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Mark Anstrom Structural option MAE/BAE AE Senior Thesis presentation Strathmore Park at Grosvenor Metro North Bethesda, MD
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Strathmore Park at Grosvenor Metro
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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
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Strathmore Park at Grosvenor Metro 1 2 3 4 5
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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
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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?
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Considerations Maintain same architectural appearance as original design (such as ceiling height) Preserve acoustical quality Check trusses for vibration Ensure proper fire ratings Check ponding on flat roof
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Presentation preview Wood design summary Steel design summary Breadth – fire, acoustics, vibration, cost Comparison – wood, steel, and concrete Conclusion
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Wood Design 16” Wood trusses with dimension lumber bearing walls
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The design
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The design floor 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)
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The design bearing 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.
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The design beams, 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
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The design Lateral 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
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Architectural modifications Columns can be eliminated Floor depth increases to ~18”, but… Dropped ceilings for MEP can be eliminated Wall thickness changes (unnoticeable)
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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
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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
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Composite steel Design w-shape Steel beams, girders and columns
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The design
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The design Floor 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
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The design Lateral 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 elevator
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The design Connections Single angle welded-bolted or bolted-bolted connection A36 steel L4x4x3/8 with (2) A325N bolts
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The design Connections 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.
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Architectural modifications 16 columns eliminated Floor depth Increases to ~22”, but… Most dropped ceilings for MEP eliminated Smaller columns
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Advantages Less columns = less steel Drops are not always needed for MEP No formwork – no stripping Lighter – less base shear
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Disadvantages No money saved Long lead time for steel shapes Welding shear studs pre-composite deflection Must fireproof steel Vibration must be addressed
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Breadth study fire protection, acoustics, vibration, estimating
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Breadth fire 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
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Breadth acoustics 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
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Breadth vibration 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
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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)
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A Comparison Concrete, wood, and steel
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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
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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
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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
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Conclusion Lessons learned
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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
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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
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Wrap up Acknowledgements
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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
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Thank you Mike Stansbury, P.E. Dr. Boothby Dr. Hanagan Dr. Ling Any Questions?
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The design
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The design roof 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
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The design
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