Joints and Structural Members
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TITLE 5. JOINTS AND STRUCTURAL MEMBERS CHAPTER XIV. JOINTS Section 55. General 55.1. Bases Joint connections in a structure should be designed to fulfil the intended level of safety, serviceability and durability, and the ability to withstand at least the stresses provided for them in the global analysis of the structure. 55.2. Manufacture and assembly Joints should be designed for ease of use and safety. Special attention shall be paid to providing the space required to: Assemble components safely. Tighten the bolts. Need for access for welders. Accessibility for people in charge of performing protection and maintenance treatments and inspection work, and their teams. The effect of the length and angle tolerances between the faces of a single part on the fit with contiguous parts should also be taken into account. 55.3. Transmission of forces The arrangement of each joint shall be studied to ensure that, with a minimum of members, the existing forces are transmitted under the best possible conditions and so as to minimise secondary forces. 55.4. Nodes in triangular structures In the case of triangular structures, compliance with the above condition shall be facilitated if the axes of the bars to be joined in a node come together in a point and when the angle formed by contiguous bars is between 30º and 150º. If both conditions are met, it may be assumed that the bars are joined in the node. This assumption may not be made if the second condition is not met, or if any loads are applied to intermediate points of the bar other than its own weight or the direct action of the wind, or if the ratio of span to height of the structure is less than 6. If the first condition is not met, the corresponding eccentricity shall be taken into account in the calculation, notwithstanding the provisions of Section 64 for certain joints between tubular sections. TITLE 5 page 1 55.5. Splices Splices are extension joints of bars or profiles of the same or very similar section. Splices shall only be permitted if included in the design or workshop drawings duly approved by Project Management. 55.6. On-site joints The number of joints made on site shall be reduced to a minimum. For this purpose, it is advisable for the Designer and Constructor to work together to resolve transport and assembly issues that such a reduction may cause. It is advisable to follow best building practice of designing joints to be made on site bolted together. 55.7. Hybrid connections Hybrid joints are joints in which two or more different joining means (welding or bolting) are used together to transmit a given force between two separate parts. This does not include the transmission of a given force from one part to another using a given joining means, and from this second part to a third part using a different means. Section 56. Determination of forces in joints and distribution between components 56.1. Forces in joints The forces applied to joints are determined by means of a global analysis of the structure, carried out in accordance with the provisions of Chapter II (Project Bases) and Chapter V (Structural Analysis) of this Code. This global analysis shall take specific account of secondary effects and structural imperfections, where relevant, and the flexibility of joints in all cases. Joints shall be dimensioned to withstand at least the forces applied to them, calculated as indicated above. Under no circumstances shall the forces NEd, MEd and VEd be less than: Half of the plastic axial force of the part section, NEd = 1/2 Np = 0.5Afy in parts subjected predominantly to axial forces, such as columns, ties and lattice parts. Half of the elastic moment of the part section, MEd = 1/2 Mel = 0.5 Wefy and one third of its plastic shear force, VEd = 1/3 Vp ≈ 0.2Awfy in internal points of flexed parts. If the joint is located less than two heights away from the anticipated location of a plastic hinge, half of the elastic moment Mel shall be replaced with the full plastic moment, MEd = Mpl = 2Syfy, subject to detailed study. One third of the plastic shear force of the part section VEd = 1/3Vp ≈ 0.2Awfy in articulated ends of flexed parts. Unless said forces have been determined precisely and cannot be increased by introducing new members in the construction or by the presence of members not taken TITLE 5 page 2 into consideration, it is advisable to dimension the joints for the maximum forces that the parts can transmit, depending on the working method to be used on them. 56.2. Stress distribution The distribution of forces between the different elements of the joint, based on an elastic linear analysis of the joint, shall be permitted in all cases. Alternatively, where expressly authorised in this Code and on the basis of the principle of minimum plasticity, distributions based on non-linear analyses shall be permitted. Any distribution of forces between the different members of the joint that meets the following conditions shall be accepted as correct: The sum of the forces and moments assumed for each of the different elements of the joint are in equilibrium with the external forces applied to it. Each element of the joint is able to withstand the forces allocated to it in the distribution. Each element of the joint has sufficient deformation capacity to make the proposed distribution physically possible. The distribution of forces should be realistic with regard to relative stiffnesses with the joints elements involved, preferably transmitted through the joint via the most rigid areas. If the fracture line method is used to justify a particular distribution, it must be checked by testing. The exceptions expressly given in this Code shall apply. Stress distribution may not be made using plastic methods in the cases indicated in section 58.10. Section 57. Classification of joints subjected to bending moment 57.1. General This section looks at joints between two parts (such as beam-column joints or beam splices) that are essentially intended to transmit bending moments. 57.2. Moment-rotation characteristic A joint intended to transmit moments is defined once the relation between the moment applied to it and the relative rotation it permits between the parts to be joined is known. The graphical representation of this relation is known as a moment-rotation characteristic. In general, this diagram has a linear branch, Figure 57.2.a (1), a plastification start zone, Figure 57.2.a (2), and a major deformation branch, Figure 57.2.a (3). TITLE 5 page 3 Figure 57.2.a Design Moment-rotation characteristic These characteristics can be obtained by testing or using numerical methods that consider the non-linearity of the different materials, structural steel, weld metal, bolts, etc., used in the joint. For calculation purposes, they may be replaced by simplified, two- or three-line diagrams obtained from the real diagram, with the sole condition that all of its points be below it, Figure 57.2.b. Real Actual Simplificado Simplified Figure 57.2.b Simplified moment-rotation characteristic A simplified diagram is defined by three parameters, Figure 57.2.b: Moment resistance MRd, determined by the highest y-axis value of the diagram. Rotation capacity, ΦCd, determined by the highest x-axis value of the diagram. Rotational stiffness, Sj, depending on the linear branch passing the origin. 57.3. Classification by strength Joints are classified as follows, depending on their strength relative to the strength of the parts to be joined: Nominally pinned joints are joints that should be incapable of transmitting significant moments (greater than 25 % of the plastic moment of the parts to be joined), which might adversely affect the performance of individual parts of the structure. They should be capable of accepting the rotations provided for in the global analysis. TITLE 5 page 4 Full strength joints, whose ultimate moment is equal to or greater than that of the parts to be joined, MRd ≥ Mpl.Rd. Partial-strength joints are joints that do not meet the criteria for nominally pinned or full-strength. Their ultimate moment may not be less than that determined in the analysis, MRd ≥ MEd. In any case, the rotation capacity of the joint shall be such as not to limit the formation of the plastic hinges provided for in the analyses. The rotation capacity of a joint must be demonstrated experimentally or by numerical means that take account of the non-linearity of the performance of the materials and elements involved, except where this Code provides simplified methods for calculating it, as in Section 62. Specifically, if the ultimate moment of a full-strength connection is at least 20 % greater than the plastic moment of the largest part being joined, MRd ≥ 1.2Mpl.Rd the rotation capacity need not be checked, this being deemed sufficient. 57.4. Classification by stiffness Joints are classified as follows, depending on their rotational stiffness Nominally pinned joints which are joints whose stiffness meets the following condition: El S b j,i n i 2L b where Ib and Lb are the second moment of area and the length of the connected beam. Rigid joints or abutments are joints whose deformation has no significant influence on the laws of global forces of the structure or its general deformability. Joints whose initial stiffness Sj,ini in their moment-rotation characteristic complies with the condition below shall be placed in this category: E l S k b j,i n i L b where k shall be 8 if the beam is part of a non-translational portal frame, or 25 if it is part of a translational portal frame.