Executive Summary: The MITRE 3 building is an eight story office building. Its current frame is composed of A992 wide flange shapes. The lateral resisting system is a moment frame. This moment frame consists of the exterior frame of the building, and a centrally located frame. The building has three main bays, the outer two at 42 feet in length, and the middle bay at 43 feet in length. There are three alternative structural systems that will be investigated in this thesis. All three will include a shortened bay length in order to reduced member sizes. Due to the cost of moment connections, the three alternate systems involve replacing the existing moment frame. The first alternative is to replace the existing moment frame with shear walls. This may come across problems with space to position them. The second alternative will be to have a braced frame. This alternative will require the rearrangement of floor spaces in order to accommodate for the new members. The third alternative will be a combination of the shear walls and the braced frame. These alternatives will require additional work to other systems within the building. The mechanical system will be looked at the most thoroughly. Many of the elements of the system will need to be moved in order to place new members. Also, if the new structure changes the floor to ceiling heights, the mechanical system will need to be analyzed to determine if it will be efficient for the new space. An alternate façade will also be added to reduce the weight of the building, and the mechanical system will again need to be analyzed for this change.
Background: The MITRE 3 building is an eight story 180000 square foot office building located in McLean Virginia. Its frame consists mainly of A992 wide flange steel shapes, with certain areas of the building having concrete members. A typical floor to floor height is 13.5 feet, with the first floor being greater at 16 feet. A composite metal deck with 3.25 lightweight concrete cover is used. There are three main bays in this building, the outer two spanning 42 feet, while the central bay spans 43 feet. The beams are spaced at 10 o.c typically. There are two elevator shafts, two stairwells, and one mechanical shaft in the building (see Figure 1). The façade of the building has both precast concrete panels and composite metal panels. The lateral loads are resisted by a moment frame. This moment frame consists of the exterior frame of the building, and one centrally located frame (see Figure 2). Figure 1: Red: Elevator Shaft Green: Stairwells Blue: Mechanical Shaft
Figure 2: Highlighted Moment Frame Foundation walls are 5000psi 28 day strength concrete, and range in height from 1 2 to 1 10. Spread footings are 4000psi concrete and are supported on soils reinforced with geopiers due to poor soil conditions on site. Problem Statement: Due to the lengths of the bays, the options for alternate, possibly more cost efficient floor systems were limited. Large steel beams are needed to span that distance, in most cases the large size was due to deflection. Concrete beams are feasible, although the sizes would be considerable due to the lengths.
The lateral resisting system is a moment frame, which can become costly with the moment connections. Although certain moment connections are needed, for example, in the cantilevered sections, many can be eliminated if a different lateral system is used. Proposed Solutions: In order to find a floor system that can possibly reduce cost, and result in a lighter building, the bays will be shortened. The outer two bays, which are currently 42 feet in length, will be shortened to lengths determined by partition placements in order to reduce obstructions due to structural elements. The central bay will also be reduced to a smaller length. In the central bay there is a lot of mechanical equipment in the plenum space that will need to be taken into consideration while placing the new columns. Different lateral systems will also be looked at. The first will be incorporating shear walls around the existing vertical shafts in the building (see Figure 1). This solution may encounter problems with space for the walls. The second alternative for the moment frame is a braced frame. Although rearrangement of the floor space may need to be considered to allow for the diagonal elements. The third alternative would be a combination of the previous two. Breadth Options: Of the options present here, two or more will be decided upon at a later date: 1. With the addition of columns the architecture will need to be changed slightly to allow for them not to obstruct any areas. Also, since the building has a more open floor plan, the architecture will need to be changed to allow for the braced frame. These structural elements will be placed in a way to limit the amount of change needed to the layout of the floor space, but there will need to be some changes.
