Conceptual Design of Buildings (Course unit code 1C2)

Transcription

1 (Course unit code 1C2) Module C Design of Steel Members J.P. Jaspart (University of Liège) CZ-ERA MUNDUS-EMMC

2 Bolts are the main type of fasteners used in steel joints. The main geometrical and mechanical properties (in accordance with EC3-1-8 and EN ) are described in the following tables. Bolt diameter Tensile area A s (mm 2 ) Nominal clearance d 0 M d + 1 mm M M d + 2 mm M M M d + 3 mm M A s d L17.. Execution procedures. 2

3 Bolts for non-preloaded joints manufactured in accordance with standard EN Bolts for preloaded joints manufactured in accordance with standard EN Eurocode 3, part 1.8 provides also design rules for other types of fasteners: rivets, injection bolts, anchor bolts, pins, flow drill bolts, HRC bolts, among others Rivets Injection bolts Flow drill bolts L17.. Execution procedures. 3

4 Bolt behaviour - non-preloaded applications Bolt in shear Bolt in tension Bolts in shear the resistance is governed by shear in the bolt or bearing of the bolt or the plate, whichever is weaker. Bolts in tension the resistance is governed by tension in the bolt or punching of the plate. L17.. Execution procedures. 4

5 Bolt behaviour - preloaded applications Slip resistant joint - resistance result from the friction between connected plates, clamped with a force equal to the bolt preload. P B Preloaded joints provide high stiffness and high fatigue resistance. That s why they are used in bridges and other structures subjected to cyclic loading. These joints are more expensive as they require high strength bolts (8.8 or 10.9) and more stringent execution processes, in particular to control the two key parameters - tightening methods and surface treatment. L17.. Execution procedures. 5

6 Bolt behaviour - preloaded applications Bolts in tension the stiffness is increased but the ultimate resistance is the same as in a non-preloaded bolt. T b (preloaded) T b (non-preloaded) K B N 1 PB PB PB 1 KC K K B C k B /k C = 5 to 10 % N in current applications L17.. Execution procedures. 6

7 Positioning of holes for bolts or rivets (in accordance with clause 3.5 of EC3-1-8) to prevent: - corrosion; - local buckling (in compression zones); - easy installation. e, d 1 e e1, e2 40mm 4 t p d0 p d 0 p p min 14 t, 200mm 1, 2 L17.. Execution procedures. 7

8 Shear resistance of a bolt (per shear plane) Single shear L17.. Execution procedures. Double shear 8

9 Bearing resistance L17.. Execution procedures. 9

10 Block shear resistance L17.. Execution procedures. 10

11 Tension resistance of a bolt - Tension resistance: - Punching resistance: F t. Rd B p. Rd 0.9 f ub M 2 A s 0.6 d f ub and f u are the ultimate tensile strength of the bolt and the plate, respectively; A s is the tensile area; d m is the average diameter of the bolt head or the nut; t p is the thickness of the plate under the bolt head or the nut. m M 2 t p f u Bolts in combined shear and tension F F v. Rd L17.. Execution procedures. Ft. Ed 1.4 Ft. v. Ed Rd

12 Slip resistance of a preloaded bolt k n s F s, Rd Fp, C M 3 F p, C 0. 7 f ub A s k s n = 1.0 for bolts in holes with nominal clearance; for other holes see clause 3.6 of EC3-1-8; is the slip factor (with values between 0.5 and 0.2 for classes A to D, in accordance with Table 7 of EC3-1-8 and EN ); is the number of shear planes; F p,c is the preloading force; M3 = 1.25 is the partial safety factor. Slip resistance of a preloaded bolt, reduced due a tension force F t,ed F s. Rd L17.. Execution procedures. k s n F p. C M F t. Ed 12

