Mitre III Building

1. Mitre III Building McLean VA Debra Schroeder Structural Option

2. Overview • Depth: • Looked at different floor systems • Investigated alternate lateral systems • Performed a vibration analysis on redesigned floor systems • Breadth: • Cost analysis of new systems • Alternate façade investigated

3. Presentation Outline • Existing Conditions • Depth Work -Floor systems/vibration -Lateral systems • Alternate Façade • Conclusions

4. Existing Building

5. Existing Building • Located in McLean VA • 8 Story Composite Steel Frame office building • Height = 120 ft • 180,000 SQ FT • Façade: • Mostly Precast Concrete Panels

6. Existing Building Structure • Existing Floor System: • 3 Bays • Outer two 42’ in length • Central 43’ in length • Typical Member Sizes: (not included in Moment Frame) • Beams: W21X50 • Girders: W18X35 • W21X50 • Columns: W12X65 • W14X283 • Foundation: • Poor soil conditions on site required geopiers to be used to support structure

7. Existing Building Structure Lateral Resisting System: Moment Frame Locations of Moment Frames

8. Proposed Changes to Structure • Floor System • Investigate the effects of Shortening spans • Alternate lateral systems • Shear walls • Braced Frames

9. Reducing Bay lengths Possible Outcomes: - Lower Costs - Decrease amount of steel in building - Decrease weight of building

10. Reducing Bay Lengths • Shortened Spans: • Outside Bays reduced to ½ original length • Central Bay reduced to a 14 foot section and a 29 foot section

11. Results • Members smaller than in existing building • Vibration Analysis: Design not sufficient to control vibration • Options: 1. Increase Concrete deck depth 2. Use Normal Weight Concrete instead of Lightweight Concrete 3. Change Member Sizes

12. Results • New Member Sizes: • Beams: W16X31 • Girders: W18X40 • Cost of new structure: $2,200,000

13. Conclusion • New floor system did not produce outcomes that were expected • Redesign floor system with long spans

14. Existing Bay Lengths Results • Member sizes reduced from existing structure • Lighter façade • Elimination of moment frame • Vibration not a controlling factor • Typical sizes of new members: • Beams: W21X44 • Girders: W18X35 • Cost of new system: $2,225,000

15. Conclusion • The best option for the floor system is the longer spans • Allows for unobstructed office layout • Difference in the amount of material Minimal • Possible increase in construction costs with shorter spans

16. Lateral Systems

17. Overview • Designed shear walls and braced frames for new structure • Wanted to eliminate moment frame

18. Alternate Lateral System • Placement of Shear walls and Braced Frames • Double angles used for braced frames

19. Alternate Lateral System Results • Shear Walls • Cost: $304,000 • Best at controlling deflections • Braced Frames • Cost: $145,000 • Original design not adequate • Changed to X braces in order to control deflection • Some walls needed to be moved in order to accommodate braces

20. Conclusions • Used braced frames: • Connections to shear walls would be difficult • Cost of shear walls much greater than braced frames

21. Overview

22. Overview • Total Cost of new System: $2,370,000 • Cost of existing system: $2,907,000

23. Alternate Façade

24. Alternate Façade • Goals: • Maintain Existing appearance • Improve upon existing system • Cost • Function • New Material: • Fiber Reinforced Concrete Panels

25. Alternate Façade • Benefits of Fiber Reinforced Concrete Panels: • Does not require thick concrete cover so panels can be up to 66% lighter • Can reduce structural member sizes • Can reduce construction costs • Increased R-Values • Able to maintain existing appearance • Cost increase from regular panels

26. Conclusions • Floor Systems: • Keep existing long spans • Lateral Systems: • Braced Frames • Alternate Façade: • Fiber Reinforced Concrete Panels

27. Questions?