Engineering for Stability in Bridge Construction: A New ...sp.bridges.transportation.org/Documents/2015 SCOBS...Engineering for Stability in Bridge Construction: A New Manual and Training

FHWA OFFICE OF BRIDGES AND STRUCTURES

1

Engineering for Stability in Bridge

Construction:

A New Manual and Training Course

by FHWA/NHI

AASHTO SCOBS T-14 Meeting

Saratoga Springs, NY

April 20, 2015

Brian Kozy, PhD, P.E.

Federal Highway Administration


Problem: • It is unacceptable for a bridge to collapse

• Such events (and near misses) are too common

during erection and/or demolition

• The majority of engineering effort in projects is

being placed in final condition rather than const

• There is general lack of criteria and guidance

– Global stability complicated

• No standard of care


Groat Road

Edmonton (March)

I-75 Cincinnati (Jan)

Recent Incidents in the news…


It’s not just a steel problem!


Course Goals

• Guidance to assist designers and construction

engineers and owners

• Establish standard of care for erection plans and

supporting engineering calcs.

• Improve bridge structural stability and safety

• Cover issues not handled by AASHTO LRFD BDS

or the AASHTO NSBA Collaboration Standard S10

and PCI Bridge Design Handbook


Designer’s Decisions Affecting Stability: • Girder proportions

• Girder spacing

• Crossframe details (spacing, strength and stiffness, fit

condition)

• Deck pour sequence

• Lateral bracing

• Erection scheme (required for complex bridges)


Contractor’s Decisions Affecting Stability: • Erection plans and procedures

• Temporary supports and/or bracing

• Means and methods

• Limits on work activities (weather, traffic, crane

release, etc.)


Reference Manual Table of Contents: • Chapter 1: Introduction

• Chapter 2: Construction Failure Case Studies

• Chapter 3: Typical Bridge Construction Practice

• Chapter 4: Stability Fundamentals

• Chapter 5: Stability in Bridge Erection

• Chapter 6: Analysis for Stability

• Chapter 7: Engineering Criteria

• Chapter 8: Erection Plans and Procedures

• Chapter 9: Major and Unusual Bridge Construction


• General

• Analysis

• Load Combs and Factors

• Loads

• Girder Lifting

• Stability

• Concentrated Loads

• Deflection Control

• Connections

Engineering Criteria • Falsework

• Bearings

• Deck


• Girder-bridge superstructures shall be constructed in

such a way that strength and stability is maintained at

all intermediate stages until completion.

• All members shall be lifted, supported, connected,

and braced in such a way that no limit states are

violated at any time and damage such as yielding,

buckling and/or concrete cracking is avoided.

• Stability shall include local, member, system and rigid

body (rollover) stability.

General


• Analysis methods used shall be sufficiently refined to

accurately evaluate the applicable force effects and

limit states for each stage of girder erection and deck

placement.

• A global stability analysis shall be conducted to verify

adequate stability when any procedures are being

utilized for which the stability condition is not known,

by either engineering judgment or documented

experience on bridges of similar span, slenderness,

lateral stiffness, and bracing.

Analysis Requirements


Wind Loads

– Construction wind load not the same as final

• ASCE 7-10 (site specific)

• Consider exposure duration

• Considers drag coef. for open girders

– Alternatively, engineer may place limits on

erection drawings


Wind Loads

COMPONENT TYPE CONSTRUCTION CONDITION FORCE COEFFICIENT (Cf)

I-Shaped Girder Superstructure Deck forms not in place 2.2*

Deck forms in place 1.1

U-Shaped and Box-Girder

Superstructure Deck forms not in place 1.5

Deck forms in place 1.1

Flat Slab or Segmental Box-Girder

Superstructure Any 1.1

Construction

Duration Velocity Modification Factor

0 – 6 weeks 0.65

6 weeks – 1 year 0.75

1 year – 2 years 0.80

2 years – 5 years 0.85


Load Combinations and Load Factors

DC CDL CLL CW

Strength I 1.25 1.25 1.5 —

Strength III 1.25 1.25 — 1.0

Strength * 1.40 1.40 — —

Service 1.00 1.00 1.00 0.7

Uplift 0.90/1.35 0.90/1.35 — 1.0

* Steel structures for only the case of placing the deck on the fully erected steel. Use

Strength I or III for intermediate steel conditions, as applicable. CW: for appropriate duration CLL: include dynamic effects

Load Factors and Combinations


Stability • Steel girders shall be evaluated for local, member, and

global stability as specified herein and the bracing

details shall be defined at each construction stage

under investigation.

• The stage of completeness of all bolted connections

shall be considered when evaluating the strength and

stability of the steel during erection.


Global Stability • Check for elastic lateral torsional buckling. This is

similar to new BDS spec, but not based on limiting

amplification


Global Stability • Slender bridges and intermediate stages may require

refined global (system) buckling analysis


Deflection control (optional) – to reduce field problems

– dLateral < span/150 after release


Deflection control (optional) Differential jtwist < 2° at field splices


• The 3 ½ Day Course option includes 8 hours of

application activity

• Participants solve real world problems using

freeware programs like VT spreadsheets, MASTAN,

UTLIFT, & UTBRIDGE on laptops

Hands on practicum


Girder Lifting • Steel girders should be lifted near their quarter points

• Where a spreader beam is employed, the line of support,

or line running through the girder lifting points, should

pass through the center of gravity of the member, and the

lifting reactions at each pick point should be equal.


Girder lifting analysis w/ UTLIFT


• Generate step by step results for deflections, stresses,

reactions, eigen values, etc.

• Explore influence of partial bracing, temp shoring, hold

cranes, etc.

Girder erection analysis w/ UTBRIDGE









Results and behaviors are discussed


Temp support measures explored


Thank you for your attention…

To schedule a course, go to National Highway Institute

(NHI) at http://www.nhi.fhwa.dot.gov

http://www.nhi.fhwa.dot.gov/

