Thesis 2005
Cira Centre – Philadelphia
Structural Redesign of
Lateral Force Resisting System
Andrew Kauffman
Structural Option
Presentation Outline
Introduction
Building Description
Structural System
Problem Statement
Solution Overview
Structural Redesign
Mechanical Redesign
Conclusion
INTRODUCTION
Introduction
Adjacent to 30th Street Train Station - Philadelphia, PA
291,000 s.f., 28 story high rise office building
Convention center, restaurants and retail space
Tallest building in Philadelphia, outside Center City
Scheduled for completion – October 2005
Total Projected Cost – $200 million
BUILDING
DESCRIPTION
Building Description – Project Team
Architects – Cesar Pelli and Assoc./Bower Lewis Thrower
General Contractor – Turner Construction Co.
Structural Engineer – Ingenium Inc.
Civil Engineer – Pennoni Assoc.
MEP Engineer – Jaros Baum and Bolles
Lighting Design - Cline Bettridge Bernstein
Acoustic Consultant - Cerami and Associates
Curtain Wall Consultant - Israel Berger and Associates
Building Description- Architectural Features
725,000 s.f. rentable space
Open plan office levels: 727,725 s.f. (average)
9 ft. floor to ceiling heights
Pedestrian bridge connecting to 30th Street train station
Single point of entrance in main lobby, added security
Highly reflective glass curtain wall
Building Description – Building Systems
Electrical –
13.2 KV primary voltage
480Y/277 volt, 3 phase, 4 wire
Secondary system
Mechanical – Fan powered, VAV system
Includes 4 cooling towers
located in top mezzanine
Conveying – 14 high speed traction elevators
Low-rise, mid-rise, high-rise
Configuration
STRUCTURAL
SYSTEM
Structural System – Overview
Steel frame super-structure
Composite floor system
Drilled pier foundation
Lateral System: Combination of
braced and moment frames
Structural System – Floor System
Fully composite, 5 ¼ in. floor system, with LW concrete, metal
decking, 50 ksi steel framing members
W18x35 and W24x76 typical beams and girders, 30’x30’ bays, typ.
7'-11"
30'
30'
30'
30'
30'
30'
30'
7'-8"
12'-6"
A
30'
B
N
30'
C
30'
D
12'-6"
E
F
1
2
3
4
5
6
7
8
9 10
Structural System – Vertical Framing
Drilled concrete piers with up to 21.5’
penetration into bedrock
Large built-up column sizes including
W36x1202 wide flange members
and 829 lb/ft. built-up box sections
Forking Columns
Leaning Columns
Structural System – North/South Building Section
7'-11"
30'
30'
30'
30'
30'
30'
30'
7'-8"
12'-6"
A
30'
B
N
30'
C
30'
D
12'-6"
E
F
1
2
3
4
5
6
7
8
9 10
North/South Section
Structural System - East/West Section
7'-11"
30'
30'
30'
30'
30'
30'
30'
7'-8"
12'-6"
A
30'
B
N
30'
C
30'
D
12'-6"
E
F
1
2
3
4
5
6
7
8
9 10
East-West Section
MECH
ROOM
MACH
ROOM
MACH
ROOM
CONFERENCE
Structural System – Lateral System
East/West - Located in building core
Combination of braced frames and moment connections
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural System – Lateral System
East – West Direction
Along column lines C & D
Located in structural core
Exterior braced frames
Interior moment frames
Structural System – Lateral System
North/South – Located in building core
Combination of braced frames and moment connections
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural System – Lateral System
North - South Lateral System
Along column lines 4 & 7
Located in Structural Core
Exterior Moment Frames
Interior Braced Frames
Structural System – Lateral System
North/South – Located along exterior frames
Only moment frames
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural System – Lateral System
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
North - South Lateral System
Along column lines 1 & 10
All moment frames
Varying stiffness
PROBLEM
STATEMENT
Problem Statement – Overview
Complicated Structure to Analyze
•
Varying Floor Geometry
•
Large built-up members
Complicated Lateral System
1.
Combination of braced and moment frames
2.
Lateral frames with varying stiffness
Problem Statement – Lateral Load Assumptions
Lateral Loads used in actual design were
developed using a wind tunnel analysis
Wind Tunnel results yielded 65% of total shear
and 75% of the overturning moment, compared
to ASCE7-02, analytical method.
Strength considerations did not control the
original design of the building.
Torsional acceleration at corner offices was
the limiting factor that controlled the design
SOLUTION
OVERVIEW
Solution Overview – Lateral System Redesign
Develop wind and seismic loads based on ASCE7-02
Redesign Lateral System based on these loads.
Compare cost of redesign to cost of original structure
Solution Overview – Design Goals
Gain a better understanding of lateral force
resisting system design for steel buildings
Investigate alternative lateral system configurations
Meet the design criteria of IBC 2003.
