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Lexington II at Market Square North, Washington D.C.

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.

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Lexington II at Market Square North, Washington D.C.

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  1. Lexington IIat Market Square North, Washington D.C. Alexis Pacella – Structural Option

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

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

  4. Building Summary

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

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

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

  8. 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???

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

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

  11. 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

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

  13. 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.

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

  15. 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’

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

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

  18. 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

  19. Lateral System 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

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

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

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

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

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

  25. 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 Typical 7th floor connection

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

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

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

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

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

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

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

  33. Construction Management • Construction Management Issues: • Site Layout • Cost • Scheduling

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

  35. Construction Management

  36. 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

  37. Construction Management Scheduling: 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. Total: 18 weeks

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

  39. Mechanical • Mechanical Considerations: • HVAC • Acoustics

  40. Mechanical HVAC: Fresh air requirements are met by windows

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

  42. Mechanical Exhaust Supply

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

  44. Mechanical Acoustics: Acoustic Tiles on suspended ceiling Assembly Issues

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

  46. 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

  47. Questions?

  48. Building Summary 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 • Key Players • Architecture • Façade • Site/Zoning • Other Systems 12 Story Residential Tower Market Square North Complex 3 below grade levels 72,000 sq ft Completed in 2002

  49. 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

  50. 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

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