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Shear Connector Inspection - a Tutorial

Steel Inspection News, January/February 1993 issue

Shear connectors embedded in concrete floor slabs, or perhaps in concrete-encased columns for composite frame construction, require inspection prior to embedment. The UBC special inspection provisions make this statement clearly. The BOCA code requires special inspection of shear connectors if you properly interpret shear connector attachment as structural welding, or perhaps consider it to be a "special case".

Shear connectors, commonly called "studs" in the trade and also by AWS, are to be welded by a stud-welding gun, an automatic welding device. In this device, the stud base is surrounded by a ceramic ferrule. The stud base is placed in contact with the steel. When the gun's trigger is pulled, the stud is lifted from the steel, the electrical flow starts and an arc is made between stud and steel. After a short period, the stud is then plunged into the now molten pool of steel base metal and the electrical flow is cut off. The welding is complete, except for removal of the ferrule and inspection.

 "Ring Test"

The traditional method for stud inspection has been the "ring" test, simply tapping each stud with a small sledge hammer ("beater") and listening for a ring, as compared to a dull clunk or watching for the stud to break off. In a seminar I was conducting in Denver, I learned that this method had reached a new level of sophistication. Local engineers were stating in their specifications that all studs were to receive "acoustical inspection".

Unfortunately, this is not a code-approved method of inspection, regardless of how sophisticated it sounds.

Stud Inspection Steps

Stud inspection takes place in several steps, as does conventional weld inspection. Weld inspection begins with the use of approved welding procedures and qualified welders. After the welding is performed, a visual inspection is performed. After this, non-destructive testing may be performed.

The same holds true for stud inspection. The welding process is established and verified, the welders are qualified, the studs are welded and visually inspected, and a certain number of studs may be subjected to nondestructive testing, namely bend tests.

Let's look at each step, using AWS D1.1 - Structural Welding Code - Steel, Section 7, Stud Welding. The steps performed prior to actual stud welding are called pre-production testing, operator qualification and application qualification. After stud welding, we have fabrication / erection inspection and verification inspection.

Stud Base Qualification

Stud bases must be qualified. This is performed by the manufacturer in accordance with AWS D1.1, Appendix IX. Studs from known manufacturers have been tested under these provisions. Both studs welded to bare steel and studs welded through deck, in the flat position, may be tested. A total of 60 studs are welded, 20 subjected to tension tests and 40 subjected to repeated bend tests in alternating directions until failure. You should ask for a copy of the Manufacturer's Qualification Test Data, which includes drawings and descriptions of the product tested, the setup for which the tests were conducted, and the test results. Settings for welding voltage, current, time, gun lift and plunge will be given. These values serve as the basis for field setup.

Stud Application Qualification

Three applications are listed in Section 7.6 for Stud Application Qualification Tests. These are:

(1) studs welded to non-planar (curved) surfaces, or studs welded in other than the flat position (0^o to 15^o to the horizontal),

(2) studs welded through decking, and

(3) studs welded to steels other than Groups I or II in Table 4.1. The steels needing application qualification tests are structural steels with specified yield strengths of 60 ksi and higher.

The second provision regarding metal decks seems to be an apparent duplication with manufacturer's stud base qualification tests performed under Appendix IX, which may be performed using decks representative of those to be used on the project. Many times, the Application Qualification Test is waived by the Engineer when previously performed by the manufacturer and documentation is submitted. Particular note should be taken of deck thickness, deck coating and coating thickness. In some applications, such as lapped non-composite form deck, studs may be shot through two layers of deck. Stud welding procedures should be verified at the jobsite using the Application Qualification Test.

The Application Qualification Test consists of welding 10 studs consecutively on a representative sample (through deck, in same position, on same curved base, on same high-strength steel, etc.). Testing of headed studs can be made by either bend testing or tensile testing. In bend testing, the stud is bent 90^o from its original position. In tensile testing, a fixture to apply controlled tension to the stud may be used. Should the stud fracture in its shank, or the base metal fail, the stud weld is accepted. Should the stud weld itself fracture, the stud weld is inadequate, the weld procedure setup must be adjusted, and 10 additional studs tested using the new setup.