2. The façade of the building will be changed to a different system. This is in order to lessen the overall weight of the structure. Also, if an energy efficient material is chosen, this can be compared to the original to see the effects that it has on the mechanical system of the building. 3. Due to the changes in the structure of the building, the mechanical system will need to be rearranged to accommodate the new structure. Also, if the floor to ceiling height changes with the new structure, the mechanical system will need to be analyzed to determine if it will be sufficient for the new volumes. In addition to that, the new façade s effects on the mechanical system will be determined. 4. For each system, including the existing, a cost analysis will be determined in order to compare the different options and arrive at a conclusion as to which system should be used. Solution Method: The design of the steel members will be based on the LRFD manual. The gravity and lateral loads, and the loading combinations will be determined using ASCE 7-02. Initial members will be found using gravity loads. For the analysis of the frame a model using initial members will be built and analyzed using RAM. The braced frame will be built and designed using RAM. The shear wall design will be completed by hand once research in methods is completed. Tasks and Tools: Alternative 1: Shortened spans and Shear Walls Task 1: Gravity member sizes
a) Determine Loads b) Locate where additional columns will be added c) Build RAM model of new structure d) Obtain initial gravity members e) Check deflections f) Redesign as needed Task 2: Design of Shear wall a) Preliminary design of Shear walls b) Determine stiffness of walls c) Distribute Lateral Loads based on stiffness of walls d) Check strength e) Check Drift Task 3: Determine effect new structure has on mechanical system Task 4: Prepare a cost analysis for this system Task 5: Compare to existing structure and alternatives Alternative 2: Shortened spans and Braced Frame: Task 1: Gravity member sizes a) Determine Loads b) Locate where additional columns will be added c) Build RAM model of new structure d) Obtain initial gravity members e) Check deflections f) Redesign as needed Task 2: Design Braced Frame a) Locate where braced frames will be added b) Obtain initial members with RAM c) Check Strength
d) Check drift using e) Redesign as needed Task 3: Determine effect new structure has on mechanical system Task 4: Prepare a Cost analysis of this system Task 5: Compare to existing structure and alternatives Alternative 3: Shortened spans, and a combination of Shear walls and a Braced frame Task 1: Gravity member sizes a) Determine Loads b) Locate where additional columns will be added c) Build RAM model of new structure d) Obtain initial gravity members e) Check deflections f) Redesign as needed Task 2: Design braced Frame a) Locate where braced frames will be added b) Obtain initial members with RAM c) Check Strength d) Check drift using e) Redesign as needed Task 3: Design Shear Walls a) Preliminary design of Shear walls b) Determine stiffness of walls c) Distribute Lateral Loads based on stiffness of walls d) Check strength e) Check Drift f) Redesign as needed
Task 4: Redistribute lateral Loads based on Stiffness a) Determine Stiffness of braced frame b) Determine stiffness of shear walls c) Redistribute lateral loads based on stiffness d) Check member sizes for strength and drift e) Redesign as needed Task 5: Determine effect new structure has on mechanical system. Task 6: Prepare a Cost analysis of this system Task 7: Compare to existing structure and alternatives
Timetable: January 2005 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 1 2 3 4 5 Alternate facades researched 6 7 8 9 10 Placement of new columns determined 11 12 Alternate façade and it s load determined 13 14 final loads 15 16 17 18 19 20 21 RAM Model built for Gravity members 22 Final Gravity members obtained 23 24 25 26 Initial Shear wall Designed 27 28 Final Shear Wall Designed 29 30 31 Location of braced frames determined
February 2005 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 1 2 3 4 5 6 7 RAM model of braced frame done 8 9 Final members of braced frame determined 10 11 12 13 14 Combined shear wall and braced frame analyzed 15 16 Final members of combined determined 17 18 19 20 21 Cost analysis of existing structure 22 23 24 25 26 27 28 Cost Analysis of Alternatives
March 2005 Sunday Monday Tuesday Wednesday Thursday Friday Saturday 1 2 Compare 3 4 Final 5 Alternatives with existing option for building structure decided 6 7 Spring Break! 8 9 10 11 12 13 14 Determine effects change in structure has in mechanical layout 20 21 Prepare final report 15 16 17 18 Analysis of mechanical system complete 22 23 24 25 26 19 27 28 Prepare for final presentation 29 30 31