Non-linear Analysis of Circular Composite Columns

in Burgess, Ian (Ed.) Special Issue: Proceedings of Eurosteel 2021: Sheffield - Steel's coming home (2021, September)

Composite columns provide many advantages for the application in multi-story and high-rise buildings. However, EN 1994-1-1 does not provide rules for the design of circular concrete sections with fully ... [more ▼]

Composite columns provide many advantages for the application in multi-story and high-rise buildings. However, EN 1994-1-1 does not provide rules for the design of circular concrete sections with fully encased steel profiles according to the simplified methods. Normally, the simplified method for the design of composite columns is based on a normal force-moment interaction diagram which assumes plastic cross-section resistance. However, the moment resistance is to be reduced by a reduction factor M, considering among other the difference between strain-limited and plastic moment resistance. This reduction factor is developed and calibrated for composite compression members with concrete encased sections, partially encased sections and concrete filled rectangular and circular tubes. Comparison of plastic moment resistance to results based on strain limitation show that the application for circular section with encased steel section but without a steel tube may lead to unsafe results. The paper discusses different approaches for the determination of the N-M Interaction curve and the design on cross-section level. The results obtained from these methods are then compared to each other for different dimensions of cross-sections of circular columns; and for different ratios of reinforcements, various steel grades and for normal and high strength concretes. Furthermore, the investigation focuses on the validation of the reduction factor alfa_M and shows for which section types the values according to Eurocode 4 cannot be applied. [less ▲]

Non-linear design for composite structures in steel and concrete in reference to Eurocode 4, Part 1-1

Learning material (2021)

The design of composite structures in steel and concrete for buildings follows the rules according to Eurocode 4: EN 1994-1-1. In the last years more and more individual composite members and cross ... [more ▼]

The design of composite structures in steel and concrete for buildings follows the rules according to Eurocode 4: EN 1994-1-1. In the last years more and more individual composite members and cross-section types have been developed. Those sections are not always fulfilling the requirements for the application of simplified design methods given by EN 1994-1-1 and therefore plastic resistance cannot always be applied. Furthermore there is an increasing demand for optimization in the design by using more advanced design methods. This optimization is concentrating on the use of individual shaped cross-sections and their efficiency, the composite shear connectors and consideration of their ductility and flexibility, investigation in the sequence of loading, time effects as well as concrete cracking and yielding of steel within the analysis. In addition it is to be mentioned that the draft for the second generation of Eurocode 4 provides more opportunities for advanced numerical methods. At least, the demand for more advanced numerical design methods is permanently increasing also due to the availability of powerful hardware and software applications. The target of this chapter is to introduce the advanced numerical design for composite structural members in steel and concrete and to explain the boundary conditions given by Eurocode 4 and related Eurocodes. Thereby the focus is more on the member and not on the global analysis. The computer-aided global analyses for the determination of action effects are well-known for years while the possibilities for nonlinear analysis based on shell and volume elements considering physical and geometrical nonlinearity becoming more important and will be further developed in future. However, there is not so much information available explaining the application of e.g. the general method in detail and guidance is missing by the code. Therefore the general method acc. to EN 1994-1-1/prEN1994-1-1 is the main focus of this contribution. In the following, some principles for the design according to EN 1994-1-1 are explained. Thereby the basics for the classification of cross-sections as well as the determination of inner forces for continuous beams will be pointed out. However, the focus is on the general method for the design of composite compression members. While in the case studies additional information for the structural analysis and the numerical investigation in composite beams is provided. This chapter will not repeat well-known explanations from the literature, which have been published many times but limits itself to the most important normative fundamentals, rather concentrate on the application of numerical methods for the design of composite structures. More details for global structural analysis and the design according to EN 1994-1-1 [1] are provided by the literature, e.g. [8], [40] and will not be further commented here. It should be noted that the here referred methods according to prEN 1994-1-1, prEN 1993-1-14 and prEN 1992-1-1 do not represent officially introduced design standards, until the introduction of the second generation of Eurocodes modification is not finalized. Therefore this references are to be understood as informative. Until the introduction of the second generation of Eurocodes further changes and adjustments may also to be expected. [less ▲]

MOMENT AND LONGITUDINAL RESISTANCE FOR COMPOSITE BEAMS BASED ON STRAIN LIMITED DESIGN METHOD

Doctoral thesis (2020)

The bending and longitudinal shear design of composite beams of steel and concrete follows often the plastic design method, which is a simplification based on rectangle stress blocks. The application of ... [more ▼]

