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Mark Anstrom Structural option MAE/BAE

This presentation provides an overview of the Strathmore Park at Grosvenor Metro project in North Bethesda, MD. The proposal explores the redesign of the existing concrete structure using wood and steel construction methods. The advantages, disadvantages, and architectural modifications of each system are compared and contrasted.

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Mark Anstrom Structural option MAE/BAE

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  1. Mark AnstromStructural option MAE/BAE AE Senior Thesis presentation Strathmore Park at Grosvenor Metro North Bethesda, MD

  2. Strathmore Park at Grosvenor Metro

  3. Project Overview • Located in North Bethesda, MD. A Washington, D.C. suburb. • 3 identical buildings – same design structurally • 4 identical floors of luxury condominiums over a 1 story parking garage • 2 hydraulic elevators • 2 stair towers • Individual HVAC units per Condo • High end finishes, fixtures and appliances • Very spacious 2-3 bedroom units • Building has sprinkler system

  4. Strathmore Park at Grosvenor Metro 3 2 1 4 5

  5. Current Construction • Completed December, 2002 • 10” CIP concrete flat slab f’c = 4 ksi • Punching shear reinforcement provided by steel studrails • Lateral forces resisted by centrally located shearwalls

  6. Thesis Proposal • Redesign Strathmore Park using: • Wood construction • Steel construction • Why? – Many reasons, these were the most viable systems for the layout • Compare and contrast the three systems • What are the advantages and disadvantages?

  7. Considerations • Maintain same architectural appearance as original design (such as ceiling height) • Layout and design members for optimal performance • Preserve acoustical quality • Check trusses for vibration • Ensure proper fire ratings • Check ponding on flat roof

  8. Presentation preview • Wood design summary • Steel design summary • Breadth - fire protection and acoustics • Comparison – wood, steel, and concrete • Conclusion

  9. Wood Design 16” Wood trusses with dimension lumber bearing walls

  10. The design

  11. The design

  12. The designfloor system • 16” deep single 2x4 top and bottom chord plate-connected floor trusses • Based on deflection, moment of inertia and span • Simple span condition throughout building • Topped with 23/32” T&G structural panels and ¾” cementitious topping (UL # L521) • 12” transfer slab at first floor (From ADOSS)

  13. The designroof system • 32” deep 2x6 single top and bottom chord metal plate-connected floor trusses • Based on deflection, moment of inertia, and span • Simple span condition throughout building • Rigid insulation on roof sloped to drain to avoid ponding • Bottom chord bearing

  14. The designbearing wall system • 2x4 SPF #2 dimensional lumber • Designed for combined axial and wind load • Double top plate per code • Spacing ranges from (1) stud @ 24” o.c to studs at 12” o.c. doubled under trusses.

  15. The designbeams, headers, and columns • Built-up SPF #2 dimensional lumber members. (2)2x6 to (2)2x12 • Structural composite lumber members used where required. 1.9E microllam (2)1 ¾” x 9 ½” LVL to (2)1 ¾ x 14” LVL • Columns are built-up 2x4s, worst case (6) studs

  16. The designLateral system • Bearing walls sheathed in Structural Panels or Gyp Board for lateral resistance • Wind loads distributed to walls by tributary area • Unit separation walls and corridor walls act as shear walls • Overturning not an issue

  17. Architectural modifications • Columns can be eliminated • Floor depth increases to ~18”, but… • Dropped ceilings for MEP can be eliminated • Wall thickness changes (unnoticeable)

  18. Advantages • Wood is less expensive • ~$109/sf vs. ~$165/sf total costs • Wood construction is faster • The design doesn’t require a great deal of skilled labor • No intrusive concrete or steel columns • No ceiling drops – plenum • No formwork – No stripping • Shorter lead time for trusses than steel

  19. Disadvantages • Because these are high end condos, the owner chose concrete over this option because of the durability of concrete over wood • Direct relationship between architectural form and structural form • Transfer slab required at garage level

  20. Composite steel Design Steel trusses with w-shape steel girders and columns

  21. The design

  22. The design

  23. The design

  24. The designFloor system • Topping slab – 5” thick 4000 psi concrete slab on 1 ½” B-lok composite metal deck • ¾”x 4 ½” shear studs • 50 ksi W-Shape beams, girders and columns • Beam and girder Sizes range from W8 to W16 • Column sizes W8 or W10 in some cases

