Review of advanced modelling methods for lattice steel towers

Abstract : The analysis of transmission line steel lattice towers is usually performed with simplified linear numerical methods. However, the actual behaviour of bolted lattice towers is complex and may be affected by different factors such as rotational stiffness of connections, bolt slippage, eccentricities at the connection, initial deformations of the members, etc. For this reason, most utilities perform full-scale tests for the qualification of new design of steel lattice transmission towers. These tests are expensive and add delays in the planning of the construction of new transmission lines. Advanced numerical methods were developed over the years and they may effectively provide more insight into the force distribution scheme and loading condition of individual members found in the steel structure and hence, help to optimize the use of full-scale experimental tests. The objective of this paper is to review the main strategies that were identified in order to model accurately the behaviour of steel lattice towers. It reviews a number of documents that were published on this subject and presents recent advances made with research projects performed at Université de Sherbrooke. First, the choice of elements is reviewed and traditional methods using truss elements, beam elements, or a combination of truss and beam elements are discussed. The possibility to use shell and 3D elements is also evaluated. Second, the behaviour of bolted connection is studied and strategies for including the effect of slippage, rotational stiffness and eccentricities are discussed. Third, a review of research works using static and dynamic analyses is presented. Finally, a novel hybrid simulation method combining experimental tests with numerical modelling is explained. In summary, many options are available for improving modeling of the complex behaviour of steel lattice towers. One needs to select hypotheses carefully to make sure that the model is compatible with the problem studied and that its complexity is minimized. Future works needed include the simplification of model building methods, further development of hybrid testing techniques, and validation of modeling techniques with full-scale lattice tower tests

Experimental investigation of multi-material aluminum-to-steel and GFRP-to-steel bonded and bolted/bonded connections

Abstract : Through an experimental study, this paper describes the behavior of single-lap bonded, bolted and bolted/bonded connections for configurations with minimum geometric parameters proposed in design references. Two types of multi-material connections are considered: Glass Fiber Reinforced Polymer (GFRP)-to-steel and aluminum-to-steel. At first, the behavior of multi-material bonded connection using methacrylate and epoxy adhesives is evaluated. Then experimental results of bolted connections are presented. Finally, the contribution of adhesive in GFRP-to-steel and aluminum-to-steel bolted connections is investigated. Test results show that on single-lap simply bonded joints, failures mostly occur at the substrate to adhesive interface. Sanding the GFRP plate was found to improve the connection strength. Despite their lower elastic modulus, methacrylate adhesives with larger elongation provides better strength due to their high resistance in peeling. For bolted/bonded joints, the adhesive was found to improve the elastic behavior and the strength of GFRP-steel joints while its effect for aluminum-steel joints was not apparent due to reduce bonded surface and the high strength performance of the bolted plates

Testing steel lattice towers with a hybrid (numerical/ experimental) method

Abstract : The analysis of transmission line steel lattice towers is usually performed with linear numerical methods. However, the actual behaviour of bolted lattice towers is complex and may be highly affected by different factors such as rotational stiffness of connections, bolt slipping, eccentricities in the connections, initial out-of-straightness, etc. For this reason, most power utilities perform full-scale tests for the qualification of novel steel lattice transmission tower designs. These tests are expensive and add delays in the planning of the construction of new transmission lines. Over the past years, advanced numerical methods were developed and they may effectively provide more insight into the force distribution found in lattice structures. However, the actual capacity and failure modes remain difficult to identify numerically and experimental tests are often required. On the other hand, over the last decades, the civil engineering field has seen the development of a completely new testing technique called hybrid testing that combines both experimental and numerical methods. This testing technique involves the experimental testing of a substructure and the interaction during the test with a numerical model of the remainder of the structure. This technique was mostly used for the dynamic analyses of buildings and bridges under the action of seismic loads. To adapt hybrid techniques to lattice towers, a methodology needs to be put in place to identify the substructure of interests to be tested experimentally. The work presented here aims at developing a hybrid testing technique for the evaluation of the failure modes and structural capacity of lattice towers. The long term objective of this research program is to provide a new testing technique that would be an advantageous alternative to advanced numerical methods and reduce the need for full-scale tests. This paper presents hybrid tests that were performed on reduced-scale lattice tower sections. Firstly, an existing 32 m lattice tower was adapted to be able to build a reduced 1:4 scale model. Secondly, a numerical model was developed to identify load cases and substructures that could allow to obtain similar capacities and failure modes to those obtained for the complete structure model. Then, experimental tests on complete 1:4 reduced scale towers were performed to serve as references. Finally, hybrid tests were performed on a 1:4 reduced scale substructure corresponding to a critical section of the tower. The hybrid tests yielded results that were in agreement with the complete tower experimental tests. The test results were also coherent with the numerical model’s results