The bending and longitudinal shear design of composite beams of steel and concrete follows often the plastic design method, which is a simplification based on rectangle stress blocks. The application of the plastic design method requires cross-section to have enough rotation capacity allowing most parts of the critical cross-section reach plastic at failure. There are different types of compact composite beams, such as the slim-floor beams. For them, the neutral axis position often gets deeper at failure, which reduces the rotation capacity and brings questions to the bending resistance and longitudinal shear design according to the plastic design resistance. For a composite beam with deep neutral axis position, advanced numerical methods such as strain-limited design and FEM simulations can provide more accurate results than the plastic cross-section resistance. However, they are challenging to perform for general design engineers. In this work, simplified non-linear strain-limited design approaches, a strain-limited design software "SL.com" and an Abaqus add-in "CivilLab" have been developed to simplify the numerical calculations. They have also been applied in other chapters of this work to check the conventional plastic design results and to provide simplified design rules through parametric studies. With full shear connection, a deep neutral axis position in composite beam under sagging bending may cause an important part of the steel section not to reach plastic at concrete failure. In this case, plastic bending resistance calculated based on rectangle stress blocks can result in an overestimation of the resistance and therefore leads to unsafe design. Thus, according to EN1994-1-1 [22], a reduction factor β on plastic bending resistance (Mpl,Rd) needs to be applied for cross-sections with steel grade S420 and S460 and the relative compression zone height (zpl/h) is over 0.15. However, with the developments in industry as well as the second generation of Eurocode, this reduction factor still needs to be updated to consider new types of composite beams and wider ranges of steel grades. While the conventional plastic design method has its limitations and only applicable when the beam cross-section has enough rotation capacity to allow full plastic development, the more advanced strain-limited numerical calculation and FEM can be used for a much wider range regardless of the position of the neutral axis. The investigations in this dissertation through comparing the plastic bending resistance with advanced numerical calculation results, have confirmed that besides the cross-sections with high steel grades (S420, S460), also certain cross-sections with lower steel grades can have an overestimated plastic bending moment resistance. At least this effect is more important for compact cross-section types such as slim-floor sections or composite beams with asymmetrical structural steel profiles or with a small concrete slab effective width. Therefore vast amount of parametric studies based on strain-limited method and FEM have been developed to check the topics, such as limitation of plastic design methods for different types of composite beams. Furthermore new reduction β functions on Mpl,Rd for engineering practice considering much wider variates of composite beam cross-sections have been deviated. For the design with partial shear connection, the partial shear diagram developed based on plastic analysis has been widely used. As discussed above, the plastic design may not be suitable when the position of neutral axis is too deep, similar problems can occur for the partial shear diagram. This problem is especially significant for slim-floor beams, for which due to the compact cross-section, the relative compression zone height (zpl/h) is usually much higher than conventional composite beams. Thus the limitation of using the partial shear diagram for slim-floor beams is provided, and additional simplified engineering design rules are proposed. Plastic development inside the cross-section increases the longitudinal shear force in the plastic zones, furthermore with ductile shear connectors and respecting the minimum degree of shear connection, the non-linear redistribution of longitudinal shear force allows equal distance arrangement of shear connectors by the conventional design. For which, the full plastic development of the cross-section allowing plastic bending moment resistance and ductile shear connectors allowing non-linear longitudinal shear force distribution are the two fundamental conditions. The deep neutral axis position brings questions directly to the first assumption, as full plastic development of crosssection may not be able to reach. Thus the impact of a deep neutral axis position in the composite beams on longitudinal shear force distribution has been analysed. For which, the influence of plastic development inside beam cross-sections on longitudinal shear force with full shear interaction is theoretically explained. The different stages of nonlinear distribution of longitudinal shear force due to shear connectors are investigated through FEM parametric studies. Based on the theoretical and numerical calculation, the design suggestions of composite beams with deep neutral axis position are given. [less ▲]

PLASTIC DESIGN FOR COMPOSITE BEAMS - ARE THERE ANY LIMITS?

in Proceedings of the 9th International Conference on Steel and Aluminium Structures ICSAS19 (2019, July)

Due to the demand for sustainable constructions, composite structures have become more important and lead to slim and economic solutions with a low structural self-weight. EN 1994-1-1 differentiate four ... [more ▼]

