Adisorn Owatsiriwong
Understanding D-Regions
1. D-Region (Disturbed Region):
• The D-region is a portion of the structure where the stress distribution is significantly disturbed and does not follow the typical linear stress distribution assumed in beam theory. These regions are characterized by complex stress patterns and require special attention in design.
• Examples of D-Regions: Areas around concentrated loads, supports, abrupt changes in cross-section, openings, and anchorage zones of prestressed members.
• In this case, the local zone directly behind the anchor is a very specific part of the D-region where the prestressing force is introduced into the concrete. The general zone surrounding this local zone, where the stress field is still disturbed but not as highly concentrated, is also part of the D-region.
2. Local Zone vs. General Zone within the D-Region:
• Local Zone: The immediate area behind the anchor, where the prestressing force is applied, is the most critical part of the D-region. This is where the bearing stresses are highest, and where confinement reinforcement is most necessary to prevent splitting and crushing failures. Local zone spreads about 1-2 times of the width of the anchor plate(b).
• General Zone: This is the area extending outward from the local zone. It can start from 2-4 times of the width of the anchor plate. While the stress disturbance is still present, it becomes less intense as the distance from the anchor increases. In this zone, the stress pattern transitions from the highly concentrated forces near the anchor to a more diffused state. This area still requires careful design, but the demands may not be as severe as in the local zone.
3. Compressive Stress Transfer in the Local Zone:
• Stable Local Zone: When the concrete within the local zone (directly behind the anchor) is stable and well-confined (e.g., by helical reinforcement), the compressive stress can be effectively transferred through the local zone. In this scenario, the stress paths remain within the plane of the prestressing force and do not need to divert to the upper and lower parts of the slab (like the flanges of an I-beam).
• In-Plane Stress Transfer: If the local zone is stable, the compressive stresses follow a direct path, maintaining their orientation within the plane of the applied force. This helps in maintaining the integrity of the structure, as the stresses are managed within the local zone, reducing the likelihood of out-of-plane stress transfer that could lead to additional complications.
Figure 1 Installation of helical reinforcement at anchorage zone (Posteck Prestressing, 2024)
4. Bearing Failure in the Local Zone:
• Bearing Failure Consequences: If the concrete in the local zone fails in bearing (due to exceeding its compressive strength or insufficient confinement), the stress must find alternative paths to redistribute. Since the direct path through the local zone is compromised, the stress will be diverted to the surrounding concrete, often moving towards the upper and lower parts of the slab (flanges in the I-beam analogy).
• Stress Diversion to Slab Flanges: When the stress diverts to the slab flanges, it introduces additional tensile forces in these regions. Since concrete is generally weak in tension, this diversion can lead to vertical tensile splitting as the slab tries to accommodate the redirected stress.
Figure 2 Bursting at anchorage zone due to insufficient bearing strength of concrete (Posteck Prestressing, 2024)
5. Vertical Tensile Splitting and Bursting:
• Vertical Splitting in General Zone: Upon bearing failure in the local zone, the diverted stresses create vertical tensile forces in the slab flanges, which can propagate cracks from the local zone into the general zone. This splitting is exacerbated if the slab is thin or lacks sufficient reinforcement to handle these redirected stresses.
• Progressive Collapse Risk: If the vertical tensile splitting is severe enough, it can lead to a progressive failure where the cracks propagate through the slab, potentially leading to a local or even global collapse (bursting). This is particularly concerning in prestressed slabs where the high forces involved can cause rapid and extensive damage once the initial failure occurs.
6 Experimental Observations:
• Experimental Consistency: Experiments on prestressed concrete structures often show that when the local zone fails in bearing, the stresses do indeed divert to other parts of the structure, causing splitting and potential bursting. The exact pattern of this failure depends on factors like slab thickness, reinforcement details, and the magnitude of the prestressing force.
• Bursting Phenomena: In experiments, bursting typically manifests as cracking that radiates outward from the failed local zone, aka. vertical tensile splitting. This is particularly observed in cases where the local zone is not adequately confined or where the prestressing forces are very high relative to the slab’s capacity.
7. Conclusion:
• Stable Local Zone: If the local zone is stable and well-confined, compressive stresses can be transferred effectively without needing to divert to the slab flanges.
• Bearing Failure and Stress Diversion: Upon bearing failure in the local zone, the compressive stresses must divert to other parts of the slab, leading to vertical tensile splitting, particularly in the slab flanges.
• Risk of Bursting and Progressive Collapse: This diversion can lead to vertical tensile splitting and, in severe cases, cause a progressive collapse due to bursting.
