1. Mitre III Building
McLean VA
Debra Schroeder
Structural Option

2. Overview Depth: Breadth: 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 Alternate Façade
-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: Cost of new structure: $2,200,000
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 System Amount of materials used Pros: Cons:
Estimated cost for materials
1. Shortened spans
817 tons
-Smaller, lighter members
-Additional columns do not allow for unobstructed floor plan
-Additional Cost for construction
-Typically does not satisfy floor vibration criteria
$2,200,000
2. Long spans
824 tons
-Allows for unobstructed floor plan
-Less costly to erect
-Slightly larger members
$2,225,000
3. Shear Wall
3073cy
-Stiffer than braced frames
-Less expensive than moment frame
-Difficult to connect to diaphragms
-More costly than Braced frames
$304,000
4. Braced Frame
54 tons
-Connections easier
-Less expensive than moment frame and shear wall for material and construction
-Require some rearrangement of architecture
-Allows slightly larger deflections than shear wall
$145,000

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

23. Alternate Façade

24. Alternate Façade Goals: New Material: 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: Lateral Systems: Alternate Façade:
Keep existing long spans
Lateral Systems:
Braced Frames
Alternate Façade:
Fiber Reinforced Concrete Panels

27. Questions?