Jobsite Conditions

Following the satisfactory completion of the Stud Application Qualification Tests of 7.6, it is time to determine if stud welding should be performed. Several guidelines are presented by AWS stating when stud welding should not be performed. The studs should be of good quality, free of injurious cracks, seams, bends, rust, scale, pits, oil, and moisture. Bursts of the stud head are permitted, provided the depth of the burst does not extend more than half the distance from the stud periphery to the stud shank. The steel to which the stud will be welded should be free of scale, rust, moisture or other injurious materials, particularly coatings. Coated decking may be welded through, but coatings on the steel itself may cause problems. Steel beams should have the top flange unpainted when studs are to be applied.

Should the situation arise when studs are shot through decking onto painted steel, additional testing should be performed. It may be necessary to burn holes in the deck, remove the paint, reweld the deck and shoot the stud through the burned hole.

Another troublesome situation is moisture from rain, snow or condensation trapped between the deck and steel beam below. Not visible, the condition is unknown until stud weld problems are discovered. If in doubt, studs should be checked frequently by the installer. If moisture is suspected, a heating torch may be used prior to stud welding to remove such moisture prior to welding.

The ferrules should be kept dry. Any ferrules where moisture is apparent, or when they have been exposed to moisture from dew, rain or snow, should be dried in an oven at 250^o F for two hours prior to use.

Welding should not be performed when the steel temperature is below 0^o F, or when the surface is exposed to rain or snow.

Pre-Production Testing

Now that we have verified that the condition of the materials and the steel surfaces are suitable for application, pre-production testing takes place. Pre-production testing is performed on a daily basis, or at the start of each shift for multiple shifts. This testing may take place on the actual work, or may be done on a representative sample of the actual work.

Two studs are welded. Both are visually inspected for flash , verifying its presence about the entire perimeter of the stud base. After the studs have cooled, they are bent approximately 30^o. This bending may be done with a sledge hammer, or by using a pipe slipped over the stud. When the temperature is below 50^o F, the slow bending using the pipe is preferred. If the stud weld does not crack or fail, the stud passes and actual production may begin. If the stud weld fails the test, the setup is adjusted and two more studs are tested.

The settings used during the pre-production testing should be recorded. If, at any time during the day, there is a change in the power source, gun lift and plunge settings, welding lead length, current settings in excess of 5% or time settings in excess of 5%, the pre-production testing shall be re-performed.

Operator Qualification

The pre-production test also serves as the operator qualification test. If more than one person welds studs, then each welding operator shall shoot two studs and have them subjected to the same tests as required for pre-production testing above.

Fabrication Inspection

Probably a better term to use would be installation inspection. The stud installer removes the ferrules and performs a visual inspection of each completed stud. The stud should show flash about the entire perimeter of the stud, 360^o. If an individual stud fails to show 360^o flash, then the stud may be bent over, using hammer or pipe, approximately 15^o in the direction opposite the missing flash. If the temperature is below 50^o F, then the pipe bending method should be used.

It should be noted that this testing should be performed by the welding operator, not the inspector. The role of inspector is defined in 7.8.2, which states, "The Verification Inspector, where conditions warrant, may select a reasonable number of additional studs to be subjected to the tests specified in 7.8.1." Section 7.8.1 was simply the visual inspection for flash and the 15^o bend test for those with missing flash. I interpret this section to indicate that additional studs may receive the 15^o bend test.

One additional provision for Fabrication and Verification Inspection is seemingly lost back in 7.5.4. When the steel temperature is below 32^o F, the 15^o bend test shall be performed on at least 1 stud in 100, even if all studs exhibit 360^o flash.

A common question is whether studs bent to 15^o or 30^o should be bent back to their original position. For studs embedded in concrete, this is not necessary. In fact, 7.8.3 states that they "shall be ... left in the bent position". Tests have indicated that they develop adequate shear capacity in the bent position.

Repair Welds

In lieu of bending a questionable stud welded during production, the option is provided to hand weld the stud using the SMAW (Shielded Metal Arc Welding), or "stick" process. Low hydrogen electrodes of given size are required, and a 5/16" leg must be made for the common 3/4" stud. A 15^o bend test must be performed, bending away from the repair weld. The repair weld must extend at least 3/8" beyond each end of the missing flash. If the stud is welded through metal decking, the decking should be cut away from the stud to ensure good fusion to the base metal. The decking will need an additional puddle weld near the stud, if the stud weld was used to replace a puddle weld.

Recommended Reading

Read thoroughly AWS D1.1, Section 7, the Commentary for Section 7, and also Appendix IX. The AWS Welding Handbook, Eighth Edition, Volume 2, contains a helpful Chapter 9 on stud welding. A third reference is AWS's C5.4, Recommended Practices for Stud Welding.