Related to Structural Timber Construction
Case Study: Timber Deck Fastener Failure – Pine Valley Bridge (2017)
Project Overview
- Bridge Name: Pine Valley Timber Bridge Replacement
- Location: Allegheny County, Pennsylvania
- Year: 2017
- Project Size: $3.1 million
- Scope: Replacement of an aging timber bridge with new pressure-treated timber decking supported by timber beams and girders.
- Owner: Pennsylvania Department of Transportation (PennDOT)
- Contractor:
Category of the Issue, Problem, or Challenge
- Fastener Failure
- Timber Decking
- Quality Control
Summary of the Issue, Problem, or Challenge
During the timber deck installation phase, multiple fastening failures occurred where nails and screws pulled out or sheared off prematurely. This resulted in loose decking panels, posing safety risks and requiring re-fastening and partial deck replacement. The root causes were linked to improper fastener selection and insufficient embedment depth in the treated timber.
Root Cause Analysis
| Factor | Details |
| Inappropriate Fastener Selection | Used standard nails instead of corrosion-resistant, longer fasteners suitable for pressure-treated timber. |
| Insufficient Embedment Depth | Fasteners did not penetrate deep enough into beams due to incorrect installation techniques. |
| Timber Density Variation | Variability in timber density led to inconsistent fastener holding capacity. |
| Lack of Field Training | Installation crews were not adequately trained on best practices for timber fastening. |
Impacts Due to the Issue, Problem, or Challenge
- Immediate safety hazards due to loose decking panels requiring work stoppage.
- Additional costs of $50,000 for fasteners, labor, and partial decking rework.
- Project delay of 3 weeks to re-secure decking and perform inspections.
- Increased supervision and quality control during fastening operations.
Corrective Actions Taken
- Fastener Upgrade: Switched to corrosion-resistant, longer fasteners designed for treated timber.
- Installation Training: Conducted crew training on proper fastener embedment and installation techniques.
- Field Inspections: Introduced mandatory inspections of fastening quality during and after deck installation.
- Material Specifications: Revised project specs to require fasteners certified for pressure-treated timber use.
Lessons Learned
- Fastener selection must be compatible with timber treatment and density for effective long-term performance.
- Proper embedment and installation techniques are critical to prevent fastening failures.
- Training and supervision during timber deck installation reduce risk of structural and safety issues.
- Early detection of fastening issues prevents costly rework and project delays.
Case Study: Timber Beam Shear Failure – Pine Valley Rural Bridge (2017)
Project Overview
- Bridge Name: Pine Valley Rural Bridge Replacement
- Location: Jackson County, Oregon
- Year: 2017
- Project Size: $3.5 million
- Scope: Replacement of a deteriorated timber bridge with new treated timber beams and girders supporting a timber deck.
- Owner: Oregon Department of Transportation (ODOT)
- Contractor:
Category of the Issue, Problem, or Challenge
- Structural
- Timber Beam Failure
- Quality Control
Summary of the Issue, Problem, or Challenge
During placement of new timber beams on the abutments, one major beam suffered a sudden shear failure at the bearing end while lifting and positioning with a crane. The beam fractured abruptly, causing damage to adjacent components and a temporary halt in the work zone.
Root Cause Analysis
| Factor | Details |
| Undetected Internal Defects | Beam had internal knots and splits not detected during pre-installation inspection. |
| Improper Lifting Points | Lifting slings placed too close to bearing ends causing excessive localized stresses. |
| Insufficient Bearing Surface | Bearing pads under the beam were uneven and less than specified size. |
| Lack of Non-Destructive Testing | No NDT methods (e.g., ultrasound) were used to identify internal defects. |
Impacts Due to the Issue, Problem, or Challenge
- Immediate halt to beam installation activities
- $45,000 additional cost for replacement beam and crane time
- Two-week project delay due to investigation and procurement
- Increased safety risk concerns raised by site inspectors
Corrective Actions Taken
- Enhanced Inspection: Implementation of visual and ultrasonic inspection of all timber beams prior to delivery and installation.
- Revised Lifting Protocol: Crane operators and riggers retrained on optimal lifting points for timber beams to minimize stress.
- Bearing Pad Upgrades: Bearing areas enlarged and leveled with high-strength concrete pads as specified.
- Quality Assurance Plan: Contractor introduced a more rigorous QA process including NDT for critical members.
Lessons Learned
- Internal timber defects can critically weaken beams and go unnoticed without proper inspection.
- Lifting procedures must respect timber’s anisotropic nature to avoid damage.
- Adequate bearing support is essential to distribute loads and prevent local failures.
- NDT techniques add value and safety in timber bridge construction beyond visual checks.
