Prestress losses in spruce timber

Prestress losses have a direct impact on concrete stress development and deflection behavior of highway bridge members. A poor estimate of prestress losses can result in a structure in which allowable stresses are exceeded or camber and deflection behavior is poorly predicted, such that the serviceability of a structure may be adversely affected. This paper reports the prestress losses observed throughout fabrication, shipment, erection, and the first 2 years of service for the first high-performance superstructure concrete bridge in Missouri. The prestress losses investigated included prerelease losses, elastic shortening losses, relaxation losses, creep losses, and shrinkage losses. Results from the study were compared with eight commonly used loss estimate models for total prestress losses, including AASHTO and Prestressed Concrete Institute methods. Recommendations were proposed by the authors for the most appropriate methodology to use to predict prestress losses in high-strength concrete girders accurately.

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Long-Term Characteristics of Prestressing Force in Post-Tensioned Structures Measured Using Smart Strands

Applied Sciences ◽

10.3390/app10124084 ◽

2020 ◽

Vol 10 (12) ◽

pp. 4084 ◽

Cited By ~ 1

Author(s):

Sang-Hyun Kim ◽

Sung Yong Park ◽

Se-Jin Jeon

Keyword(s):

Prestressed Concrete ◽

Mechanical Strain ◽

Prestress Losses ◽

Prestressing Force ◽

Eurocode 2 ◽

Study Results ◽

Girder Bridge ◽

Creep And Shrinkage ◽

Term Monitoring

The proper distribution of prestressing force (PF) is the basis for the design of prestressed concrete (PSC) structures. However, the PF distribution obtained by predictive equations of prestress losses has not been sufficiently validated by comparison with measured data due to the poor reliability and durability of conventional sensing technologies. Therefore, the Smart Strand with embedded fiber optic sensors was developed and applied to PSC structures to investigate the long-term characteristics of PF distribution as affected by concrete creep and shrinkage. The data measured in a 20 m-long full-scale specimen and a 60 m-long PSC girder bridge were analyzed by comparing them with the theoretical estimation obtained from several design equations. Although the long-term decreasing trend of the PF distribution was similar in the measurement and theory, the equation of Eurocode 2 for estimating the long-term prestress losses showed better agreement with the measurement than ACI 209R and ACI 423.10R did. This can be attributed to the more refined form of the predictive equation of Eurocode 2 in dealing with the time-dependency of the PF. The study results also confirmed the need to compensate for the temperature variation in the long-term monitoring to derive the actual mechanical strain related to the PF. We expect our developed Smart Strand to be applied practically in PF measurement for the reasonable safety assessment and maintenance of PSC structures by improving several of the existing drawbacks of conventional sensors.

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Structural Design of High-Performance Concrete Bridge Beams

Transportation Research Record Journal of the Transportation Research Board ◽

10.3141/1696-59 ◽

2000 ◽

Vol 1696 (1) ◽

pp. 171-178

Author(s):

Xiaoming (Sharon) Huo ◽

Maher K. Tadros

Keyword(s):

Structural Design ◽

High Performance ◽

High Performance Concrete ◽

Highway Bridges ◽

Major Effect ◽

Minor Effect ◽

Bottom Flange ◽

Prestress Losses ◽

Bridge Beams ◽

Creep And Shrinkage

Recently high-performance concrete (HPC) has been used in highway bridges and has gained popularity for its short-term and prospective long-term performances. Benefits of using HPC include fewer girder lines required, longer span capacity of girders, reduced creep and shrinkage deformation, less prestress losses, longer life cycle, and less maintenance of bridges. Research has been conducted on several issues of structural design of HPC bridge beams. The topics discussed include the effects of section properties of prestressed concrete girders, allowable tensile and compressive stresses, creep and shrinkage deformations of HPC, and prediction of prestress losses with HPC. The results from a parametric study have shown that a section that can have a large number of strands placed in its bottom flange is more suitable for HPC applications. The use of 15-mm-diameter prestressing strands allows the higher prestressing force applied on sections and can provide more efficiency in HPC bridges. The research results also indicate that the allowable compressive strength of HPC has a major effect on the structural design of bridges, whereas the allowable tensile stress has a minor effect on the design. Equations for predicting prestress losses based on the experimental and analytical results are recommended. The recommended equations consider the effects of lower creep and shrinkage deformations of HPC.

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Evaluation of Prestressing with a Shape Memory Alloy

MRS Proceedings ◽

10.1557/proc-503-287 ◽

1997 ◽

Vol 503 ◽

Cited By ~ 1

Author(s):

Arup K. Maji

Keyword(s):

Shape Memory ◽

Prestressed Concrete ◽

Strain Energy ◽

Event Rate ◽

Martensite Phase ◽

Nickel Titanium ◽

Prestress Losses ◽

Higher Temperature ◽

Prestressed Concrete Bridge ◽

Niti Wires

ABSTRACTThe paper addresses the possibility of using shape-memory effect in Nitinol (Nickel-Titanium) alloy to remedy prestress losses in aging concrete beams. Straight annealed NiTi in the low temperature martensite phase can undergo large plastic deformations. This deformation and strain energy is recoverable when the material transforms to the higher temperature (about 55° C) austentite phase. This phenomena can be exploited to induce curvature, hence prestressing, by embedding NiTi wires and ropes in a structural member. Depending on the type of wire used its actuation effect can be reversed as necessary, creating the possibility of a smart prestressed concrete bridge of the future.Materials aspect studied in this project include the bond between NiTi wires and the host composite material, through mechanical testing and microscopy. An evaluation of the phase transformation is conducted using Acoustic Emission (AE) technology. AE event rate, locations and characteristics were evaluated during the actuation process.

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Time-Dependent Prestress Losses in Prestressed Concrete Girders Built of High-Performance Concrete

Transportation Research Record Journal of the Transportation Research Board ◽

10.3141/1594-07 ◽

1997 ◽

Vol 1594 (1) ◽

pp. 64-72 ◽

Cited By ~ 1

Author(s):

M. Myint Lwin ◽

Bijan Khaleghi

Keyword(s):

Prestressed Concrete ◽

High Performance ◽

High Performance Concrete ◽

Time Dependent ◽

Step Method ◽

Time Step ◽

Prestress Losses ◽

Concrete Girders ◽

Prestressed Concrete Girders ◽

Creep And Shrinkage

The Washington State Department of Transportation is one of several state departments of transportation involved in demonstration projects to acquire information and data on the design, fabrication, and construction of prestressed concrete bridges with high-performance concrete (HPC). Predicting the time-dependent prestress losses due to the creep and shrinkage of HPC and the relaxation of prestressing steel is difficult because of the limited experience with and data on the creep and shrinkage properties of HPC. The AASHTO load resistance factor design specification approach to estimating prestress losses is discussed, and the time-step method and the modified rate-of-creep method are introduced as ways of predicting more accurately the time-dependent prestress losses. A design example is used to compare and discuss numerically the prestress losses computed by the various methods. The modified rate-of-creep analysis method yields the lowest losses. The modified rate-of-creep method is a desirable, comprehensive, applicable, and practical method for estimating time-dependent prestress losses in composite and noncomposite prestressed concrete girders with HPC. This method is suitable for manual and electronic computation.

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