9/28/ :22 PM. Chapter 9. Welding and the Design of Permanent Joints. Dr. Mohammad Suliman Abuhaiba, PE

Transcription

1 9/28/ :22 PM Chapter 9 Welding and the Design of Permanent Joints 1

2 2 9/28/ :22 PM Chapter Outline

3 3 9/28/ :22 PM Welding Symbols Welding symbol standardized by AWS Specifies details of weld on machine drawings Fig. 9 4

4 4 9/28/ :22 PM Welding Symbols Arrow side of a joint is the line, side, area, or near member to which the arrow points The side opposite the arrow side is the other side Shape of weld is shown with the symbols below Fig. 9 2

5 Welding Symbol Examples Weld leg size of 5 mm Fillet weld Both sides Intermittent and staggered 60 mm long on 200 mm centers Leg size of 5 mm On one side only (outside) Circle indicates all the way around

6 Welding Symbol Examples Fig. 9 5

7 Welding Symbol Examples Fig. 9 6

8 Welding Symbols Fig. 9 1

9 9 9/28/ :22 PM Tensile Butt Joint Simple butt joint loaded in tension or compression Stress is normal stress Throat h does not include extra reinforcement Reinforcement adds some strength for static loaded joints Reinforcement adds stress concentration and should be ground off for fatigue loaded joints Fig. 9 7a

10 10 9/28/ :22 PM Shear Butt Joint Simple butt joint loaded in shear Average shear stress Fig. 9 7b

11 11 9/28/ :22 PM Transverse Fillet Weld Joint loaded in tension Weld loading is complex Fig. 9 8 Fig. 9 9

12 Transverse Fillet Weld Summation of forces 12 9/28/ :22 PM Fig. 9 9 Law of sines Solving for throat thickness t

13 Transverse Fillet Weld Nominal stresses at angle q 13 9/28/ :22 PM Von Mises Stress at angle q Fig. 9 9

14 14 9/28/ :22 PM Transverse Fillet Weld Largest von Mises stress occurs at q = 62.5º with value of s' = 2.16F/(hl) Maximum shear stress occurs at q = 67.5º with value of t max = 1.207F/(hl) Fig. 9 9

15 15 9/28/ :22 PM Experimental Stresses in Transverse Fillet Weld Figure 9 10: Stress distribution in fillet welds: (a) stress distribution on legs as reported by Norris; (b) distribution of principal stresses and maximum shear stress as reported by Salakian

16 16 9/28/ :22 PM Transverse Fillet Weld Simplified Model No analytical approach accurately predicts the experimentally measured stresses. Standard practice is to use a simple and conservative model Assume the external load is carried entirely by shear forces on the minimum throat area. By ignoring normal stress on throat, the shearing stresses are inflated sufficiently to render the model conservative. By comparison with previous max shear stress model, this inflates estimated shear stress by factor of 1.414/1.207= 1.17.

17 17 9/28/ :22 PM Parallel Fillet Welds Same equation also applies for simpler case of simple shear loading in fillet weld Fig. 9 11

18 18 9/28/ :22 PM Fillet Welds Loaded in Torsion Fillet welds carrying both direct shear V & moment M Primary shear Fig Secondary shear A is throat area of all welds r is distance from centroid of weld group to point of interest J is second polar moment of area of weld group about centroid of group

19 Example of Finding A and J Rectangles represent throat areas. t = h 19 9/28/ :22 PM Fig. 9 13

20 20 9/28/ :22 PM Example of Finding A and J t 3 terms will be very small compared to b 3 and d 3, Usually neglected Leaves J G1 and J G2 linear in weld width Can normalize by treating each weld as a line with unit thickness t Results in unit second polar moment of area, J u Since t = 0.707h, Fig J = 0.707hJ u

21 Table 9 1: Common Torsional Properties of Fillet Welds

22 Table 9 1: Common Torsional Properties of Fillet Welds

23 Example 9 1 Fig. 9 14

24 Example 9 1 Fig. 9 15

25 Example 9 1 Fig. 9 15

26 Example 9 1 Fig Shigley s Mechanical Engineering Design

27 Example 9 1

28 Example 9 1 Fig. 9 16

29 Example 9 1 Fig. 9 16

30 Stresses in Welded Joints in Bending FBD of beam would show a shear-force reaction V and a moment M The shear force produces a primary shear in the welds of magnitude 30 9/28/ :22 PM Moment M induces a horizontal shear stress component in the welds. Treating the two welds as lines, the unit second moment of area to be Fig. 9 17

