Thermal Prestressing Method Research Articles

Experimental load-carrying capacity evaluation of thermal prestressed H beam with steel brackets

To improve the load-carrying capacity of steel structures there are mainly two methods of using cover-plate which are the preflex method and multi-stepwise thermal prestressing method. However a prestress applied to the top flange of H beam is less than the bottom flange because these two methods have small eccentricity. It is necessary to develop an advanced feature of prestressing method which efficiently increases load-carrying capacity and prestressing efficiency. Therefore this study proposes a cover-plate prestressing method with increased sectional stiffness and prestressing efficiency, called multi-stepwise thermal prestressing with steel brackets. To evaluate the prestressing efficiency and load-carrying capacity of the proposed method, prestressing application tests and static loading tests were conducted on three H beams. The multi-stepwise thermal prestressing method with the steel brackets had higher displacement induced to the H beam and higher tensile stress in the top flange than in a case without the steel brackets. Also, when prestress is applied, the tensile stress in the cover-plate significantly decreased with the installation of the steel brackets. The static loading tests showed that the installation of the brackets increased the stiffness, the yield load, and ultimate load of the structure, compared with an unreinforced girder, which meant higher load-carrying capacity than a conventional thermal prestressed structure without brackets.

A verification of simplified analysis method for thermal prestressing in continuous composite bridge

The use of thermal-prestressing method (TPSM) prevents the occurrence of tensile transverse crack in concrete decks located in negative bending moment regions of continuous composite bridges. An accurate representation of the structural response can be obtained by means of refined analyses using three-dimensional finite element models to carry out both heat transfer and thermo-elastic calculations. So, a simplified approach was proposed for easily using the TPSM effect in design to reduce the amount of computational work of the refined analysis, but its application to real structure was not confirmed. Therefore, in this study, the verification of a simplified analysis approach has been performed to explain TPSM effects in the analysis of continuous composite bridges. For this purpose, experimental results on 2-span thermal prestressed continuous composite bridge specimens were compared with simplified analysis and refined analysis results as well as analytical approach on 2-span and 3-span continuous composite bridges. From the verification results, the proposed simplified TPSM analysis methodology shows the good predictions of the thermal prestressing effect to apply design of TPSM applied continuous composite bridges.

Plate Girder Bridge Strengthened with Multi-Stepwise Thermal Prestressing Method

The strengthening of a plate girder bridge with external prestressing tendons is a commonly used method of upgrading existing bridges. Indeed, it has been known to offer advantages such behaviors as increased elastic behaviors under higher loading, increased ultimate resistance, and reduced deflection under service loads. However, this method has notable disadvantages such as stress concentration at anchorages and inefficient live load-carrying capacity. The thermal prestressing method (TPSM) for steel bridges proposed in this study uses thermal expansion and contraction of a cover-plate to provide a combination with the prestress effects of an external prestressing tendon and the section enlargement benefits of the external bonding method. In this study, the basic concepts of the proposed strengthening protocol using multi-stepwise TPSM are presented. The existence of the strengthening effect is well substantiated and the proposed analytical approach is also rigorously verified. The significant strengthening effects on a simply supported plate girder bridge are analyzed.

Experimental Study on Application of Thermal Prestressing Method to Continuous Composite Girder Bridges

Experimental Study on Application of Thermal Prestressing Method to Continuous Composite Girder Bridges

Life-cycle cost analysis of a TPSM applied continuous composite girder bridge

Transverse cracks in composite girder bridges are repeatedly reported because the tensile stresses in the concrete deck induced by the negative bending moment at the intermediate supports of continuous composite girder bridges. The occurred transverse cracks rapidly deteriorate the concrete deck, giving rise to frequent need for maintenance work. Prestressing methods are usually utilized in this kind of situation for the concrete structures, however no effective and feasible method has yet been proposed for steel-concrete composite structures. A new type of prestressing method entitled Thermal prestressing method (TPSM) has been proposed for innovative construction of continuous composite girder bridges, as effective prestressing method to prevent the occurrence of the tensile transverse crack of the concrete deck at the negative bending moment regions. In this study, the methodology for the economic analysis of TPSM applied bridges and case studies are introduced based on the life-cycle cost, considering initial construction cost and maintenance cost, to demonstrate the financial viability of the TPSM applied bridges compared to conventional continuous composite girder bridges.

