Steel Bollard Weld Penetration

Hi, long time no post!

Should steel bollards on a tug boat always have a partial or full penetration weld when welded directly to the deck plating?
For example, would you always prefer a single bevel with fillet reinforcement over a larger fillet weld of equivalent theoretical strength? I feel like the answer is yes because of fatigue, but I'm having trouble justifying this myself. Am I on the right track?

Appreciate any input at all.

 

Referring the bollards only to the deck can cause significant problems. It would be better to secure them to the primary structure of the ship.
If they are welded only to the deck, of course, it must be done with full penetration and appropriately reinforcing the deck below it.

 

Thanks for the response. I guess to expand a little bit, I already have design drawings with the weld prep being a 3/8" single bevel with reinforcing fillet 3/8" fillet. Bollard material thickness is 3/4".

So with regards to my question above, would you always prefer a single bevel with fillet reinforcement over a larger fillet weld of equivalent theoretical strength? If yes, why? Fatigue reasons?

Again, appreciate the input
Thanks

 

Well, that depends what YOU mean by partial and full.

The welding of the bollard to the deck can of course only be fillet weld, unless it is a through the deck structure type of arrangement.
Assuming it is a conventional bollard, that being a base plate with the bollard on the plate (and not through the deck), the base plate can only be attached by a fillet weld.

Thus one would not perform a chain or staggered weld. It would be a continuous weld, in that sense, of the base plate to the main deck.
So, this must be a structural weld - this then means the weld must be fully effective.
So, why would one consider a "partial" weld?...(again - what ever your definition of that is..).
So this weld must be a standard full penetration weld.

And that brings us to your next point:

Neither!

The fillet weld, can only carry, or transfer the load from the base plate to the main deck, via the weld.
So, if the base plate is say 10mm thick, (or 3/8"), if you have a weld of throat thickness of say 5mm...this can only transfer 1/2 the amount of theoretical load possible.
If for example, you decide to perform a fillet weld of say 15mm throat thickness, are you transferring 50% more load via the weld?
Why?...well, you now have 50% more thickness available...so that's seems logical...yes?

Well, no!

You can only transfer a load related to the thinnest material that is being joined, in this case the 10mm.
So less than 10mm, means the weld is transferring less than possible. More than 10mm means, well, you are not getting any gain or benefit, as the max possible is still 10mm, even if you have a 15 or even 20mm fillet weld.

And this brings us to the final point:

If you create an oversized fillet, or weld reinforcement as you note, what does this do?

Well, we have already established a fillet weld that has a throat thickness greater than the thinnest material it is being attached to, has no gain.
So, what is the point of the reinforcement then?

That's a good question...and it often refers to fabricators (and some designers) who "think" more metal deposited is a good idea.

It is not..quite the opposite, especially in terms of fatigue.

Apart form the excessive heat input that performing an oversized fillet weld does, and the concomitant change in metallurgical effects, it is all about the geometry.
And over sized fillet weld creates a stress concentration simply because the weld is now stiffer than the would otherwise be....this is because the geometry of the weld, locally, is greater than is needed and in doing so, makes that joint stiffer. And any structure that has locally high stiffness, attracts the load - in very simplified terms.

So, in terms of fatigue strength it is weaker with a weld reinforcement, than if one performs a correct throat thickness fillet weld.

 

Firstly, thank you for your thoughts and response.

By partial penetration weld, I mean any groove weld that does not penetrate the full thickness of the plate with edge prep. For example, a 3/4 plate single beveled 3/8". By full, I mean the full thickness of the plate/part.."

Why must these welds of course only be attached by a fillet weld? Why not a partial or full penetration weld of the bollard of base plate to the deck plating?

Given my definition of a partial penetration weld, if say the thinnest material is the deck plating at 1/2" (~13mm), what about a continuous partial penetration weld of 3/8" single bevel with a reinforcement fillet of 3/8". Together, the weld throats add up to a little over 1/2" (I get that any extra doesn't help but just for an example). Would one prefer a weld like this over just a continuous fillet with a weld throat of 1/2"? This is really what I've been trying to ask through my attempts.

Appreciate the metallurgical perspective here. I had not considered heat input, but if the weld is not "oversized" as I think I've explained above, would this still be the case vs a single bevel + fillet reinforcement with equivalent weld throat by summation?

Again thanks for what you do. Look forward to hearing from you. Cheers.

 

Okay, you’re defining a “standard” weld that is not necessarily aimed at being a fully effective structural weld.

Well, that depends whether the base plate is effectively a doubler – like most – or, whether the base plate is an insert. Two examples, one of each shown here:

Fillet:-


Insert - butt joint:-


The base plate as a doubler, welded directly onto the deck...can only be a fillet weld.
The base plate that forms part of the deck, is an insert, and is a butt weld.

Again, that really depends what type of weld connection you are referring to.

Again, that really depends what type of weld connection you are referring to.

It really comes down to what type of bollard you have and how you are fixing it to the deck!

 

I agree with Ad Hoc; whatever the weld is it should be 100% efficient. For what that is, see the following two guidance manuals. (yeah, old Navy stuff, but it is right)
Mil-Std 22d https://navytribe.com/wp-content/uploads/2015/11/mil-std-22d.pdf
Mil-Std 1628 MIL-STD-1628 TEST METHOD FILLET WELD SIZE STRENGTH http://everyspec.com/MIL-STD/MIL-STD-1600-1699/MIL-STD-1628_8636/

If you really want to get into it, try this paper.
https://deepblue.lib.umich.edu/bitstream/handle/2027.42/169809/hqlu_1.pdf?sequence=1

 

Can you show those?