Developments and Future Needs in Welding Coldformed

Transcription

1 Missouri University of Science and Technology Scholars' Mine International Specialty Conference on Cold- Formed Steel Structures (1988) - 9th International Specialty Conference on Cold-Formed Steel Structures Nov 8th Developments and Future Needs in Welding Coldformed Steel R. E. Albrecht Follow this and additional works at: Part of the Structural Engineering Commons Recommended Citation Albrecht, R. E., "Developments and Future Needs in Welding Cold-formed Steel" (1988). International Specialty Conference on Cold- Formed Steel Structures This Article - Conference proceedings is brought to you for free and open access by Scholars' Mine. It has been accepted for inclusion in International Specialty Conference on Cold-Formed Steel Structures by an authorized administrator of Scholars' Mine. This work is protected by U. S. Copyright Law. Unauthorized use including reproduction for redistribution requires the permission of the copyright holder. For more information, please contact scholarsmine@mst.edu.

2 Ninth International Specialty Conference on Cold-Formed Steel Structures St. Louis, Missouri, U.S.A., November 8-9,1988 DEVELOPMENTS 'AND FUTURE NEEDS IN WELDING COLD-FORMED STEEL By: R. E. Albrecht, P.E.(1) In 1978, the Structural Welding Code-Sheet Steel AWS D was adopted for use in designing structures using Cold-formed steel as a structural component and for use in controlling and qualifying the welding processes and the welding mechanics. Concurrently, the parts of the above Code concerned primarily with the design strength of the various types of welds for cold-formed steel were included in the American Iron and Steel Institute Specifications for the Design of Cold-Formed Steel Structural Members, 1980 Edition. These design specifications for arc welding of cold-.formed steel (sheet steel and strip steel) filled a need that had existed for many years. For the most part, these specifications have proved to be quite adequate to cover most conditions encountered in this field since the first publication of them in However, during this intervening period several needs have become known which, if solutions were provided would enhance these Codes and improve the practicality of their use. Some of these needs and the developments available for solving these needs will be discussed in the following text. A. Arc Spot Weld Connections In Tension An arc spot weld connection in tension probably most commonly occurs when there is a wind uplift force (suction force) on light gauge cold-formed steel roof deck attached by arc spot welds to the flanges of steel beams or open web steel joists. Neither the AWS Code nor the AISI Specification presents any design information on this condition. However, a series of tension tests of arc spot welds was performed for the Canadian Steel Industries Construction Council (CSICC) under Industry Research Project 175, and as reported by C. Fung, Westeel-Rosco Limited, in 1978, "Final Report on CSICC Industry Research Project 175, Strength of Arc-Spot Weld in Sheet Steel Construction". In the series of tests investigating arc spot welds in tension, the following conditions and variables were included: 1. Sheet Steels Specified ASTM A446, G-90, Grade A (minimum yield 33 KSI and m1n1mum strength 45 KSI) and Grade D (minimum yield 50 KSI and minimum strength 65 KSI). The actual measured tensile yields varied between 43 KSI and 57.7 KSI and tensile strengths varied between 50 KSI and 68.5 KSI. The steel sheet core thickness varied between.031 inches and.072 inches. (1) Manager, Engineering Development, H. H. Robertson Company, Pittsburgh, PA. 573

3 Plate Steels Specified CSA G W (minimum yield 44 KSI) which is comparable to ASTM A572, Grade 42, in thicknesses ranging from inches to 1.0 inch, and SAE 1010 with a minimum yield of 45 KSI in thicknesses ranging from inches to inches. Two different surface coatings were provided on the steel plates to which the sheet steel was welded. One type of surface coating was hot dip galvanizing per ASTM A525 G-90 (approximate coating thickness of inches). The other type of surface coating was a paint primer, air dried red oxide epoxy primer with a dry film thickness of approximately inches. 3. Welding Electrode One type of welding electrode, AWS Classification E6010, of 5/32 inch diameter, was used (probably the most commonly used electrode class for construction jobs). 4. Specified Weld Diameters Weld diameters specified were 0.50 inches, 0 inches, and 1.00 inch, with the majority of welds specified to have a diameter of 0 inches. The actual measured diameter of welds varied between 0.47 inches and 0.94 inches. 4. Air Gap Between Sheet and plate The majority of specimens had no set air gap (which is the common condition for most welding situations). However, pre-set air gaps of 0.06 inches, 0.10 inches and 0.13 inches were included in the testing array. 5. Weld Tension Test Specimens Each of the test specimens was fabricated by attaching a short length of a channel formed from sheet steel to a square steel plate, with a single arc-spot weld at the center common to the sheet steel channel and the steel plate. To apply the tensile force to the arc spot weld, a steel pin connected the two flanges of the sheet steel channel, through holes in those flanges, to the top part of the test jig, and the steel plate was attached to the bottom part of the test jig with four bolts. The entire assembled test JLg was then placed in a tensile test machine and the force applied. A drawing of this test specimen is shown on Figure No.1. Quoting from the "Report of Project 175", the following occurred during the test, "Although a force is applied normal to the plane of the web, the stresses around the weld is not uniform due to the peeling effect. As the applied force increases, the sheet channel begins to straighten out, subjecting the portion of the weld adjacent to the flange to a higher stress. Since the condition simulates the actual uplift condition of the deck under wind action, no attempt was made to subject

