Frank BurkeStructural Option

Sallie Mae HQReston, VA

Outline

•Introduction•Structural Overview

•Thesis Goals•Structural Depth

•CM Breadth•Mechanical Breadth

•Conclusion•Acknowledgements

Introduction

• Project cost: 41 million • Design-Bid-Build• April 2003- September 2004

• 9 story Office Building– 22,000 SF/Floor

• 5 story Parking Garage– 75,000 SF/Floor

• Glass and Precast Facade

Structural Overview

• Parking Garage1. Below grade2. 10’ high stories3. 28’ x 28’ bays4. Two-way concrete slab5. 9” & 10” elevated slabs6. Square Footings

• Office Structure1. Above Grade2. 14’ Stories3. 42’(+) x 28’ Bays4. Steel Framing5. 3 ¼” lightweight concrete over 2”

deep x 18 gage composite metal deck

Structural Overview

W27’s

W24’s

Central Core

Structural Overview

Thesis Goals

• To create a post-tensioning design that considers constructability and is simplistic

• Investigate the savings in post-tensioning

Design Considerations• Type of System?

One-way slab

• Type of Concrete? 6000psi Normal Weight Concrete

• Unbonded or bonded tendons? Unbonded Tendons

• Design Class U, T or C? Slab = Class U Beams = Class T

• Roof Structure? Original Steel Framing

Vertical Members

• Columns:– 20” x 20” – 24” x 24”– 24” x 30”

• Shearwalls are 12” thick and are located in the elevator core and stairwells

Column Pedestal Detail

• Slab is 8” thick• Slab tendons are in bundles of 4 and spaced at 5’

Gravity Members

Gravity Members• Prestressed beams are 26” and 30” deep• The majority of beam tendons are in bundles of four• 4” cover at midspan & supports / 6” cover at anchorage

zones

Midspan

Supports

Penthouse

Slab tendons

Nonprestressed Area

Prestressed Beams

Regular Beams

8th Floor

8th Floor / Typical Floor

30” Beams

Columns from above

CM Breadth

• Benefits of my post-tensioning design

– The PT strands can be placed quickly– Jacking can occur from one side– Facade can be reduced by 6”per floor

CM Breadth

• Downfalls of my post-tensioning design

– Heavier building– Larger Foundations– Longer schedule

CM Breadth

Specific Task Working Days Needed

Construct Columns and Walls 4

Form Slab, Beams 3

Place Rebar 2

Place PT Strands 2

Pour Slab, Beams 2

Cure Slab, Beams 3

Stress PT Strands 2

Construct 8 floors 72

• Schedule

Construct 1 floor 9

CM BreadthFloors $1,013,200

Columns $481,600

Walls $264,800

Formwork $946,200

General Conditions Increase $50,000

Foundation Increase $188,400

Facade Savings -$152,600

Cost Index .91

Total $2,540,356

SF analyzed 166,000

Price/SF $15.30/SF

PT Concrete Steel

$15.30/SF $17.79/SF

Mechanical Breadth

• Post-tensioning allows savings in ceiling depth

• Mechanical ducts can be enlarged from 12” to 18” in depth to be more efficient and save sheet metal cost

Mechanical Breadth

Steel Ceiling SectionConcrete Ceiling

Section

Mechanical Breadth

Savings

Sheet metal 10400 LBS

Schedule 4 days

Total Cost $41,000

Conclusions

Lead- in- time

Constructability

Safety

Cost / Weight

Foundation

Gravity System

Lateral System

Mechanical System

Fire Rating

Vibration

Deflection

Acoustics

Alterations

Overall Cost

Concrete Steel LegendGood

Bad

Conclusions

• Sallie Mae is solely a headquarters building; no tenants

• If Sallie Mae had renters, depending on the lead-in-time, steel or concrete might be the better alternative

• However; due to the facade savings post-tensioning is the cheaper system

Acknowledgements

Cagley and Associates / Frank Malits

Professor Boothby, Professor Parfitt, Professor Hanagan, Professor Ling, Professor Lepage

Structural Mentors

Bryan Quinn, Lourdes Diaz

Family and Friends

Questions

?

Introduction

Architect

▼ Boggs & Partners

Structural

▼ Cagley and Associates

MEP

▼ GHT Limited

Contractor

▼ Hitt Contracting Inc.

Typical Floor