Last reviewed 30 Sept 2026 · 6 min read
Why the end of a prestressed member is critical
In post-tensioned members the whole tendon force is applied at the ends through a small anchorage plate. The concrete just behind it is subjected to huge local compressive stress, and as the force spreads out into the full depth of the member, transverse tensile stresses develop that can split the concrete. A large number of failures during stressing come from poorly reinforced anchorage zones, so the design of the end block is as important as the design of the beam.
Pre-tensioned members transfer force by bond over the transfer length, and the equivalent problem is smaller (the force enters gradually), but bond splitting and end-cracking near the ends must still be checked.
Zones at the end
- Local zone — the concrete immediately behind the anchor plate; it is subject to very high bearing pressure and needs confinement (spiral or closely spaced hoops, or a special anchor casting).
- General zone — the region in which the concentrated force spreads to a linear stress distribution; it extends about one member depth from the end (from Saint-Venant's principle). Here the transverse tension develops.
Types of transverse tension
- Bursting tension — in the centre of the block, at some distance from the anchor plate, the compressive trajectories spread out and create transverse tension along the line of the force. It is the largest and most damaging effect.
- Spalling tension — near the loaded face, away from the anchor, tension at the end face occurs when the anchor is eccentric or the section is flanged; it can cause the concrete surface to peel off.
- Edge (longitudinal) tension — when the anchor force is eccentric, the far edge of the end block can go into tension along the length as the stress flow adjusts.
Bearing pressure under the anchor plate
The bearing stress on the concrete under the plate is limited to
= concrete strength at transfer; = loaded (plate) area; = maximum area of the similar (geometric) figure that can be inscribed in the concrete cross-section, concentric with the plate. Confinement by spiral reinforcement raises the allowable pressure. Anchor manufacturers give tested values for their anchorage devices and the code defines their acceptance tests.
Guyon's bursting tension
Guyon's analysis of a symmetrically loaded end block gives the bursting tension as
= the tendon force (jacking force, with a load factor at ultimate), = half the side of the loaded (anchor) area, = half the side of the end block (the two dimensions in the plane considered). When the anchor plate covers a large fraction of the end face, is large and the tension small; a small plate gives a large bursting tension.
The maximum bursting stress occurs at a distance of about 0.5–1.0 from the end face, and the tension zone extends about 2 into the member. The reinforcement is distributed over this length.
A tendon force = 1000 kN is applied by a square plate of 150 mm side to an end block of 300 mm × 300 mm (symmetrical).
= 75 mm, = 150 mm → = 0.5.
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Using mild-steel bars at a design stress of about 140 N/mm² (permissible stress design), mm² in each direction — for example, 4-legged 10 mm stirrups at close spacing over the bursting length. (Codes using the limit state method specify a higher load factor and .)