4 575 the weld in pure tension. Under load, the sheet channel began to straighten out subjecting the weld to a peeling action. The stress was highest at the locations which were closest to the legs of the channel. Failure commences with tearing of the sheet material at these locations and gradually propagated along the circumference of the weld." 6. Analysts of Test Results The test specimen variables as well as ultimate strength by test of each weld, Put, are listed on Table No. 1. This Table lists the results of 103 individual tension tests of arc spot welds. It includes all of those tests reported in the "Report of Project 175" except those with pre-set air gaps of 0.10 inches and 0.13 inches (but does include those with pre-set air gaps of 0.06 inches). Also on Table No. 1 the test results are compared to a proposed expression for the strength in tension of arc spot welds attaching steel sheet members to steel plate members. This proposed expression for the strength in tension of arc spot welds is in terms and form similar to those expressions presently used for shear strength of arc spot welds in the 1986 Edition of the AISI Cold Formed Specification. The proposed equation is: P nt Where: P nt d da t Fu Fxx Put 0.9 t da Fu nominal strength in tension of an arc-spot weld, lbs. visible diameter of outer surface of arc-spot weld, inches. average diameter of the arc-spot weld at mid-thickness of t, [where da = (d-t) for a single sheet], inches. steel thickness (exclusive of coatings) of the sheet involved in tension transfer, inches. tensile strength of the sheet steel, [where Fxx~ Fu~60,OOO PSI], lbs. per square inch. stress level designation in AWS electrode classification, PSI. test strength in tension of an arc spot weld, lbs. As shown on Table No.1, the values and the test values. ratio of calculated strength deviation was 0.352, and the correlation was good between the calculated For the 103 test values listed the average to test strength was 1.198, the standard coefficient of correlation was Table No. 2 is a summary of the tests of arc spot welds in tension, and groups the test specimens by sheet thickness and by specified variable conditions. The plot of these groupings of sheet steel thickness (t) versus ratio of test strength to calculated strength (put/pnt) is shown on Figure No.2.

5 576 The AISI Cold-Formed Specification recommends a factor of safety of 2.50 for arc welded connections. Thus, the design tension load on each weld is determined as follows: Where: Pa Sl. u Pnt design load for a weld, lbs. factor of safety for arc we lded connections nominal strength in tension of an arc spot weld, lbs. Referring again to the plot on Figure No.2, it is noted that for the groupings represented by 82 out of 103 weld tests, the ratio of test strength divided by calculated strength is at or above 1.00, indicating a safety factor maintained equal to or greater than 2.5 for design load. Also, note that for all of the groupings, the ratio of test strength divided by calculated strength is above 0.84, indicating a safety factor of greater than 2.1 for design loads. Figure No. 3 is a plot of tested strength versus calculated strength for arc spot welds in tension. This also illustrates, that for the majority of tests, the calculated values are conservative, and that for those that are not as conservative the resulting safety factor will mostly be greater than 2.0. As noted by Orner W. Blodgett, Design Consultant, The Lincoln Electric Company, in his "Report on Proposed Standards for Sheet Steel Structural Welding", Fourth International Specialty Conference on Cold-Formed Steel Structures, 1978, "With sheet steel welding, the variables cannot be as closely defined as with plate welding, and empirical consideration overshadows theoretical analysis." 7. Cold-Formed Steel Structural Members--Canadian Standard CAN3-S136-M84 The test data presented above was used to arrive at an expression for arc spot weld connections in tension for the Canadian Standard. The equation used in that Standard is: Where: 3!/Ic l0 (5.6t - 1) Tr!/Ic t the factored tensile resistance of an arc spot weld, Newtons. resistance factor for connections thickness of sheet, mm.

