Analysis of Short-Term Prestress Losses in Post-tensioned Structures Using Smart Strands

The proper estimation of prestressing force (PF) distribution is critical to ensure the safety and serviceability of prestressed concrete (PSC) structures. Although the PF distribution can be theoretically calculated based on certain predictive equations, the resulting accuracy of the theoretical PF needs to be further validated by comparison with reliable test data. Therefore, a Smart Strand with fiber optic sensors embedded in a core wire was developed and applied to a full-scale specimen and two long-span PSC girder bridges in this study. The variation in PF distribution during tensioning and anchoring was measured using the Smart Strand and was analyzed by comparison with the theoretical distribution calculated using the predictive equations for short-term prestress losses. In particular, the provisions for anchorage seating loss and elastic shortening loss were reviewed and possible improvements were proposed. A new method to estimate the amount of anchorage slip based on real PF distributions revealed that the general assumption of 3–6-mm slip falls within a reasonable range. Finally, the sensitivity of the PF distribution to a few of the variables included in the equation of the elastic shortening loss was examined. The study results confirmed that the developed Smart Strand can be used to improve the design parameters or equations in PSC structures by overcoming the drawbacks of conventional sensing technologies.

Full-Scale Field Test On Retaining Structure Enhanced With Soil Nails And Prestressed Anchors

Retaining structure enhanced with soil nails and prestressed anchors is found good at constraining the horizontal displacement and therefore ensuring the stability of the foundation pit during excavation. Based on these advantages, such retaining structure is widely used in foundation excavation practice. This paper presents results of a series of in-situ tests conducted to investigate the mechanical behaviors of retaining structure enhanced with soil nails and prestressed anchors. Behaviors of three different retaining structures enhanced with i) soil-nails; ii) soil-nails and prestressed anchors without unbonded part; iii) soil-nails and prestressed anchors with a 2.5m unbonded length, were monitored during staged excavation to investigate the influences of i) the prestressing force and ii) unbonded length of the prestressed anchors on the performance of the entire retaining system. It was found that the affecting the stress and deformation of composite retaining system, which is in agreement with the other published results in the literature. The variation of the magnitude and distribution of soil nail force responding to the anchor prestressing force however showed no systematic trend. The unbonded length of anchors, which is suggested to be the main factor affecting the structural stability in dense materials in the literature, is found to have little influence in loose fill materials used in this study. Studies presented in this paper are useful for the rational design and serviceability analysis of the composite soil-nailed retaining structure enhanced with prestressed anchors.

Parabolic tendons in prestressed concrete - how accurate are equivalent loads?

The mechanical effects of a parabolic tendon can be modeled replacing the tendon by external loads applied to the concrete. The intensity of these loads depends on the prestressing force P and curvature of the tendons. These two factors are also interrelated because the losses of prestress vary with the curvature. The structural analysis can be simplified by approximating that the line load against the tendon, able to maintain the initial parabolic form of the tendon and called equivalent load, is constant, perpendicular to the centroidal axis of the beam and equal to P/R where R is the radius of curvature of the parabola at its vertex. This approximation is one of the key issues in the textbooks but normally not properly justified. In this paper, the mathematical background for the approximation is formulated. Some typical tendon layouts are analyzed to evaluate the approximation error. The error proved to be insignificant for simple beams. For cantilever and continuous beams more accurate methods in the final design are recommended.

Nonlinear Finite Element Modelling and Parametric Study of Prestressed Steel Box Girder

The prestressing technique is easy to apply and is generally used to strengthen steel bridges and controls their gross deflection. ANSYS has been used to establish a numerical model for the mechanical behavior of a steel box girder and prestressed by external tendons. In this paper, steel plate girders with and without strengthening technique was tested to assess the effectiveness of this technology. The results showed that prestressing improves the mechanical behavior of a girder and that its effect is proportional to magnitude of the applied external load. The results of the numerical model showed good agreement with the experimental data. A full-course simulation was conducted with ANSYS for a parametric study to analyze the influence of prestressing force magnitude, span-height ratio, and tendon configurations in increasing the effectiveness of prestressed technique.

COMBINING EBR CFRP SHEET WITH PRESTRESSED NSM STEEL STRANDS TO ENHANCE THE STRUCTURAL BEHAVIOR OF PRESTRESSED CONCRETE BEAMS

In this study, a combined strengthening technique is used to improve the flexural performance of prestressed concrete beams using CFRP sheets as EBR and prestressed steel strands as NSM. Seven prestressed beams were tested under four-point loading with one control specimen, one EBR CFRP sheet strengthened specimen, one NSM steel strand without prestress strengthened specimen and four specimens strengthened with a combination of EBR CFRP sheet and NSM steel strands prestressed from 0% to 70% of their tensile strength. The flexural responses and failure modes of the specimens were investigated and the variations due to the level of prestressing force in the PNSM steel strands were also assessed. A finite element model (FEM) was developed using ABAQUS to verify the flexural responses of the strengthened specimens. The test results revealed that the combined strengthening technique remarkably enhanced the flexural performance of the specimens. The serviceability, first crack, yield, and ultimate load capacities improved up to 44%, 49%, 55% and 70%, respectively when compared with the control specimen. The combined technique also ensured the flexural failure of the specimens with significant enhancement in stiffness and energy absorption. The results of the FEM model exhibited excellent agreement with the experimental results.