Due to the demand for sustainable constructions, composite structures have become more important and lead to slim and economic solutions with a low structural self-weight. EN 1994-1-1 differentiate four different cross-section classes for the determination of moment resistance. For the plastic design of the moment resistance it is assumed, that each cross-sectional fibre can plastify without any limitation of the strains. For standard composite beams in case of sagging moments and a high-lying plastic neutral axis, plastic design and strain-limited design give similar results. In the case of sections with a large compression zone height xpl and limited rotation capacity, a concrete failure in the compression zone can occur before the plastic moment resistance Mpl,Rd, is reached. The strain limit design becomes decisive. This paper points out the impact of the concrete part on the design of composite beams. A parametric study comparing plastic and strain limited moment resistance of a considerable variety of cross-sections has been carried out to develop the beam design methods considering high compression zone height of next generation of Eurocodes. [less ▲]

Slim-floor beam bending moment resistance considering partial shear connection

in Wald, Frantisek; Jandera, Michal (Eds.) Stability and Ductility of Steel Structures 2019 (2019)

Having the advantage of flat lower surface, high stiffness and integrated fire resistance, slim-floor composite beams are widely used and favoured in many design solutions. The current bending design ... [more ▼]

Having the advantage of flat lower surface, high stiffness and integrated fire resistance, slim-floor composite beams are widely used and favoured in many design solutions. The current bending design methods are mainly derived from plastic design methods for classical composite beam with consideration of the special features for slim-floor beams such as the transverse bending of bottom flange when used as support for slabs. Alternatively, more advanced strain-limited design method or FE-method can be used. In the case of full shear connection, with deep position of neutral axis and great compression zone height, there is a risk that plastic design method may overestimate the bending resistance of the cross section compared to the strain-limited design method. In the case of the partial shear connection, shear design diagram for slim-floor beams obtained by means of the strain-limited design can also differ significantly from the one obtained by plastic design method, thus further research on slim-floor beams is still necessary. [less ▲]

Particularities for plastic design of composite beams with deep plastic neutral axis

in 12th Japanese-German Bridge Symposium (2018, September 04)

The demand for sustainable constructions increases the importance of composite structures as they lead to slim and economical solutions with a low self-weight of the structure. The determination of the ... [more ▼]

The demand for sustainable constructions increases the importance of composite structures as they lead to slim and economical solutions with a low self-weight of the structure. The determination of the moment resistance for composite beams follows the rules stated in EN 1994-1-1. Based on the slenderness c/t of the compressed parts of the steel-section, composite cross-sections are classified into four cross-section classes. This classification indirectly reflects the rotation capacity and susceptibility to local buckling. For class 1 and 2 cross-sections, the plastic moment resistance of the crosssection may be considered. Otherwise, an elastic design (for class 3) or an elastic design considering local buckling effects (for class 4) is necessary. If the plastic resistance of the cross-section is assumed, it is considered that each cross-section fibre may plastify without limitation of the strain values. For normal composite beams subjected to the sagging moments and with a high plastic neutral axis, the real moment resistance is quite greater than one obtained by the method of the plastic design. For sections with a large compression zone, xpl, a concrete failure in the compression zone can happen before the plastic moment resistance of the composite cross-section, Mpl,Rd, is reached. Strain limit design, therefore, becomes critical. EN 1994-1-1 provides a limitation of the plastic design resistance only for sections with steel grades S420 and S460. However, there is no guidance given for lower steel grades or the determination of the corresponding concrete compression force. This paper points out, that the rotation capacity of a composite section is dependent on the slenderness of steel crosssection parts and on the behaviour of the concrete part. A comparison of plastic and strain limited moment resistances as well as the analysis of partial shear diagram-based strain limited design results in new findings for the limits of plastic design methods. [less ▲]

Comparison of Eurocodes and the Chinese standards - Design basics and Design of Composite structure

Presentation (2018, August 31)

Comparison of design for composite columns in steel and concrete according to Eurocode 4 and Chinese design codes

in Romero, Manuel (Ed.) Proceedings of the 12th International Conference on Advances in Steel-Concrete Composite Structures (ASCCS 2018) (2018, June 27)

This paper compares the design of composite columns in steel and concrete based on EN1994-1-1 and Chinese JGJ138-2016. First, the application ranges of the codes are pointed out. Both codes contain the ... [more ▼]