6 577 The following limitations concerning mechanical and physical properties are specified for arc spot welds in the Canadian Standard: a. Electrodes to be E410XX or E480XX (E60XX or E70XX per U.S.A. standard designation). b. Visible nominal diameter of the weld to be 20rnrn (0.787 inches). c. The thickness of the structural supporting element shall be at least 2.5 times the steel sheet thickness. d. The minimum edge distance measuring from the centerline of the weld to the end or boundary of the connected member shall not be less than 25mm (0.984 inches). e. The steel shall be of weldable quality and shall have a yield strength of 230 MPa (33,360 PSI) or greater. f. The factored resistances apply only to individual sheet thicknesses from 0.70 to 1.67mm ( to inches) inclusive. The specified load factor for live loading is ~ L = Expressing the resistance factor, 0 c ' and the load factor,oll, in terms of a safety factor as used in the AISI Cold-Formed Specification, results in a safety factor for welds of 2.24 (i.e., 1.50/0.67 = 2.24), as compared to 2.5 for the AISI Specification. Transforming the Canadian Standard equation to pound-inch units results in the following equation: Tnt (142.24t - 1) Where: Tnt t nominal strength in tension of an arc spot weld, lbs. steel thickness, inches. And using a safety factor of 2.24 the design tension load on each weld is determined as follows: Where: Ta J\T Tnt design load for a weld, lbs. factor of safety for arc welded connections nominal strength in tension of an arc spot weld, lbs. A comparison of the Canadian Standard design equation and the design equation proposed in this document is provided in the next item.

7 Design Comparison--Canadian Standard to Proposed Equation For use in the design strength comparisons, the following mechanical and physical properties are used: a. Strength of sheet steel, Fy = 37 KSI and Fu Grade B. 52 KSI, ASTM A-446, b. Welding electrode E60XX or Fxx = 60 KSI. c. Visible diameter of weld, d = 0.80 inches. The design strengths of arc spot welds in tension for the two equations are as listed below: SHEET STEEL ARC SPOT WELD IN TENSION USS Thickness DESIGN LOAD, LBS. Ga. Inches Ta(CS) Pa (Proposed) Summary Test data on a significant number of arc spot welds in tension has been presented and analyzed. The test data had a significant number of variable conditions included in its array. Some of these were sheet steel strengths of from 50 KSI to 68.5 KSI, steel sheet thicknesses ranging from inches to inches, galvanized steel sheet, prime painted and galvanized steel plate, and visible diameter of welds ranging from 0.47 inches to 0.94 inches. The proposed equation for strength of arc spot welds loaded in tension and the resulting design values (i.e. Pa = Pnt/2.5, where Pnt = 0.9 tdafu) when compared to test data are conservative for most of the test points and reasonably safe for the remainder when considering a safety factor of 2.5 for welds and comparing it to that for bending, etc. It is hoped that the above testing and analysis can be the basis for introduction into the AISI Cold-Formed Specification a means of determining the load capacity of arc spot welds loaded in tension.

8 579 References 1. American Welding Society, Inc., Structural Welding Code--Sheet Steel, AWS D1.3-81, Second Edition. 2. American Iron and Steel Institute, Cold-Formed Steel Design Manual, August 1986, Edition. 3. Canadian Steel Industries Construction Council (CSICC), "Final Report on CSICC Industry Research Project 175, Strength of Arc-Spot Weld in Sheet Steel Construction", by C. Fung, Westeel-Rosco Limited, Blodgett, Orner W., "Report on Proposed Standards for Sheet Steel Structural Welding", Fourth International Specialty Conference on Cold-Formed Steel Structures, St. Louis, Missouri, June 1-2, 1978, Volume II of Proceedings, published by Department of Civil Engineering, University of Missouri-Rolla. 5. Canadian Standards Association, Cold-Formed Steel Structural Members, CAN3-S136-M84, December, 1984.

