Lecture07.doc

Structural Steelwork Eurocodes Development of a Trans-National ApproachFrame Design approaches Traditional and modern design approachesStructural Steelwork Eurocodes Development ofA Trans-national ApproachCourse: Eurocode 3 Module 3 : Frame design approachesLecture 7 : Traditional and modern design approachesSummary: The traditional approaches to the design of frames are concisely described: -continuous framing with rigid joints and /or simple framing with pinned joints,-the so called “wind moment” method-the “partial strength” approach-rigid-plastic design. The modern approach to frame design, i.e. semi-continuous framing using semi-rigid joints, is then outlined; how it is to be distinguished from the traditional approaches is explained and the potential benefits (scientific and economic) for its use are raised. A consistent design process is described in which joint behaviour is accounted for in the global analysis from the outset. It is shown how to modify the simple framing or continuous framing approaches to be more in line with a consistent design process. It is explained that a consistent design process can take different forms which depend on the assumptions about joint behaviour in the global analysis, who is responsible for joint design and/or the degree of collaboration between the parties (designer and fabricator). Design practices are identified which show how design responsibility is, or can be, shared. It is explained that an understanding of the sharing of design responsibility is essential in order to modify current practice so as to allow a consistent design process to be used.Pre-requisites: A knowledge of the fundamentals of the theory of resistance of materials (for beams and tension members) and of structural analysis. A knowledge of the elastic and plastic design of simple members. Module 1 for loading and definitions of limit states. Lecture 4 “ Frame idealisation and analysis”. Lecture 5 “ Frame classification and joint representation”.Notes for Tutors: This material comprises one 45 minute lecture.Objectives: The student should: Understand the different approaches to frame design, both traditional and modern. Have an appreciation of the potential benefits of using a consistent design process which best accounts for joint behaviour. Understand the consequences that the sharing of design responsibility may have on the subsequent design. Know how to put a consistent design process into practice.References: 1 Anderson, D., Reading, S.J., The Wind-moment design for unbraced frames, SCI publication P-082, 1991.2 Anderson D., Colson A., Jaspart J.-P., Connection and frame design for economy, ECCS/TC10 publication N, 1991 (also published in a number of national journals).3 Maquoi, R., Chabrolin B., Frame design including joint behaviour, Report EUR 18563 EN, ECSC/European Commission, 1998.4 ENV 1993-1-1: Design of steel structures: Part 1-1: General rules and rules for buildings.1 Traditional approaches to design1.1 Pinned-Rigid joint approachUp to now, the typically used process of designing building structures involved the following successive steps: frame modelling including the choice of rigid or pinned joints initial sizing of beams and columns then, for each ultimate limit state (ULS) and serviceability limit state (SLS) load combination:- evaluation of internal forces (load effects)- check of ULS and SLS criteria iteration, if needed, on member sizes until all checks are satisfactory. at the final satisfactory stage after the previous iterations:- design of the joints to resist the relevant members end forces in accordance with the initial assumptions (frame modelling) about the joint stiffness.The approach is given in flow chart form in Figure 1. Since the joints are considered as either pinned (simple framing- no moment assumed to be transferred) or rigid (continuous framing- rigid moment-carrying), their design becomes a separate task from that of the design of the members. It is often performed at a later stage in the overall design process by other personnel (usually the fabricator).5.2.2.25.2.2.3This approach is suited for framed structures which are classified as braced non-sway where most of the beam to column joints are not required to transmit moment. Other joints may form part of a bracing system where they would be required to be rigid and have high resistance. The use of moment-carrying joints in frames of any height or number of storeys may often provide an economic alternative to the case when bracing alone must provide lateral stability. Because floor depths are reduced, the overall volume of the building for a given floor space is less. Eliminating bracing gives an increased freedom for use and results in aesthetically pleasing designs. For medium to high multi-storey buildings, it is often necessary to have moment carrying beam to column joints so as to provide obstruction free office floor space which can be rapidly re-organised at least cost. However, assuming such joints as rigid may not be the most economic solution, and consideration should be given to the semi-rigid joint alternative.The most common example of a steel frame in which rigid moment-carrying beam to column joints are used is the one storey pithed-portal frame industrial building. While still keeping the joint in the “rigid joint” classification, design to Eurocode 3 Part 1-1 permits less costly joint designs than those usually adopted. More economic designs, as compared to common practice, should be possible.Annex J. Figure 1 Traditional design process of a steel frame for rigid and/or pinned joints.1.2 Wind moment methodIn the wind moment method 1, the beam to column joints are considered to transmit no moment for vertical loading cases