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Mitre III Building Structural Enhancement Overview

Explore the structural options for the Mitre III Building in McLean, VA, focusing on floor and lateral systems. Delve into redesigning floor systems, analyzing costs, and investigating alternate façade options.

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Mitre III Building Structural Enhancement Overview

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  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?

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