If you blink, you might miss the Canyon Padre Bridge in Coconino County. But this unassuming structure was once a very important part of transportation on Route 66, about 22 miles east of Flagstaff.

The Canyon Padre wasn't very deep or wide, but it was a major problem for traffic on the new Santa Fe Highway. In 1913, state engineer Lamar Cobb decided to build a bridge across the canyon. After a bid was accepted and construction began, the bridge was completed in April 1914.

It's true that the tightly-curved approaches to the bridge were dangerous... and yet, the Canyon Padre Bridge carried mainline traffic on the Santa Fe Highway for more than 20 years until its replacement in 1937. It now carries some local traffic on the Navajo reservation.

This bridge is notable for being Arizona' first Luten (or horseshoe) arch, a patented bridge design only used in around 4,000 bridges nationwide. It's also one of the few still-intact portions of the Santa Fe Highway (or National Old Trails Highway). The highway was rerouted before it was paved along this stretch, leaving the road and bridge in basically original condition.

Seeing this bridge today, you get a very good idea of what it was like to travel our state's highways in the 1920s and 1930s.

"The spandrel of an arch bridge is the area between the arch ring and the roadway. ... Open-spandrel arches have columns resting on the arch ring that support floor beams, which in turn carry the roadway."

We know what you're thinking: Uh, OK, what does that mean in plain English? Let's show you an example: Cienega Creek Bridge in Pima County.

Known as Arizona's longest open-spandrel bridge, the Arizona Highway Department -- a precursor to the Arizona Department of Transportation -- began construction of Cienega Creek Bridge began in 1920 as part of the new Borderland Highway (US 80) across southern Arizona. The bridge is located a few miles east of Vail, Ariz., and near a railroad.

Construction was finished in 1921. At 146 feet, Cienega Creek Bridge's open-spandrel arch is comprised of two tapered ribs that are anchored in concrete foundations with spread footings. 

The bridge was part of U.S. 80 from 1926 to 1955 and was put on the National Register of Historic Places in 1988. It is no longer a state highway and, today, carries local traffic on Marsh Station Road. 

On State Route 73 in the Fort Apache Reservation, alongside the North Fork White River in some of the prettiest country around, ADOT is in the process of replacing an old bridge deck. 

By the standards of the region, the Post Office Canyon Bridge, built in 1968, isn't quite a senior citizen at 53 years. But if it could talk, it would tell you the top song that year was "Hey Jude," by The Beatles, minimum wage was $1.60 an hour and a Big Mac was 49 cents.  

The single span, steel girder bridge rises more than a mile above Post Office Canyon at 5,912 feet. 

It is located nearly half-way down an 11-mile stretch of SR 73, between Indian Pine near State Route 260 and the Alchesay National Fish Hatchery, that was designated in 1993 as the White River Scenic Road. The route is a U-shaped loop, about 40 miles long with one end on US 60 north of Carrizo and the other near Indian Pine and State Route 260.

According to the state-published Arizona Memory Project, SR 73 started out as little more than a foot trail: 

“With the establishment of a military camp in 1870 that would be renamed Fort Apache in 1879, the development of rough trails into passable packed-earth roads became necessary. In 1929, the old trail that would become SR 73 allowed horse-riding soldiers and mule-drawn freight wagons to reach the remote garrison from both the north and the south.“ 

The Post Office Canyon Bridge was probably named after the canyon it crosses, the Post Office Canyon, said the ADOT engineer who is overseeing the deck replacement, Navaphan Viboolmate (Noon) of ADOT’s Bridge Design Section. 

Before 1968, an earlier bridge across the canyon had a deck built from timber, Viboolmate said. It was torn down to make way for the current bridge.

“It was an open design,” he said. “Driving on it you could see side rails and cross rails.” 

Located in Navajo County between mileposts 348 and 349, the bridge sits on a section of SR 73 that is classified as a Rural Major Collector Road. 

“It connects to local communities, including Fort Apache, Whiteriver and Cedar Creek,” Viboolmate said. “It connects State Route 260 and US 60.” 

By ADOT engineering measurements, the bridge is safe, but because ADOT specifications and standards have evolved, it’s time to replace the current deck with reinforced concrete 8-inches thick and to replace deck drains, deck joints and bridge barrier transitions, among a host of other improvements. The project will reconstruct and significantly extend the life of the bridge

The project is in full swing and crews are demolishing and replacing the old decks.

