Cranes; Part I (3/3/16)
Construction projects start slowly. Site preparation involves only destruction rather than the promise of a new beginning. Utility replacement is usually an annoyance and the excavation and foundation work seems to last forever, revealing an ever deepening pit surrounded by unsightly wood planks and piles.
To a casual observer, the anticipation that greeted the announcement of the project fades and boredom sets in as the seemingly interminable work on the substructure drags on. We continue to walk past the site every day, but now our eyes look down or gaze outward. Our pace quickens and the sidewalk brings no joy. See, Construction Notes, August 18, 2015,http://www.law.georgetown.edu/campus-services/facilities/construction-info/index.cfm We no longer think about the buildings that will one day change the landscape of our neighborhood and, perhaps, the rhythms of our lives.
And then one day, structure begins to appear. Walls and floors seem to rise out of nowhere as the pace of construction quickens. Our eyes once again are directed toward the project and anticipation begins anew as we start to get an inkling of what is to come. We begin to fantasize about life in and around the structure; we dream of new people entering our neighborhood and of new shops and entertainment venues that will divert us from the life we now lead. As we walk, our pace is slower and we gaze again at the site, as the future becomes real and the past recedes. We sometimes stop and linger, contemplating possibilities that we have never considered before.
Such is the case with the Capitol Crossing project. Since April of 2014, we have watched the slow progress of utility and foundation work. Sometimes, but with only great imagination, we got a sense of what this project might look like. But today, the future is revealed. The 2ndStreet off-ramp has taken form as the road surface and the walls are being installed. The columns, standing silently for months like a concrete brigade, now assume their purpose as the first steel girders and precast concrete slabs give form to the tunnel that will, by the end of this year, cover the “highway to nowhere” that I wrote about in June of 2014. Construction Notes, June 2014, http://www.law.georgetown.edu/campus-services/facilities/construction-info/index.cfm
Building the tunnel begins with the steel girders — American steel — fabricated in the Carolinas by the Owens Company, one of America’s premier steel fabricators founded in 1936. http://www.owensteel.com/ The girders themselves are a marvel. Most are 69 feet long although one of the longest will be 88 feet long. Each weighs up to 88 tons (176,000 pounds) and is 8 feet 8 inches tall, taller than the men who set it in place.
Once fabricated, the girder is transported by train to a storage yard in Baltimore. It is then brought to the site by a huge, 18-wheel flatbed truck that actually telescopes to accommodate the length of the beam. Getting the beams to the site requires the service of an immensely skilled driver who enters the highway along the closed southbound lane at New York Avenue. But because of the size of the crane that will soon lift the girder, the driver must back the 90-foot flatbed truck down the lane from New York Avenue to Massachusetts Avenue, a distance of nearly one mile.
Once the girder arrives it is hoisted by the 300-ton Manitowoc crane that I mentioned in my last Construction Note onto the columns on either side of the lane. There are predrilled holes in the girder and precast anchors in the columns. The girders slide down onto the anchors and then the columns and the girders are fastened together by bolts. Seven precast concrete slabs, each 8 inches thick, are laid between two girders to form the roof of the tunnel and the sub-surface of the platform. The seams between the slabs are filled with grout and a four-inch topping slab covered by insulation is placed on top of the original slabs. At a later date, an architectural slab will be poured on top of the tunnel roof which will become the visible surface of the platform.
As the roof of the tunnel is being built, the walls of the tunnel are rising alongside the lanes. The walls themselves are not load-bearing, that is, they do not support the structures above them. They are built primarily to divide the highway lanes. The braces between the tops of the columns, called BR 1s, help support the lateral loads and maintain stability if a catastrophe occurs in the tunnel. The walls are made from three tiled, precast concrete slabs that are hung between the columns and welded to plates in the column. Clips hold the panels to one another. Once the tunnel is reopened, the walls will be protected by an F shaped barrier, often but erroneously called a Jersey barrier, upon which it was modeled. The barrier is not shaped like an F. It takes its name from a series of crash tests on various types of barriers labeled A through F. This barrier performed better than the others and is now becoming the standard throughout the industry, although the Jersey barrier still meets safety standards and is still in use. Once the barrier is in place, the space between it and the wall will be backfilled with gravel. Should an accident occur, the barrier will deflect the cars away from the tunnel wall. Thus the tunnel, the platform above it, and the building on the platform will all be protected from serious damage.
The steel and the precast concrete will be hoisted into place by various cranes. Cranes are ubiquitous in modern American cities and we have seen many different kinds of cranes as the project progressed. They come in various forms and sizes, but most are equipped with a hoist rope, wire ropes or chains, and sheaves that are used to raise and lower materials or to move them horizontally. Cranes are mainly used for lifting heavy things and transporting them to other places. Although we urban dwellers usually see them used to erect buildings, they are also used to load and unload ships, trucks, and trains; move and unload materials in factories; and to assemble heavy equipment. Indeed, cranes are often used to erect other cranes. Right now, a telescoping crane and a track crane are on the site.
