TOWERS CRESCENT BUILDING F Introduction

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TOWERS CRESCENT BUILDING FIntroduction

• - 199 Speculative Office Building
• - 304,880 Square Feet of Office Space
• - 368,770 Square Feet of Parking Space
• - Original Structural Design by KCE Redesigned
  by SKA
• - Hensel Phelps Construction Management
• - Design-Assist Project
• - Estimated Cost 81 Million and Falling

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TOWERS CRESCENT BUILDING FArchitecture

• - Semicircular tower over subterranean parking
  levels
• - Central core area containing elevators,
  bathrooms, stairways, electrical and mechanical
  rooms, etc., otherwise open plan
• - Façade employs mixture of glass, stainless
  steel and aluminum, and brick veneer no brick on
  North side
• - Painted steel spire and large architectural
  ornament which resembles metal fan spread over
  building.
• - Plaza with greenery at base of tower

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TOWERS CRESCENT BUILDING FExisting Structural
System

• Caisson Foundation, Concrete Columns, Flat Slab,
  Shear Walls

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TOWERS CRESCENT BUILDING FFloor System Redesign

• - Post-tensioning investigated as means to
  reducing slab 8 to 6
• - 24x14 supporting edge beams to produce 1-way
  behavior and contribute to lateral stiffness
• - Columns moved, 1 added
• - Live load reduced from 100 psf to 70 psf

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TOWERS CRESCENT BUILDING FPost-Tesnsioned Design

• Longitude Tendons

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TOWERS CRESCENT BUILDING FPost-Tesnsioned Design

• Latitude Tendons

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TOWERS CRESCENT BUILDING FPost-Tensioned Design

• - All slab areas pass
• - Certain beams fail, are upsized
• - Alleged punching shear failures and some
  alleged beam failures determined to be erroneous

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TOWERS CRESCENT BUILDING FPost-Tensioned Design

• RAM Designs Reinforcement

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TOWERS CRESCENT BUILDING FLateral Analysis

• - 3-D model represents spatial distribution of
  mass and stiffness in the structure
• - Makes use of heretofore neglected lateral
  stiffness elements such as curved frame
• - More accurate distribution of loads
• - Allows accurate determination of fundamental
  period
• - Introduction of edge beams stiffens structure
  and simplifies analysis

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TOWERS CRESCENT BUILDING FLateral Analysis

• - Orthogonally intersecting shear walls serve as
  web and flange for one beam section
• - Mass and center of mass determined in detailed
  spreadsheets
• - Columns modeled as rectangular sections, beams
  as T sections
• - Flexural stiffness of beams reduced by 50 and
  walls by 30 to account for
• cracking

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TOWERS CRESCENT BUILDING FLateral Analysis

• - 26 deep shear ties added between shear walls
• - Dramatically increases stiffness in East-West
  Direction by joining two sections into
• one section about 3x as deep
• - Increases torsional stiffness by providing
  logical path for shear flow
• - Requires reduction in elevator lobby ceiling
  height from 10 to 9

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TOWERS CRESCENT BUILDING FSeismic Analysis

• Occupancy Category III - Seismic Use Group II, I
  1.25
• Seismic Site Classification D
• Seismic design category B ? 1.0
• Shear wall-frame interactive system with ordinary
  reinforced concrete moment frames and ordinary
  reinforced concrete shear walls.
• R 5.5 O 5.5 Cd 4.5
• Spectral Response Acceleration
• Sds 0.208 Sd1 0.112
• Loads calculated by ELF, applied N-S E-W
• Building passes 1.5 story drift criteria

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TOWERS CRESCENT BUILDING FWind Analysis

• - Wind loads calculated in accordance with the
  provisions of ASCE 7-02 Section 6.5 for dynamic
  structures
• - Load factor of 1.6
• - Applied loads at 100 strength at center of
  area and 75 strength at eccentricity determined
  by Eq 6-21

(1) North-South, non-eccentric loading dmax
2.69 (2) North-South, eccentric loading dmax
8.09 (3) East-West, non-eccentric loading dmax
3.24 (4) East-West, eccentric loading dmax
3.98
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TOWERS CRESCENT BUILDING FWind Analysis

• - Deflections within industry standard tolerance
  of l/400 except North-South eccentric loading
• - North-South eccentric loading deflects l/295
• - Load applied in this analysis much larger than
  the load which would be applied in an actual
  windstorm, due to the 16 perforated panel wall
  at the top of the structure. Code requires (ASCE
  7-02, Sec. 6.5.2.2) that one treat air permeable
  cladding as solid wall, unless approved test
  data or recognized literature demonstrate lower
  loads for the type of air permeable cladding
  being considered.
• - Since dynamic structure, eccentricity of
  applied loading amplified by Equation 6-21.
  Results in eccentricity of 62.1, which is higher
  than actual.
• - Wind tunnel test would have been helpful
  currently one must treat semicurcular face of
  building as projected rectangle

