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Ice Arch 2007

The 2007 Ice Arch was a steel-reinforced arch. The design consisted of three arches, one over the top of another, intersecting at 60-degree angles. The Arch was designed similarly to a steel-reinforced concrete beam.

The arch was poured on the ground forms similar to that of a concrete structure; the main difference is that the ice had to be formed in ¼-inch to 1-inch lifts in order to minimize cracking due to the expansion of ice as it freezes. The problem of forming the arch on the ground and them lifting it into place is that as the arch is lifted, it acts as a beam and not as an arch. So the arch had to be designed to carry the bending moment created by its own weight while it was being hosted into place.

The arch was designed using concepts from the ASD concrete design with a cracked section analysis. It was assumed that the ice would crack as the arch was lifted into place and that the steel would have to carry all of the tensile force in the bottom of the beam as the arch was lifted into place.  Also a very large safety factor was applied to the strength of ice as the range for the strength that was achieved during testing was so variant. During minimal testing (six samples) values for the compressive strength ice ranged from 600 psi to 2000 psi so design strength used for the construction of the arch was taken as 300 psi.

Formwork for the arches was built inside in modules and then transported outside to the arch location. Forms were connected and level foundations were poured. Great care was taken so that the arch forms were level and the base of the arch was perpendicular to foundation so that when the arch was tipped up into place it would stand level. Lake water was poured into the forms in ¼-inch – 1-inch lifts depending on temperature. If the temperature was warm, four ¼-inch lifts were poured instead of one 1-inch lift. This allowed the ice to freeze faster.

Lake water was used as it was thought that if lakes form clear ice, that lake water would form a clear arch. The idea didn’t work. In a lake as the ice freezes, the water expands and the ice simply lifts and there is basically no air on the freezing front of a lake. On the arch, every lift of water is exposed to air and a white ice was formed.

It took about two and a half weeks to fill the forms to a depth of two feet. When the forms were removed, it was found that ice was cracked clear through structure due to thermal expansion that resulted from large temperature swings as the arch was formed.  This was not considered a problem as the arch was designed using a crack section method and the tensile strength of the ice was considered zero.

The arches were lifted into place by GHEMM Co., who generously donated crane time for the lift.  During the lift there was no noticeable deflection in the arch, although there was one very large pop at about 45 degrees. As the arch went vertical, the center of gravity of the arch moved from outside of the arch to inside of the arch. As this happened, the weight of arch came all the way off of the crane and the arch rocked into place. There was no cracking of the arch legs as a result of this but if the arch was larger this might have been an issue. Water was poured under the arch legs to bond it to the foundation.  The arch stood until the beginning of April at which time it was deemed structurally unsafe.

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