Effect of geometric parameters on the behavior of bolted GFRP pultruded plates

Abstract : This paper presents the effect of geometric parameters on the behavior of bolted GFRP pultruded plates for civil engineering applications. After a literature review, results of an experimental analysis investigating the behavior of GFRP-to-steel single-lap bolted connections are presented. Then, a finite element analysis validated by experimental data is used to evaluate the effects of the end-distance, side-distance, gage, pitch and plate properties on the strength and failure mode of the connection. A critical examination of geometric recommendations proposed in design references is presented. Bearing failure caused by contact of the bolt on the GFRP plate is usually defined as the preferred failure mode. With highly orthotropic plate, this type of failure was found to be less likely to occur when loading is applied in the pultruded direction. The investigation showed that the minimum end-distance and pitch-distance recommended by design references usually produce a connection with the maximum capacity. However, it was found that the minimum side-distance recommended by these references does not necessarily lead to the maximum capacity for one-bolt and for two-bolt in a column connections

Effect of geometrical parameters of aluminum-to-steel bolted connections

Abstract : Through experimental and numerical studies, this research work aims to provide directions on the optimal geometric configuration for single-lap and double-lap bolted connection between aluminum alloy 6061-T6 and steel. From experimental test results, the effects of different geometric parameters on the joint strength were discussed. These parameters include the end-distance, the side-distance, the pitch-distance, the plate thickness and the joint eccentricity. Then, the experimental results were compared to predicted results using design references and geometric recommendations proposed by design references were critically examined. The experimental study was complemented by finite element (FE) analysis to extend the study to a larger range of parameters. In addition to the analysis of the geometric parameters listed above, the effects of the gage-distance on the joint strength were studied in the FE analysis. The experimental and finite element results show that a careful selection of geometric parameters can result in the high improvement of the connection strength and failure mode. Limiting the side-distance to the minimum recommended value was found to limit the strength of a connection with two bolts in a column to that of the one-bolt connection. In most cases, bearing was found to govern the strength of the connections. The calculated bearing strengths were found to underestimate significantly the connection strength. Based on these analyses, maximum geometric parameters beyond which there is no further increase of the joint capacity were evaluated and optimum geometric parameters were proposed

Comportement post élastique de poteaux en béton à haute performance confinés par des étriers rectangulaires

Ce rapport présente une étude expérimentale réalisée entre le mois d'avril 1993 et le mois de juin 1994 sur six poteaux de taille réelle en béton à haute performance confinés par des étriers rectangulaires. La résistance du béton utilisé variait entre 92 et 105 MPa. L'espacement des étriers et le niveau de charge axiale ont été étudiés sous une sollicitation en flexion composée. Les membrures en béton à haute performance ont longtemps été considérées comme fragiles. Toutefois, les résultats de cette recherche montrent que les poteaux suffisamment bien confinés peuvent se comporter de manière très ductile. Ainsi, deux d'entre eux ont atteint des ductilités structurales de l'ordre de 8.5. Seulement deux spécimens se sont fracturés de manière fragile. L'influence du niveau de charge axiale et celle de l'espacement des étriers ont été nettement observées. Différents blocs de contraintes ont été évalués, de même que trois modèles de comportement de membrures en béton armé. Certains blocs ont clairement montré leurs limites. D'autres ont très bien prévu la résistance des différentes membrures. Un seul des trois modèles de comportement a bien prédit les réponses. Finalement, une équation permettant de calculer l'armature transversale minimum de membrures en béton à haute performance armé a été validée, et des valeurs pour les blocs équivalents de contraintes ont été proposées