Limit interstory drift
Limit overall building drift to L/400 criteria
Achieve an economically feasible design
Optimization of original design was not a goal
Solution Overview - Procedure
Develop wind loads using Analytical Procedure
Model 2-D lateral frames using GT Struddle
Determine relative stiffness based on virtual loads
Distribute loads based on stiffness and torsion analysis
Analyze frames for deflection and interstory drift
Redesign lateral frames based on drift criteria - iterative
Compare cost of redesign to original structural system
Solution Overview – Mechanical Breadth Study
Analyze feasibility of adding enthalpy wheels to the original
mechanical system.
Goal: Utilize the properties of building exhaust to save $$$
STRUCTURAL
REDESIGN
Structural Redesign – Stiffness Analysis
Created model of each lateral frame in GT Struddle
100k virtual load at top of each frame to measure relative stiffness
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural Redesign – East/West Lateral Frames
Column Lines C & D
Equal Stiffness
Distribute half of total story load to
each frame
Equal distance from center of plan
Torsion had minimal effect in this
direction
Structural Redesign – Load Distribution
Structural Redesign - Results
Total Deflection: 13.25”
L/400 = 13.08”
Acceptable based on
occupancy comfort
Structural Redesign – North/South Direction
Modeled lateral frames along Column Lines 1,4,7,10
Applied Virtual Load at levels 28,20,10
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural Redesign – North/South Direction
CL 1
CL 4/CL 7
CL 10
Structural Redesign – North/South Direction
Relative stiffness varied with height.
Applied uniform 10 kip load to verify stiffness
Plotted results and fit equation
Solved equation for stiffness in terms of height
Relative Lateral Frame Stiffness
400
y = 23584x - 2049.3
350
Building Height (ft.)
300
CL4
250
CL10
CL1
200
Linear (CL1)
y = 2088.6x - 122.41
Linear (CL10)
150
Linear (CL4)
100
y = -3839.6x + 1640.3
50
0
0
0.1
0.2
0.3
K - Relative Stiffness
0.4
0.5
Structural Redesign – North/South Direction
Performed torsion analysis at each level based on center of rigidity
Included 5% eccentricity per code, and determined loads on frames
Level 20
A
B
N
C
Centre
of
Rigidty
21'
90 Kips
D
1,900 Ft. Kips
E
F
1
2
3
4
5
6
390 PSF
7
7
8
10
Structural Redesign – North/South Direction
Applied load to models in GT Struddle and analyzed results
Structural Redesign – Results
Each lateral frame deflected equal amounts.
All frames deflected well over the L/400 limit.
19.99”
19.92”
17.16”
Structural Redesign – Solution
Alleviate interstory drift problems
Limit overall building drift to 12”
Started with exterior frames
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural Redesign – North/South Direction
Analyzed several bracing configurations using iterative procedure.
Eliminated interstory drift problems, limited total drift to 12”
Structural Redesign – North/South Direction
Used same procedure for interior lateral
frames along column lines 4 & 7
Could not limit drift to 12”
A
B
LATERAL
FRAMES
ELEVATORS
STAIR
TOWERS
N
C
LATERAL
FRAME
D
E
F
1
2
3
4
5
6
7
8
9 10
Structural Redesign – North/South Direction
Increased stiffness of exterior lateral frames
W14x145 bracing members
Additional chevron braces to these frames
Limited total drift to 9”
Changed bracing of interior frames
W14x159 bracing members
Increased stiffness of girders to W33x221
Reapplied stiffness analysis and torsion calculations
Calculated new story loads
Results
Structural Redesign – Cost Analysis
Used R.S. Means to estimate total cost of original structure
20% total building cost = $ 40 million
Performed take-off to calculate cost of lateral system redesign
Compared additional cost to overall structural cost
Cost Increase = 0.6% Structure Cost
Cost Increase = 0.1% Structure Cost
Mechanical
Redesign
Mechanical System Redesign
System Description
Fan powered VAV system
Supply Air: 80% return, 20% outdoor air
Exhaust: based on 150 cfm/toilet, 20 cfm/sink
Typical Air Handler Size: 23,500
Procedure
Use ASHRAE Bin data to analyze a full year cycle
Based on 2 design condition: on peak – business hours,
off peak – evenings and weekends
Calculated total building load with/without use of enthalpy wheel
Compared loads and calculate savings
Mechanical System Redesign – Typical Floor
Total Savings = Sensible Load savings + Latent Load savings
Enthalpy wheels turned off when no energy is saved
Additional energy can be saved by modulating wheel
Mechanical System Redesign - Results
Using Peco Energy Rates:
Total Energy Savings:
671,236 kwh
Total Cost Savings:
$28,506/year
CONCLUSION
Conclusion
Based on lateral load assumptions used for this
analysis, the lateral frames in the North/South
direction should be designed with increased stiffness
based on occupancy comfort criteria.
Redesign of lateral force resisting system is an
economic solution compared to overall cost of structure
Enthalpy wheels should be utilized by the mechanical
system to increase overall efficiency and save $$$.
Acknowledgments
AE Faculty and Staff
Dr. Memari and Dr. Geschwindner
Dr. Hanangan and Prof. Parfitt
Jeff Weinstein and Andy Bush, Brandywine Realty Trust
Dr. Banavalker, Ingenium Inc.
Peter Jennings, Jaros, Baum, and Bolles
My Family and Friends
My wife Nicole