JOINTS IN CONSTRUCTIONAL STEELWORK, RECENT DEVELOPMENTS IN THE NETHERLANDS S U M M A R Y

Transcription

1 Martin STEENHUIS( 1 ) and Nol GRESNIGT ( 2 ) JOINTS IN CONSTRUCTIONAL STEELWORK, RECENT DEVELOPMENTS IN THE NETHERLANDS S U M M A R Y Recently, two reports about the design of joints in structural steelwork were published in The Netherlands. These reports focus on a rational treatment of the design of joints, leading to more economical forms of construction. This paper will pay attention to both publications, which deal with nominally pinned joints and moment connections. Both reports provide additional guidance to Eurocode 3. Published in the Proceedings of the 8th MASE International Symposium, ed. by T. Nikolovski (1999) pp 18/1-18/11. 1 Department of Structural Design, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands, Phone , Fax , c.m.steenhuis@bwk.tue.nl 2 Faculty of Civil Engineering and Geosciences, Delft University of Technology, P.O. Box 5048, 2600 GA Delft, Phone , Fax , a.m.gresnigt@ct.tudelft.nl

2 I N T R O D U C T I O N In The Netherlands, labour costs increase substantially every year, whereas material costs remain more or less constant. This is especially the case in the steel construction industry. Until recently, however, detailing of connections in structural steelwork was very much based on traditional practice, aiming at minimising material consumption rather than reducing overall costs. Consequently, the detailing of the joints is often too complex. As an alternative, the size of members can be increased, and joints simplified (e.g. expensive stiffeners could be avoided). The Technical Committee 10a (SG/TC10a) of the Dutch Staalbouwkundig Genootschap (association for constructional steelwork), produced two reports about the design of steel joints. In these reports, attention is paid to economical forms of construction. The committee felt that standards like Eurocode 3 [1], [2] provide a good basis for determination of mechanical properties of steel joints. However, for economic design, attention should be paid to the conceptual design of the joints, i.e., the choice of the layout of the joints in relation to the choice of the members. In the meantime, two reports have been published: a report on nominally pinned connections, also referred as simple connections [3] and a report on moment connections [4]. The difference is that simple connections usually can be assumed as nominally pinned connections and thus transferring only shear and normal forces, whereas moment connections also transfer significant bending moments. In The Netherlands, traditionally, two parties are responsible for the design of steel frames: the engineer designs the beams and columns and the steel fabricator designs the connections. In this design practice, the engineer specifies the mechanical requirements of joints. The steel fabricator designs the joints to fulfil these requirements. The fabricator also considers manufacturing aspects. In the reports prepared by the Staalbouwkundig Genootschap, this sharing of the design task between engineer and steel fabricator has been taken as a starting point. Since steel fabricators usually designs simple connections, they are supposed to be the readers of the report on simple connections. For moment connections, the situation is different, since the choice of the members has an influence on the layout of the joints. In that case, also guidance is given to the engineer in making the conceptual decisions on joints. S I M P L E C O N N E C T I O N S The development of a design guide for simple connections in The Netherlands is not a unique event. Also elsewhere in the world, attention is paid to economic design of simple connections. Design guides [5] - [9] are developed to facilitate the fabrication and erection simple connections. Most of these design guides are based on the following basic points: using bolting rather than welding on the erection site. On the erection site, installation of bolts is faster and better under control than welding; a preference for snug tightened bolts rather than preloaded bolts in predominantly static loaded structures. Tightening and checking procedures of preloaded bolts are time consuming during erection and thus expensive;

3 standardisation of bolts (diameter and strength). With bolt standardisation, less bolts need to be kept in stock, machines can be more productive since less time is needed to change drills, quantum savings can be gained from the bolt manufacturer and there is less risk of faulty installed bolts; the use of fully threaded bolts. With the use of fully threaded bolts, the number of different bolts lengths can be reduced. This means less different bolts in stock and on the erection site, see Table 1 for an example. Fully threaded bolts show good ductility and only slightly less tensile capacity than bolts with short thread lengths [10]. Table 1 shows preferred bolt lengths of fully threaded bolts. Table 1: Preferred bolt lengths of fully threaded bolts (mm) [1]. M16 M20 M standardisation of plates and angle cleats (geometrical properties and strength). In this case, less plate and cleat materials need to be kept in stock. There is a tendency toward adopting steels with higher strengths (European steel grade S355, yield strength of 355 MPa) as standard plate and cleat material. This material is slightly more expensive than lower grade standard structural steel, but in general, design will result in smaller plate thickness, requiring less welding or drilling; rationalisation of dimensions to improve design capacities. With proper dimensions of plates, welds, bolts and bolt spacing, connections can show ductile behaviour and at the same time show good strength characteristics; rationalisation of beam and column elements [5]. In the workshop it is preferred to have only welding or drilling instead of welding and drilling on one single element. This saves internal transportation of the element through the workshop. Therefore, the choice of the connection depends also on other connections at the same element. For example, if a primary beam connected to a column needs to be drilled, secondary beams may be connected with partial depth end plates or angle cleats. Figure 1 shows some detailing to be preferred and other details to be avoided. Preferred: Only sawing and drilling Preferred: Only sawing and welding Preferred: Only cutting and drilling Preferred: Only cutting and welding Avoid: Cutting and drilling and welding Avoid: Cutting and drilling and welding Figure 1: Rationalisation of beam elements.