According to the project website, the following restrictions are in place from 6 a.m to 6 p.m. seven days a week through September:

• SR 73 is narrowed to one lane of alternating north- and southbound travel near the Post Office Canyon Bridge (mileposts 348-349).
• A temporary traffic signal helps drivers travel through the construction area.
• 12-foot vehicle width restriction in the work zone.
• Reduced speed limit in the work zone.

When you cross the newly refurbished span, you now know you will be one mile high, on the White River Scenic Road and a Rural Major Collector Road --  and on a passage whose history began in the 1800s.  

We're a pretty diverse lot here at ADOT, but collectively as an agency we do love bridges and great photography. And a little bit of 1950s B-movies, which is maybe why we glommed on to this particular shot.

As you probably sussed at a glance that's the Hoover Dam in the background, demarcating the state line between Arizona and Nevada. But up there above it looks like the handiwork of the largest spider ever, with long strands branching out from anchor points to support the bottom of a magnificent arch. We almost have the screenplay written in our heads: A spider is mutated from (insert your favorite reason here), grows to monsterous proportions and terrorizes Las Vegas. The climax will be a battle at the spider's lair spanning the Colorado River. The movie poster's tagline will be "To IT, we are all bugs!" 

Okay, so those are just silly flights of fancy. In reality this is obviously part-way through construction of the Mike O'Callaghan-Pat Tillman Memorial Bridge, which created a bypass so US 93 traffic no longer had to go over the dam. Construction of the actual bridge (which is 900 feet above the river!) ran between 2005 and 2010, and when completed was the highest and longest arch concrete bridge in the Western hemisphere. 

Another fun fact – due to how hot it gets in that particular spot, concrete had to be poured from mid-air at night and then cooled with tubes full of liquid nitrogen. That also potentially explains why we were able to capture this shot of crews working on the bridge this particular night. 

Just something to think about the next time you drive to or from Las Vegas. 

Now, if you'll excuse us, we are going to pitch our monster movie idea to Hollywood.

A bridge is often an awesome sight to behold: Thick pillars, massive abutments, maybe lots of shiny steel tubing soaring into the sky above a canyon. Or, it could be as simple as a concrete structure crossing a small wash (like the Lowell Arch Bridge near Bisbee).

You may have wondered, if you've passed by or driven on one recently: How did this thing get designed and built?

Wonder no more: Bill Downes, an ADOT design engineer, is here to help explain the process. He says a bridge design engineer must consider three very important points:

1. Geometry: "How long does the bridge need to be, what does it go over and how high is it," said Downes. "The bridge must be high enough to allow traffic or flood water to pass under it."
2. The number of spans: "Essentially how many piers will be needed. Where can piers be placed is also a major factor in this decision. You cannot place piers in a roadway. Where is the roadway below the bridge, and where will it be in the future?"
3. Traffic: "This is not much of a problem on a new bridge on a new roadway, but it is very important on a bridge replacement. Can the new bridge be built next to the existing bridge and the roadway moved over? Can the road be closed/detoured during construction? Often traffic must continue even while trying to build a new bridge."

In addition to all of these, Downes says, the engineer must also take into account utilities near the proposed site; how and where construction crews will access the site and store their equipment; and whether accelerated bridge construction (ABC) methods can be used.

"Although ABC reduces traffic disruptions, they generally cost more and require special planning," he says.

The topography and soil of the proposed bridge site are also important.

"What type of soil is in the area, and what types of foundations can hold up the bridge. Can shallow foundations like a spread footing be used, or are deep foundations such as drilled shafts required?"

Then comes the review.

"As each item is looked at, it may change the choices made earlier," says Downes. "For example, the ability to ship girders to the worksite may change how many piers you will need to have."

After all the data gathering and review, it's time to start designing!

"This is the point where we start the math to determine how strong each bridge element needs to be," he says.

Although no one can predict future traffic patterns, engineers must allow for that, too.

"Bridges historically were designed to last 50 years," he says. "Generally we now design for a 75-year life and many will be expected to continue to carry traffic for over 100 years with only little maintenance or rehabilitation." 

Once the design is completed and approved, Downes says, plans must be made for contractors; and once a contractor is chosen, the bridge design engineer will need to work with the contractor throughout the construction process.

The most important point of all?

"Although bridges are beautiful things they don’t exist in isolation," Downes says. "Building a bridge cannot be accomplished without a great deal of coordination and work with roadway, geotechnical, traffic, district, right-of-way, utility, environmental, material, drainage and other engineers."

We here at ADOT like the details. 

