brane23-
When the metals get heated from the welding process and then cool - they distort. Imagine welding an angle to the top of a WF. If you only welded one side of the angle, you can imagine how it would want to "bend" up, lifting the back (unwelded end) off of the WF. These forces and residual stresses can be alleviated somewhat by staggering the welds.
One reason to use skip welds in exterior exposure is to minimize the trapping of water in the joint. Unless you can really keep it sealed it is best make sure that the water has a way out.
= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection
In the bridge world, an intermittent fillet weld, in addition to being a fatigue problem, is seen as an invitation to trapping water in the joint. Close fitup doesn't drain all that well.
I typically used staggered intermittent welds on thin material where buckling is a concern. I use chain intermittent welds on thicker material where angular distortion is a concern.
The staggered intermittent welds on thin material reduce the spacing between opposite welds and reduces the unattached (unwelded) lengths between welds that can buckle.
The chain intermittent welds on thicker materials, where buckling isn't as much a problem, balance the residual forces by having welds directly opposite each other which reduces the angular distortion.
A small continuous weld on both sides of the joint (provided they are large enough to meet the code requirements such as AWS D1.1 based on material thickness and hydrogen levels) are usually a better alternative to intermittent fillet welds. For the same strength, continuous welds typically require less weld volume, result in less distortion, have fewer starts and stops, thus fewer discontinuities, and less likely to initiate fatigue cracks.
General statements like the one about usually bite you in the “you know what”, but I said it anyway.
Consider the weld volume; assume you are welding 2 inches on four inch centers, i.e., 50% of the joint length is welded. A continuous weld would have 1/4 the weld metal volume for the same load carrying ability, thus less volume, requires less heat input, and results in less angular distortion.
Other points already stated are also valid points to consider.
gtaw-
I don't know about you, but typically when I design an intermittent weld it is because a continuous weld of the minimum size will be grossly over capacity. When you are already using the code minimum size weld, your only option is to use intermittent fillet welds.
I don't think anyone was advocating using a 3/8" fillet 6" long at 12" o.c. when a 3/16" continuous fillet weld will do. I, and probably everyone else, meant for it to be used when the code minimum (based on thickness of the thicker, or thinner part (depending on which AISC manual you are using)) is say 3/16" and a continuous fillet provides 2.78K/in (for ASD), but your analysis says you only need 0.5K/in. Since you can't decrease the weld size, you use an intermittent weld of 3/16" fillet - 3" long at 12" o.c.
No disagreement from me on that point. If the welds are already the minimum size permitted by the AWS structural code, then by all means, staggered intermittent fillet welds are reasonable.
The point I was trying to make was that a continuous fillet weld that meets the minimum size requirement is better rather than a larger intermittent fillet weld.
As for distortion, it is related to the thermal history, i.e., a multipass weld will have more angular distortion than a single pass fillet of the same size.
