vertical reaction at each support.
- +1Top-node equilibrium is 2C sin45 degrees = 600.
- +1Each strut force is 424.3 kN compression.
- +1Each horizontal component is 424.3 cos45 degrees = 300 kN.
- +1The bottom tie carries 300 kN tension.
- +1The two vertical components total the applied 600 kN.
Key terms
- B-region
- A region with smooth stress flow where ordinary beam assumptions are reasonably applicable.
- D-region
- A disturbed zone near concentrated actions or geometry changes where a truss idealisation can represent load transfer.
- Strut
- A concrete compression field represented by a compressive member in the equilibrium model.
- Tie
- A tensile force path generally carried by reinforcement and anchored through the connected nodes.
- Nodal zone
- A finite concrete volume that transfers forces among struts, ties and external actions.
Strut and Tie Model Foundations FAQ
Where is a strut and tie model used?
It is suited to disturbed concrete regions near supports, concentrated loads, openings, corbels, deep beams and abrupt changes in geometry where linear beam stress assumptions are unsuitable. The truss represents how the boundary resultants flow through concrete and reinforcement. Equilibrium selects the component forces. The final truss must reproduce the boundary resultants.
Is the equilibrium truss unique?
No. Several load paths may satisfy equilibrium. A useful model is direct, stays inside the concrete, permits practical reinforcement and anchorage, and can accommodate the required strut and node dimensions.
What is the difference between a strut and a tie?
A strut represents compression carried through concrete, while a tie represents tension usually carried by reinforcement. Their capacity rules and detailing needs therefore differ even when force magnitudes are similar.
Why are nodes finite zones rather than points?
Real forces enter through bearing faces, compression fields and anchored bars over physical dimensions. The nodal zone must transfer those resultants without exceeding its capacity or conflicting with geometry.
How are D-region boundary forces obtained?
Cut the disturbed region from the surrounding structure and replace the adjoining B-regions by their resultant axial force, shear and moment, together with direct loads and reactions. The internal truss must reproduce those resultants, so the global boundary equilibrium is the first and last model check.
What makes one load path preferable?
Prefer a direct equilibrium path that stays within the concrete, avoids unnecessary tension, permits practical bar placement and leaves room for strut and nodal widths. Because several trusses can balance the same boundary actions, constructability and credible stress flow help choose among them.
Exam move
Mark every concentrated load, support reaction, opening and geometry change on the concrete outline, then identify the D-region boundary resultants. Place nodes at physical load-entry and support areas. Draw direct compression paths and the reinforcement ties needed to close equilibrium. Solve every node with force arrows before thinking about capacity.
Finally overlay the force path on the real member and ask whether strut widths, nodal faces and tie anchorage can fit; if not, change the truss and solve again. Practise two alternative equilibrium models for the same region and compare tie force, path length and constructability. The exercise makes clear why equilibrium alone does not select a design and why a small change in node location changes every resolved force.