  25. The designLateral system • 8” thick 4000 psi concrete shear walls reinforced with #4 bars @12” o.c. each face, each way • No drift problems (< H/400) • Accidental eccentricity of 5% x width Stairwell Stairwell elevator

  26. The designConnections • Single angle welded-bolted or bolted-bolted connection • A36 steel L4x4x3/8 with (2) A325N bolts

  27. The designConnections • Beam connected to concrete shearwalls using steel plate embed w/ anchor bolts • Uses same connection as beam – girder connection • Slab connected to shearwalls with #4 dowels @ 12” o.c.

  28. Architectural modifications • 16 columns eliminated • Floor depth Increases to ~22”, but… • Most dropped ceilings for MEP eliminated • Smaller columns

  29. Advantages • Less columns = less steel • Drops are not always needed for MEP • No formwork – no stripping • Lighter – less base shear

  30. Disadvantages • No money saved • Long lead time for steel shapes • Welding shear studs • pre-composite deflection • Must fireproof steel • Vibration must be addressed

  31. Breadth study acoustics, fire protection, vibration, estimating

  32. Breadthacoustics FHA: STC > 50, IIC > 52 Wood Design • Unit separation wall – STC 56 • Floor assembly – STC 52, IIC 52 (72 at carpet) Steel Design • Unit Separation Wall – STC 56 • Floor Assembly – STC 50, IIC 53

  33. Breadthfire ratings BOCA Type 5A construction: 1hr for floors, unit sep. walls 2 hr for exits, garage slab Wood Design • Unit separation wall – UL#U301 1 hr • Floor assembly – UL#L521 1 hr Steel Design • Unit separation wall – UL#U420 1 hr • Floor assembly – UL#D902 1 hr

  34. Breadthvibration Wood Design • Floor Truss frequency >15 Hz optimal • Actual worst case Frequency 14Hz Steel Design • According to Design Guide 11: • floor acceleration < 0.5%xG • Actual floor acceleration = 0.288%xG

  35. Cost estimate Concrete • Actual cost including Land, Development, and construction - $33.8M • =$165/sf Steel • Using R.S. Means sq. ft est. for construction and including land and development • =$170/sf Wood • Using estimate from owner and including land and development • =$109/sf (These are estimates)

  36. A Comparison Concrete, wood, and steel

  37. concrete Advantages & disadvantages Pros • Durability and Strength • Inherent Fire Protection, Vibration control, Sound Transmission control Cons • No Plenum for MEP • Cost > Wood • Columns are necessary

  38. Wood Advantages & disadvantages Pros • Least Expensive of the three • No columns necessary • Less lead time than steel • Plenum – No drops Cons • Transfer Slab necessary • Lacks permanence of Concrete, Steel • Close Architectural and Structural relationship

  39. Steel Advantages & disadvantages Pros • Durability and strength • Fewer columns than concrete • Plenum space • Lighter – less base shear • not critical here Cons • No money saved • Long lead time for steel shapes • shear studs Welding • steel Must fireproofed

  40. Conclusion Lessons learned

  41. Is there a best option? After weighing all of the designs… Concrete is the best option Why? • Its advantages outweigh its disadvantages • It is traditional for this type of project • It is common for the area • Cost is not much of an issue • Durability is a greater requirement

  42. a larger view This study only applies to a single project, but it shows that: • There is a multitude of different criteria for different projects • There is a very close relationship between architectural development and structural scheme • The least expensive design is not always the best

  43. Wrap up Acknowledgements

  44. Design team • Owner – Eakin Youngentob Associates, Inc. • General Contractor – Clark Construction • Architect – Torti Gallas and Partners CHK, Inc. • Structural Engineer – Cates Engineering, Ltd. • MEP Engineer – Schwartz Engineering, Inc. • Civil Engineer – Loiderman Associates, Inc. • Landscape Architect – Parker Rodriguez

  45. Mike Stansbury, P.E. Dr. Boothby Dr. Hanagan Dr. Ling Thank you Any Questions?

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