Simple CJP Weld Design Question 1

[bpiermat]

Ok, so maybe I have not had enough coffee yet....But here is a simple question.

If I am designing a CJP Weld and Table J2.5 states that the "Strength of the Joint is controlled by the base metal", do the checks of the weaker base metal need to be per Chapter J4? i.e. 0.9 Fy for tension?

Thanks!

 

[MotorCity]

Assuming the weld (electrode) strength is greater than the strength of your base metal, you need to check the basemetal for whatever type of force is being delivered by the weld.......most likely it will be tension, but it could be shear. Just depends on the load path and the basemetal. You need to check all basemetals connected by the weld. The basemetal strengths could vary depending on their thickness and yield strength.

 

[gtaw]

AWS D1.1 Structural Welding Code/Steel requires the strength of the filler metal match the strength of the "weaker" base metal when the load is in tension or shear. If the joint is subject to compression, undermatching the filler metal is permitted. It is of no benefit to over match the filler metal because the allowable unit stress of the base metal would be the limiting factor as far as determining the ultimate strength of the CJP grooved joint.

It is generally accepted that the deposited weld metal will "gain" strength as a result of strain hardening is the weld yields, so a 70 ksi filler metal will gain about 10 ksi once plastic strain has occurred.

Best regards - Al

 

[Ron]

It is of no benefit to over match the filler metal because the allowable unit stress of the base metal would be the limiting factor

gtaw....pretty rare that I feel compelled to comment on your posts...they're always spot on. My only comment here is that filler metal is almost always overmatched to the base metal and part of that reason is to cause the base metal to be the limiting factor....it is more predictable and controlled than the filler metal in terms of its metallurgical and physical properties, thus it's limitations are more predictable.

 

[dik]

It is generally accepted that the deposited weld metal will "gain" strength as a result of strain hardening is the weld yields, so a 70 ksi filler metal will gain about 10 ksi once plastic strain has occurred.

didn't know that... thanks

Dik

 

[dik]

OP: can you avoid a CJP weld by using something 'lesser' that works... CJP's are costly.

Dik

 

[gtaw]

Bolts work.

As for the predictability of welds; if the welding is performed in accordance with a qualified WPS or when AWS D1.1 is the governing document, prequalified WPSs, are used by qualified welders, the resulting welds are quiet predictable.

The unpredictibility comes about when the welder isn't provided with the proper work instruction or they are not properly trained and qualified. Quality isn't free and therein lies the problem. Owners will agree to forgo the qualification requirements if they can "save money".

I've been involved with a number of failure investigations and I can say without hesitation that in each case the welders were not properly qualified and rarely did they have a WPS that provided them with the information needed.

The owner, through the Engineer bears, some responsibility in ensuring the contractor and the welders follow the applicable codes. All too often the system breaks down when the owner can save money by not enforcing the qualification of the welders, welding procedures, and follow through with meaningful quality assurance.

Doubling the size of a weld or over matching the filler metal rarely makes the weld "better" if the individual doesn't know how to weld.

I find it troubling when I receive drawings with notations such as "weld here", "weld as required", or when the welding symbol doesn't follow the conventions of the industry standard, i.e., AWS A2.4. I still see welding symbols that follow the conventions used prior to 1976, the date when welding symbols were revised extensively.

There's enough blame to pass around. We know how to deposit sound welds with properties equal to or exceeding the properties of the base metal, but it takes a team effort by people that understand the technology and are willing to makes sure each party involved does their part.

Just today I sent a memo to a detailer informing him that the welding symbols used for intermittent fillet welds were incorrect and we typically don't use filler metal having a tensile strength of 700 ksi to join carbon steel.

Best regards - Al