Thesis 2005Cira Centre – Philadelphia

Structural Redesign ofLateral Force Resisting System

Andrew KauffmanStructural Option

Presentation Outline

Introduction

Building Description

Structural System

Problem Statement

Solution Overview

Structural Redesign

Mechanical Redesign

Conclusion

INTRODUCTION

IntroductionAdjacent to 30th Street Train Station - Philadelphia, PA291,000 s.f., 28 story high rise office buildingConvention center, restaurants and retail spaceTallest building in Philadelphia, outside Center CityScheduled for completion – October 2005Total 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 Features725,000 s.f. rentable spaceOpen plan office levels: 727,725 s.f. (average)9 ft. floor to ceiling heightsPedestrian bridge connecting to 30th Street train stationSingle point of entrance in main lobby, added securityHighly 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 systemIncludes 4 cooling towers located in top mezzanine

Conveying – 14 high speed traction elevatorsLow-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 SystemFully composite, 5 ¼ in. floor system, with LW concrete, metaldecking, 50 ksi steel framing membersW18x35 and W24x76 typical beams and girders, 30’x30’ bays, typ.

1 3 4 5 6 8

D

C

B

A

2 7 9 10

E

F

7'-11" 30'

12'-6

"30

'

7'-8"

12'-6

"30

'30

'

30' 30' 30' 30' 30' 30'

N

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

1 3 4 5 6 8

D

C

B

A

2 7 9 10

E

F

7'-11" 30'

12'-6

"30

'

7'-8"

12'-6

"30

'30

'

30' 30' 30' 30' 30' 30'

N

North/South Section

Structural System - East/West Section

1 3 4 5 6 8

D

C

B

A

2 7 9 10

E

F

7'-11" 30'

12'-6

"30

'

7'-8"

12'-6

"30

'30

'

30' 30' 30' 30' 30' 30'

N

East-West Section

MECHROOM

MACHROOM

MACHROOM

CONFERENCE

Structural System – Lateral SystemEast/West - Located in building core

Combination of braced frames and moment connections

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

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 SystemNorth/South – Located in building core

Combination of braced frames and moment connections

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

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

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

North/South – Located along exterior frames

Only moment frames

Structural System – Lateral System

234567891011121314151617181920

22232425262728

21

North - South Lateral SystemAlong 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 System1. 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 wasthe 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 StudyAnalyze 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

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

Structural Redesign – East/West Lateral Frames

Equal Stiffness

Distribute half of total story load to each frame

Equal distance from center of plan

Torsion had minimal effect in this direction

Column Lines C & D

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

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

Structural Redesign – North/South Direction

CL 1 CL 4/CL 7 CL 10

Relative stiffness varied with height.Applied uniform 10 kip load to verify stiffnessPlotted results and fit equationSolved equation for stiffness in terms of height

Structural Redesign – North/South Direction

Relative Lateral Frame Stiffness

y = 23584x - 2049.3

y = 2088.6x - 122.41

y = -3839.6x + 1640.3

0

50

100

150

200

250

300

350

400

0 0.1 0.2 0.3 0.4 0.5

K - Relative Stiffness

Build

ing

Heig

ht (f

t.) CL4CL10CL1Linear (CL1)Linear (CL10)Linear (CL4)

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

1 3 4 5 6 7

D

C

B

A

2 7 8 10

E

F

N

Level 20

Centre of

Rigidty

390 PSF

21'

1,900 Ft. Kips

90 Kips

Structural Redesign – North/South Direction

Applied load to models in GT Struddle and analyzed results

Structural Redesign – ResultsEach lateral frame deflected equal amounts.All frames deflected well over the L/400 limit.

19.99”17.16”

19.92”

Structural Redesign – SolutionAlleviate interstory drift problemsLimit overall building drift to 12”Started with exterior frames

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

Structural Redesign – North/South DirectionAnalyzed several bracing configurations using iterative procedure.Eliminated interstory drift problems, limited total drift to 12”

Structural Redesign – North/South DirectionUsed same procedure for interior lateral frames along column lines 4 & 7

Could not limit drift to 12”

2 7 9 10

E

F

NELEVATORS STAIR TOWERS

LATERAL FRAMES

LATERAL FRAME

1 3 4 5 6 8

D

C

B

A

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 AnalysisUsed 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 redesignCompared additional cost to overall structural cost

Cost Increase = 0.6% Structure CostCost Increase = 0.1% Structure Cost

Mechanical Redesign

Mechanical System Redesign

Fan powered VAV systemSupply Air: 80% return, 20% outdoor airExhaust: based on 150 cfm/toilet, 20 cfm/sinkTypical Air Handler Size: 23,500

System Description

ProcedureUse ASHRAE Bin data to analyze a full year cycleBased on 2 design condition: on peak – business hours, off peak – evenings and weekends

Calculated total building load with/without use of enthalpy wheelCompared 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