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TOWERS CRESCENT BUILDING FStrength Check

• - West shear wall under N-S ecc. Loading carries
  1,265k shear, 67,275 ft-k overturning moment
• - ?Vn 0.6(5744)(2(80001/2) 0.002045(60000))
  1039.4k lt 1264.8k (no good)
• - 100(4p) 20(1)(4p) 15(2)(4p) 1885k lt
  2293.5k (no good)

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TOWERS CRESCENT BUILDING FShear Tie Design

• - Critical tie beam load applied to bottom tie
  beam of the second floor under East-West
  eccentric wind loading
• - Maximum moment 521 ft-k, maximum shear 120k
  above design resists 512 ft-k moment, 120k shear

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TOWERS CRESCENT BUILDING FColumns and Foundations

• - Reduction in weight allows reduction in column
  and caisson sizes
• - Consider 1.2D1.6L0.5S and 1.2D1.0L1.6W0.5S
• - Column 34 currently specified as 24x24
  section roof to 4th floor, 24x30 section from
  4th floor to level P4, 24x36section from P4 to
  the base
• - PCA Column shows 18x24 w/ (12) 9 vertical
  reinforcing rods suffices to 4th floor, 24x24
  w/ (12) 10 suffices to base
• - Caisson could not be reduced

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TOWERS CRESCENT BUILDING FArchitectural Design

• Rose, black, and white granite

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TOWERS CRESCENT BUILDING FArchitectural Design
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TOWERS CRESCENT BUILDING FMechanical Adjustments

• - Reshape ducts so as not to increase frictional
  losses
• De 1.3(ab)5/8/(a b)1/4 1.3(1212)5/8/(12
  12)1/4 13.12
• 1.3(10 b)5/8/(10 b)1/4
• b 15
• De 1.3(1610)5/8/(16 10)1/4 13.73 1.3(8
  b)5/8/(8 b)1/4
• b 21
• De 1.3(1412)5/8/(14 12)1/4 14.16 1.3(9
  b)5/8/(9 b)1/4
• b 20

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TOWERS CRESCENT BUILDING FMechanical Adjustments
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TOWERS CRESCENT BUILDING FDiscussion and
Recommendations

• - Laying out post-tensioning difficult due to
  irregular floor plan results are only moderately
  successful
• - Reduction of slab from 8 to 6 saves materials
  expenses
• - Benefit would be offset by additional labor and
  time of construction required to build
  post-tensioned slab
• - Was not able to reduce floor to floor height
• - In some places I have had to make the floor
  system deeper
• - Weighing advantages and disadvantages of my
  post-tensioned slab against current conventional
  reinforced slab, current system better

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TOWERS CRESCENT BUILDING FDiscussion and
Recommendations

• - Do recommend the addition of shear tie beams to
  connect the shear walls in the East-West
  direction
• - Shear ties will dramatically increase the
  stiffness of this building in the East-West
  direction
• - Adding these small members allow large
  reductions in the size of the lateral system
  elsewhere
• - Unfortunately, not able to prove this SKA
  must have determined the stiffness of the
  structure more rigorously, and hence less
  conservatively, than I
• - According to my calculations, structure fails
  deflection criterion (maximum building deflection
  under eccentric wind loading in the North-South
  direction) even with addition of shear ties
• - Also, in my calculations, one wall, with the
  caissons underneath it, fails strength criteria

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TOWERS CRESCENT BUILDING FDiscussion and
Recommendations

• - Wind tunnel testing would be helpful would
  decrease one particular load (eccentric
  North-South wind load) which is especially
  conservative, and which is greatest impediment to
  downsizing lateral system
• - Performance of this test, combined with
  addition of shear ties and more rigorous
  determination of building stiffness, would allow
  reduction in thickness of shear walls and thus
  achieve significant materials savings
• - Also, would recommend decreasing live load on
  office floor from 100 psf to 70 psf, which would
  make it possible to downsize columns
• - Furthermore, would recommend more rigorous
  analysis of bearing capacity of caissons
• - Current stipulation of the plans, caissons
  designed for an allowable bearing
• pressure of 100,000 psf plus skin friction of
  1,000 psf between elevation 440.00 and 420.00,
  and 2,000 psf for elevation below 420.00
• - Allowable stresses could be determined more
  accurately by a number of accepted theoretical
  and empirical methods

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TOWERS CRESCENT BUILDING FAcknowledgements

• Dr. Boothby, Dr. Lepage, Cynthia Milinichik, Joe
  Uchno, John Logue