Lexington II at Market Square North, Washington D.C. Alexis Pacella – Structural Option

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Building Summary 12 Story Residential Tower Market Square North Complex 3 below grade levels 72,000 sq ft 49 Apartment Units Completed in 2002

Building Summary

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Existing Structure Gravity System: 2 Way Flat Plate Slab 8 inches Irregular Column Layout Small Bays Lateral System: Shear Walls Foundation: MAT

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Proposal Current design restricted by height limit: 130 feet (current height = 125 ft w/ penthouse) Only structures with a thin floor sandwich, such as flat plate and pre-stressed were considered When height restriction is not a factor, Can A More Economic System be Found???

Proposal How will a steel structural system effect… Other building systems Time COST

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Gravity System Composite: Steel composite deck on steel beams and columns Relatively shallow floor sandwich No large change in stiffness and vibration Steel is usually most cost effective Additional cost and labor for fireproofing and connections

Gravity System New column layout required for steel system to be economical Does not interfere w/architecture Windows moved One closet obstructed

Gravity System Composite Decking: Span is too long, bisect each bay with a beam Unshored saves on labor and material Unshored will use extra concrete Residential Levels: 2” LOK floor deck with 4.5” slab Public Levels: 3” LOK deck with 5.5”slab Decking chosen from USD catalog.

Beams, columns, and shear studs were designed for gravity loads using RAM software. Gravity System RAM criteria set to LRDF 3 rd Edition

Gravity System Final Design: 4.5” deck + slab 12” beam = 16.5” Total floor sandwich depth Increase of 8.5” per floor  8.5’ total New Height = 133.5’ > Allowable 130’

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

3 possible Lateral Systems: Shear Walls Moment Frames Braced Frames Located around elevator shaft Architectural Feature of the building Will not obstruct bays Costly connections High strength and stiffness Easy to erect Lateral System

Shear Walls: Check Existing Walls ETABS Results: Surround Elevator Shaft Steel  Lighter Weight  Less Seismic Load 8.5’ height increase  Slightly Greater Wind Load

Braced Frames: Find location in building Distribute Loads by Rigidity Check ASCE 7 wind cases Design, check on STAAD Design was controlled by allowable stress Lateral System

STAAD Design: Chevron Braces Member Stresses Controlled Size Range: W12x65 – W12X170 Total Deflection < 3.6” allowable

Final Design: Some columns controlled by biaxial bending  increased size Sizes: W12x65 – W14x342 Final Deflection = 2.7” < 3.6” allowable Lateral System

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Are there other issues with the structure to be considered? Connections Substructure Foundation Other Structural Factors

Beam to Column Connections: Stiffeners Required Typical 7 th floor connection Other Structural Factors

Heavy Bracing Connection: Uniform Force Method Same beam to column connection 27”x27”x1/2” gusset plate 3”x3”x1/2” angles Other Structural Factors Typical 7 th floor connection

Other Structural Factors The Substructure: Concrete is better in subterranean conditions Additional building weight Concrete is a more durable parking structure

Other Structural Factors Substructure: One way joist floor ~easy to construct ~works with new bay sizes 20” pans; 5” ribs; 25” o.c.

Other Structural Factors Substructure: Girders: 24”x15” top steel = 7 #9’s bottom steel = 4 #8’s Stirrup PlacementReinforcement Cutoffs Girder Section

Other Structural Factors Substructure: Columns: Below the Braced Frames: Shear walls were used, shear controlled

Other Structural Factors Substructure: Connections ~ a base plate sunk into floor slab at each column

Other Structural Factors Foundation: Existing MAT foundation Punching shear Overturning Forces affecting overturning No Problems

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Construction Management Construction Management Issues: Site Layout Cost Scheduling

Construction Management Site Layout: Needed spaces: offices, circulation paths, storage, etc… Steel Design ~ Crane ~ Lay Down Area

Construction Management

Cost : Concrete $1,526,000 Steel ………………………………...$1,455,600 Saves $70,000 This does not however account for cost due to extra façade material or fireproofing Building Scale is not large enough to develop full economy associated with a steel structure

Construction Management Each level is built in the sequence: 1 day for columns and beams 2 days for deck Next level is started Slabs poured when beam, column, and deck construction is 3 stories ahead. Scheduling: Total:18 weeks

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Mechanical Mechanical Considerations: HVAC Acoustics

Mechanical HVAC: Fresh air requirements are met by windows

Mechanical HVAC: MEP Systems can be moved from soffits into ceiling sandwich. Beams should not be intersected.

Mechanical SupplyExhaust

Mechanical Acoustics: Original System: Concrete ceilings with sprayed acoustical sealant NRF =.75 Steel System: Suspended ceiling with acoustical tiles NRF =.5 to.95

Mechanical Acoustics: Acoustic Tiles on suspended ceiling Assembly Issues

Presentation Outline Building Summary Existing Structure Proposal Gravity System Lateral System Other Structural Factors Construction Management Mechanical Conclusion

Conclusions Structural design was controlled by height restriction Composite: Larger Bays Works well with other integrated systems Full Economy of Steel Savings was not achieved

Questions?

Building Summary Key Players Architecture Façade Site/Zoning Other Systems Owner Square 407 LP Architect Studios Architects Structural Engineer Thornton Tomasetti MEP Engineering Design Group General Contractor Clark Construction Transportation 2 elevators and stairwells in building core Mechanical Water source heat pump; Boiler, pumps, cooling tower Electric/Lighting 1 phase, 3 wire; 120/208 V Fluorescent Lights 12 Story Residential Tower Market Square North Complex 3 below grade levels 72,000 sq ft Completed in 2002

Existing Structure Materials Concrete: Columns and MAT ~ 5000psi Slab and Shear Walls ~ 4000psi Reinforcement: Slab ~ bottom #4 every 12” Shear Walls ~ #4 every 12” MAT ~ #8 bars every 9” Columns ~ varies by size Top and extra reinforcement is placed as needed

Gravity System Loads: Dead Load: Self Weight of System Superimposed = 30psf Live Load: Residential = 60psf Public = 100psf Partitions = 20psf Loads taken from ASCE 7 02

Construction Management Cost: Steel vs. Concrete Steel Costly Connections Fireproofing Concrete Formwork Rebar Waste Accessories SteelConcrete Costly ConnectionsFormwork FireproofingReinforcement Extra FaçadeWaste Accessories

Excavation:$23,600 Foundation:$159,000 Concrete Design Cost: Sub Grade Floors:$671,000 Super Structure Floors:$154,000 Columns:$395,000 Shear Walls:$123,900 ______________________________________ Total $1,526,000 Construction Management Prices taken from R.S. Means

Construction Management Excavation:$23,600 Foundation:$159,000 Steel Design Cost: Sub Grade Levels:$741,200 Steel Levels:$467,200 Braced Frames:$156,400 Connections:$51,300 ___________________________________ Total:$1,455,600 Prices taken from R.S. Means

Construction Management Scheduling: Excavation Backhoe.5 day Sheathing.5 days Hauling 1 days Sheathing 3 days Total per story 4 days x 3 stories 2.4 weeks

Construction Management Scheduling: Foundation Cast in Place MAT 1day Curing4 days Total5 days 3 weeks

Scheduling: Substructure Columns 1.1 day Grade walls2 days Shear walls.5 days 1-way Joist5 days Curing 4 days Total per story 11 days x 3 stories 9.2 weeks Construction Management

Scheduling: Super Structure Columns.5 day Beams.5 day Deck 2 days Slab1 day Curing 1 day Total per story 5 days x 12 stories 18 weeks

Mechanical Fire Suppression: Sprinkler Layout