lexington ii at market square north, washington d.c
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Lexington IIat Market Square North, Washington D.C.
Alexis Pacella – Structural Option
Presentation Outline
Building SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
Existing Structure
Gravity System:
2 Way Flat Plate Slab8 inches
Irregular Column LayoutSmall Bays
Lateral System:
Shear Walls
Foundation:
MAT
Presentation Outline
Building SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
Gravity System
Composite:Steel composite deck on steel beams and
columns
Relatively shallow floor sandwichNo large change in stiffness and vibrationSteel 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 3rd 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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
3 possible Lateral Systems:Shear WallsMoment FramesBraced 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
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 RigidityCheck ASCE 7 wind casesDesign, check on STAAD
Design was controlled by
allowable stress
Lateral System
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
12
Mny
Muy
Mnx
Mux
Pn
Pu
Lateral System
Presentation Outline
Building SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
Are there other issues with the structure to be considered?
Connections
Substructure
Foundation
Other Structural Factors
Beam to Column Connections:
Stiffeners Required
Typical 7th floor connection
Other Structural Factors
Heavy Bracing Connection:
Uniform Force MethodSame beam to column connection
27”x27”x1/2” gusset plate
3”x3”x1/2” angles
Other Structural Factors
Typical 7th 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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
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
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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
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 SummaryExisting StructureProposalGravity SystemLateral SystemOther Structural FactorsConstruction ManagementMechanicalConclusion
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 PlayersArchitectureFaçadeSite/ZoningOther Systems
OwnerSquare 407 LP
ArchitectStudios Architects
Structural EngineerThornton Tomasetti
MEPEngineering Design Group
General ContractorClark Construction
Transportation
2 elevators and stairwells in building core
MechanicalWater source heat pump;
Boiler, pumps, cooling tower
Electric/Lighting
1 phase, 3 wire; 120/208 VFluorescent 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 Concrete
Costly Connections
Formwork
Fireproofing Reinforcement
Extra Façade Waste
Accessories
Excavation: $23,600Foundation:
$159,000
Concrete Design Cost:Sub Grade Floors:
$671,000Super Structure Floors:
$154,000Columns:
$395,000Shear Walls:
$123,900______________________________________Total $1,526,000
Construction Management
Prices taken from R.S. Means
Construction Management
Excavation: $23,600Foundation: $159,000
Steel Design Cost:Sub Grade Levels: $741,200Steel Levels: $467,200Braced Frames: $156,400Connections: $51,300
___________________________________Total: $1,455,600
Prices taken from R.S. Means
Construction Management
Scheduling:
Excavation
Backhoe .5 daySheathing .5 daysHauling 1 days
Sheathing 3 daysTotal per story 4 days
x 3 stories 2.4 weeks
Construction Management
Scheduling:
Foundation
Cast in Place MAT 1dayCuring 4 days
Total 5 days
3 weeks
Scheduling:
Substructure
Columns 1.1 dayGrade walls 2 daysShear walls .5 days
1-way Joist 5 daysCuring 4 days
Total per story 11 days x 3 stories 9.2
weeks
Construction Management
Construction Management
Scheduling:
Super Structure
Columns .5 dayBeams .5 dayDeck 2 days
Slab 1 dayCuring 1 day
Total per story 5 days
x 12 stories 18 weeks
Mechanical
Fire Suppression:
Sprinkler Layout
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