13 0.5 F Ed 0.5 F Ed L17.. Execution procedures. F Ed FEd MEd b 4c 2a 8 13

14 L17.. Execution procedures. 14

15 Bolts force distribution Elastic analysis of groups of bolts in shear P Example 1) In a bolt group with n bolts and the transferred force P applied in the centroid of the group, the acting force on each bolt is given by: P F i n (if all bolts have the same cross section area) F i P Ai A i (different bolts with cross section area A i ) Example 2) If the transferred force P is applied with an eccentricity e, the bolts are subjected to additional forces given by: P where M = P.e. F i ri M 2 r i L17.. Execution procedures. 15

16 Bolts force distribution Long joints Where the distance L j between the centres of the end fasteners in a joint is more than 15 d, the design shear resistance F v,rd of all the bolts should be reduced by multiplying it by a reduction factor β Lf, given by: Examples of elastic and plastic distribution of bolt forces F e.h.d 0.5 F v.h.d F v.ed F v.ed 0.5 F v.h.d M Ed F v.ed V Ed M Ed F v.h.d F v.v.e.d. V Ed F v.ed V Ed F F v.h.d =M Ed /5p F v.v.d =V Ed /5 F F F M 6 p. v Ed Ed v. h Ed. v. v. v Ed Ed L17.. Execution procedures. 16

17 Tightening of non-preloaded bolts (in accordance with EN ) i) Each bolt assembly shall be tightened at least to a snug-tight condition, with special care being given to avoid over-tightening. ii) The tightening process shall be carried out from bolt to bolt of the group, starting from the most rigid part of the connection and moving progressively towards the least rigid part. To achieve a uniform snug-tight condition, more than one cycle of tightening may be necessary. NOTE 1: The most rigid part of a cover plate connection of an I section is commonly in the middle of the connection bolt group. The most rigid parts of end plate connections of I sections are usually beside the flanges. NOTE 2: The term "snug-tight" can generally be taken as that achievable by the effort of one man using a normal sized spanner without an extension arm, and can be set as the point at which a percussion wrench starts hammering. NOTE (of EC3-1-8): If preload is not explicitly used in the design calculations for slip resistances but is required for execution purposes or as a quality measure (e.g. for durability) then the level of preload can be specified in the National Annex. L17.. Execution procedures. 17

18 Methods of bolt preloading (in accordance with EN ) i) Direct tension indicator method the preload is achieved when the gap measured with a feeler gauge on the indicating washers reach the limits established in EN ii) Torque method preload is applied with a torque wrench. The relation between the applied torque M p and the preloaded installed in the bolt (F p,c )is given by: M P k d F p, C, where d is the bolt diameter and k is a parameter supplied by the bolt manufacturer or calibrated by tests. L17.. Execution procedures. 18

19 Methods of bolt preloading (in accordance with EN ) iii) Combined method comprises: - a first tightening torque using a torque wrench; - a second tightening step in which a specified nut rotation is applied. iv) Bolts HRC the bolt is tightened using a specific shear wrench with two co-axial sockets, which react by torque one against the other, causing the failure of the spline end when the required bolt preloading is attained. L17.. Execution procedures. Tightening system 19

20 general execution procedures (EN ) Bolts and nuts shall not be welded. For non-preloaded bolts, at least one full thread shall remain clear between the bearing surface of the nut and the unthreaded part of the shank. For preloaded bolts according to EN , EN and EN , at least four full threads shall remain clear between the bearing surface of the nut and the unthreaded part of the shank. Nuts shall run freely on their partnering bolt, which is easily checked during hand assembly. Nuts shall be assembled so that their designation markings are visible for inspection after assembly. Generally washers are not required for use with non-preloaded bolts in normal round holes. If required (to reduce local damage to metal coatings), it shall be specified whether washers are to be placed under the nut or the bolt head, whichever is rotated, or both. L17.. Execution procedures. 20

21 general execution procedures (EN ) For single lap connections with only one bolt row, washers are required under both bolt head and the nut. Plain washers shall be used for preloaded bolts as follows: - for 8.8 bolts a washer shall be used under the bolt head or the nut, whichever is to be rotated; - for 10.9 bolts washers shall be used under both the bolt head and the nut. Washers used under heads of preloaded bolts shall be chamfered according to EN and positioned with the chamfer towards the bolt head. Washers according to EN shall only be used under nuts. Taper washers shall be used if the surface of the constituent product is not perpendicular to the bolt axis. L17.. Execution procedures. 21