Multi-stepwise thermal prestressing using a cover-plate in steel structures

This paper introduces an innovative prestressing method for steel structures utilizing a multi-stepwise thermal prestressing method (M-TPSM). The proposed prestressing method is founded on a simple concept of thermal expansion and contraction of steel. A cover-plate, a high strength steel cover-plate used as a post-tensioning member in this method, is heated by electrical heaters before it is attached to the main member. After the cover-plate is attached to the main member by using high tension bolts at several points or by welding, the electromagnetic induction heaters are removed, and the resulting contraction force of the cover-plate induces eccentric axial forces to the main member. In this study, a fundamental investigation involving analytical and experimental study is carried out to evaluate the feasibility of the M-TPSM for steel structures. Also, static loading tests were performed on the rolled beam specimens for assessment of the behavior of the steel beam prestressed with M-TPSM.

Innovative Key-Segment Closing Method Using Thermal Prestressing Technique for Partially Earth-Anchored Cable-Stayed Bridges

This paper proposes an innovative key-segment closing method for partially earth-anchored cable-stayed bridges erected using free cantilever construction method, and examines its feasibility by performing a construction sequence analysis. The proposed method is based on the thermal prestressing technique, which gives an artificial heat to the girder in a cantilever state, in order to introduce the closing force required for connecting the key-segment to the adjacent segment by means of thermal expansion. The new method not only provides a feasible closing method for partially earth-anchored cable-stayed bridges, but also has a significant effect in reducing the axial force in the girders. A construction sequence analysis of an example bridge shows that the axial forces in the girders and the uplift forces at the bearings are reduced by a considerable amount due to application of the thermal prestressing method, whereas the member forces of cables and pylons are slightly changed. Therefore, it is verified from this study that the structural capacities of the girders and the bearings of the partially earth-anchored cable-stayed bridges can be effectively enhanced by application of the proposed method.

An analytical investigation of thermal prestressing method for continuous composite girder bridges

The negative bending moment near the intermediate supports of continuous composite girder bridges induces tensile stresses in the concrete deck. Although a relatively high ratio of reinforcement is present in these regions, tensile cracks are repeatedly reported. The tensile cracks rapidly deteriorate the concrete deck, giving rise to frequent need for maintenance work. Also owing to the tensile stresses, the concrete deck sections at the negative bending moment regions are ignored in the design process and this kind of design concept is a huge drawback regarding effective use of materials. Prestressing methods are usually utilised in this kind of situation for the concrete structures, but no effective and feasible method has yet been proposed for steel–concrete composite structures. In the current study, a new type of prestressing method entitled the thermal prestressing method (TPSM) is proposed for the innovative construction of continuous composite girder bridges. Basic concepts of the TPSM are introduced and a methodology for computing the TPSM effect is presented with illustrative examples.

An experimental investigation of thermal prestressing method for continuous composite steel girder bridges

This paper describes an experimental investigation conducted to verify and substantiate the preceding analytical study, which was focused on the development of the thermal prestressing method (TPSM) including the basic concepts of the method and the methodology for numerical analysis of the thermal prestressing effect. The experimental results from this research, especially the initial stress measurement from a prototype bridge, prove that the method can be successfully used for prestressing continuous composite girder structures. Also, the experimental and analytical results exhibit good agreement, which implies that the thermal prestressing effect can be predicted with reasonable accuracy with the previous proposed methodology. It is concluded that the TPSM can be successfully applied to both the design and construction of continuous composite girder structures, and the authors are optimistic about the TPSM becoming an innovative solution for prestressing continuous composite girder bridges.

A STUDY ON STRENGTHENING OF STEEL BRIDGES BY THERMAL PRESTRESSING METHOD

近年，過去の高度経済成長期に建設された多くの道路橋が，高齢化の時期を迎えている．したがって，これらの橋梁を，貴重な社会資本として，延命化させることが重要な課題である．既設鋼橋の補強工法としては，単なる添接補強材工法と外ケーブル工法とが挙げられるが，その補強効果に，長短がある．筆者らは，既設鋼橋の新しい補強・補修工法として，熱プレストレス工法を提案してきた．この工法は，耐荷力が不足している橋梁でも，B活荷重対応橋梁として，格上げすることができる，同時に，耐久陸も向上させることができる．本文では，この熱プレストレス工法による既設連続桁の補強・補修効果について検討した結果を報告する．

An Experimental Study on Reinforcement of Steel Bridge Piers by Using Thermal Prestressing Method

An Experimental Study on Reinforcement of Steel Bridge Piers by Using Thermal Prestressing Method

Thermal Prestressing of Structural Steel

The thermal prestressing method discussed in this article consists of shop welding heated high strength steel cover plates to the bottom flanges of beams and girders. Heat is produced with a simple device, such as a gas torch; elongation of the plate is measured with a dial indicator. After cooling, the plate creates predetermined favorable stresses in the beam which in effect permit less steel to do more work. Considering all costs, the savings over conventional construction are shown to amount to about 6 percent. Advantages, limitations and proposed applications of the method are discussed. Underlying theory is limited to end results.