31 31 9/28/ :22 PM Stresses in Welded Joints in Bending The model gives the coefficient of 1.414, in contrast to the predictions of Sec. 9 2 of from distortion energy, or from maximum shear. The conservatism of the model s is not that it is simply larger than either or 1.207, but the tests carried out to validate the model show that it is large enough. The second moment of area in Eq. (d) is based on the distance d between the two welds.

32 Table 9 2: Bending Properties of Fillet Welds

33 Table 9 2: Bending Properties of Fillet Welds

34 Table 9 2: Bending Properties of Fillet Welds

35 35 9/28/ :22 PM Strength of Welded Joints The matching of electrode properties with those of parent metal is usually not so important as: Speed Operator appeal Appearance of the completed joint. Select a steel that will result in a fast, economical weld. Best results will be obtained if steels having a UNS specification between G10140 & G10230 are chosen. These steels have a tensile strength in the hot-rolled condition in the range of 60 to 70 kpsi.

36 Table 9 3: Minimum Weld-Metal Properties AWS specification code numbering system for electrodes E prefixed to a four- or five-digit numbering system First two or three digits designate approximate tensile strength Last digit includes variables in the welding technique, such as current supply Next-to-last digit indicates welding position: flat, or vertical, or overhead

37 37 9/28/ :22 PM Strength of Welded Joints American Institute of Steel Construction (AISC) code for building construction: Permissible stresses are based on yield strength of material instead of ultimate strength The code permits the use of a variety of ASTM structural steels having yield strengths varying from 33 to 50 kpsi The code permits the same stress in the weld metal as in the parent metal. For these ASTM steels, Sy = 0.5Su Table 9 4 lists formulas specified by the code for calculating these permissible stresses for various loading conditions

38 Table 9-4: Stresses Permitted by AISC Code for Weld Metal For tension, n = 1/0.60 = For shear, n = 0.577/0.40 = 1.44, using the distortion-energy theory.

39 39 9/28/ :22 PM Strength of Welded Joints The electrode material is often the strongest material present. If a bar of AISI 1010 steel is welded to one of 1018 steel, the weld metal is actually a mixture of the electrode material and the 1010 and 1018 steels. A welded cold-drawn bar has its cold-drawn properties replaced with the hot-rolled properties in the vicinity of the weld. Check the stresses in the parent metals.

40 40 9/28/ :22 PM Strength of Welded Joints The AISC code, as well as the AWS code, for bridges includes permissible stresses when fatigue loading is present. For structures covered by these codes, the actual stresses cannot exceed the permissible stresses; otherwise the designer is legally liable. Codes tend to conceal the actual margin of safety involved. The fatigue stress-concentration factors listed in Table 9 5 are suggested for use. These factors should be used for the parent metal as well as for the weld metal. Table 9 6 gives steady-load information and minimum fillet sizes.

41 41 9/28/ :22 PM Fatigue Stress-Concentration Factors K fs appropriate for application to shear stresses Use for parent metal and for weld metal Table 9 5: Fatigue Stress-Concentration Factors, K fs

42 Example 9 2 Fig. 9 18

43 Example 9 2

44 Example 9 2

45 Example 9 3 Fig. 9 19

46 Example 9 3

47 Example 9 3

48 Example 9 3

49 Example 9 3

50 Example 9 4 Fig. 9 20

51 Example 9 4

52 Example 9 4

53 Example 9 4

54 Example 9 5 Fig. 9 21

55 Example 9 5

56 Example 9 5

57 Example 9 6 Fig. 9 22

58 Example 9 6

59 Example 9 6

60 60 9/28/ :22 PM First Exam On Tuesday 1/10/2013 at 11:00 Tested Material: Chapter 9

61 61 9/28/ :22 PM Practice Problems 1, 5, 9, 16, 20, 24, 28, 34, 45, 51