22 - The process of joining together two pieces of metal so that bonding accompanied by appreciable interatomic penetration takes place at their original boundary surfaces. is carried out by the use of heat or pressure or both and with or without added metal. Advantages: Disadvantages: simplicity; execution and control processes more higher stiffness; stringent; better appearance. fracture and fatigue problems. processes (covered electrode and submerged arc) Other welding processes flux-cored welding, MIG (metal inert gas) welding, MAG (metal active gas) welding. L17.. Execution procedures. 22

23 Types of welds: fillet weds, butt welds, fillet welds all around; plug welds, among others. Fillet welds Butt welds Throat thickness of a weld a height of the largest triangle that can be inscribed within the fusion faces and the weld surface, measured perpendicular to the outer side of this triangle. L17.. Execution procedures. 23

24 Some execution disposals and defects (clause 4 of EC3-1-8): i) Pieces connected by fillet welds should form an angle between 60º and 120º. ii) The effective throat thickness of a fillet weld should not be less than 3 mm. iii) Weld may be continuous or discontinuous. iv) Eccentricities should be avoid. v) Residual stresses should be reduced (ex. pre-heating). vi) Defects should be avoid, such as: cracks, lack of fusion, lack of penetration, porosities, etc, Cracks Eccentricities Porosities Lack of fusion Lack of penetration Geometrical defects L17.. Execution procedures. 24

25 Control of quality of welding: Destructive processes microscopic examination, hardness and tensile tests; Non-destructive processes (NDT) visual analysis, liquid penetrant testing, magnetic particles inspection, radiographic testing and ultrasonic testing. Liquid penetrant test Tensile test L17.. Execution procedures. Ultrasonic testing 25

26 Analysis and design of fillet welds Geometrical dimensions of a fillet welds: a - throat thickness; l w - fillet weld length. Directional method (clause of EC3-1-8) the forces transmitted by a unit length of weld are resolved into components parallel and perpendicular to the longitudinal axis, along the throat plane. Simplified method (clause of EC3-1-8) (conservative method) the design resistance of a fillet weld may be assumed to be adequate if, at every point along its length the resultant of all forces per unit length transmitted by the weld (F w,ed ) satisfy the following criteria: F W.Ed < F w.rd where F w,rd is the design weld resistance per unit length. L17.. Execution procedures. 26

27 Analysis and design of fillet welds Directional method: f u 2 M 2 2 // w f u M 2 w is a correlation factor given in Table 4.1 of EC Simplified method: F W.Ed < F w.rd F w. Ed F 2 1 F L F 2 3 Throat plane F w. Rd fu w 3 M 2 a L17.. Execution procedures. 27

28 Analysis and design of fillet welds EXAMPLE: Simplified method F w. Ed NEd fu 3 Fw. Rd 2 L W w M 2 a N Ed N Ed 1.15 w fu M 2 a L W Directional method 2 2 a LW 2 a L L17.. Execution procedures. W 2 2 N Ed N Ed 2 2 N Ed a L w W fu 1.41 M 2 a L W 28

29 Analysis and design of butt welds and fillet welds - The design resistance of a full penetration butt weld should be taken as equal to the design resistance of the weaker of the parts connected. - In a lap long joint (L w > 150 a), the design resistance of a fillet weld should be reduced by multiplying it by a reduction factor Lw, given by the following expression, to allow for the effects of non-uniform distribution of stress along its length. Lw L W 150 a L w L17.. Execution procedures. 29

30 This lecture was prepared for the Edition 1 of SUSCOS (2012/14) by RUI SIMÕES (UC) and FLOREA DINU (UPT). Adaptations brought by J.P. Jaspart (Ulg) for Edition 2 of SUSCOS The SUSCOS powerpoints are covered by copyright and are for the exclusive use by the SUSCOS teachers in the framework of this Erasmus Mundus Master. They may be improved by the various teachers throughout the different editions.

31 Thank you for your attention