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June 30, 2020

Structural Welds – FAQ How Do You Determine Weld Size?

Structural welds

A recent question landed in my inbox from a regular client.

Kyle wondered if we take much notice of weld sizes, or leave it to the fabricator, since in our markups we don’t often mention weld details.

Quick answer – we leave nothing to chance!

We use standards to minimise the amount of repetitive work and communication, so on our standard drawing notes we use this old chestnut:

“All welds shall be 6mm CFW or full penetration butt welds UON”.

Any question asked is worth a decent explanation, so let’s dive in.

First up, some definitions.

Fillet Weld

Structural weld - Fillet weld

Fillet welds are defined by the size of the leg length e.g. a 6 mm CFW is a continuous fillet weld having a leg length of 6 mm.

Structural welds - weld size

Butt Weld

Structural welds - Butt weld

Butt welds are typically the same size as the welded material, stronger due to the weld material having a greater tensile strength and as such are called “full penetration butt welds” or “full strength butt welds”. If a size is nominated on a butt weld symbol, then it is a “partial penetration butt weld” – used when you don’t need full strength and want to reduce the cost and time associated with welding.

AS 4100 is “the code” for steel structures and defines strength and other requirements for welds. There are minimum and maximum sizes for fillet welds, for example. For plates less than 6 mm thick, the maximum weld size is the thickness of the plate. For plates 6 mm and thicker, it’s 1 mm less than the plate thickness. Minimum sizes are as per this table:

Minimum size of a fillet weld

But it’s not just about size – welds (obviously) need to be strong enough to hold the joints together.

To give you some idea, a “standard” 6 mm fillet weld has a capacity of 1kN/mm.

In layman’s terms – a normal-sized car weighs around 1700 kg, so you could pick it up with 17 mm of weld!

There’s a lot more to it of course, but needless to say most welds are OKBI (okay by inspection).

Structural Design capabilities

We use our own in-house design criteria and Aussie Standards to ensure our designs are compliant but not excessively so.

We design using latest software and technologies – and always challenge the status-quo.

Check out the structures we design that involve careful attention to connection design.

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