Mark Anstrom Structural option MAE/BAE AE Senior Thesis presentation Strathmore Park at Grosvenor Metro North Bethesda, MD

Strathmore Park at Grosvenor Metro

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

Strathmore Park at Grosvenor Metro

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

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?

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

Presentation preview  Wood design summary  Steel design summary  Breadth – fire, acoustics, vibration, cost  Comparison – wood, steel, and concrete  Conclusion

Wood Design 16” Wood trusses with dimension lumber bearing walls

The design

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)

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) 24” o.c to studs at 12” o.c. doubled under trusses.

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

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

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

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

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

Composite steel Design w-shape Steel beams, girders and columns

The design

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

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

The design Connections  Single angle welded-bolted or bolted-bolted connection  A36 steel L4x4x3/8 with (2) A325N bolts

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 12” o.c.

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

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

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

Breadth study fire protection, acoustics, vibration, estimating

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

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

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

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)

A Comparison Concrete, wood, and steel

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

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

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

Conclusion Lessons learned

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

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

Wrap up Acknowledgements

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

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

The design

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

The design