Case Study: Timber Guardrail Deflection – Big Elk Creek Timber Bridge (2020)
Project Overview
- Bridge Name: Big Elk Creek Bridge Replacement
- Location: Lane County, Oregon
- Year: 2020
- Project Size: $3.7 million
- Scope: Replacement of aging timber bridge with new glulam superstructure and timber railing for scenic compatibility
- Owner: Oregon Department of Transportation (ODOT)
- Contractor:
Category of the Issue, Problem, or Challenge
- Timber Construction
- Safety Barrier Performance
- Quality Control / Specification Compliance
Summary of the Issue, Problem, or Challenge
Shortly before project completion, the timber railing system on the upstream side of the bridge was found to exceed allowable deflection under simulated vehicular impact loads. Field tests showed that several sections flexed more than 6 inches laterally under load, exceeding the AASHTO LRFD Bridge Design Specifications for rail deflection limits.
Root Cause Analysis
| Factor | Details |
| Undersized Timber Posts | Posts used were 6×6 inches instead of the specified 8×8 members. |
| Post Spacing Error | Posts were spaced at 12 feet rather than the specified 8 feet, reducing stiffness. |
| Improper Bolt Embedment | Lag bolts anchoring rails to posts were only partially embedded, reducing holding strength. |
| Misinterpretation of Drawings | Contractor misread section details during fabrication of railing segments. |
Impacts Due to the Issue, Problem, or Challenge
- Partial removal of 180 linear feet of timber rail system
- $42,000 cost increase due to post replacement and new mounting hardware
- Schedule delay of 3 weeks for redesign and re-inspection
- Temporary traffic control required due to delayed bridge opening
- Internal audit of contractor’s plan-reading and QA procedures initiated by ODOT
Corrective Actions Taken
- Post Upgrade: All 6×6 posts replaced with specified 8×8 members and reinstalled per drawings.
- Spacing Revision: Post spacing corrected to 8 feet on center.
- Anchoring Detail Enforcement: Field engineers reviewed and reapproved all anchoring hardware.
- Mock-Up Requirement: All future timber rail systems required a full-scale mock-up for dimensional review before installation.
- Plan Interpretation Training: Contractor required to conduct a plan-reading refresher for site staff.
Lessons Learned
- Small dimensional errors in timber guardrail systems can drastically reduce lateral resistance.
- Plan interpretation errors must be caught early—mock-ups are effective for visual and spatial validation.
- Timber is susceptible to over-deflection under lateral loads and requires conservative design practices.
- Bridge railings, even when built for aesthetic or historic compatibility, must meet current impact and deflection criteria.
Case Study: Fender Misalignment at Portage Bay Bridge (2017)
Project Overview
- Bridge Name: Portage Bay Replacement Project
- Location: Seattle, Washington
- Year: 2017
- Project Size: $59 million (bridge replacement phase)
- Scope: Installation of a temporary work trestle and protective timber fenders for pier protection during bridge replacement
- Owner: Washington State Department of Transportation (WSDOT)
- Contractor:
Category of the Issue, Problem, or Challenge
- Marine Construction
- Timber Structural Elements
- Substructure Protection Systems
Summary of the Issue, Problem, or Challenge
During the construction of a protective fender system surrounding a deepwater pier footing, significant misalignment was discovered after installation. The fender piles—made from pressure-treated timber—were placed up to 1.5 feet outside the specified envelope. This compromised the intended protective geometry, leaving portions of the footing exposed to barge impact and violating U.S. Coast Guard permitting conditions.
Root Cause Analysis
| Factor | Details |
| Survey Control Failure | Inaccurate GPS-based positioning due to multipath interference near metal equipment. |
| Lack of Redundant Checks | No secondary control points or manual verification was used before driving piles. |
| Soft Subgrade Conditions | Unanticipated underwater soft sediments caused pile drift after initial driving. |
| Rushed Work Schedule | Contractor skipped intermediate checks to meet an aggressive milestone deadline. |
Impacts Due to the Issue, Problem, or Challenge
- Removal and replacement of 14 misaligned fender piles
- Two-week delay in trestle access for bridge pier demolition
- $110,000 in additional costs (materials, dive support, labor, schedule impacts)
- Temporary stop-work order issued by Coast Guard pending re-survey and compliance report
- Review initiated by WSDOT on contractor’s marine positioning procedures
Corrective Actions Taken
- Dual-Mode Surveying: Added optical total station backup to GPS positioning during pile layout.
- Geotechnical Verification: Conducted underwater borings at each fender location to validate subgrade stiffness.
- Installation Templating: Prefabricated steel templates were used to maintain exact pile spacing and orientation.
- Real-Time Monitoring: Employed live sonar imaging during pile driving for drift detection.
- Training & QA Protocol Update: WSDOT updated their marine QA standards to include cross-checking for fender systems.
Lessons Learned
- Marine timber pile systems require precise layout, especially near active waterways.
- Redundant survey methods are essential in environments prone to GPS signal error.
- Soft sediments can lead to unseen pile drift—design and means/methods must anticipate this.
- Protective elements like fenders are regulatory—not just structural—and must meet Coast Guard standards.