Most lifting techniques began as animals or humans, usually slaves, raised blocks either by muscle or an inclined ramp to higher levels. “The Ancient Egyptians used ramps to move the 2.3 million seven-ton stone blocks that comprise the Great Pyramid of Giza. Anywhere from 20,000 to 50,000 men built the 6.5-million-ton pyramid over the course of 23 years, meaning they set a stone every 2.5 minutes every day of the week starting in 2540 B.C. The result was a 449.5-foot-high pyramid with a base the size of 10 football fields.”
Although I am not a great fan of Wikipedia, it actually has one of the most readable descriptions of early cranes and so I will quote liberally from its entry. The crane for lifting heavy loads was invented by the Ancient Greeks in the late 6th century BCE. Archaeologists have found stone blocks in Greek temples dating to 515 BCE with distinctive cuttings for both lifting tongs and lewis irons. A lewis iron a lifting device used by stonemasons to lift large stones into place with a crane, chain block, or winch. It is inserted into a specially prepared hole, or seating, in the top of a stone, directly above its center of mass. It works by applying principles of the lever and utilizes the weight of a stone to act on the long lever-arms. Because these holes are found either above the block’s center or in pairs equidistant from a point over the center of gravity, archaeologists believe that they reveal the existence of some sort of a lifting “machine.”
The Parthenon and other Greek temples of the classical age are made from stone blocks weighing around 15–20 metric tons but sometimes less. As the Greeks abandoned ramp lifts for the crane, skilled construction teams replaced large numbers of unskilled labor. These specialist teams were able to assemble the buildings quicker and at less financial and human cost. By the 3rd century BCE, Aristotle was writing about compound pulley systems that were used to hoist large marble and other stone blocks.
The Romans used cranes extensively during the years of the empire as buildings soared in height and mass. The Romans adopted the Greek crane but developed their own tread-wheel crane. The simplest Roman crane, the trispastos, consisted of a single-beam jib, a winch, a rope, and a block containing three pulleys. “Heavier crane types featured five pulleys (pentaspastos) or, in case of the largest one, a set of three by five pulleys (Polyspastos) and came with two, three or four masts, depending on the maximum load. The polyspastos, when worked by four men at both sides of the winch, could readily lift 3,000 kg (3 ropes x 5 pulleys x 4 men x 50 kg = 3,000 kg). If the winch was replaced by a treadwheel, the maximum load could be doubled to 6,000 kg using only half the crew since the treadwheel possesses a much bigger mechanical advantage due to its larger diameter…However, numerous extant Roman buildings which feature much heavier stone blocks than those handled by the polyspastos indicate that the overall lifting capability of the Romans went far beyond that of any single crane. At the temple of Jupiter at Baalbek, for instance, the architrave blocks weigh up to 60 tons each,” and one corner cornice block weighs over 100 tons. All of them were raised to a height of about 19 meters. In Rome, the capital block of Trajan’s Column weighs 53.3 tons. It had to be lifted to a height of about 34 meters.
As with much of Roman technology, mechanical cranes disappeared in the early Middle Ages. However, the Roman Polyspastos powered by a treadwheel crane was reintroduced in France no later than 1225. Harbor cranes were also in use in Utrecht in 1244, in Antwerp in 1263, in Bruges in 1288, and in Hamburg in 1291. The treadwheel was not reintroduced into England until the 14th Century. The medieval treadwheel was a large wooden wheel turning around a central shaft with a treadway wide enough for two workers walking side by side. Treadwheel cranes were used to build Gothic cathedrals but illustrations at the time show that less sophisticated and more labor-intensive lifting methods were still used.
I am going to leave the history of cranes for now so you can spend some time on the Mass Avenue Bridge watching the walls and girders go up. The best time is at night between the hours of 10:00 P.M. and midnight. You will see the crane roll out onto the highway and the truck back down the highway from New York Avenue until it is parked in front of the crane. Then the industrial ballet begins. The Entrée begins on the ground as the workers dance along the girder and the truck, fastening the cables in preparation for the aerial ballet to follow. Then the Adagio commences, as the crane lifts the beam, slowly turning it 90 degrees as it rises. Variations begin as the crane lowers the beam and the workmen assume their positions for the final act. The Coda occurs as the crane lowers the beam onto the columns where the workers will finish connecting the two together — the end to an industrial pas de deux, sure to entertain and delight.
Enjoy your Spring break. When you return, more of the highway will be covered and I will continue my discussion of these fabulous cranes. If your appetite for knowledge of ancient cranes has been whetted, you can visit the Wikipedia entry at https://en.m.wikipedia.org/wiki/Crane_%28machine%29 or take on some of the more complex reading by Shapiro and Verschoof noted below.
Shapiro and Shapiro, Cranes and Derricks, McGraw Hill, 4th ed. 2011.
Verschoof, Cranes, Professional Engineering Publishing, 2nd ed. 2002
Wazee Crane Company, http://wazeeco.com/news/how-cranes-built-civilization-ancient-world-modern-era/
Wikipedia, Cranes, https://en.wikipedia.org/wiki/Crane_%28machine%29
Wikipedia, F shaped Barriers, https://en.wikipedia.org/wiki/F-Shape_barrier
Owens Steel Company, http://www.owensteel.com/