Museum Towers
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 One
of the newest high-rise luxury residences in the Boston area is
the Museum Towers at North Point in Cambridge.
Located across
from the Museum of Science and overlooking Boston's historic Back
Bay and Charles River, Museum Towers consists of two 24-story towers
attached at the base with a central lobby, one six-story building
that is joined to one of the towers, and a three level post-tensioned
parking garage.
The buildings
house 435 one- and two-bedroom apartments, penthouse apartments,
and a full-service health club with an Olympic-size lap pool. One
of the most important factors that contributed to the overall success
of this project was the collaboration between the general contractor,
subcontractors, and design team that resulted in cost-effective
and timesaving solutions, as outlined below. Another critical element
was familiarization and training of a skilled labor force with a
unique forming system.
Structural
System
The overall plan dimensions of a typical tower floor are 101 ft
by 101 ft (30.8 m by 30.8 m), with spans varying between 13 and
24 ft (4.0 and 7.3 m). A conventionally reinforced 8-in. (200-mm)
thick concrete slab was chosen for the floor system. Twenty-four
in. (610-mm) square columns are used for the full height of the
tower, with varying amounts of reinforcing steel.
The lateral
force resisting system consists of 14-in. (355-mm) thick shear walls
in a 40 ft by 40 ft (12.2 by 12.2 m) core area, and four 12-in.
(305-mm) thick outrigger walls that extend from the core walls to
the exterior of the building. The outrigger walls, used to reduce
the drift of the building, are located between the 2nd to 4th, 13th
to 15th, and 21st to 23rd floors.
Normal weight concrete with a specified compressive strength of
4,000 psi (25 MPa) is utilized for the framing members, except for
the columns and shear walls in the lower 8 floors which are 5,000
psi (35 MPa).
The foundation consists of 14 in. by 14 in. (355 mm by 355 mm) precast
concrete piles with a 120 ton (1,070 kN) capacity.
Design Loads
The buildings
are designed in accordance with the 5th edition of the Massachusetts
Building Code. It is important to note that seismic forces governed
the lateral design, with the seismic base shear equal to 3.5% of
the weight of the building.
Construction
Data
Four to
five days was a typical cycle for each floor. The shear walls were
constructed first, three floors ahead of the slabs and columns.
The walls were formed with the Peri Formworks system, supplied by
Peri North America, Baltimore. Skydeck, the high-strength lightweight
aluminum slab formwork which was also supplied by Peri, was used
for the slabs. The Skydeck allowed for removal of the form panels
and beams without removal of the shores, thereby eliminating the
need to reshore. Strength accelerators were used in the concrete
mix for the floor slabs in order to achieve 1,500 psi (10 MPa) within
24 hours. This strength was needed in order to allow safe stripping
of the forms.
In order to familiarize the local labor force with the Peri systems,
the Reinforced Concrete Construction Committee (RC3), Boston, worked
with the local unions in acquiring test systems of the formwork.
This training proved extremely valuable when the actual project
began.
To achieve a four to five day cycle, seven levels of shoring would
normally be required. Weidlinger Associates, Inc., the structural
engineer for the project, performed a finite element analysis of
the floor slabs and shoring and determined that only four levels
of shoring would be needed. The engineers worked in conjunction
with the University of Vermont to assure the accuracy of the finite
element analysis. Measurements obtained from load cells located
on judiciously chosen shoring members compared very closely to the
results of the analysis. The collaboration between the contractors
and the structural engineer resulted in significant savings in time
and money.
A typical cycle was as follows:
Day 1 - place
the floor forms;
Day 2 - place the slab reinforcing steel;
Day 3 - pour the slab;
Day 4 - erect the column reinforcing steel and pour the columns
to the next floor.
Minimizing crane
time was key to construction scheduling. Time-to-completion was
enhanced due to the fact that the Peri systems move up the tower
at all times and never have to be brought back to the ground. The
contractor also took advantage of the fact that there was two identical
towers, and cycled work crews between the two so that both towers
would be completed at the same time.
Concrete
Versus Steel Framing
Both reinforced
concrete and structural steel framing schemes were considered for
the towers in the preliminary design stages of the project. The
concrete system was clearly the most advantageous alternative for
the following reasons.
Although local zoning limits the height of construction to less
than 85 ft (25.9 m), the developer worked with the City of Cambridge
to obtain a tower height of 234 ft (71.3 m). By using the flat plate
system, the floor-to-floor height is 8 ft - 8 in. (2.6 m), compared
to 10 ft (3.0 m) that would have been required for the steel system.
Thus, by using the concrete scheme, two additional levels were added
to each of the towers for the same building height, resulting in
a greater number of rentable units per tower.
An additional significant cost savings was realized by using the
flat plat system, since the underside of the slab was used as the
finished ceiling for the floor below. By working closely with the
precast concrete contractor, simplicity of formwork was further
achieved by supporting the precast panels directly on the columns,
thereby eliminating the need for perimeter edge beams.
The overall time-to-completion of the project was a clear advantage
of the concrete system. Excavation and pile driving began in late
November of 1996, and both towers were completed in October of 1997.
The faster completion time translates into earlier tenant occupancy,
and, thus, an earlier return on the developer's investment.
Credits
Developer: Congress Group Ventures, Cambridge, MA
Architect: Jung/Brannen Associates
Structural Engineer: Weidlinger Associates, Inc., Cambridge,
MA
General Contractor: Beacon Skanska/Congress Group Construction
Corp. (joint venture)
Concrete Contractor: S&F Concrete Contractors
Concrete Supplier: Boston Sand & Gravel
Reinforcing Steel Supplier: Barker Steel
Reinforcing Steel Placer: Bart Lund Steel Services
Precast Concrete Contractor: Consolidated Precast
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