Architect: Philip Johnson and Architects Design Group
Size: 500,000 square feet
Cost: $23,000,000
Completion: 1972

LeMessurier Consultants provided structural engineering services from schematic design to final working drawings and construction inspection for this project. The site, in the Copley Square area of Back Bay, Boston, has streets on three sides and adjoins the historic 1895 Central Library, designed by McKim, Mead & White and considered a landmark of American architecture.

Some features of the $23 million dollar structure include:
-7 above grade and 2 below grade floors, a total of 500,000 sq. ft.
-Flat concrete slab upper floors three through six are hung from a main grid of story-depth interconnecting trusses, most of which also span the building's nine basic 58-foot bays. Three of the hanging floors will carry book stacks, while a fourth is for library administrative offices. The 16 foot deep trusses are supported on steel box columns rising from the basement foundation slab 27 feet below street level, while some will be supported on 9-foot-deep welded plate girders spanning 58 feet. The lower level of the trusses (bottom chord) carries the seventh floor for large mechanical equipment (up to 150 pounds per square foot of live load) and for additional book storage. In floors three through six, there are similarly high live loads ranging up to 120 psf in book stack areas. The upper level of trusses (top chord) is designed to support the sloping steel-framed roof - maintaining almost the same shape as the existing library roof.
- Welded built-up 32-inch-deep by 10-inch-wide composite plate girders span 58 feet at the second floor to carry post-tensioned two-way waffle slabs 60 feet square.
- The engineering incorporates for the first time in buildings, so far as is known, "orthotropic" pedestrian bridges. These low-profile bridges, only 19-¼ inches deep and either 6-½ feet or 13 feet wide as required, span 58-foot open modules at the interior mezzanine level. The term orthotropic generally refers to the design of a steel plate bridge deck which serves the double purpose of being a structural member of the bridge as well as the roadway surface. The library addition bridges incorporate a 2-¾ inch concrete walking surface held by welded stud shear connectors to ¼-inch steel plate on 16-inch beams. Among other advantages, orthotropic design permits construction of very shallow-depth bridges spanning long distances, as was required in the library addition to provide necessary headroom between the street and second floors.
- An unusual construction technique was used starting the building from the bottom, then erecting the main columns and 7th floor trusses and proceeding from the top down. Temporary bracing was used to tie the columns together and resist lateral forces until the 2nd through 6th floors were completed. Because the basement of the new building is right next to the present library - yet is 18 feet deeper - the subsurface soil strains was monitored by the use of special instruments. Settlement movement of both the old and new library buildings and the city streets during construction was also constantly monitored.
Heavy 7-foot-thick reinforced concrete spread footings up to 40 feet square are used for foundations. Interconnecting them is a 3-foot pressure slab designed to resist water uplift of 1,000 pounds per square foot. The sequence and size of foundation excavations to the clay bearing stratum was carefully specified and controlled during construction.

Another important problem that had to be solved was the presence of extensive ground water. Many buildings near the construction site, including such historic structures as Trinity Church, Old South Church and the present central library, rest on wooden piles. These foundations must be kept submerged to prevent rot. To maintain the existing ground water level, steel sheet piling was driven around the entire construction site and an elaborate recharging system was set up. The sheet piling wall resisted intrusion of the surrounding subsurface water into the pumped-out excavation. Since the sheet piling was not completely watertight, ground water lost through it was replaced by pumping clean water into the soil by means of a conventional well point system installed outside of the sheet piling. Numerous observation wells around the Copley Square area were checked twice weekly to see that water levels remain high.