Lecture 07.docx

1、Lecture 07Structural 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 rigi

2、d 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 traditio

3、nal 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 appro

4、aches 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 partie

5、s (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 use

6、d. 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 idealisatio

7、n 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

8、 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

9、 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 18

10、563 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

11、 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 nee

12、ded, 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

13、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

14、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

15、 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 o

16、f 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 offic

17、e 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 jo

18、ints 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 pos

19、sible.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 h

20、as 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 a

21、nd 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 v

22、alid 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

23、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 f

24、loors 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 Applica

25、tion 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, econo

26、mic 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 o

27、n 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

28、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 applie

29、d 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 loa

30、ding 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 maj

31、or 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 rea

32、sonable 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, th