Week of Aug. 14, 2007

Bridge inspections support safe travel throughout King County

Inspecting a bridge is more like inspecting a house than a car. There are no tires to kick or hood to look under. But, there is a foundation to keep an eye on, structure to preserve, components to weatherproof, and other support systems to repair.

With responsibility for more than 258 bridges, King County takes its inspection duties for those spans, trestles, and crossings very seriously. Not just because it is required to do so by the federal government, but primarily because the public relies on safe and reliable bridges to travel throughout the county’s road network.

“Bridges need attention just like homes, and inspections can reveal the need to repair a couple parts, rehab the bridge with widespread improvements, or possibly replace the entire structure,” said Tim Lane, a supervising engineer with the Bridge & Structural Design Unit of the King County Road Services Division.

Of the 258 bridges that the Roads Division inspects and inventories:

• 181 are wholly owned by the Roads Division;
• Six are co-owned with local cities;
• 61 are owned by a city that contracts with Roads for bridge inspection;
• 10 are owned by the King County Department of Natural Resources and Parks, and travel over or under roadways requiring inspection by the Roads Division.

Those bridges vary in length, width, height, structural materials, age, condition, the feature they cross, and the volume of traffic that crosses them. The busiest bridge is the Bear Creek Bridge on Avondale Road with a daily traffic volume of more than 26,000 vehicles. Then, there is the Tye River Bridge near Stevens Pass, which is only open to foot, mountain bike, snowmobile, and horse traffic.

The National Bridge Inspection Standards (NBIS) mandates that public agencies inspect and report on all bridges at least once every two years. Under these standards, King County is required to document and report the current condition of each bridge, determine the degree of wear and deterioration, and recommend repairs or needed services.

”Inspection times can vary from less than 30 minutes on simple, newer spans to several days on older, complicated, multi-span bridges,” said Lane.

Certain bridges require special inspections in addition to a routine review. These special inspections are designated as Under Bridge Inspection Truck (UBIT), Fracture-Critical, and Underwater Inspections.

• Bridges with elements that are not accessible from the ground or from a ladder require an UBIT to properly inspect the bridge. These inspections are performed on 43 bridges every two to six years.
• Bridges with steel elements under tension that, if fractured, would result in partial or total collapse of the bridge require a fracture-critical inspection. These inspections are required every two years on 17 bridges and coincide with the routine inspections.
• Bridges with piers that cannot be visually inspected from above water require underwater inspections. Divers perform these inspections on six bridges once every five years, except for the South Park Bridge, which is inspected every three years due to the poor condition of the in-water piers.

Next week, it will be the Mt. Si Bridge’s turn for inspection. The 90-year-old bridge – located on Mt. Si Road near North Bend – is slated for replacement in 2007. Yet, it must still be inspected on a regular basis until it is permanently closed and the new bridge opened.

The inspection of the Mt. Si Bridge will probably take four days to complete and requires routine, fracture critical, detailed timber, and UBIT inspections. Due to the narrow deck and restricted-load capability, traffic may be restricted across the bridge for up to two days while the UBIT equipment occupies the deck. Special inspection techniques are necessary for this bridge – the fracture critical pins that connect the parts of the truss require special ultrasonic testing and its timbers are checked with a pulse-echo instrument that detects rot.

The county has its own inspection schedule for vulnerable bridges, such as the South Park Bridge between Seattle and Tukwila. The condition of the 75-year-old span on 14th/16th Avenue South has severely deteriorated in recent years, and was made worse during the 2001 Nisqually Earthquake. Using a Federal Highway Administration (FHWA) rating scale, the bridge rates a 4 out of a possible 100 points, one of the lowest ratings of any bridge in the state of Washington.

In recent years, the aging bridge has been difficult to operate due to long-term movement of the supporting piers in the Duwamish River, which causes a misalignment of the moveable drawbridge spans. Lane said a routine inspection of the bridge earlier this month showed that the piers continue to move and more repairs will be needed to realign the moveable spans.

Bridges deficient in their condition require more frequent inspection, as do those bridges with a deteriorating timber structure. Although King County no longer builds bridges out of wood, it still has more than 48 bridges with timber components to maintain and keep operational.

As bridges begin to age, certain components require repair. The county’s maintenance program to repair and replace worn or broken components extends the life of the bridge inventory, and corrects any immediate safety deficiencies. The goal of the repairs is to remove hazards and provide for preservation of infrastructure in a cost-efficient manner. Common repairs include replacing cracked concrete, rotted timber, corroded steel, or other deteriorated components.

Careful monitoring of deficiencies such as rot, cracks, and erosion around supports also allows engineers to more accurately assess the rate of deterioration and provide assurance that the bridge is capable of supporting traffic loads. In addition, monitoring helps the engineers better track the rate of change of a condition, which provides more lead-time to schedule repairs that minimize traffic disruptions.

Every year, new bridge deficiencies are found during routine inspections, and repairs are scheduled. Each bridge is also ranked with a rating factor known as the Sufficiency Rating (SR). All of the information in contained in the annual bridge report.

The SR is a score calculated for each bridge with a multitude of ratings the inspector assigns to the bridge based on the condition of the various components of the bridge. The geometric layout, safety, traffic volume and the length of the detour route are also factored into the SR. The SR can range from zero (a bridge that is closed and cannot carry traffic loads) to 100 (a new bridge with no deficiencies). The average SR of King County’s bridges in 2005 was 68.2.

A total of 120 routine bridge inspections were conducted in 2005. When the inspection revealed a deficiency, a maintenance work order was generated and assigned a priority. Urgent structural or safety concerns were addressed as soon as possible. Some of the larger bridge repairs that were completed in 2005, include: replacing corroded sections of the steel deck on the Tolt Bridge; replacing rotted timber caps and strengthening wingwalls on the Cottage Lake Creek Bridge; and replacing deteriorated timber on the Semanski Bridge.

“Safety is always our primary concern, followed by maximizing the useful life of an existing bridge,” said Lane. “With limited revenues for building new bridges, we need to do the best job possible to preserve our existing bridge inventory.”

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