but to transmit moment when wind loading is considered. It has the advantage of allowing the use of rather simple beam to column flange joints. The design process fits well into the flow chart for the traditional pinned-rigid approachWhat is interesting about this approach is that it is a way, albeit it rather simple, of accounting for the actual stiffness and for the inherent moment carrying capacity, although low, of the joint. However, it is not clear to what extent the wind moment method can be adopted within the context of the application of EC3. The joints details have to be carefully chosen for the assumptions on their behaviour to be actually valid as regards the Eurcode Part 1-1 rquirements.1.3 Partial-strength beam to column joints Although it is not a very traditional approach, like the wind moment method, it has found some application already in industry 2. This approach fits into the traditional breakdown in design tasks as shown in Figure 1. In this case, one avoids any risk of it being difficult to design a joint with the required moment resistance and stiffness. The latter is a problem which arises regularly between the fabricator and the designer for structures designed assuming rigid joints.Usually used for the design of floors of braced non-sway frames, the method involves adopting partial strength ductile beam to column joints. The beam is designed on the basis of a simple mechanism with two joint hinges and a third hinge at the mid-span of the beam. Figure 2 shows an example of the beam mechanism. Figure 2 Application of the partial-strength approachEconomic floor beams of reduced depth, compared to the beam obtained by the usual assumption of pinned joints, can be proposed. Since the joint design moments are usually chosen at less than 40% of that of the plastic moment resistance of the attached beam, economic joint types, which are usually of the bolted flush end plate type, can be used.The design method is very simple and rapid to use.The “partial strengthapproach”, like the wind moment method, may be also be considered as a particular application of the semi-rigid approach where the accent is not on flexural stiffness but on choosing joints which have moderate or low bending resistances and are ductile.1.4 Rigid-Plastic designAs this method is as yet little used in many countries, it can be called traditional to a proportion of engineers only. Nevertheless, it has been used for some time now for the design of the most common steel structure, the one-storey portal frame.5.2.1.4 This approach has been described in more detail in the module 2 “ Frame Analysis and Design”. The “partial strength joint” method for rectangular frame structures is a simple variant of it.The method can be applied to certain types of sway frames, although most would limit its use to sway frames of only one storey high. 5.2.6.3It is much used in the UK for the design of portal frame industrial buildings. In these frames, hinges do not form in the rafter at the eaves joints since haunches are used and the loading is such that the hinge in the rafter is not at the apex. Its use for other types of building seems to be rare, although the “partial strength” approach has been used for the design of multi-storey frames. The requirement that member cross-sections be of class 1 or of class 2 is probably the major hindrance to its wider application.2. The modern approach to frame designIt is now well recognised that assuming joints to be rigid or pinned may neither be accurate nor result be economical. Simply because a joint has sufficient strength does not mean it has sufficient stiffness for it to be reasonable to model it as rigid. Many joints, often assumed to be rigid exhibit an intermediate behaviour between the rigid and pinned states. Eurocode 3 Part 1-1 has taken this fact into account and in doing so opened the way to what is now known as the semi-rigid approach (see Figure 3).5.2.2.4.In the semi-rigid approach, the behaviour of the joints is taken into account at the outset, i.e. when the components are sized at the preliminary design stage, the sizing takes account of the joint behaviour also. The initial global analysis includes an approximate estimate of the joint characteristics (stiffness, strength and rotation capacity), and which can be refined later, as one does for the member sizes, in the final analysis. The joint is usually represented as a rotational spring at the extremity of the member (usually the beam) which characterises the joint behaviour. Available models can represent the moment-rotation characteristic only, which is sufficient for the majority of structural joints in frames. Annex JModern computer methods of design and the equipment on which they are installed are particularly suited to the approach since, with such tools, repeating an analysis on a modified structure is very rapid. When a realistic model of the joints is included in the global analysis, in addition to providing a better appreciation of the structural behaviour, there is reason to believe that more economical design can be achieved in many typical structures 2. A significant part of the cost of a steel structure is related to the fabrication, of which the joint fabrication itself forms a considerable part. Without too much simplification, one can say that the greater the moment a joint must carry the greater the costs of materials, fabrication and erection man-hours. Since semi-rigid design allows lower joint moments, it can readily lead to cost savings which may outweigh extra costs incurred by increased member sizes, if any, to avoid excessive deflections. Figure 3 Procedure for analysis which is consistent with joint responseHowever, since the approach no longer allows