This paper compares the design of composite columns in steel and concrete based on EN1994-1-1 and Chinese JGJ138-2016. First, the application ranges of the codes are pointed out. Both codes contain the design of fully encased composite sections and concrete filled rectangular and circular tubes. However, there are different limitations on cross-section sizes, material strength classes, and others. JGJ138 has three separate chapters for the designs related to the three different types of columns. Eurocode 4 gives three different design methods: one general method based on nonlinear calculation, and two simplified methods based on European buckling curves or N-M iteration curves. For the materials, mechanical properties, such as design strength values, are compared based on the same material grade. For axial compression resistance and eccentrically compressive resistance, the two simplified methods from Eurocode 4 are compared with the design method according to JGJ138-2016 through theoretical and parameter studies. The influences of related parameters such as long-term effects, the buckling curves, and N-M iteration curves are also compared. For shear design, JGJ138-2016 considers mainly transverse shear resistances, while Eurocode 4 further considers shear connection and load introduction. The design transverse shear resistance is compared through theory. [less ▲]

Longitudinal Shaer in Composite Beams

in Young Engineers Colloquium Munich 2018 (2018, April 13)

Longitudinal shear has important influences on the performance of composite beams. According to Eurocode 4, with cross-section class 1 and 2 in combination with ductile shear connectors, a plastic method ... [more ▼]

Longitudinal shear has important influences on the performance of composite beams. According to Eurocode 4, with cross-section class 1 and 2 in combination with ductile shear connectors, a plastic method, considering redistribution of longitudinal shear force is allowed. The conditions therefore are full development of plastic moment resistance and sufficient slip at interface to reach the plateau of P-δ curve of the shear connectors. However, for slim-floor beams, it is to question if the above-mentioned method can be applied. To answer it, related topics still need further investigation, which is the focus of the author’s Ph.D work. This paper presents part of the first stage results on the theoretical analysis of longitudinal shear, considering the influence of plasticity of the beam. For composite beams with rigid shear connectors, non-linear shear stress distribution appears in the plastic region, which influences the shear connector arrangement. However, detailed theoretical research is still hard to be found. In this work, based on a simple beam partly in plastic zone, mathematical equations are developed to calculate the plastic zone height and longitudinal shear stress distribution. Benchmarks with ANSYS and existing tests are performed. [less ▲]

Comparison of design for composite beams in steel and concrete according to Eurocode 4 and Chinese Design Codes

in XI Conference on Steel and Composite Construction (2017, November 23)

Parallel to the investigation in the second generation of Eurocodes, the Chinese design standards are also updated. At 01/12/2016, a new Chinese Code for the design of composite structures JGJ138-2016 was ... [more ▼]

Parallel to the investigation in the second generation of Eurocodes, the Chinese design standards are also updated. At 01/12/2016, a new Chinese Code for the design of composite structures JGJ138-2016 was put in practice to replace the old JGJ138-2001. Benefit from the vast experience of composite structure applications blooming recently in Chinese, the new code covers much wider topics and is more comparable to Eurocodes 4. Generally, both codes cover topics of composite beams and columns, composite slabs and composite connections. However, JGJ138-2016 covers also the design of composite shear walls, while Eurocode 4 considers more about durability and time effects. It is necessary to compare the materials used for both of codes. In this article, it covers comparison of the mechanic properties such as elastic modulus, characteristic strength and design strength. The analysis includes concrete, reinforcement, and structural steel. The main comparison is the design procedure of composite columns. The application range of the codes will be pointed out first. Both codes allow the design of concrete encased sections and concrete filled rectangular and circular tubes. However different limitations on cross-section sizes and materials can be found. Eurocode 4 represents three different design methods: the simplified method for centric loading, based on the buckling curves; the general method based on 2nd order calculation for action effects and N-M interaction for the determination of resistances; as well as the non-linear method based on stress-strain limitation of material and FE-Analysis. Chinese code gives a simplified method of the calculation of column resistance for centric and eccentric loading. Also the impact of second-order effect are to proof if needed. According to the EN1994-1-1 the impact of creep is to consider for the denervation of the bending stiffness and for ideal buckling resistance and non-dimensional slenderness. JGJ138-2016 do not consider impact of creep for the design. Besides, JGJ138-2016 includes also design for seismic situation. The axial cross-section resistance and normal force-moment interaction curves (N-M curves) will be compared through theoretical analysis and parameter studies. JGJ138-2016 considers mainly transversal shear resistances, while Eurocode 4 further considers shear connection, load introduction and longitudinal shear outside the areas of load introduction. The design shear resistance will be compared in general and within parameter study. [less ▲]