9 Specified Specified Measured Calc. Test Variable t Fu Fxx d d da Pnt Put Specimen No. Condition in. KSI KSI in. in. in. Ibs. Ibs. Put/Pnt 2AT-I07 Plate To Sheet Thickness a:; 0 2AT-I01 Plate To Sheet Thickness AT-304 Plate To Sheet Thickness TAB L E NO. TEST RESULTS AND CALCULATIONS OF TENSION ON ARC SPOT WELDS [P nt 0.9 tdafu; 60 Ksrli!!!. Fu~ Fx)

10 TEST RESULTS AND CALCULATIONS OF TENSION ON ARC SPOT WELDS [P nt 0.9 tdafu; 60 KSI~ Fu::!:-F xx ] Spec ified Specified Measured Calc. Test Variable t Fu Fxx d d da Pnt Put Specimen No. Condition in. KSI KSI in. in. in. lbs. lbs. Put/Pnt 2AT-I04 Plate To Sheet Thickness BT-101 Plate To Sheet Thickness r:7o BT-I04 Plate To Sheet Thickness BT-107 Plate To Sheet Thickness TAB L E NO.

11 Specified Specified Measured Calc. Test Variable t Fu Fxx d d da Pnt Put Specimen No. Condition in. KSI KSI in. in. in. lbs. lbs. Put/Pnt 3CT-401 Weld Size CT-411 Weld Size I>:) 3CT-407 Weld Size DT-301 Surface Coating Primed Plate " Surface Coating (1337) -411 G90 Galv.Plate TAB L E N 0 TEST RESULTS AND CALCULATIONS OF TENSION ON ARC SPOT WELDS [Pnt 0.9 tdafu; 60 KSI=- Fu~ Fx)

12 TEST RESULTS AND CALCULATIONS OF TENSION ON ARC SPOT WELDS [P nt 0.9 tdafu; 60 KSI ~ F ~F x) u Specified Specified Measured Variable t Fu Fxx d d da Specimen No. Condition in. KSI KSI in. in. in. 3DT-404 Surface Coating Primed Plate " " " " " , Surface Coating G90 Galv.Plate " " " " ET " Ai~ Gap " " " " " " ET " Air Gap Calc. Pnt lbs x Test Put lbs AVE S DEV n COEFF. OF VAR. n Put/Pnt Cl1 00 <:c TAB L E N 0

13 AVE AVE AVE Dev. COEFF. Pnt Put OF lb.. lb.. Put/Pnt Sn VAR =.2392=.2011 CJl 00 oj:>. TAB L E NO. SUMMARY OF TENSION ON ARC SPOT WELDS [Pnt 0.9 tdafu; 60 KSI~ Fu ~Fxxl Specimen Group Number Of Items Specified Variable Condition in. Fu KSI Fxx KSI AVERAGE Specified Measured d d da in. in. in. 2AT- 13 Plt. To Sheet Th AT- Pit. To Sheet Th AT- Plt. To Sheet Th AT- PIt. To Sheet Th BT- PIt. To Sheet Th BT- PIt. To Sheet Th BT- 6 PIt. To Sheet Th CT- 6 Weld Size CT- Weld Size CT- 4 Weld Size DT- Primed Plate DT- G90 Galv. Plate DT- Primed Plate DT- e90 Galv. Plate ET- 4.06" Air Gap ET- 4.06" Air Gap TOTAL 103 WEIGHTED AVERAGES

14 585 SHEET STEEL CHANNEL 2" X 2" X 2" LONG + BOLT (TYPICAL) ARC SPOT WELD t TOP PART OF TEST JIG ARC SPOT WELDED SHEET STEEL CHANNEL 2" X 2" X 2" LONG + BOLT + BOLT BOTTOM PART OF TEST JIG FIG U R E NO. 1 ARC SPOT WELD TENSION TEST TEST SPECIMEN & TEST JIG

15 01 gs FIG U R E NO. 2 TENSION ON ARC SPOT WELDS... c '"... " '" (!) (6) <:> (6) - (,,\ 0(5) &(13) t!> ( ) ( 4) AVERAGE 0(7) <:> (9) (Put/Pnt ) ~ ~~~ (; (6) <:> (6) o (9) P = 0.9 td F nt a u & 60 KSI ~ F ~ F u -xx P = Test Ultimate u ( ) = Number Of Items TOTAL NU BER OF 11 -MS <:> (6) 0.00 o -- ~ ~.... -,,~ n n~ " n/. () nr::;. n.()'; n.07 o. 8 SHEET THICKNESS, t, INCHES

16 587 FIG U R E NO. 3 TENSION ON ARC SPOT WELDS P nt & 60 KSI ~ Fu~ Fxx ~ o<l...:l :z:" E-< '-' Z W..: E-< '" Q w E-< '" W E-<. ~ :l ~ QO o 7 / /. /1/ " :.. 1/ i,.- t......:;/ :. :... y... ::.. I.... V /. / " V o P nt, CALCULATED STRENGTH, LBS. "

17