P.O.S.T. System for Bookmen Stacks

lux_res_bookmanNestled amid the bustling freeways entering downtown Minneapolis, the Bookmen Stacks is a nine-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 feet. 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, Minnesota, 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 four-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 two and three and levels four and five), 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.

lux_res_bookman_trussMost 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-square-foot structure comprising eight stories of condominiums over one level of underground parking. The typical floor plate is 67.33-by-159.0-feet, with trusses at each column line spanning the full 67.33 feet. To complete the floor system, 12-inch thick precast concrete hollow-core spans the truss intervals ranging from 35.0- to 42.0-feet. 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-inch deep 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 percent. 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-by-13.5-feet and weigh 75,000 pounds. 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-by-12-inch flanges and 10-by-23-inch webs. A 4-inch thick lightweight concrete topping overlays the entire floor system, yielding a shallow floor depth of 16 inches 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 inches, the overall wall width is less than 12 inches. This wall width is over 25 percent thinner than the 16 inch 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, Minnesota, 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 miles per hour 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 percent 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 multi-use 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.

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JAS Apartments, Inc., Minneapolis, Minnesota

LSA Design, Inc., Minneapolis, Minnesota
James Dayton Design, Ltd., Minneapolis, Minnesota

Structural Engineer:
Ericksen Roed & Associates, Saint Paul, Minnesota

General Contractor:
Flag Builders, Inc., Minneapolis, Minnesota

Hanson Spancrete Midwest Co., Maple Grove, Minnesota