The load path within a building structure is not always easy to understand from the view- point of someone looking up at it from the street. In the case of Simmons Hall, at the
MIT campus, the designers offer assistance. Completed in 2002, this large dormitory at the edge of MIT’s campus is of overwhelming scale. The designers sought to diminish the appearance of the building by making it visually “porous.” A second goal was to keep the interior as flexible and open as possible in order to maximize social interaction among residents. The solution was to place the pri- mary structure at the perimeter of the build- ing, using a gridded, precast system dubbed “PerCon” for “perforated concrete.” The architect, Steven Holl, had always intended
to include color on the outer concrete façade. Initially, the thought was to have outside col- ors reflect inside function in the building; however, during the design process a drawing created to check steel reinforcement by Amy Schreiber, an engineer from Simpson Gumpertz & Heger Inc. Structural Engineers (in collaboration with Guy Nordenson and Associates), became the inspiration for the final design. Amy’s drawing used color pencils to identify the amount of steel reinforcement in each respective concrete panel.
This graphical diagram was translated into the aluminum cladding on the precast concrete façade panels. The load path of forces at the building perimeter is literally identified with colors on the façade.
The building structure of Simmons Hall is easily understood from the street. Similar to the colored output produced by some com- puter software programs, color is used to dif- ferentiate various stresses on the structure under loading. The building façade appears to reference the computer analysis that was used to create it. Like many modern build- ings, the design of Simmons Hall was made possible by the use of computer analysis. Computers using finite element software have provided a method for analyzing extremely difficult statically indeterminate structures. But what happened before the invention of the computers, when there was just paper and pencil?
above and right Simmons Hall at MIT. Precast concrete panels lifted into place and assembled.
right Simmons Hall at MIT. Like a Vierendeel truss, the precast concrete panels inter- connect with steel reinforce- ment to span large and small openings.
118
119
Antonio Gaudí used models of steel chains covered in clay, hung from the ceiling, to design the emorfic structure of Casa Milla, in Barcelona in 1904. Robert Maillart used static graphics to design the innovative reinforced concrete bridges at Salgina, Switzerland, in 1929. Perhaps the greatest modern masonry structural designs of the 20th
century come from the work of Eladio Dieste using tech- niques similar to those of Maillart and Gaudí. Dieste’s work was confined to the country of Uruguay, but otherwise had no boundaries. The building structures provide solutions using innovative techniques for their time and
local building experience with masonry con- struction. One of Dieste’s greatest structural forms was the self-supporting arch.
The Municipal Bus Terminal in Salto, Uruguay, completed in 1974, provides a classic exam- ple of how daring structural forms were devel- oped prior to the advent of sophisticated com- puter analysis. A single row of columns is all that supports the long self-carrying vaults with equal cantilevers of 12.2m (40 feet). These cantilevered vaults are ideal for sheltering travelers coming in and out of the buses at the station. The self-carrying vault is achieved using innovative prestressing techniques that could be simply executed by local laborers. Timber trusses provide the formwork for posi-
tioning the bricks on the vault. The bricks are evenly separated by small wood strips. Steel-reinforcing bars are placed between the bricks, and the joints are filled with mortar. right Computer model of seis-
mic loading of concrete build- ing structure in contemporary design.
Robert Maillart used static graphics to design the innova- tive reinforced concrete bridges at Salgina, Switzer- land, 1929.
above Church of Christ the Worker, designed by Eladio Dieste, Atlántida, Uruguay, 1960.
left Casa Milla, designed by Antonio Gaudí, Barcelona, 1904.
A large looped prestressed steel cable is then installed to absorb the negative bending moment on the vault. With the vault still sup- ported by formwork, the ends of the loop are cast into the vault. After the lightweight con-
crete has hardened, the prestressing cable is mechanically pinched at the mid-point of the vault. The mid-section of the vault is then covered with mesh and filled with lightweight concrete to complete the roof. Although com- plicated by contemporary developments, prestressing has been used for thousands of years. A good example of prestressing can be seen in the fabrication of an old cart wheel. The heating of the iron outer band of the wheel prior to placing it around the cart wheel provides a form of prestressing. As the steel band cools, it contracts, clamping the wood parts together. In a similar manner, Eladio Dieste developed a method of pre- stressing with the same inherent simplicity, requiring no heavy equipment. Prestressing creates much larger forces in a building element than the element would normally experience through normal use. What hap- pens if the natural forces are much greater than expected?
120
121
below Prestressing of a self- supporting arch.
1 Lightweight concrete 2 Unbraced lightweight con- crete
3 Prestress cable 4 Reinforcement anchor f Force of jack F Force of prestress The Municipal Bus Terminal in Salto, Uruguay, 1974.
right and below The Munici- pal Bus Terminal in Salto.