CIV5136 Chap.10 Strut Tie Sizing and Anchorage
Strut Tie Sizing and Anchorage
Component sizing turns an equilibrium strut-and-tie sketch into a feasible reinforced concrete load path. Strut shape and cracking influence compression efficiency, node type records the entering compression and tension components, and tie reinforcement must provide both steel area and development beyond the nodal zone.
Geometry is revised whenever required widths or anchorage do not fit.
Classify each compression field as prismatic, fan-shaped or bottle-shaped before selecting its efficiency. Spreading compression creates transverse tension and cracking that can lower effective capacity and require reinforcement.
Calculate a required strut area or width using the force, design concrete strength and appropriate efficiency, then draw that width at the critical node face rather than leaving it as a number detached from the member.
Classify nodes by the forces that bound them: CCC, CCT, CTT or TTT.
Hydrostatic geometry aligns component resultants normal to nodal faces, while non-hydrostatic geometry includes shear on one or more faces. The chosen node dimensions must accommodate bearing, connected strut widths and reinforcement. Moving a face or node changes the truss geometry and requires the forces to be solved again.
Tie design begins with required steel area but ends with force transfer.
Select a bar arrangement that fits the effective tie width, cover and spacing, then show development beyond the back of the node through bond or mechanical anchorage. If the steel or a strut does not fit, revise the load path. A capacity calculation cannot legitimise geometry drawn outside the concrete.
What this chapter covers
- 01
Prismatic, fan and bottle struts
- 02
Strut force and effective area
- 03
Tie force and reinforcement area
- 04
CCC, CCT, CTT and TTT nodes
- 05
Hydrostatic and non-hydrostatic nodes
- 06
Bar development through nodal zones
- 07
Component fit within concrete
- 08
Iterative geometry and force recalculation
Worked example · free
Size an original tension tie
- +1Use As = T divided by phi fy.
- +1Convert 300 kN to 300,000 N.
- +2As required = 300,000 divided by 0.8 times 400 = 937.5 mm squared.
- +1Four 20 mm bars provide 1,256.6 mm squared.
- +1Provided area exceeds the illustrative required area.
- +1Still verify fit, spacing, cover and development beyond the node.
Key terms
- Bottle strut
- A spreading compression field that creates transverse tension and needs an efficiency and reinforcement check.
- Fan strut
- An array of compression trajectories spreading from or converging on a nodal zone.
- CCC node
- A nodal zone bounded only by compression fields.
- CCT node
- A node where compression fields meet one tension tie.
- Development length
- The reinforcement length needed to transfer the calculated tie force through bond or anchorage.
Strut Tie Sizing and Anchorage FAQ
Why does strut shape affect capacity?
Prismatic, fan and bottle compression fields have different spreading, cracking and transverse-tension behaviour. The selected efficiency must match the physical field rather than only its axial force. Required width must then be drawn inside the available concrete geometry. Fit is part of capacity. Anchorage and node dimensions remain independent checks.
How is node type identified?
Classify the components entering the nodal zone as compression or tension. CCC contains only compression, while CCT and CTT include one or more tension ties.
Is sufficient tie steel area enough?
No. The bars must fit within the tie region, satisfy spacing and cover, and develop the calculated force beyond the nodal zone. Undeveloped reinforcement cannot complete the assumed load path.
What happens when a required strut width does not fit?
Revise node locations or choose a different equilibrium truss, then resolve forces and repeat capacity checks. Drawing the calculated width outside the concrete does not produce a feasible design.
Why is bottle-strut spreading important?
A bottle-shaped compression field widens between its ends and creates transverse tension as the force spreads. Cracking changes its effective compressive behaviour and may require transverse reinforcement. Select the efficiency and detailing rule for that field rather than treating it as a uniform prismatic block.
When must node geometry be revised?
Revise it when bearing faces, connected strut widths, tie anchorage or concrete boundaries cannot coexist at the assumed location. Moving the node changes member angles and resolved forces, so repeat equilibrium and every capacity check instead of adjusting only the drawing.
Exam move
Use a component schedule with force, type, efficiency, required dimension, provided detail and check result. Classify every strut shape and node before selecting an expression. For ties, keep force capacity and anchorage as separate rows. Overlay calculated widths and reinforcement paths on a scaled member outline.
Any geometry change alters strut angles and component forces, so loop back to equilibrium rather than recycling earlier numbers. End with a drawing that shows nodal faces, strut boundaries and developed bars. Add a revision column that records why a component failed: stress, width, bar fit, cover, anchorage or node geometry.
That label points directly to the rational change and prevents a failed design from being patched with an unrelated larger value.