NUMERICAL STUDY ON SHEAR BEHAVIOUR OF ENHANCED C-CHANNELS IN STEEL-UHPC-STEEL SANDWICH STRUCTURES

This paper firstly developed a three-dimensional (3D) finite element model (FEM) for enhanced C-channels (ECs) in steel-UHPC-steel sandwich structures (SUSSSs). The FEM was validated by 12 push-out tests on ECs with UHPC. With the validated FEM, this paper performed in-depth parametric studies on shear behaviours of ECs with ultra-high performance concrete (UHPC). These investigated parameters included bolt-hole gap (a), grade (M) and diameter (d) of bolt, core strength (fc), length of C-channel (Lc), and prestressing force ratio on bolt (ρ) in ECs. Under shear forces, the ECs in UHPC exhibited successive fractures of bolts and C-channels. Increasing the bolt-hole gap within 0-2 mm has no harm on the ultimate shear resistance, but greatly improves the slip capacity of ECs. Increasing grade and diameter of bolts improves the shear resistance and ductility of ECs through increasing the PB/PC (shear strength of bolt to that of C-channel) ratio. Increasing the core strength increased the shear resistance, but reduced the ductility of ECs due to the reduced PB/PC ratio. The ECs with Lc value of 50 mm offer the best ductility. Prestressing force acting on the bolts reduced the shear strength and ductility of ECs with UHPC. Analytical models were proposed to estimate the ultimate shear resistance and shear-slip behaviours of ECs with UHPC. The extensive validations of these models against 12 tests and 31 FEM analysis cases proved their reasonable evaluations on shear behaviours of ECs with UHPC.

Full-Scale Field Test on Construction Mechanical Behaviors of Retaining Structure Enhanced with Soil Nails and Prestressed Anchors

Soil nailing combined with prestressed anchors has a good workability and is relatively cheap in constraining the horizontal displacement. Current research on the technique, whether theoretical analyses, numerical simulations, or model tests, was conducted under ideal working conditions. However, in fact, external disturbances, such as tensioning-lagging of the anchor, are very common and play an important role on stress and displacement. Therefore, it is of great significance to carry out a field test considering the effects of external disturbances, which can obtain real and reliable data through real-time monitoring. In this paper, the impacts of the construction conditions on practical engineering are discussed based on in situ tests, and some reasonable suggestions for the upgrading of misbehaviors in the current construction situation are put forward. In particular, the influence features of soil predisturbance, excessive excavation, unloading on the surface of edges, tensioning-lagging of the anchor, and continuous rainfall on the stress–time curve of soil nails under practical working conditions are analyzed. Behaviors of three different retaining structures enhanced with (i) soil nails; (ii) soil nails and prestressed anchors without unbonded parts; and (iii) soil nails and prestressed anchors with a 2.5 m unbonded part were monitored during staged excavation to investigate the influences of (i) the prestressing force and (ii) the unbonded part of the prestressed anchors on the performance of the entire retaining system. Results show that (i) the prestressing force is the main factor affecting the stress and deformation of the composite retaining system, which is consistent with the existing literature; (ii) the variation of the magnitude and distribution of the soil nail force responding to the anchor prestressing force, however, showed no systematic trend; and (iii) the unbonded part of anchors, which was validated to be the main factor affecting the structural stability in dense materials in the existing literature, is found to have a minor influence in loose fill materials used in this study.

Verification of Actual Prestressing in Existing Pre-Tensioned Members

In the case of prestressed concrete structures, information about the actual state of prestressing is an important basis for determining their load-carrying capacity as well as remaining service life. During the service life of the prestressed concrete structure, the initial level of prestressing is inevitably reduced as a result of the actions of various factors. These reductions of prestressing force are considered as prestress losses, which are influenced by construction stages, used materials, prestressing technology, or required length of service life. Available standards enable the determination of the expected values of prestress losses. Ultimately, their calculation is part of the design procedure of every prestressed concrete structure. However, aging and often neglected infrastructure in Europe is also exposed to factors, such as environmental distress, that are not considered in standard calculations. Therefore, verified and reliable methods for determining the actual state of prestressing are needed. This paper presents an experimental program of an indirect method for the evaluation of the value of prestressing force in seven prestressed concrete sleepers. Particularly, the non-destructive saw-cut method as a pivotal object of this study is performed and assessed. Furthermore, the Barkhausen noise technique is used as a comparative method. Subsequently, the experimental campaign is supported by the numerical analysis performed in the ATENA 3D software. Finally, the experimentally determined values of residual prestressing force are compared to the expected level of prestressing according to Eurocodes.