4 a preference for connections that facilitate easy erection on site. Connections should allow for some tolerances. For example, to correct out of plumb columns due to variations in column depth, in case of connections with end plates, filler plates should be adopted. These are additional loose elements during erection. A better solution may be cleated connections and fin plate connections, which allow for a tolerance of twice the bolt hole clearance (+ 4 mm). Thus, small corrections can be made without special measures. These basic points have also been taken into consideration when developing the report on simple connections. Furthermore, a system of bolt spacing was adopted, which allows a steel fabricator to develop his own standardisation system in the workshop based on the concept of "compact", "moderate" and "wide" bolt spacing. As bolt diameters, only M16, M20 and M24 were recommended. Table 2: Recommended bolt spacing. bolt spacing M16 M20 M24 compact edge distance e 25 mm 30 mm 35 mm pitch s 45 mm 55 mm 65 mm moderate edge distance e 40 mm 45 mm 55 mm pitch s 55 mm 70 mm 80 mm wide edge distance e 55 mm 70 mm 80 mm pitch s 70 mm 85 mm 100 mm The advantage of this bolt-spacing system is that calculations to determine the resistance of the joint according to Eurocode 3 are simplified. For instance, a short end plate beam-to-column joint should be checked for the following failure modes: bearing of the end plate, shear of the beam web, bearing of the connected column flange and shear of the bolts. For each failure mode, tables are given in the report. These tables can be used as follows: assume a 10 mm end plate with ten M20 bolts, "moderate" bolt spacing in steel grade S235. In that case, the bearing capacity of the end plate is equal to 89,9 t = 899 kn, where t is equal to the thickness of the plate in mm. The value 89,9 t can be simply found in a table, see Figure 2.

5 Figure 2: Resistance end plates, taken from [1]. With the help of these tables, the design and selection of simple joints has been simplified. The report on simple joints has now been available in The Netherlands for one year and the first reactions from practice are positive. M O M E N T C O N N E C T IO N S Advantages Traditionally, moment connections in steel frames are treated as rigid. Typical examples of rigid joints are stiffened fully welded joints, (stiffened) haunched joints with full depth end plate etc., see Figure 3a. a) rigid joints b) semi rigid - partial strength joints Figure 3: Typical moment connections. There is another class of joints: unstiffened moment connections. They are rarely applied in practice, but research has demonstrated their potential economical benefit. The behaviour of unstiffened moment connections is usually such that the mechanical properties (strength, stiffness and rotational capacity) should be taken into account when performing the design and analysis of the frame. This kind of joints is also referred to as semi rigid - partial strength joints, see Figure 3b.

6 How economical benefit can be achieved from the application of semi rigid / partial strength joints is indicated in Figure 4 and Figure 5. In Figure 4, two alternatives are given for a beam to column connection in a braced frame. IPE 550 IPE 500 IPE 450 IPE 400 HE 220 A HE 220 A Figure 4: Alternatives for joints in a braced frame. Left: nominally pinned. Right: partial strength. HEB 340 HEB 400 Figure 5: Alternatives for joints in an unbraced frame. Left: rigid. Right: semi rigid. The joint on the left-hand side consists of traditional double angle cleats. The design criterion for the beam IPE 550 is that the section should be able to resist the sagging moment (for a uniformly distributed load q: M = 1/8 q l 2, where l is the beam span). The connection is assumed to transfer shear only. The joint on the right-hand side consists of a full depth end plated connection. In this connection, a hogging moment can develop. Usually, for full depth end plated or extended endplated connections, the moment capacity of the joint is about 15-30% of the resistance of the beam (this is also referred to as partial strength ). In other words, it is possible to decrease the beam section from an IPE 550 to an IPE500 because the vertical loading moment will be partly transferred through the joint and not resisted only by the mid section. A second effect is that deflections of the beam in the right-hand solution will decrease due to the restraining effect of the connections compared to the pinned solution. Economical benefit will now be gained because of the savings in material and savings due to reduced construction height will in general be more than the extra production costs for an end plate connection compared to angle cleats. Cost savings are reported of up to 13% of the bare frame costs [11]. Figure 5 shows two alternatives for knee joints in a two-storey unbraced frame. The connection on the left-hand side is a traditional rigid connection. To be sufficiently rigid, stiffening is required. The connection on the right-hand side consists of an unstiffened connection. To fulfil requirements like overall frame stability, the column size is increased from an HEB 340 to an HEB 400. However, it appeared that the beam size could reduce from an IPE 450 to an IPE 400. In the case of the unstiffened connection, the flexibility of the connection should be taken into account in the frame analysis. When this is the case, the joint is referred to as semi rigid. Economical benefits will be gained from savings in fabrication costs which are higher than the increase of material costs (in the right-hand side solution, there is no need for the fabrication of a haunch). Cost savings are reported of up to 30% of the bare frame costs [11]. Design process As already stated, the design of joints in The Netherlands is usually performed by two different parties, the engineer and the steel fabricator. A typical flow of the design process with rigid joints is given in Figure 6. With this process the following problems may arise:

7 if an engineer decides to apply a rigid joint, the joints may appear to be uneconomical during the design of the joints. This is the last step in the design process and then it is impossible to change member sizes; if an engineer would like to apply a semi rigid joint, the joint characteristics need to be introduced in the frame analysis. These joint characteristics, however, are based on the layout of the joint, which usually is assessed by the steel fabricator during the last step of the design process. This is conflicting. Design guidance for the engineer The report on moment connections published by the Staalbouwkundig Genootschap provides tools to help a structural engineer during the conceptual design of the joint. First, if the engineer decides to apply rigid joints, design graphs are given in the report to have an impression of the layout of the joint. The following example explains the use of these design graphs. STRUCTURAL CONCEPTION Frame (Geometry, Member types etc.) Joints (Rigid) ESTIMATION of loads PRE-DESIGN Choice and classification of members GLOBAL ANALYSIS (Continuous) STRUCTURAL RESPONSE Limit States (ULS, SLS) Design criteria (Sway/non Sway, Elastic) no Limit States & Design Criteria OK? yes Engineer DESIGN OF JOINTS Type of joint (Rigidity Strength) Tables, softwares Fabricator no, other members only in rare cases! Design of Joints OK? yes no, other type of joint STOP Figure 6: Flow of a design process with rigid joints.

8 Assume in an unbraced frame, an IPE 500 beam connected to a HE 340 B column. The beam span is 7200 mm. The length / breadth ratio of the IPE 500 beam is equal to: l b / b b = 7200 / 200 = 36. The ratio between the IPE 500 beam flange thickness and the HE 340 B column flange thickness is: t f;b / t f;c = 16 / 21.5 = 0,75. Then, the selection of the layout of rigid joints is as follows. A line is drawn through t f;b / t f;c = 0,75 en l b / b b = 36 (unbraced), see Figure 7. The cross point between this line and the upper axis indicates the boundary for rigid joints. All joints left from this point may be considered as rigid, the others as semi rigid. For backgrounds, refer to [3]. Rigid Semi-rigid Unbraced lb bb tf;b tf;c Figure 7: Selection of rigid joints, after [3]. When an engineer feels unsatisfied about the outcome to the conceptual design carried out with the aid of the design graphs of Figure 7, he might wish to choose an unstiffened semi rigid joint. The report on moment connections provides a simple table in which a "good guess" can be made of the expected initial stiffness of the semi rigid joint based on the joint concept only, see Table 3. This "good guess" of the initial stiffness should be used in the frame analysis. Table 3 was published before, for instance in [12], [13]. In the table, S j is the initial stiffness of the joint, E is the young modules of steel, z is the lever arm of the joint and t f,c is the column flange thickness. From the table it can be seen that the joint layout is an important parameter to determine the stiffness value. Generally, the "good guess" value of the joint, determined with Table 3, is close to the stiffness determined with Eurocode 3, so no additional frame analyses are required after the joints are designed by the steel fabricator. Design guidance for the steel fabricator

9 The design of a moment connection is a complex task. The joint must fulfil several requirements. It has to be sufficiently strong, stiff, ductile and economical. In addition there are requirements for fire safety, corrosion, erection, etc. Computer programs to determine the resistance, stiffness and rotation capacity of joints are an important design aid for a designer. These programs require the layout of the joints as input and produce a moment-rotation curve as output.

