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1、Experience with Use of Heavy W Shapes in Tension JOHN W. FISHER and ALAN W. PENSE During the last five years, a few cracking and fracture problems have been observed with welded, large rolled jumbo sections. This type of rolled section is classified as a group IV or V shape in the 8th Edition AISC M
2、anual of Steel Construction.1 They include W12 and W14 rolled steel sections equal or exceeding 210 lbs./ft. The cracking has been observed in sections equal or exceeding 370 lbs./ft. They have been observed in sections produced without special metallurgical requirements as well as in sections produ
3、ced with killed fine grain practice. This paper reviews the characteristics of these steel sections, the conditions that led to the cracking and provides recommendations for splicing and use of these members in tension applications. In addition, there are suggestions for weld splices in compression.
4、 When first introduced in the 1960s, these sections were intended as columns in structures resisting large axial loads. However, with passage of time the sections were gradually introduced in large trusses as tension members. This provided a means to develop the large forces in the chords and diagon
5、als of long-span trusses or shorter spans carrying heavier loads. Figure 1 shows the fractured chord of a large truss fabricated from these rolled sections. The cracks that developed have all been associated with groove welded splices in the flanges and webs of the steel sections. In the fracture sh
6、own in Fig. 2, the crack originated from the flame-cut copes in the web adjacent to the flange groove welds and extended into the web-flange core (i.e. the web-to-flange intersection). Other cracks have originated from the termination of groove welds in the web, as in Fig. 3 Dr. John W. Fisher is Pr
7、ofessor of Civil Engineering at Lehigh University. Dr. Alan W. Pense is Professor of Lehigh Universitys Department of Materials Science and Engineering, and Associate Dean in the College of Engineering and Physical Sciences. Fig. 1. Fractured chord of Orlando Civic Center truss Fig. 2. Close-up of f
8、racture SECOND QUARTER / 198763 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. or from the groove welded web splice, as in Fig. 4. Cracks also develop
9、ed from the flame-cut ends of large sections and extended a short distance longitudinally into the section, as in Fig. 5. The causes for each of these cracking conditions is examined in this paper along with recommendations for their correction. DETAILS AND CHARACTERISTICS AT CRACKED SECTIONS a. Fra
10、ctured Tension Chord The fracture in the bottom chord of the truss shown in Figs. 1 and 2 originated from a crack formed from the flame-cut surface of the web cope hole in a W14 370 A572 Gr.50 steel section. Figure 6 shows the fracture surface and a closeup view of the crack origin. The dark oxide-c
11、overed surface extends 1.15 in. from the edge of the cope into the flange; its maximum width 2.75 in. At the other end of the web cope, no obvious cracking was visible, as shown in Fig. 7. This area was saw-cut from the section and the flame-cut Fig. 3. Cracked column web originating at groove-welde
12、d web splice termination Fig. 4. Cracked column web originating at groove weld . surface treated with liquid penetrant, as seen in Fig. 8. This revealed a crack-like indication. No evidence of cracking was found on either outside surface of the web of this detail The cope segment was cut into two pi
13、eces by sawing at the mid- thickness of the web. This surface was polished and etched as in Fig. 9 which shows the crack originated at the flame-cut edge and extended on an angle into the web-flange core. Figure 10 shows the saw-cut surface of the section where the web was cut from the flange. This
14、crack was found to extend into the flange about the same amount observed at the dark oxide surface in Fig. 6. A martensitic layer can be seen along the flame-cut edge in Fig. 9. A photomicrograph of the crack origin is provided in Fig. 11. The martensitic layer is about 0.025 in. thick. At the crack
15、, the martensite Rockwell hardness (Rc) was 42.5 and varied between 32 and 37 elsewhere along the burned edge. An examination of other flame-cut copes in W14 455 and W14 500 sections in the structure revealed other cracks, as in Fig. 12. At the section that failed, the web- groove weld was made firs
16、t, followed by the outside halves of flange double bevel groove weld. The inside weld (a). Milled end of column at aplice (b). Longitudinal crack in web Fig. 5. Crack originating from flame-cut edge 64ENGINEERING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION 2003 by American Institute of Steel
17、Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. (a). Fracture surface of flange (b). Close-up view of initial crack at web cope Fig. 6. Crack surface of tension chord (see Fig. 1.) Fig. 7
