engineering for stability in bridge construction: a new...
TRANSCRIPT
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
Groat Road
Edmonton (March)
I-75 Cincinnati (Jan)
Recent Incidents in the news…
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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)
FHWA OFFICE OF BRIDGES AND STRUCTURES
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.)
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
• General
• Analysis
• Load Combs and Factors
• Loads
• Girder Lifting
• Stability
• Concentrated Loads
• Deflection Control
• Connections
Engineering Criteria • Falsework
• Bearings
• Deck
FHWA OFFICE OF BRIDGES AND STRUCTURES
• 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
FHWA OFFICE OF BRIDGES AND STRUCTURES
• 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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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.
FHWA OFFICE OF BRIDGES AND STRUCTURES
Global Stability • Check for elastic lateral torsional buckling. This is
similar to new BDS spec, but not based on limiting
amplification
FHWA OFFICE OF BRIDGES AND STRUCTURES
Global Stability • Slender bridges and intermediate stages may require
refined global (system) buckling analysis
FHWA OFFICE OF BRIDGES AND STRUCTURES
Deflection control (optional) – to reduce field problems
– dLateral < span/150 after release
FHWA OFFICE OF BRIDGES AND STRUCTURES
Deflection control (optional) Differential jtwist < 2° at field splices
FHWA OFFICE OF BRIDGES AND STRUCTURES
• 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
FHWA OFFICE OF BRIDGES AND STRUCTURES
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.
FHWA OFFICE OF BRIDGES AND STRUCTURES
• 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
FHWA OFFICE OF BRIDGES AND STRUCTURES
Thank you for your attention…
To schedule a course, go to National Highway Institute
(NHI) at http://www.nhi.fhwa.dot.gov