Our engineers obsess over grades, curves, speeds, traffic volume, materials and geotechnical data, while other employees busy themselves with permits, biological impacts, landscaping, and safety. 

Maybe that's why our ADOT photographers like to find fine-detailed shots like the one below. You'll probably recognize instantly that it's a series of rivets, but will need a bit of help identifying them as those holding together the bridge along the nature trail at Cochise Stronghold, southeast of Dragoon. 

This shot – and the "A325" rivets we see in it – is a good reminder that it's important to pay attention to the details. 

As a new state, Arizona didn't waste time when it came to building bridges. That's certainly the case for Chevelon Creek Bridge in Navajo County, a significant bridge from the state's early history.

The deep, rocky canyon over Chevelon Creek, east of Winslow, cuts across the Colorado Plateau, the broad, relatively flat piece of topography that makes up most of northeast Arizona. And according to a historic bridge inventory for Navajo County, this canyon formed a "practically impassible" topographic barrier to the Santa Fe Highway across northern Arizona.

The Santa Fe Highway was a portion of the longer National Old Trails Highway that entered the state at Lupton and went through to Topock. Despite expectations, the road wasn't called the Santa Fe Highway directly after the city in New Mexico. Rather it got the name because it followed the old Atchison, Topeka and Santa Fe Railroad, commonly just called the Santa Fe Railroad. And, yes, all this was in the age before highways had numbers.  

In 1912, the newly-minted Arizona State Legislature set aside $5,500 for construction of a bridge at this difficult spot. The Missouri Valley Bridge & Iron Works received the contract for the job. The final design was for a poured-in-place concrete slab bridge deck supported by steel girders with concrete curbs and steel latice guardrails.

The contract was awarded in October 1912 and by the next July, after a mere 10 months, it was accepted by the state. The contractor even came in under budget. The Chevelon Creek Bridge then began carrying mainline traffic on the Santa Fe Highway as an important crossing for one of the nation's earliest transcontinental routes.

Unfortunately, the bridge didn't enjoy its major crossing status long. A highway realignment occurred sometime between 1917 and 1924, which shifted the route north of Chevelon Creek's junction with the Little Colorado River, closer to what is Interstate 40 today. After that, the bridge and the road over it reverted to the county.

Today, this remote crossing carries relatively sparse local traffic on McLaws Road. But that doesn't mean the state was done with the bridge yet. In 2013, it was decided the bridge needed to be closed for some much needed renovations. However, it proved to be something of a logistical challenge as the bridge had been originally coated with lead-based paint and ADOT had to abate this environmentally harmful situation.

Once that was taken care of, repairs could get underway in earnest and the bridge reopened in 2014. That's the restored bridge you see in the top photo; the bottom is its appearance right before repairs started.

Despite no longer being part of an important transcontinental road, Chevelon Creek Bridge still holds a place as one of the State of Arizona's first highway structures. It was also the first "pony truss" bridge built by the state that was designed, fabricated and erected by a nationally-important bridge firm. Only the Hereford Bridge in Cochise County exceeds it for both age and span length.

And that makes it something worth remembering.

You probably saw the concrete bridge piers rise from the ground.

And you certainly noticed cranes hoist massive bridge girders onto the piers.

But you may not have thought the future bridge at I-10 and Ruthrauff looked like an actual bridge – until the recent work to pour a concrete deck atop the girders.

The concrete pour signals another important step in the process of building a new Ruthrauff Road/El Camino del Cerro bridge that will span I-10 by next year as part of a $129 million interchange reconstruction project in Tucson. It’s the first of several concrete pours that will take place through next year as crews build the structure in phases.

While you can see the concrete deck in place, few people saw the concrete pour because it took place in the middle of the night. Crews worked from about 1 a.m. to 5 a.m. on a recent day to complete pouring 290 cubic yards of concrete. 

Those hours aren’t the most fun for a lot of us, but there’s a good reason behind the schedule. The concrete is more workable during cooler nighttime hours at this time of year. And it’s less likely to crack in that temperature range because the concrete sets up more gradually.

Crews poured concrete over just half of the existing structure, and they’ll be back in about a week to finish the remaining portion of the bridge above eastbound I-10.

You’ll see the entire process again above westbound I-10 beginning in early 2021. That will require shifting eastbound and westbound I-10 traffic beneath the portion of the bridge that’s under construction now.

During the entire process, ADOT will maintain three lanes of traffic in each direction during daytime hours. And we’ll maintain access to local businesses.

For updates and traffic alerts, please visit azdot.gov/RuthrauffTI.