Bookmen Stacks
Minneapolis, Minnesota
Buildings Home
> Case Studies: Luxury Residential
> Bookman Stacks
P.O.S.T.* System for Bookmen Stacks
 Nestled
amid the bustling freeways entering downtown Minneapolis, the Bookmen
Stacks is a 9-story, 45-unit contemporary residential development
in the popular Minneapolis Warehouse District. Viewed by thousands
of drivers on a daily basis, Bookmen Stacks’ prime location
gives the structure high visibility, which generated many passerby
double takes during the rapid construction of its unique structural
system. Although, like most of the warehouses in the district, this
condominium project used concrete as the primary structural element,
Bookmen Stacks stands in stark contrast to its brick-clad neighbors.
While architecture is typically the defining measure of a structure’s
creativity, for this development, it is the innovative precast concrete
structural system that sets Bookmen Stacks apart.
Knowing that concrete construction offers the most economical and
beneficial solution for multifamily residential living, the Bookmen
Stacks design team faced a daunting challenge posed by the owner’s
project requirements. The hurdle was to find a structural system
that maintains the shallow floor-to-floor heights synonymous with
flat plate construction while achieving clear spans of up to 70
ft (21 m). The Stacks employs a new exposed structural precast concrete
truss system that allows alternating floors to remain completely
free of interior columns.
For this particular project, the driving force behind developing
a system that afforded column-free space was the owner’s need
to maximize the number of parking stalls in the underground structure.
Any columns penetrating the parking level to reduce the open-span
length would result in a reduction in the number of parking stalls,
which is directly related to the number of planned condominium units.
To optimize usable space in the Bookmen Stacks condominium project,
the interior columns in the parking space needed to be eliminated.
This open-space requirement was a high hurdle to clear, but through
problem solving, creativity, and teamwork, a new innovation for
precast concrete construction was born.
Mike DeSutter, partner with Ericksen Roed & Associates Inc.
of St. Paul, Minn., took on the challenge of finding a system to
achieve the desired clear spans for the condominium project. Formerly
an engineer in the precast concrete industry, DeSutter’s background
helped him conceive the new innovative Ericksen Roed Prestressed
Open Space Truss (ER-POST) system. He drew upon the concept of the
structural steel staggered-truss system created in the 1960s when
developing the full-story precast concrete trusses. The idea was
to invent a system to bridge the gap between the maximum allowable
height of a 4-story wood building and the economical height of a
10-story, post-tensioned, cast-in-place concrete system while improving
clearances and providing maximum space utilization for owners.
 Diverging
from the conventional staggered truss design, the patented ER-POST
system utilizes prestressed, precast concrete trusses that support
two floors simultaneously. The trusses span between the exterior
walls and are located at each column line with bottom and top chords
supporting floors. The trusses are on every other floor (for example,
between levels 2 and 3 and levels 4 and 5), allowing the odd numbered
floor levels to be completely free of structure. This alternating-level
truss pattern creates column-free spaces on an entire level, resulting
in the ultimate flexibility in interior wall layouts to accommodate
a multitude of space functions.
Most importantly, the ground level of Bookmen Stacks is completely
open, which allowed complete design freedom in the column-free space.
This open-space configuration is not only advantageous to condominium
construction. Owners of hotels, senior housing, assisted living
residences, and other buildings with high occupancy desire this
column-free arrangement for open lobbies, large banquet halls, meeting
rooms, and other group facilities. Mixed-use projects that incorporate
retail shops at the ground level with residential housing above
also benefit from a column-free main floor.
Bookmen Stacks is a 112,000 ft2 (10,400 m2) structure comprising
eight stories of condominiums over one level of underground parking.
The typical floorplate is 67.33 ft × 159.0 ft (20.5 m ×
48.5 m), with trusses at each column line spanning the full 67.33
ft. To complete the floor system, 12-in.-thick (300 mm) precast
concrete hollowcore spans the truss intervals ranging from 35.0
ft to 42.0 ft (10.7 m to 12.8 m). The longer hollow-core spans allowed
for a reduced number of trusses to additionally economize the system.
To obtain an open first-level lobby along with one parking level
below grade, the trusses start on the second level. The first floor
(grade level) is framed with 33-in.-deep (840 mm) precast concrete
double tees that span the long distance over the subgrade parking.
By eliminating the columns in the parking area, the parking capacity
increased approximately 15%. This additional space alleviated many
of the problems in parking planning and layout and provided sheltered
parking for 100 vehicles
Design Based on Vierendeel Truss
Designed as a hybrid bowstring Vierendeel truss, the POST system
marries economics and functionality. The trusses are 64.0 ft ×
13.5 ft (19.5 m × 4.1 m) and weigh 75,000 lb (34,000 kg).