10 Table 3: Good guess of joint stiffness. Configuration Extended end-plate, single sided and unstiffened S j 13 Extended end-plates, double sided, unstiffened and symmetrical Extended end-plate, single sided, stiffened in tension and compression Extended end-plates, double sided and stiffened in tension and compression, symmetrical Flush end-plate, single sided 7,5 8, Flush end-plates, double sided, symmetrical 9,5 Flush end-plate, single sided and cover plate 11,5 Flush end-plates, double sided, cover plate and symmetrical 6 Welded joint, single sided and unstiffened 11,5 Welded joints, double sided unstiffened and symmetrical E z 2 t fc 6 Usually, the assessment of the layout of the joint is not automated. The designer tries a typical layout and then determines the mechanical properties. If the design is not satisfactory, it is modified and again the mechanical properties are determined with a computer program. Typical modifications are the change of bolt diameter, the bolt positions and the addition of a haunch or stiffeners. This process is repeated until a layout has been found which fulfils all requirements.

11 In the Dutch report on moment connections, a series of tables is given aiming at providing aid in this design process of joints. The tables suggest alternatives in case the mechanical properties of a joint are unsatisfactory. An example is given in Table 4. This table can be used when for instance the column web in tension, the column flange in bending or the end plate in bending are governing the joint behaviour. Table 4: Design table for bolts in end plated joints, after [4]. Visualisation Location of the bolts weak flexible Smaller bolts more end plate bending Second bolt row strong stiff less end plate bending Remarks The strength and stiffness behaviour of an end plate connection can be positively affected by placing the bolts closer to the beam web, beam flange, column web and if present column web stiffeners. Smaller bolts can be placed more close to the beam web, beam flange, column web or column web stiffeners. In that case, less end plate bending occurs Smaller bolts should be applied only in case the bolts are sufficiently strong to ensure ductile behaviour of the connection. A second bolt row placed next to the top bolt row with a small pitch increases the capacity of the tension zone in the joint. weaker more flexible stronger stiffer C O N C L U S I O N S In the Dutch steel industry, the yearly increase of labour cost is more than the increase in material costs. Since the steelwork connections have an important influence on the final costs of a steel structure, the Staalbouwkundig Genootschap published two reports. One of them deals with simple connections, the other with moment connections. These reports aim at the design of joints in an economical and rational way and are complementary to Eurocode 3. The report on simple connections provides design recommendations for steel fabricators to reduce the production costs, for instance by minimising handling in the workshop. The report on moment connections gives design tools for both the engineer who designs the structure and makes a conceptual decision on joints (e.g. the joints are rigid or semi rigid) and the steel fabricator, who decides upon the final layout of the joint. Also in the latter case, the aim is minimising the production costs.

12 A C K N O W L E D G E M E N T The first author of this paper was employed at TNO Building and Construction Research during the writing of the two publications on connections by SG/TC10a. TNO is greatly acknowledged for providing the opportunity to do this project. The authors wish to thank the members of SG/TC10a for their contributions. R E F E R E N C E S [1] CEN. Design of Steel Structures Part 1.1. General rules for buildings. Eurocode 3 ENV Brussels: CEN, [2] CEN. Design of Steel Structures Part 1.1. General rules for buildings. Revised Annex J. Eurocode 3 ENV /A2. Brussels: CEN, [3] Technische Commissie 10a, Normaalkrachtverbindingen en dwarskrachtverbindingen, Rotterdam: Staalbouwkundig Genootschap, [4] Technische Commissie 10a, Momentverbindingen, Rotterdam: Staalbouwkundig Genootschap, [5] Australian Institute for Steel Construction. Standardized Structural Connections. Sidney: AISC, [6] BCSA/SCI. Joints in simple construction, volumes 1 & 2: design methods SCI-P-105, practical applications SCI-P-105. Ascot: Steel Construction Institute, [7] Henderson JE. Standardized Steel Connections. Design Manual. Willowdale Ontario: Canadian Institute Steel Construction, [8] American Institute of Steel Construction. Detailing for Steel Construction. Design Manual. Chicago: AISC, [9] Treiberg T. Design Handbook for Connections (in Swedish). Publications Stockholm: Swedish Steel Construction Institute, [10] Steurer A. Trag- und Verformbarheitverhalten von auf Zug beanspruchter Schrauben. Bericht 217. Zurich: Institut für Baustatik und Konstruktion EHT, [11] ECCS-TC10-WG2 Connections and frame design for economy. Publication 77. Brussels: ECCS, [12] European Commission of the European Communities. Frame design including joint behaviour, volume 1-3. Reference number 93-F6.05. Brussels: ECSC, [13] Steenhuis CM, Gresnigt AM & Weynand K. Pre-design in semi rigid joints in steel frames. In Wald F. (Ed) Proceedings of the 2d state of the art workshop. Prague: Czech Technical University, 1994.