Big Repairs Require Big Machines: How is the Bottom of a Node Milled After Installation?

Another in the Series of: How do They do That?

Down on “B” level (the basement) where the large “A” level steel support beams stretch from node to node, there is an interesting undertaking that is in the making. Several months ago, I had learned that the isolator bases bolted into the foundation floor had been constructed and installed so that if need be, they could be repaired or replaced. How could this be done? I mused. I never believed that I would live enough years to see such a scenario take place. Well, I thought wrong. This week, I toured two areas on “B” level where jacks (rated for 200-tons each) and support columns shoring up the massive “A” level steel support beams.  The isolator bases, once supporting the nodes, had been unbolted and slid out from underneath the nodes now supported by the jacks and shoring pedestals. When these two particular nodes were forged, they apparently got overheated resulting in some unevenness where the nodes to the bottom of the nodes. Such unevenness or ripple effect does not allow the isolators to bear the total load weight of the nodes as specified by the structural engineers. For the nodes to evenly distribute the bearing weight, they must rest flat on the isolators to properly distribute the weight. To remedy this imperfection, a milling company (LARON) based in Arizona, which specializes in on-site specialty milling was called to mill the bottom of these two nodes. This herculean undertaking is both tedious and time-consuming and will take approximately two weeks to complete. This apparatus will not become part of the structure. Once the project is completed, the milling machine will be dismantled and carefully extricated by crane. jacks (rated for 200-tons each) and support columns

A Hydraulic 200-ton jack, such as the one depicted in this photo, is placed on a steel pedestal next to the imperfect node pad and adjoining steel beams. Once secured, the hydraulic pumps are actuated and the jacks raise the steel node (and close surrounding beams) a fraction of an inch above the isolator to allow it and its base to be unbolted and slid out from underneath the nodes.

The arrow points to the underbelly of the node to be milled. Five of the eight isolator base pads are visible. The isolator base is to the left just out of the frame.

Note the isolator base (lower center left), which has been unbolted and pushed back from under the node. The narrow space between the isolator and the steel beam is clearly visible.

The electric hydraulic pump, as shown in this photograph, is essential for heavy/large lifting where sustainability is crucial.

In the morning of February 12, 2018, a low bed semi-truck arrived from Arizona with the milling equipment. The largest piece is this 12,000-pound rotating milling base. How will they ever maneuver this 14 ft.- 4-inch diagonal monstrosity down in the pit and past the girders? I wondered. It wouldn’t take long to find out.

The north crane slowly lifted the machine, on the diagonal, off of the low bed. As soon as the crane operator reached the required height, he carefully swung the load from the northeast corner of the pit to the northwest corner.

This video clip shows the milling machine being lifted off of the low bed and transported from the northeast corner of the pit to the northwest corner.

Carefully being maneuvered between the girders.

The master machinists are in the process of setting up the lazy susan-type milling machine. Note: the frame of the apparatus rests on wooden planks and then was welded to the massive beams.

The milling worksite under the spline beam and next to the western wall. The master machinists observe the milling progress and make adjustments via a computer resting on a makeshift stool in front of the gentleman on the right.

Down on “B” level, four stories below the where the ironworkers are wrestling with the steel, the milling machine slowly makes its rotation as the grinding wheel shaves off millimeters of steel as specified by the computer program. Note the metal shaving around the bottom of the wheels.

A closeup of the milling wheel: The bluish tint is due to the blue tarp that reduces the sunlight and heat reflected off of the west foundation wall.

This video clip shows the milling machine at work on the underside of the steel node.

With the milling machine having been relocated to the opposite side of the section made for better lighting. It will take the milling machine’s head about one hour to rotate around the node. As the head works its way closer to the center it will take less time. It is estimated that it will take about one-week to complete the job.

The tungsten carbide milling cutter blades that fit on the eight slot milling head can be rotated four times. The blade is about the size of a dime. Each blade cost approximately $30.00.

Near the milling machine was a large trash can filled to the brim and overflowing with steel shavings. I was told that before the job was done on this node, the trash can would be filled at least twice more. BTW, the trash can had been emptied before this photo was taken.

After the milling is complete a measurement will be taken to determine the thickness that was shaved off and a machined steel plate matching that thickness will be placed between the isolator and node and be bolted down.

The movie