The 100-foot long Hale Bridge was successfully moved from its piers on Friday, 28 February 2003. The two 80 foot long bridge sections had been successfully moved on 18 and 19 February 2003.

There were significant concerns over moving the 100 foot long span because of a significant rust thru at the top of the down stream tubular top arch. This problem is part of the reason that caused the Hale Bridge to be closed to traffic in 1997.

The Bowstring Bridge design and construction is unique. The design utilizes the properties as exemplified by an archery Long Bow plus the support design of spokes as used in a bicycle wheel.

An archer’s long bow consists of the curved bow and a bowstring. The string sets in a notch located at each end of the bow and is held taught by the outward force applied by the bow wanting to straighten out. In the design of the Bowstring Bridge, the tubular top arch is the bow and the bottom cord is the string. The bridge’s wood deck rests on and is supported by the bottom wrought iron cord. The tubular top arch ends “rest” in, but are not fastened too the notch located in the iron endplates that are bolted to the bottom cord.

In order for the bottom cord and the wood deck to be supported, vertical and diagonal struts run from the top tubular arch down to the bottom cord. These struts are equated to the spokes of a bicycle wheel. In the bicycle wheel, the rim is tied to the hub via spokes. You can think a bicycle wheel rim as the top tubular arch, the spokes as the vertical and diagonal struts and the hub as the bottom cord.

All stress forces on the bridge place each member in either tension or compression. The top tubular arches are in compression and the bottom cords and struts are in tension. This is why a bowstring bridge design is extremely strong for the size/weight of the structural members.

It is my understanding that the King patent Bowstring Bridge was construction sequence was basically as follows. First, the permanent limestone piers were completed and then temporary wood cribbing was built between the piers to support construction of the wrought iron bottom cord with the wood deck. Then, the top tubular arches were assembled and lifted into position by slipping the ends into the endplate’s notches. Finally, the vertical, diagonal struts and outrigger braces were installed. Since the 100-foot bridge was much larger than the 80-foot spans, additional cross braces between the top of arches were added. With the cribbing removed, we now have a free standing bowstring bridge. It was very interesting to find that the Hale Bowstring Bridge spans were just setting on their piers, without any mechanical anchor to the piers. With only gravity holding the Hale Bridge in place, it was pure luck that the many floods didn‘t push the bridge off into the Wapsipinicon River. After over a year of construction, the Hale Bowstring Bridge was opened to traffic in 1879! Now, you have a basic idea of how the King Bowstring Bridge was designed and built.

Moving the three spans of the Hale Bridge presented many unique problems and concerns. The two 80 foot long spans are in reasonably good condition. They are missing a few rivets and have some minor cracks in the tubular top arches. The 100 foot long arch’s condition is worse as the down stream top tubular arch has a significant rust thru problem. The added weight of the bridge’s wood deck placed a significant compression load on the top tubular arches.

Vose Moving Company Inc. of Waterloo was the lucky contractor selected to move the Hale Bridge to the near by staging sites. Gary was in charge of his crew from Vose Moving plus he directed all aspects of the operation. He had never lifted a bowstring bridge before. He found that the Bowstring Bridge is not as ridged as the “standard” bridge, which is a box type design. He found that unless they handled the bridge properly at all phases of the operation, rigidity and weight could become major destructive forces.

The lift on the two smaller 80-foot long spans was a learning experience. The knowledge gained was needed to be able to safely lift of the rust damaged 100 foot long span. The smaller spans weighted about 41,000 pounds each with the wood deck in place. The 100-foot long span weighed about 55,000 pounds with the wood deck in place. From 25% to 50% of the bridges weight is in the wood decking. Vose and Peterson Contractors, Inc supplied the cranes used on the initial lifts.

The first concern is the top tubular arch is “resting in“ and not fastened to the endplate. Therefore, the arch could be jerked out of the endplate notch. If this happened, the bottom cord with the deck would fall free and crash. The vertical and diagonal struts plus the outward pressure applied by the top tubular arch, are all the constraints that hold the bridge together. All three spans were lifted from their piers via cables attached to the top tubular arches about 10 feet in, adjacent to the first vertical strut. Luck prevailed, the top tubular arches stayed put on all three bridges.

The second problem encountered was how to support the bridge on the truck. This was accomplished by extending the truck body the full length of each span. The endplates at the bridge’s four corners were supported just as the bridge had been when mounted on the piers.

The third major concern was twisting the bridge as it was being trucked. This problem was solved with several hours of bulldozer work leveling and sloping the ramps at each end of the bridge. The trailer was designed to limit twisting.

The 100-foot long span was very carefully test lifted and inspected to see if the rusted area would crush. No problems were seen. Then the bridge was carefully lifted off the piers and with great precision and placed on the extended truck trailer bed. Once the bridge was on the truck bed, the bridge was supported at each endplate with some additional cribbing placed in the center. Then the bridge was chained in place.

The truck moved very slowly with great care not to put any twists on the bridge. Once the truck was on the road, it was carefully backed into the field. There was just a little creaking sound from the bridge/trailer as it moved.

Best of all, a quick inspection failed to reveal any damage as a result of moving the three spans! We have our hats off to praise the skill and care exercised by Gary and his crew from Vose Moving Company Inc. The three Hale Bowstring Bridge spans are safely moved to their staging sites for repair prior being placed at their final location. The 100-foot long span will me lifted off the truck during the week of 3 March and be placed on leveled cribbing.