When we think of bridges, the technical, historical and artistic viewpoints all come to mind. Technically we think of stress and strain, compression and tension, expansion and contraction, weight and counterweight, and many more design considerations. The many aspects of structural engineering are the key to bridge design. Among these are initial cost, low maintenance, longer life span, and high performance. Incidentally, structural engineering is not limited to building and bridge design. When I graduated from engineering college, my highest-paying job offer was from an aircraft company to design aircraft.

Other than pontoon bridges, such as those built by our troops in Europe during WWII and the Admiral Clarey Bridge, all bridges start with foundations. What's the Admiral Clarey Bridge? Imagine a bridge with a 650-foot (that's over two football fields) retractable floating span to allow ships to pass through. This is the bridge from Pearl Harbor to Ford Island in Hawaii.

Good firm foundations are supported by bedrock, piles or solid ground. We were once stopped and detoured while traveling east on the New York State Thruway between Syracuse and Albany. A spring thaw and a heavy rain had caused a river to scour the clay from under the spread footings of a reinforced concrete slab bridge. The bridge collapsed taking with it a truck and a couple of cars that were unable to stop in time. Pile driving or shaft drilling for support under these footings would have prevented this collapse. As with any bridge repair or replacement, a difficult challenge is redirecting the traffic during construction. There's also the challenge of reducing construction time.

When, what was considered the eighth wonder of the world in 1883, the Brooklyn Bridge was built, it was the deep foundations that were the greatest challenge. While laborers excavated inside the caissons to bedrock for the pier foundations, for the first time caissons disease (the bends) was recognized. Imagine the counterweight anchorages for the suspension cables at each abutment for this bridge. This project was so grand; it even included train and trolley tracks. These have since been eliminated.

Crossing the continent, we have the glory of the Golden Gate and Bay Bridge. The dramatic earthquake of 1989 overshadows the technical challenges of construction in 1937. The Golden Gate suspension bridge was designed to withstand 100-mph winds by allowing it to sway 27 feet. Nature still had the upper hand. A 15-second earthquake collapsed the Bay Bridge and many more structures in San Francisco. Since that time, the Golden Gate Bridge started a three-phase renovation to meet seismic requirements. The upgrade will allow it to withstand a quake measuring 8.3 on the Richter scale.

When traveling down our highways we can easily confirm that 80 percent of expenditures for freight transportation in the United States is spent on trucking. This encourages studies in axle weight limits, gross weight limits, length limits, and height limits. When driving throughout Europe, I've noticed that all semi-trailers have three axles. This substantially reduces axle weight which has a significant bearing on bridge and roadway surfaces. The increase in surface transportation is prompting technical advances in materials for bridge construction. One of these materials is HPS (high-performance steel). The reports are that the strength, weldability, toughness, ductility, fatigue, and corrosion resistance have all been improved. Also there is a weight reduction of the HPS of about 16 percent allowing for longer spans or shallower beams.

As we all know, the most common material for all our bridges is concrete. Michael Cullen, P.E., senior bridge engineer, Caltrans, says: "We almost always use reinforced concrete slab bridges when we have span lengths of around 25 feet; we no longer use T-beam and girder bridges at all. Design charts for traditional slabs — and for post-tensioned and voided slabs — make our bridge design very straightforward.''

This combined with high-performance concrete and epoxy-coated reinforcing steel reduces maintenance and increases the life-span of all our concrete bridges whether they're precast or cast-in-place. Adding a microsilica to the mix further enhances the high-performance concrete used for bridges. This reduces the permeability, improves the freeze-thaw durability and improves the workability of the mix. Polyurethane coatings for water repellant will further increase the lifespan of concrete bridges.

In case you have a quiz:

• The highest bridge is the new Millau Viaduct Bridge just completed in France. The highest piers are over 770 feet high (higher than the Eiffel Tower) and the masts are another 300 feet above that.
• The longest suspension bridge is the Akashi Kaikyo in Japan spanning some 6,532 feet. The anchorages on either end for this suspension bridge are some 350,000 tons of concrete. The proposed Straight of Messina Bridge is challenging this length record spanning from mainland Italy to Sicily. This single span bridge would be about 2 miles long.
• The widest cable-stayed bridge is the new cable-stayed Leonard P. Zakim Bunker Hill Bridge across the Charles River designed by Swiss engineer, Christian Menn. This bridge, at the north end of the Boston Big Dig, accommodates 10 lanes of traffic and is 183 feet wide. The bridge from Boston to Charlestown was opened on Mother's Day 2002. This record will soon be outdone by the Mississippi River Bridge in St. Louis: 222 feet wide.
• Once the longest wooden bridges in the United States, the Powder Point Bridge in Duxbury, Mass., lost its historical listing in the Guinness Book of Records when it was rebuilt in the 1980s.

I can't talk bridges without mentioning my brother's home, The Toll House, situated in South Newbury, Vt., on the Connecticut River. Starting in 1805, a ferry service was replaced by a wooden bridge between Haverhill, N.H., and South Newbury, Vt. This bridge and several to follow were swept away by floods, ice flows and windstorms. The final two-span covered bridge, the second-longest covered bridge in the country, became a toll bridge. My brother's picture window in the toll house looked straight down the length of the covered bridge. The toll gate was at his front door. Toll tickets were 2 cents per person, 5 cents for a single team and 10 cents for a double team. Passage was free from 6 p.m. to 7 a.m.