Experimental Mechanics

, Volume 43, Issue 2, pp 201–215 | Cite as

Field measured pre-stress concrete losses versus design code estimates

  • O. U. Onyemelukwe
  • P. E. Moussa Issa
  • C. J. Mills
Article

Abstract

In Florida, pre-stressed concrete bridges with a hybrid structural configuration, consisting of simply supported girders and continuous deck units providing composite action, are prevalent. In this study, an actual bridge with this configuration is instrumented with embedded vibrating wire strain gages during construction. The axial strain data are utilized to determine the time-dependent pre-stress loss variation and distribution in the pre-tensioned concrete girders used for the bridge. In this paper, however, we discuss the bridge instrumentation and monitoring technique used, and the deduced pre-stress losses obtained from field-measured strains up to 150 days, before placement of the composite slab. A comparison of the measured loss with the estimates of the Precast/Prestressed Concrete Institute (PCI) and the American Association of State Highway and Transportation Officials (AASHTO) indicates that the field-measured pre-stress loss is non-uniform across the girder depth, opposed to a uniform distribution implicitly assumed in most codes. The measured pre-stress variation is used in computing the concrete stress distribution in the girder at different depths within the given cross-section. When compared to the concrete stress from using the PCI and AASHTO suggested losses, the stress distribution resulting fron using the field-measured loss is found to be non-linear, and in most cases higher.

Key Words

Pre-stressed concrete bridge pre-stressed concrete girder pre-stress losses time-dependent pre-stress losses nonlinear concrete stresses bridge instrumentation bridge monitoring vibrating wire strain gages axial strain measurements modified AASHTO type VI girder 

Nomenclature

Cconcrete

Location of concrete girder centroid measured from bottom of girder

Ec(t)

Time-dependent value of modulus of elasticity of concrete

Ec(28)

Value of the modulus of elasticity of concrete on the 28th day

Es

Modulus of elasticity of the pre-stressing strands

fck

Characteristic strength of concrete

fcm

Mean compressive strength of concrete

Δfpre-stress

Change in pre-stress

Iconcrete

Moment of inertia of concrete girder

M0

Bending moment due to girder self weight

s

Cement type correction factor

t

Time in days

Tt

Temperature at any time,t

Tt−1

Temperature at preceding time,t−1

y

Depth of interest measured from bottom of girder

βE

Age correction factor used for determining the modulus of elasticity of concrete

βɛtemperature

Relative change in strain due to temperature changes

βɛweight effects

Relative change in strain due to girder weight and time dependence of the modulus of elasticity of concrete

ε

Strain

ɛt

Measured strain at any time,t

ɛ0

Measured strain at reference time,t o

Δε

Absolute change in strain

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References

  1. 1.
    Value Engineering Report, “Finley McNary/Janssen Spaans—State Road No. 600 Westbound Gandy Bridge (Value Engineering Contract),” Tallahassee, FL (1995).Google Scholar
  2. 2.
    American Association of State Highway and Transportation Officials (ASSHTO), “AASHTO-LRFD Bridge Design Specifications,”1st edition.AASHTO, Washington, DC (1994).Google Scholar
  3. 3.
    Geokon Inc., “Instruction Manual: Models VCE-4200/4202/4210 Vibrating Wire Strain Gages,” Lebanon, New Hampshire (1995).Google Scholar
  4. 4.
    CEB-FIP 1990, “Model Code for Concrete Structures (MC-90),” Thomas Telford, London (1993).Google Scholar
  5. 5.
    Ghali, A. andFavre, R., “Concrete Structures: Stresses and Deformations,”2nd edition.E & FN Spon, London (1994).Google Scholar
  6. 6.
    American Concrete Institute (ACI), “Building Code Requirements for Reinforced Concrete ACI 318-89 (Revised 1992) and Commentary ACI 318R-89 (Revised 1992).”ACI, Detroit, Michigan (1992).Google Scholar
  7. 7.
    Precast/Prestressed Concrete Institute (PCI), “PCI Design Handbook: Pre-cast and Pre-stressed Concrete,”4th edition.PCI, Chicago, IL (1992).Google Scholar
  8. 8.
    Nilson, A. H., “Design of Pre-stressed Concrete,”Wiley, New York (1987).Google Scholar

Copyright information

© Society for Experimental Mechanics 2003

Authors and Affiliations

  • O. U. Onyemelukwe
    • 1
  • P. E. Moussa Issa
    • 2
  • C. J. Mills
    • 3
  1. 1.CEE DepartmentUniversity of Central FloridaOrlando
  2. 2.TY Lin InternationalChicago
  3. 3.HNTBOrlando

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