This case study is from 2012. All project details have been intentionally anonymised to protect confidentiality. I am grateful to the former Superintending Consultant for sharing this valuable reference.
NCR No.: NCR-HWY-...-.... | Date Raised: 202x-0X-xx
Area: Bridge Superstructure : Box Girder Segment B2,
Chainage CH 12+300 to CH 12+320, Gridline C1–C2
Clause Reference: JKR Bridge Works Clause 7.3.2
Severity: Medium | Disposition: Rework (Realignment)
1.0 PROJECT CONTEXT
Highway bridge works involving cast-in-situ reinforced concrete Box Girders.
During routine inspection of the box girder segment placement prior to post-tensioning, an alignment deviation was detected.
2.0 DESCRIPTION OF THE NON-CONFORMANCE
The horizontal alignment of Box Girder Segment B2 deviated +45 mm from approved design centreline after formwork stripping.
2.1 Tolerance Reference
JKR Specification: Civil Works Clause 7.3.2 – Alignment tolerance for bridge superstructure: ±30 mm,
BS 5400 / Eurocode / AASHTO: Similar ±25-40 mm depending on span and structure class.
JKR QA/QC Plan (Project Specific): Alignment control and surveying tolerances.
This deviation exceeds the allowable tolerance (±30 mm)
2.2 Evidence/Attachments
Site photographs,
Survey records,
Inspection checklist,
Test results
Sketch/marked-up drawing
Classification : Medium Risk NCR (not severe to halt works but must be corrected before post-tensioning).
3.0 FINDINGS
4.0 REFERENCE
JKR Specification for Bridge Works,
JKR Standard Drawings (SJ / SK / SP series),
BS 5400 / Eurocode 2 / AASHTO LRFD (for reference),
Project QA/QC Plan & Inspection Test Plan (ITP)
Survey Control Method Statement
5.0 CALCULATIONS/SURVEY VALIDATION
Survey Data (Before & After)
Design Line: X = 1,250.000 m
Measured After Stripping: X = 1,250.045 m
Deviation: Δ = 0.045 m ⇒ 45 mm
5.1 Tolerance Check
| 0.045m | =45mm>30mm (JKR tolerance)
= OUT OF TOLERANCE
5.2 Effect on Post-Tensioning
Misalignment may induce eccentricities, resulting in:
Stress redistribution
Potential cracking
Post-tensioning stressing inefficiencies
A quick simplified check:
6.0 METHOD FOR CORRECTIVE ACTION (Corrective Realignment of Box Girder Segment B2)
6.1 Activities
Set up precision total station and re-establish control points.
Verify existing alignment and mark deviation.
Loosen temporary supports (if any) under segment B2.
Apply controlled jacking (using hydraulic jacks) to shift girder laterally to design centreline.
Monitor moves in real time (surveyor continuously records).
Lock off and resecure formwork anchors and supports.
Re-survey final position and record in NCR corrective action.
Pre-stress only after verification by QA/QC and Resident Engineer.
6.2 Resources
Hydraulic jacks and load cells
Total station and Optical targets
Trained survey crew
Supervision by Structural Engineer
6.2 Acceptance Criteria
Final alignment within ±20 mm (stricter than JKR tolerance for safety)
Record updated survey logs
7.0 ROOT CAUSE ANALYSIS
oot Causes
Survey control points not adequately protected during casting = drifted,
Inadequate formwork bracing = lateral movement under concrete pressure.
8.0 CORRECTIVE ACTIONS
Immediate:
Realign girder segment (as per method statement),
Reestablish and secure all survey control points,
Resurvey all adjacent segments.
9.0 ROLES, RESPONSIBILITIES AND AUTHORITIES
10.0 PREVENTIVE ACTIONS
Update Survey Control Method Statement,,
Install redundant benchmark targets,,
Daily verification of bracing and formwork stability,
Pre-pour check sign-off with 2-person independent verification,
Toolbox talk for crew on tolerance requirements
11.0 IMPACT ASSESSMENT AND RISK REGISTER UPDATE
No comments:
Post a Comment