One of the most complicated operations of constructing the new North Wing is shoring the wall which is in between the Museum and the addition. The wall is an old one made from brick and concrete block. Brick walls are very strong as long as the forces on them are vertical (straight down from the top). If the forces are horizontal (side to side), the bricks need to be held in place. Such a horizontal force might be wind load: big walls can act as a very large sail and catch a lot of wind load in extreme weather.
The first step in shoring is to add strength to the wall by adding stiffbacks.
In this case, stiffbacks are channels. Holes are drilled in the channels and into the brick and block behind. Inserted into these holes is a plastic sleeve with plastic mesh. It is slightly smaller than the hole.
The sleeve is filled with epoxy, and a length of all-thread rod is inserted. When the epoxy sets the washer and nut can be installed and the channel secured tightly to the wall.
Stiffbacks are against the walls, and the other side of the shoring is secured to existing columns. Plates were welded to the columns to accept the diagonal bracing. The plates at the bottom of the columns will be encased in concrete. Angle bracing is extensive and prevents movement from front to back, side to side and up and down. In order to make sure the base of the columns do not move, additional weight is added to the bottom of the columns. Six cubic yards of concrete each weighing between 2,500 and 4,000 pounds were poured at the base to produce a deadweight of between 15,000 and 24,000 lbs. This is only a fraction of the amount of concrete that will be poured this winter forming the walls of the new addition, which is why the shoring is so important.
Because the concrete exerts a powerful force pushing the form outward, steel wire is used to hold the form together. Typically, snap-ties are used in standardized wall thicknesses like 8” or 12”. In our case, the form is much thicker, and we are using a special thick wire called pencil rod.
Pencil rod is inserted in between studs on opposite sides of the form, threaded through cast iron escutcheons called catheads. The pencil rod is secured by a set screw. The carpenters secure a cathead to one end of the pencil rod. A second cat head is placed loosely on the opposite side of the form. The pencil rod is pulled tight by hand. A tightener is also place over that end.
The carpenters pour three forms at a time. In order to reduce the outward pressure on the forms, each form is poured one third full from the first truck load. That way the concrete can begin to set before it is all added. The second truck adds a second third to all forms and so on.
The concrete truck does not enter the building, to prevent the possibility of a buildup of fumes in the area and also to make sure no fumes enter the Museum. A large forklift uses a half-yard concrete hopper to deliver the concrete to the form.