Inspectors Snoop Under Bridges to Check Conditions

An under bridge inspection unit, also known as a snooper truck, is an odd looking vehicle that causes many who see it to do a bit of a double take. On May 27, there were many spectators at Table Rock Lake in Branson, MO that did just that. While they were watching from the lake front, dam observatory and occasionally from idling boats, a four-man MoDOT crew was assisting the Army Corps of Engineers in the inspection of the dam.

The under bridge inspection unit on location in Branson that day is one of only three in the state. With so few units statewide, each truck and crew covers a large portion of Missouri and its bridges. All three crews are part of the Bridge Management Section of the Bridge Division.

The trucks and the crews that operate them are an invaluable asset when it comes to making sure the bridges of Missouri are safe for motorists. Every bridge in Missouri is inspected regularly for safety. If a disaster happens, a bridge must be inspected immediately before motorists will be allowed to drive on it for their safety.

In order for the crew to inspect under the bridge, the snooper truck can extend its arm allowing the operator to go off the side of the bridge. A counter weight on the truck prevents the unit from tipping over. The arm is then maneuvered into position underneath the bridge for inspection. The truck’s arm can lower itself and reach a total of 62 feet across the width of a bridge for the inspector to check for cracks, damage and deterioration.

If a crack is found, it is measured and evaluated to make sure it is not compromising the integrity of the bridge. Not all cracks are equal. While bridges will have cracks, the crew is looking for cracks in areas of stress, or cracks that show growth from a previous inspection. They also are testing the thickness of the steel in areas of section loss caused by rust to make sure it is still in good condition.

According to the bridge inspection crew, it is not uncommon to close up to two bridges a month in need of immediate repairs. The bridge deterioration can be caused by numerous things. It can be from the salt and abrasives added during winter to help keep ice off the bridges that cannot be reached to wash away, or simply from fatigue over time caused by heavy trucks passing over it.

In Missouri, there are more than 10,400 state bridges. A total of 208 bridges are considered “major” meaning they exceed 1,000 feet in length. Currently 47 of those major bridges are considered to be in poor condition while an additional 97 are only considered to be in fair condition. Overall a total of 6,598 bridges are in fair to poor condition.

Bridges Bounce



The wonderful thing about bridges,

Is bridges are flexible things.
The structure is made to move up and down,
Sometimes it feels they’re on springs.

Bill Emerson Bridge in Cape Girardeau, Mo.

My apologies to Tigger and his fans. I just couldn't help myself. 

The idea for the verse popped in my head one evening as I waited for a crash to be cleared from my lane on a Missouri River bridge. As cars and commercial trucks rumbled by, I felt a bounce that was a bit unnerving. It wasn't the first time I'd felt the bounce. It happens on any type of bridge - overpasses and elevated lanes included.

Luckily, several bridge experts work just a short elevator ride from my desk. They helped me to understand that bridges are designed with some flexibility built in. They’ve got to perform in temperature extremes – contracting and expanding with cold and heat – in still and windy conditions, and under differing weight loads.

That flexibility means there will always be some movement. In fact – the Golden Gate Bridge in San Francisco is built to swing up to 27 feet mid-span in the unlikely event of a 100 mph broadside wind!



Setting the supports for the Miami Bridge.

It's hard to understand how something that looks so stable and solid could move up and down, but have you ever walked on a board laid across two supports? The further apart the supports are, the more vibration you feel as you walk across the board. This vibration, called resonance, also plays a part in bridge movement as vehicles move across a span.



Designers and engineers have to strike a careful balance of distances and supports, because a too-stiff bridge has its own issues.

If you are ever stopped on a span and become uneasy with the bouncing movement, maybe this will help - MoDOT's bridge inspection teams, who sometimes use a special truck to view the undersides of bridges, sheepishly tell stories of seasickness suffered when they spend too much time in the bucket attached to the flexible arm of the truck parked on a bouncing bridge. They might be uncomfortable, but they’re never scared. They know why bridges bounce. And now, so do you!


Inspectors examine the underside of the
Hurricane Deck Bridge using a snooper truck.