joints to be designed separately, it will require a change in the manner of sharing the responsibility for design between the parties. Often, joint design is carried out by the steel fabricator on the basis of a structure previously designed and analysed by a consulting engineer. While improved communication with, and early involvement of, the fabricator at the design stage is essential, the use of the “good guess” approach can be a useful tool for joint stiffness estimation to guide the designer towards a satisfactory design (see reference 3 and lecture 8 “Practical application of modern design approaches”).3 Consistent design processThe design procedure for frames using a global analysis which is consistent with the joint response ( Figure 3) is somewhat different from the traditional procedure as given in Figure 1. The main differences are as follows: Structural conception: The mechanical behaviour of the joints is modelled in the global analysis. Preliminary design: In the pre-design phase the designer chooses a joint based on his or her experience. This choice provides the basic dimensions for the joint components: end-plates or cleats, location of bolts, number and diameter of bolts, sizes of column and beam flanges, thickness and depth of column web etc. Determination of the mechanical properties: In step 4 of Figure 3, the complete mechanical response of the selected members and joints is determined. The joint characterisation is usually simplified to a linear or a bi-linear response.It is evident that this procedure is only applicable when both members and joints are designed by one party, since the mechanical properties of the joint are included in the frame analysis.Since in this procedure one can consider any type of joint behaviour, it is applicable to when one wishes to use so called semi-rigid joints. In fact, almost all typical joints used in frame structures are semi-rigid to some degree, although some may be considered as pinned and others considered as rigid. It is recalled that the term “semi-rigid” relates to the stiffness of a joint. In the case of rigid joints (continuous framing), since the stiffness of the joint is assumed to be infinite, the global analysis of the frame will indicate that the beams and the columns have the same rotations at the interconnecting nodes.In the case of semi-rigid joints (semi-continuous framing), the beams and columns will have different rotations at the interconnecting nodes (see lecture 5 “Frame classification and joint representation”). It is recalled that, when using the spring model for the joint in the global analysis, the difference obtained between the rotation of a column and that of a beam connected to it is equal to the rotation in the joint connecting them.5.2.2.35.2.2.4The procedure given in Figure 3 can also be applied when the joints are taken as pinned (simple framing) or rigid (continuous framing). In the case of rigid and/or pinned joints, the inclusion of joint properties in the global frame analysis is obviously simpler than in the case of semi-rigid joints, because there is no need to determine the joint characteristics for the modelling.5.2.2.23.1 Intermediate forms of approachesThe two design approaches given above, i.e. the procedure for pinned or rigid joints and the procedure for semi-rigid joints respectively, are actually two extremes. Intermediate forms of the approaches can be used. For example, the procedure given in Figure 1 can also be applied for semi-rigid design. In that case, during the first pass through the design process, the joints are assumed to behave as rigid or pinned. Then, during the second pass (after step 8) the “real” joint properties are included in the global frame analysis. The frame analysis is performed with account being taken of the “real” properties of all the components, i.e. of the beams and column but also of the connections. The design process then becomes similar to that indicated in Figure 3.4 Design practice and its implicationsIn practice, the design activity can be performed by one or by two parties: by a combination of an engineering office (engineer) and a steel fabricator (fabricator), by an engineering office only, by a steel fabricator (fabricator/designer) only.Table 1 shows the share of responsibilities for design and fabrication for these three basic cases.RoleCase ACase B1Case B2Member design EngineerEngineerFabricatorJoint design FabricatorEngineerFabricatorFabricationFabricatorFabricatorFabricatorTable 1 Parties and roles in the design and fabrication processes of a steel structure The objective of the design process is to design a structure, which fulfils architectural requirements, is safe, serviceable and durable at minimum fabrication, erection, protection and maintenance costs. Furthermore, the parties involved in the design activity have an economic interest in limiting the costs of the design activity itself. Case AIn case A, an engineer designs the members and the steel fabricator designs the joints. In this case the engineer specifies the mechanical requirements of joints and the steel fabricator designs the joints to fulfil these requirements. The fabricator also considers manufacturing aspects. However, due to the fact that the engineer specifies the mechanical requirements of the joints, the design solution found by the fabricator may be sub-optimal. The design will depend essentially on the beam and column dimensions chosen by the engineer. The engineer may decide to choose minimum beam and column dimensions with the consequence that the joints need to be stiffened to fulfil the requirements for safety and serviceability. When the engineer decides to choose larger beam dimensions for instance, the joints may be simplified