18、. Cope on east side of flange-groove weld (see Fig. 2.) Fig. 8. Flame-cut cope with liquid penetrant Fig. 9. Polished and etched section through center of web (see Fig. 8.) showing crack and flame cut edge SECOND QUARTER / 198765 2003 by American Institute of Steel Construction, Inc. All rights rese
19、rved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. Fig. 10. Liquid penetrant indication showing crack extension into web-flange core Fig. 11. Photomicrograph showing crack martensite and grain structure 40x Fig. 12. Crack in
20、dications at cope enhanced by magnetic particle halves of the flanges were completed last. At other sections that were field-welded, the flange groove welds were made first and the web groove weld last. In a number of the sections welded in this latter fashion, cracks developed in the web from the f
21、lame-cut edge of the cope. Nearly all welded splice locations had cracks at the cope, as in Fig. 12, which did not appear to depend on the weld sequence. The rolled sections in these truss members were all A572 Gr.50 steel material. They had not been supplied as killed, fine-grain practice steel sec
22、tions. b. Groove Welded Column Splices The cracks that originated from the web groove weld termination shown in Fig. 3 occurred in W14 730 A572 Gr.50 steel sections. However, this material was supplied as killed, fine-grain practice steel. Figure 13 shows a schematic of the column splice at the crac
23、ked section. Partial penetration welds were placed in the flange and a double bevel groove weld in the web. At the weld ends, the bevel was radiused to the surface. Production of the web groove welds creates high tensile residual stresses in the longitudinal and transverse directions of the weld as
24、it shrinks and cools. As a result, the weld termination resides in a region of yield point residual tensile stress in the web of the rolled section. The cracks developed in these sections occurred even when preheat and postheat treatments were used. At the time cracks formed, ambient temperature was
25、 between 25 and 40F. The crack shown in Fig. 4 likely occurred from an embedded defect on the fusion line of the rolled section. No core was removed at the crack origin so this was not verified by examination. Fig. 13. Schematic showing joint geometry, residual stress orientation and crack 66ENGINEE
26、RING JOURNAL / AMERICAN INSTITUTE OF STEEL CONSTRUCTION 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. c. Longitudinal Cracks at Flame-Cut Edges The c
27、racks detected in the milled end of the column in Fig. 5 were observed to form from the flame-cut edge. These cracks extended about in. into the length direction of the section. These columns were also W14 730 A572 Gr.50 steel sections. As apparent from Fig. 11, a thin martensitic layer can be expec
28、ted at the flame-cut edge. Also, residual tensile surface stresses will develop as the cut is made and the material transforms from a liquid to solid state and cools. Measurements have demonstrated that yield point residual stresses will develop at a flame-cut edge.2 CHARACTERISTICS OF THE JUMBO SEC
29、TIONS The material properties of the fractured and cracked sections shown in Figs. 1, 3 and 7 will be examined together with additional tests on several large sections that have been recently evaluated. This includes their tensile properties, microstructure, chemical composition and fracture charact
30、eristics (Charpy V-notch and fracture toughness tests). a. Tensile Properties The tensile tests of the two W14 370 A572 Gr.50 steel sections adjacent to the fracture in Fig. 1 provided an average yield point of 61.3 ksi for the fractured section and 60.6 ksi for the adjacent one, with similar cracks
31、 in the flame-cut cope. The corresponding tensile strengths were 94.6 ksi and 91 ksi. The percent elongation in 2 in. varied from 20 to 26% and the reduction in area was about 55% for both sections. For the W14 730 A572 Gr.50 sections in Figs. 3 and 4, the yield point was 61.7 ksi, the tensile stren
32、gth 86.1 ksi, and the elongation 29%. Additional tests carried out on the webs of W14 550, W14 605 and W14 730 steel sections from a third supplier and manufactured to killed fine-grain practice had web-yield points between 54 and 58 ksi. These sections had similar tensile characteristics ranging be
33、tween 87 and 92 ksi. b. Chemical Analysis of Steel Sections Chemical analyses were carried out on the W14 370 sections from Supplier A that were taken from the cracked chord. The W14 730 sections from Supplier B that experienced cracks at column groove welds were also tested. Tests were carried out
34、on web samples from the W14 550, W14 605 and W14 730 sections evaluated from Supplier C. The test results are summarized in Table 1. When compared with the ASTM Specifications, it was found none of the elements were out of the specification limits for the applicable grade. For the fractured W14 370
