Thesis Proposal December 5, 2003

Transcription

1 Executive Summary Thesis Proposal December 5, 2003 Brennan Hall is a five-story office/classroom building. It is located on a slightly sloping site, surrounded by other structures and a small green space. Floor to floor heights are typically 14 feet, with the roof topping off around 70 feet. The top of the tower reaches to nearly 90 feet above grade. Gravity loads are supported by composite concrete slabs which act compositely with wide flange steel beams. Lateral loads are resisted by a series of rigid moment frames. Because of the expense of moment resisting connections, three alternate lateral force resisting systems will be studied in this thesis. The first system consists solely of steel braced frames. CMU shear walls resist the lateral forces in the second alternative. The final alternative combines steel braced frames with CMU shear walls enclosing the elevator shafts. Steel design will be based on Load and Resistance Factor (LRFD). RAM, STAAD, and possibly other computer modeling programs will be used to aid in the analysis of each lateral system. Research will be conducted to acquire a better knowledge of the design and analysis of CMU shear walls. One breadth area study that will be undertaken is a rearrangement of Brennan Hall s interior spaces in order to devise a layout that will better accommodate the new lateral system. The building s mechanical system will also be checked and adjusted as needed to meet the heating and cooling needs of the new architectural layout. Page 1 of 6

2 Background Brennan Hall is a five-story, steel-framed building housing high-tech classrooms, faculty offices, and an executive conference center with catering facilities. It is located on a slightly sloping site, nestled into a typical city block, surrounded by other structures and a small green space. Floor to floor heights are typically 14 feet, with the roof topping off around 70 feet. The top of the tower reaches to nearly 90 feet above grade. Gravity loads are carried by 5 thick normal weight concrete slabs which act compositely with 2 metal floor deck. Shear connectors are used to ensure composite action between floor slabs and the wide flange steel beams and girders that support them. Single angle and double angle shear connections are used to connect beams to girders. Lateral loads are resisted in both directions by moment frames. Girders are connected to columns with flange plate bolted/web bolted moment connections at interior locations and shear connections at exterior locations. Because Brennan Hall is L-shaped and features some large open spaces there are many different frame sizes and configurations. The prominent architectural feature of Brennan Hall is the tower on the West corner of the building. The tower is prominent inside as well, consisting of two spaces that are open to the floor above and an executive board room on the fifth floor. The sides are more subtle, further emphasizing the tower, which suits the classrooms and offices that they house. To minimize traffic through the upper levels of the building, the classrooms are located on the first two floors. While the building is not actually symmetrical, from certain angles it appears so. This apparent symmetry is protected by the placement of additional stairs and elevators in the rear of the building where they are less likely to be seen. This building uses a variable air volume system with hot water reheating coils. It is serviced by two 1892 MBH cast iron boilers which also serve the adjacent dormitory that was constructed at the same time. Water is cooled by two rotary liquid chillers connected to cooling towers. The chillers sit on concrete pads and are separated from the structure by vibration insulators. All ductwork is treated with ½ acoustical lining and is mounted to auxiliary steel with spring hanger rods. Problem Statement Moment resisting connections are very expensive compared to shear only connections. Since Brennan Hall s lateral system consists of multiple moment frames, considerable time and money on both the design and construction sides could possibly be saved by eliminating most, if not all, of the moment connections. Therefore a new lateral system must be devised that will transfer lateral loads into the ground without employing connections designed to resist moment. However, such systems require diagonal braces Page 2 of 6

3 or shear walls which cannot be easily concealed in the current floor layouts. In order to implement one of these systems, a rearrangement of the interior spaces will likely need to be undertaken. Proposed Solution Three alternate lateral force resisting systems will be examined. Structural efficiency, economy, constructability, and impact on architectural layout will be considered while determining the preferred solution. Alternative #1: This alternative employs steel braced frames which will be placed mostly around the perimeter of the building with the possibility of a few located in the interior (see figure 2 below). Alternative #2: In this alternative, the CMU walls framing the elevator shafts (see figure 1 below) will be used as shear walls to resist lateral loads. Additional shear walls will be located throughout the structure as necessary to adequately support the building. Alternative #3: The final alternative will be a combination of the previous two alternatives. CMU elevator shafts will act as shear walls and be supplemented by steel braced frames as needed to resist controlling lateral loads. Figure 1. Location of elevator shafts Figure 2. Possible locations for shear walls or braced frames Page 3 of 6

4 Solution Method Both gravity and lateral loads will be determined using ASCE The design of steel braced frames will be based on AISC s Load and Resistance Factor (LRFD) Specification for Structural Steel Buildings. Braced frames will be entered into STAAD with preliminary sizes in order to determine the stiffness of each frame. These stiffnesses will then be used for the distribution of lateral loads and for building torsion considerations. RAM and/or some other computer modeling programs may be utilized to analyze the entire structure. Before undertaking the design of the shear walls, research will be conducted in order to better understand both shear walls and the design of CMU walls. Tasks and Tools Below are step-by-step breakdowns for the design and analysis of each alternative. The tasks for the second and third alternatives are preliminary as the design procedures for CMU shear walls are not fully understood at this time. I. Alternative #1: Steel Braced Frames a) Finalize all loads b) Determine locations for braced frames c) Establish preliminary member sizes d) Model frames in STAAD to determine frame stiffnesses e) Distribute lateral loads relative to frame stiffnesses f) Use computer models to conduct strength and drift checks g) Compare with other alternatives II. Alternative #2: CMU Shear Walls a) Finalize all loads b) Determine locations for shear walls c) Research shear walls d) Research design of CMU walls e) Establish preliminary wall sizes f) Analyze walls to determine stiffnesses g) Distribute lateral loads relative to wall stiffnesses h) Conduct strength and drift checks i) Compare with other alternatives III. Alternative #3: CMU Shear Walls with Steel Braced Frames a) Finalize all loads b) Research CMU and shear walls c) Determine locations for braced frames d) Establish preliminary sizes Page 4 of 6

5 e) Determine stiffnesses and distribute lateral loads f) Conduct strength and drift checks g) Compare with other alternatives Timetable January Start of Finalize Loads Semester Braced Frame CMU Shear Wall Research and February CMU Shear Wall Research and Combined Lateral System Compare Alternatives Architectural 29 March Architectural Mechanical Spring Break Mechanical Write Report Presentation Preparation Page 5 of 6

6 April Presentation Preparation Presentation Preparation Presentations Presentations Presentations Breadth Topics Architectural: Mechanical: Featuring large interior spaces and a fair amount of exterior glazing, the architectural layout of Brennan Hall does not provide enough adequate locations for shear walls and braced frames. Therefore, the interior spaces will be rearranged to accommodate the new lateral system designed in the depth portion of this thesis. The new architectural layout will still be functional for the building tenants and will meet applicable code requirements such as egress. Rearranging the interior spaces potentially changes the heating and cooling loads imposed upon the building s mechanical system. In addition, the ductwork and air distribution will need to be rearranged throughout the building. Calculations will be performed to determine whether the current mechanical system is adequate to service the new architectural layout. Adjustments to the mechanical system will be made as necessary. Page 6 of 6