Review of advanced modelling methods for lattice steel towers

Abstract : The analysis of transmission line steel lattice towers is usually performed with simplified linear numerical methods. However, the actual behaviour of bolted lattice towers is complex and may be affected by different factors such as rotational stiffness of connections, bolt slippage, eccentricities at the connection, initial deformations of the members, etc. For this reason, most utilities perform full-scale tests for the qualification of new design of steel lattice transmission towers. These tests are expensive and add delays in the planning of the construction of new transmission lines. Advanced numerical methods were developed over the years and they may effectively provide more insight into the force distribution scheme and loading condition of individual members found in the steel structure and hence, help to optimize the use of full-scale experimental tests. The objective of this paper is to review the main strategies that were identified in order to model accurately the behaviour of steel lattice towers. It reviews a number of documents that were published on this subject and presents recent advances made with research projects performed at Université de Sherbrooke. First, the choice of elements is reviewed and traditional methods using truss elements, beam elements, or a combination of truss and beam elements are discussed. The possibility to use shell and 3D elements is also evaluated. Second, the behaviour of bolted connection is studied and strategies for including the effect of slippage, rotational stiffness and eccentricities are discussed. Third, a review of research works using static and dynamic analyses is presented. Finally, a novel hybrid simulation method combining experimental tests with numerical modelling is explained. In summary, many options are available for improving modeling of the complex behaviour of steel lattice towers. One needs to select hypotheses carefully to make sure that the model is compatible with the problem studied and that its complexity is minimized. Future works needed include the simplification of model building methods, further development of hybrid testing techniques, and validation of modeling techniques with full-scale lattice tower tests

Testing steel lattice towers with a hybrid (numerical/ experimental) method

Abstract : The analysis of transmission line steel lattice towers is usually performed with linear numerical methods. However, the actual behaviour of bolted lattice towers is complex and may be highly affected by different factors such as rotational stiffness of connections, bolt slipping, eccentricities in the connections, initial out-of-straightness, etc. For this reason, most power utilities perform full-scale tests for the qualification of novel steel lattice transmission tower designs. These tests are expensive and add delays in the planning of the construction of new transmission lines. Over the past years, advanced numerical methods were developed and they may effectively provide more insight into the force distribution found in lattice structures. However, the actual capacity and failure modes remain difficult to identify numerically and experimental tests are often required. On the other hand, over the last decades, the civil engineering field has seen the development of a completely new testing technique called hybrid testing that combines both experimental and numerical methods. This testing technique involves the experimental testing of a substructure and the interaction during the test with a numerical model of the remainder of the structure. This technique was mostly used for the dynamic analyses of buildings and bridges under the action of seismic loads. To adapt hybrid techniques to lattice towers, a methodology needs to be put in place to identify the substructure of interests to be tested experimentally. The work presented here aims at developing a hybrid testing technique for the evaluation of the failure modes and structural capacity of lattice towers. The long term objective of this research program is to provide a new testing technique that would be an advantageous alternative to advanced numerical methods and reduce the need for full-scale tests. This paper presents hybrid tests that were performed on reduced-scale lattice tower sections. Firstly, an existing 32 m lattice tower was adapted to be able to build a reduced 1:4 scale model. Secondly, a numerical model was developed to identify load cases and substructures that could allow to obtain similar capacities and failure modes to those obtained for the complete structure model. Then, experimental tests on complete 1:4 reduced scale towers were performed to serve as references. Finally, hybrid tests were performed on a 1:4 reduced scale substructure corresponding to a critical section of the tower. The hybrid tests yielded results that were in agreement with the complete tower experimental tests. The test results were also coherent with the numerical model’s results