The overall layout of the vertical and diagonal truss members has
enough flexibility to align with architectural openings. Passage
locations for doors (and for corridors in residential units) are
not obstacles for this framing system because the diagonal members
can be eliminated in predetermined sections. With the generous column
spacing, the trusses often enclose an entire unit layout and generally
do not interfere with future tenant modifications.
The precast concrete hollow-core floors are supported by prestressed
top and bottom chord inverted T-beams with 20 in. × 12 in.
(510 mm × 300 mm) flanges and 10 in. × 23 in. (255 mm
× 660 mm) webs. A 4-in.-thick (100 mm) lightweight concrete
topping overlays the entire floor system, yielding a shallow floor
depth of 16 in. (410 mm) for the long-span, column-free space that
is difficult to match.
The top and bottom chords extend slightly into the open living
space with minimal soffit protrusion. The vertical components are
generally contained within the walls that separate the living units.
A typical wall width comprises the thickness of the vertical concrete
truss members and the room finish. Because the typical member width
is 10 in. (250 mm), the overall wall width is less than 12 in. (300
mm). This wall width is over 25% thinner than the 16 in. (410 mm)
equivalent steel staggered truss width, which requires additional
width for the necessary fireproofing requirements.
Fire Rating, Sound Transfer, and Strut-and-Tie
Model
The truss system yields a two- to three-hour fire rating, which
lowers insurance costs. Without the need for fireproofing material,
there was no additional schedule allowance required for another
subcontractor to perform this work.
Sound attenuation is excellent, with a sound transmission class
(STC) of greater than 50 inherent in the material. The impact insulation
class (IIC) ratings for the truss system are above 50 when using
a sound mat.
The top chord of each truss is designed primarily as a column because
the design loads place this component in compression. In the bottom
chord, the prestressing strand is designed to resist all of the
design tensile forces and mild reinforcing steel is used for local
stress reversals.
The trusses generally have an open-web configuration, but depending
on structural capacity requirements, some truss segments may require
a solid web section or additional diagonal struts.
Casting a solid web panel is not difficult, but the overall truss
weight may be contingent on the capacity of the crane used for erection.
Thus, using the open-web truss layout is more economical despite
the increased congestion at the connection regions. The truss connections
are designed using the strut-and-tie model (STM) approach. The STM
approach provides a more realistic load path for the safe transfer
of load throughout the truss and to the supporting vertical members
and allows for easier visualization by the designer. The use of
self-consolidating concrete and the high level of quality control
at the Hanson Structural Precast–Midwest Inc. facility in
Maple Grove, Minn., virtually eliminate any problems associated
with the highly congested reinforcement areas in the trusses.
Lateral loads are transferred at each level through the precast
concrete plank diaphragm to a concrete shear core. The core for
Bookmen Stacks is located at the bank of elevators near the building
center, and the connections use standard precast concrete details.
For areas where the governing lateral force is the minimum 90 mph
(145 km/h) wind load, the lateral design generally does not control
the system design for structures up to 12 stories. This building
system is practical for structures up to 25 stories in areas of
low seismicity.
Precast concrete also provides a stiffened structure, which significantly
reduces the vibration and deflection issues that plague structural
steel structures and the shrinkage/movement issues associated with
wood construction (racking of doors and windows). With concrete
as the material of choice, the tenants of the Bookmen Stacks will
experience high-end loft living in this durable and low-maintenance
structure.
Bookmen Stacks is one of the newest condominium developments to
hit the trendy Minneapolis Warehouse District. These condominium
units are not like the rest. They boast plenty of open interior
spaces and some of best views of the downtown skyline and have the
distinct advantage, with respect to Minnesota’s frigid winters,
of tenant parking in an enclosed and heated precast concrete parking
structure. Using a precast, prestressed concrete truss system cut
20% off the structure’s original package costs, shaved two
months off the construction schedule, and eliminated all of the
interior columns on alternating floors.
The original goal was not to find an inexpensive building material;
it was to find a solution to the challenges of building mid- to
high-rise multiuse housing that meets owners’ needs. The Bookmen
Stacks design team discovered that not only did the truss system
work, it saved considerable money over conventional designs, gave
builders and designers more freedom in construction, and created
open spaces to accommodate any need.
This project is a great example of how teamwork and ingenuity evolve
into an innovative solution.
Click here for a
pdf of the complete article from the PCI Journal.
|