35、section, the carbon and manganese were near the upper limit and provided a carbon equivalent of at least 0.55. This indicated a high potential for cracking in heat-affected zone areas such as flame-cut edges and welds. Table 1 Chemical W14370 Core Supplier A* W14730 Web Supplier B W14550 Web Supplie
36、r C W14605 Web Supplier C W14730 Web Supplier C Carbon0.220.180.270.250.24 Manganese1.351.230.971.000.87 Phosphorus0.01 0.022 0.028 0.025 0.017 Sulfur 0.022 0.014 0.040 0.032 0.027 Silicon0.010.280.240.240.19 Vanadium0.080.07 0.049 0.061 0.059 Chromium 0.103 0.078 0.077 0.079 Nickel 0.091 0.041 0.04
37、4 0.100 Columbim(Nb)0.010.03 Titanium0.01 Copper0.028 Aluminum 0.003 *Not killed, fine-grain practice. c. Charpy V-Notch Test Results Charpy V-notch tests were carried out on the web and flange material of the fractured section shown in Fig. 2 (W14 370). Tests were carried out at the standard quarte
38、r thickness locations in the web and flange, the web-flange core and the surface layers. Figure 14 shows a schematic with these locations. Orientation of the notch is indicated for the various specimen locations. The test results are summarized in Fig. 15. The quarter thickness results are shown as
39、plus (+), the web-flange core region and centerline as solid dots () and the surface layers as open dots (o). It can be seen the web-flange core and the flange quarter-point specimens provided comparable levels of absorbed energy up to 130 F. The quarter thickness web specimens were similar to the f
40、lange-web core below 100F. However, at and above 100 F, web specimens tended to exhibit higher levels of energy and scatter. The surface specimens with notches within 0.2 in. of the surface all provided significantly higher levels of absorbed Fig. 14. Schematic showing CVN test locations for W14 370
41、 sectionSupplier A SECOND QUARTER / 198767 2003 by American Institute of Steel Construction, Inc. All rights reserved. This publication or any part thereof must not be reproduced in any form without the written permission of the publisher. Fig. 15. Charpy V-Notch test results for W14 370 section Sup
42、plier A energy. Few of these specimens were less than 20 ft-lbs. down to 32 F. Hence, the outer skin of the section had adequate levels of absorbed energy for welded surface attachments and shallow discontinuities. The characteristics of the W14 730 sections shown in Figs. 3 and 4 were also examined
43、. The only material available from these sections was a small segment cut between the end of the section and a lifting hole, as in Fig. 16a. The CVN specimens adjacent to the web surfaces had their notches extending 0.5 in. into the web. Hence, no shallow surface notches were examined. Several tests
44、 were also available on other sections tested by the manufacturer. These specimens were at the quarter thickness of the web and flange and at the center of the flange as sketched in Fig. 16b. Test results for web specimens fabricated from the small segment shown in Fig. 16 are plotted in Fig. 17. Th
45、e specimens were tested at 40 F, 70 F and 100 F. These test results are shown as solid and open dots and a cross. The tests by the manufacturer were only carried out at 100 F. The test results can be seen to be similar to the absorbed energy provided by the W14 370 section plotted in Fig. 15. One si
46、de of the web surface provided higher levels of energy than its companion on the other side. The test results indicate that providing steel to killed fine-grain practice has not resulted in a significantly higher level of absorbed energy for this W14 730 section. Its level of absorbed energy is near
47、ly identical to the W14 370 section that was not furnished as killed steel. Additional tests carried out on W14 550, W14 605 and W14 730 A572 steel sections were limited to segments cut from the web, shown schematically in Fig. 18. The surface layer specimens were notched so that the notch tip was w
48、ithin 0.25 in. of the web surface. The other specimens were near the point and Fig. 16. Schematic showing CVN test specimen locations for W14 730 sectionsSupplier B Fig. 17. Charpy V-notch test results for W14 730 section Supplier B the centerline. The test results are plotted in Figs. 19,20 and 21
49、for the three section sizes. It can be seen that the centerline of each of the web sections examined were not much different than the W14 370 and W14 730 sections shown in Figs. 15 and 17. The results from the W14 605 section were lower at one end than at the other. However, both ends fell within the scatterband provided by the W14 370 section. d. Fracture Toughness, Kc Compact tension specimens were fabricated from the flanges of the W14 370 section in Fig. 1 and the web segm
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