TOWERS CRESCENT BUILDING F
Introduction
- 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 SK&A
- Hensel Phelps Construction Management
- Design-Assist Project
- Estimated Cost $81 Million and Falling
TOWERS CRESCENT BUILDING F
Architecture
- 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
TOWERS CRESCENT BUILDING F
Existing Structural System
Caisson Foundation, Concrete Columns, Flat Slab, Shear Walls
TOWERS CRESCENT BUILDING F
Floor System Redesign
- Post-tensioning
investigated as means to
reducing slab 8” to 6”
- 24”x14” 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
TOWERS CRESCENT BUILDING F
Post-Tesnsioned Design
Longitude Tendons
TOWERS CRESCENT BUILDING F
Post-Tesnsioned Design
Latitude Tendons
TOWERS CRESCENT BUILDING F
Post-Tensioned Design
- All slab areas pass
- Certain beams fail,
are upsized
- Alleged punching
shear failures and
some alleged beam
failures determined to
be erroneous
TOWERS CRESCENT BUILDING F
Post-Tensioned Design
RAM Designs
Reinforcement
TOWERS CRESCENT BUILDING F
Lateral 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
TOWERS CRESCENT BUILDING F
Lateral 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
TOWERS CRESCENT BUILDING F
Lateral 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’
TOWERS CRESCENT BUILDING F
Seismic Analysis
Occupancy Category III - Seismic Use
Group II, I = 1.25
Seismic Site Classification D
Seismic design category B; ρ = 1.0
Shear wall-frame interactive system with
ordinary reinforced concrete moment
frames and ordinary reinforced concrete
shear walls.
R = 5.5; Ω = 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
TOWERS CRESCENT BUILDING F
Wind Analysis
- Wind loads calculated
in accordance with the
provisions of ASCE 702 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)
(2)
(3)
(4)
North-South, non-eccentric loading: δmax = 2.69”
North-South, eccentric loading: δmax = 8.09”
East-West, non-eccentric loading: δmax = 3.24”
East-West, eccentric loading: δmax = 3.98”
TOWERS CRESCENT BUILDING F
Wind 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
TOWERS CRESCENT BUILDING F
Strength Check
- West shear wall under N-S ecc. Loading carries 1,265k shear, 67,275 ft-k
overturning moment
- ФVn = 0.6(5744)(2(80001/2) + 0.002045(60000)) = 1039.4k < 1264.8k (no good)
- 100(4π) + 20(1)(4π) + 15(2)(4π) = 1885k < 2293.5k (no good)
TOWERS CRESCENT BUILDING F
Shear Tie Design
- Critical tie beam load applied to bottom tie beam of the second floor under EastWest eccentric wind loading
- Maximum moment 521 ft-k, maximum shear 120k; above design resists 512 ft-k
moment, 120k shear
TOWERS CRESCENT BUILDING F
Columns and Foundations
- Reduction in weight allows reduction in
column and caisson sizes
- Consider 1.2D+1.6L+0.5S and
1.2D+1.0L+1.6W+0.5S
- Column 34 currently specified as 24”x24”
section roof to 4th floor, 24”x30” section
from 4th floor to level P4, 24”x36”section
from P4 to the base
- PCA Column shows 18”x24” w/ (12) #9
vertical reinforcing rods suffices to 4th floor,
24”x24” w/ (12) #10 suffices to base
- Caisson could not be reduced
TOWERS CRESCENT BUILDING F
Architectural Design
Rose, black, and white granite
TOWERS CRESCENT BUILDING F
Architectural Design
TOWERS CRESCENT BUILDING F
Mechanical Adjustments
- Reshape ducts so as not to increase frictional losses
De = 1.3(ab)5/8/(a + b)1/4 = 1.3(12*12)5/8/(12 + 12)1/4 = 13.12
= 1.3(10 *b)5/8/(10 + b)1/4
b = 15”
De = 1.3(16*10)5/8/(16 + 10)1/4 = 13.73 = 1.3(8 * b)5/8/(8 + b)1/4
b = 21”
De = 1.3(14*12)5/8/(14 + 12)1/4 = 14.16 = 1.3(9 * b)5/8/(9 + b)1/4
b = 20”
TOWERS CRESCENT BUILDING F
Mechanical Adjustments
TOWERS CRESCENT BUILDING F
Discussion 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
TOWERS CRESCENT BUILDING F
Discussion 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; SK&A 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
TOWERS CRESCENT BUILDING F
Discussion 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
TOWERS CRESCENT BUILDING F
Acknowledgements
Dr. Boothby, Dr. Lepage, Cynthia
Milinichik, Joe Uchno, John Logue