Mitre III Building McLean VA Debra Schroeder Structural Option

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

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

Existing Building

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

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

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

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

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

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

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

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

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

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

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

Lateral Systems

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

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

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

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

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

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

Alternate Façade

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

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

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

Questions?