Local timber bridges under watch: training, tools, and critical findings
By: Dave Conkel, State Aid Bridge Engineer
Minnesota’s local timber bridges are getting unprecedented attention lately. We’re receiving critical finding notifications and new load postings almost weekly. A critical finding denotes a structural or safety related deficiency that can lead to a full or partial bridge closure. Other actions following a critical finding can include immediate load postings and/or repair work.
This may sound alarming, but it reflects great progress in how local bridge owners, consultants, and our State Aid Bridge Team Leaders are inspecting timber bridges. This article provides a synopsis of how we’re collectively managing our aging local timber bridge infrastructure.
Today’s local timber bridge landscape
According to our new Advanced Timber Bridge Inspection Manual Supplement, as of January 2025, Minnesota had approximately 1,560 bridges with documented primary timber elements—whether in the deck, superstructure, abutment, pier, or some combination thereof.
Although roughly three or four new timber bridges are built in the state each year, most of the current inventory was built between 1950 and 2000. A closer look reveals that approximately 1,239 bridges had pile cap elements showing localized areas in condition state 4 (severe). The bulk of these bridges were built between the 1960s and 1980s and are reaching the end of their useful life. Along with recent failures, this reality highlights the need for in-depth timber bridge inspections, advanced inspection techniques, and updated load ratings.
Statewide SHV load rating contracts
How are we supporting local timber bridge owners with this growing need? One avenue has been the popular Special Hauling Vehicle (SHV) load rating consultant contracts managed annually by State Aid Bridge. Over the last decade, these large consultant contracts have helped local agencies with advanced timber bridge inspections and new load ratings.
The SHV load rating contracts are subsidized by federal funds and the State Bridge Inspection and Signing Account (BISA). The BISA is governed by MN Statute 169.86, subd. 5b, which establishes the account and defines eligible uses. SHV Contract 10 is currently being developed and, like SHV Contract 9, will focus on local timber bridges in poor to severe condition.
Please work with Moises Dimaculangan of State Aid Bridge to include timber bridges in your inventory that may warrant an advanced inspection.
Advanced Timber Bridge Inspections.
Back in 2014, we collaborated with the UMD Natural Resources Research Institute and the USDA Forest Service on efforts to develop advanced timber bridge inspection tools, inspection protocols, inspection manual, and statewide training. These efforts later connected us with Iowa State University to develop cost-effective timber bridge repairs, culminating in a practical timber bridge design document for designers, owners, and builders.
With the 2022 National Bridge Inspection Standards (NBIS), we realized an update to the Advanced Timber Bridge Inspection Manual was necessary to align with the Specifications for the National Bridge Inventory (SNBI). To support this effort, State Aid Bridge commissioned SRF to assist with this update and to provide statewide training in 2026. It will be a half-day seminar which will walk participants through a timber bridge inspection, demonstrate advanced tools on timber samples, and cover field documentation, NBIS ratings, and responses to critical findings. Blending technical guidance with hands-on practice, it equips local engineers and inspectors to better assess and manage timber bridge safety.
State Aid Bridge, along with input from the County Engineers Bridge Committee and the MnDOT Bridge Inventory Management Unit, may consider additional timber inspection checklist items for the Structure Information Management System (SIMS) to further mitigate risks.
State Aid Bridge Regional Team Leaders
State Aid Bridge has four Team Leaders on staff to assist with timber bridge inspection needs. From advanced inspection tool requirements to on-site training, they are available to help your Team Leaders and Program Administrators meet the new NBIS and SNBI requirements.
As Northwest Regional Team Leader Randy Aamodt says, "Getting up close and personal is critical. Some timber bridge issues sneak up on you, and these advanced inspection tools let us catch them before they become serious.”
Rehab and replacement efforts
Local timber bridge owners have stepped up, collaborating with State Aid Bridge, consultants, and MnDOT to ensure timber bridges are properly load posted. Annually, local agencies are replacing approximately 45 timber bridges with either culverts or bridge structures.
Many counties also apply remedial repairs in-house. Advanced inspections can result in low or unacceptable load postings, or even bridge closures. Since local bridge funding is limited, owners must prioritize, often applying repairs to manage risks until replacement is feasible. Resources on cost-effective timber bridge repairs are available, don’t hesitate to contact State Aid Bridge for guidance.
Looking ahead
Minnesota’s approach to timber bridge inspection combines local action with statewide support, ensuring the aging timber bridge inventory is properly monitored, maintained, and load rated/posted. With available resources, advanced inspection tools, advance inspection training, and inspection findings guiding load posting and repairs, local communities can rely on their timber bridges to remain safe and functional.