BRBF Steel Design and Bozeman’s Mill Street LoftsFeb 17 2016 · 0 comments · Mixed-Use, NISHKIAN MONKS, Steel-framed ·0
During the past 18 years, Buckling-Restrained Braced Frames (BRBFs) have been used extensively in the United States as part of the seismic force-resisting system for buildings in regions of high seismicity. One of the first new construction projects in Montana that employed BRBFs was the Mill Street Lofts building located in north of downtown Bozeman. The Mill Street Lofts building construction was the first phase of a multi-phase urban renewal projects to revitalize an older, industrial sector of the city. The vision that the owner/developer, Mill District Partners, LLC had for the greater project was a main street lined with buildings, which paid tribute to the industrial sector while creating the look and feel of a downtown main street. Comma-Q Architecture, Inc. took on the role as lead designers and project managers for this project. Nishkian Monks served as the structural design consultant providing development and design of the structural system, construction administration, and special inspection of the building design collaborating with general contractor Martel Construction, Inc.
The Mill Street Lofts building was designed as a three-story live/work structure which had commercial first floor space and four residential lofts above the first level. Each loft was two levels with access to a private roof top deck. Comma-Q’s design team developed a building where the structural system was to be part of the architecture; expressed and appreciated instead of being concealed. Nishkian Monks was asked to develop a structural system that followed the vision for the project. For an industrial look and feel, Nishkian Monks chose a steel framed building with composite floor systems (metal deck with concrete fill over steel wide-flange beams). To further provide visibility to the structural system, a lateral force resisting braced frame system was chosen.
The uniqueness of the structural design of this project was centered on the braced frame system. Comma-Q and Nishkian Monks placed the braced frames in locations that were highly visible and functioned efficiently. Comma-Q’s goal was to have the frames visible but simple in design. At the time of construction new provisions to the Building Code mandated that connections of steel braces have the ability to withstand significant inelastic demands from stretching and buckling braces during a seismic event. These demand forces lead to large, heavy brace connections at the beam and column joints. Larger frame beams and columns also resulted from the changes to the seismic code.
In order to satisfy the architectural team’s vision of the braced frames, Nishkian Monks chose the Buckling-Restrained Braced Frames (BRBF’s). BRBF’s had just been added to the seismic code as an alternative to the traditional braced frame systems at the time of design. BRBF’s have a superior performance during a seismic event than the traditional braced frames. During an earthquake, traditional braced frames start to loose strength and stiffness as the braces buckle and stretch repeatedly from the structure swaying back and forth. The BRBF’s are designed to eliminate brace buckling during an event. With buckling eliminated the frame’s strength and stiffness is not reduced, therefore increasing the likelihood of the structure surviving a very strong earthquake. Since the performance of the BRBF’s is so much more reliable than the traditional braced frame systems, the Building Code allows for the structural engineer to design the lateral force resisting system for much less seismic force. The smaller force results in smaller connections, beams, columns, and foundation elements; all which save the owner money on the cost of their project. The BRBF system provided the project with very small, inexpensive connections in the frames which satisfied the Architect, all-the-while providing a very high level of confidence for performance during a strong earthquake.
Nishkian Monks chose a manufacturer of BRB’s, Star Seismic of Park City, Utah for assistance in the design process. Star Seismic produces BRB’s with simple pinned end connections which Comma-Q saw as an elegant structural connection.
The design process with BRBF’s was challenging due to its iterative nature. Properties of the BRB were entered into the computer model of the building to analyze the structure during a seismic event. The brace forces from the analysis and BRB property assumptions were submitted to Star Seismic for their review. Star Seismic would then make any required changes to the BRB property assumptions, the model would be revised based on their changes, and the process would repeat until Star Seismic did not need to adjust the BRB properties anymore.
Through the design process, Star Seismic found that the standard BRB’s that they manufacture would be significantly stronger than would ever be needed during a seismic event on the Mill Street Lofts building. To provide economy for the project and meet the vision Comma-Q had for the braces, Star Seismic actually designed, manufactured, and tested a completely new BRB exclusively for this building project. The new brace was compact in cross-section, was adequately proportioned for the design seismic forces, and had a simpler/cheaper connection at the joint.
By utilizing BRBF’s for this project Nishkian Monks paved the way for development of low demand BRB’s to be utilized in small steel building projects prevalent in Montana. Further use of BRBF systems will first and foremost provide superior protection of life in buildings during an earthquake, and save precious resources by decreasing the code prescribe lateral forces on structures.
RENDERING COURTESY OF COMMA-Q ARCHITECTURE, INC.