24, 60 or 72 Months? How Your Bridge's Inspection Interval Is Set
By Cheryl Ansel, DOT bridge program engineer, NHI-qualified team leader. Reviewed by Amanda McCallister, editor.
Underwater Bridge Inspection | Cathodic Protection & Anodes I've stood on a lot of bridge decks waiting for a dive team to surface. The one I remember best was a 1976 welded girder over the Missouri River, where the pilot asked if we could push the underwater survey to 72 months because the budget off-ramp was dragging. I told him yes, but only if the previous two cycles showed zero scour movement and the owner could document a stable riverbed profile. That conversation is exactly what 23 CFR 650.311 is about. The rule isn't a one-size-fits-all. It's a framework that lets the state set an interval based on risk, condition, and history. Too many bridge owners think 24 months is the only option for routine inspections and 60 is the only option underwater. That's the baseline, not the ceiling. But getting past the baseline takes documentation, not just a signed form.
The Baseline: 60 Underwater, 24 Above
Start with the numbers that actually appear in 23 CFR 650.311. Paragraph (b) sets the underwater inspection interval at a baseline of 60 months. Routine above-water inspection? That's 24 months. So the default for most bridges is a dive every five years and a visual walkthrough every two. Those aren't optional or arbitrary – they're the starting point. But here's the part that surprises people: the 24-month above-water number is not a hard maximum, and neither is the 60-month underwater number. The rule says the interval shall be established based on the conditions in the regulation. So the state owner – usually me, in my office – sets it. The baseline applies unless you justify something else. And that justification isn't a one-page memo. It's a documented engineering decision that goes into the bridge file. If the state gets audited by FHWA, you better have every sonar image, every dive log, every bed elevation survey to back up that 72-month call.
Stretching to 72 Months Underwater
The 72-month underwater interval shows up in 650.311(b)(1)(iii)(A) and (b)(2). It's not an informal FHWA nod. The regulation says you can extend to 72 months if the bridge meets the stated conditions: no known deficiencies in the underwater members, a history of satisfactory inspections, and a stable, documented stream bed. The phrase 'documented' is doing a lot of work here. You need the dive reports, the sonar data, the bed change analysis. I've signed off on 72-month intervals maybe a dozen times in 25 years. Every one of those bridges was a low-risk river crossing with redundant structure, no ice damage, no scour red flags. And even then, I kept the above-water at 24. The underwater extension doesn't change the visual schedule. It's a different clock. But don't for a second think 72 is a reward for doing nothing. It's a reward for doing everything right, consistently, and proving it to me on paper.
Above-Water Intervals Aren't a Hard 24 Either
Now the above-water side. The same regulation allows 48-month and even 72-month intervals for routine inspection under the stated conditions. So a bridge with a good track record, no fatigue-prone details, and low traffic can go two or three years between walkthroughs. Some states do this for certain structures, especially low-volume rural crossings. But don't read that as permission to stretch everything. If the bridge has fracture-critical or non-redundant members, you're likely to stay at 24. The regulation separates that into 650.311(c) under the label 'Non-Standard Tolerable Movement' – what used to be called fracture critical. That's not about underwater, though. It's about the upper structure and the special inspection that goes along with it. And a 48-month routine interval still requires an annual or two-year check of critical details like bearings, welds, and pin hangers. The longer interval applies to the overall inspection, not to the specific elements that can kill you if they let go.
Who Signs and Who Dives
One thing that doesn't change with the interval is who's qualified to do the work. Inspector qualifications are in 23 CFR 650.309, not 650.311. The team leader has to be an FHWA-NHI qualified team leader. That's the certification from the National Highway Institute – the two-week course, the field exam, the reputation piece. I've been one since the late '90s. The diver in the water deserves the same scrutiny. A PADI or NAUI card is a recreational credential. It does not mean you can inspect a bridge foundation. We use commercial divers with ADCI consensus standards training and real in-water experience. And don't get me started on the guy who shows up with a dive shop certification and a wetsuit. That's a digression, but it's one I make every time a contractor tries to cut corners. The interval is only as good as the team that executes it. You can set 60 months, 72 months, whatever the rules allow – but if the inspection itself is sloppy, you'll miss the scour hole or the undermining that takes a pier down. That's not a paperwork problem. That's a bridge failure waiting to happen.
Frequently Asked Questions
Does the 60-month underwater baseline mean a dive every five years, without exception? Not exactly. The baseline is 60 months, but you can go to 72 if the bridge meets the conditions in 23 CFR 650.311(b)(1)(iii)(A) and (b)(2). You cannot go shorter than 60 without a specific finding, and you can't go longer than 72. Is the 24-month routine inspection interval a hard maximum? No. Routine above-water inspection can be extended to 48 or 72 months under the rule, but only if the bridge qualifies based on condition, risk, and documented history. The default is still 24. Who ultimately decides whether a bridge goes from 60 to 72 months underwater? The state DOT bridge owner, but the FHWA-NHI qualified team leader has to sign off. It's not a one-person call; it's a documented engineering decision based on the factors in 650.311. Does the 72-month underwater interval apply to all bridges? No. It's limited to bridges that meet the stated conditions in the regulation. If you've had scour issues, known deficiencies, or a history of poor dive reports, you'll stay at 60 or shorter.