Butt Joint Weld Questions

[Steel5]

I'm hoping someone can help me with some confusion I have on welds. Let's say I have a butt joint between equal thickness plates and the welding process is SMAW. I want to avoid CJP and I'd like to weld from one side, so I'm considering one of 3 options between B-P1c, BC-P2, and BTC-P4, as shown on pages 8-52, 8-53, and 8-55 of the AISC Manual 14th edition, respectively.

Let's say I have 1/4" thick plates, the largest Weld Size (E) I can get out of each is:

B-P1c:
Weld Size (E) = T1/2 = 1/4" /2 = 1/8"

BC-P2:
f = 1/32" min
S = 1/4" - 1/32" = 7/32"
Weld Size (E) = S = 7/32"

BTC-P4:
f = 1/8" min
S = 1/4" - 1/8" = 1/8"
Weld Size (E) = S - 1/8" = 0"

Let's say I have 5/16" thick plates, the largest Weld Size (E) I can get out of each is:

B-P1c:
N/A, not permitted for thickness over 1/4"

BC-P2:
f = 1/32" min
S = 5/16" - 1/32" = 9/32"
Weld Size (E) = S = 9/32"

BTC-P4:
f = 1/8" min
S = 5/16" - 1/8" = 3/16"
Weld Size (E) = S - 1/8" = 1/16"

First of all, is my understanding of these tables and the equations correct? If so, here's my confusion:

Why is BTC-P4 weaker than B-P1c? In both situations the plates are butted up against one another and welded, but one has a bevel. Intuitively I would think any bevel would provide better weld strength than no bevel.

Also, while I understand why BC-P2 is stronger than PTC-P4, why is it by such a large margin? In 5/16" thick plates BC-P2 is 4.5 times stronger than BC-P2, and in 1/4" plates BTC-P4 provides zero strength, which I also find hard to believe.

My ultimate goal is to specify a non-CJP weld that can be done from a single side that provides reasonable strength while minimizing unnecessary fabrication.

As it stands, it seems that I should be specifying B-P1c for 1/4" plates and thinner, and BC-P2 for plate thicknesses of 5/16" and thicker. BTC-P4 just doesn't seem to have a place until you get to much thicker plates such as 1/2" where the weld size would be at least half that at 1/4". I just don't like that I have to add the fabrication cost of beveling both plates, I wish I could use just one bevel but these tables and equations suggest I get little to no strength out of that. So if I have a 5/16" plate, am I really stuck beveling both plates?

I hope all of this makes sense. This isn't for a specific project or anything, I'm really just looking for general guidance/wisdom on best practices for butt joints like this. Thanks for any help.

 

[BMart006]

It appears you are reading the tables correctly. The differences arise from the root opening. Yes, a bevel typically allows for more penetration, but not when the root opening is significantly smaller than that of the butt joint. With regards to the single V (double bevel) versus single bevel joints, consider that a bevel weld's strength only comes from where there is solid (sound) weld. When you bevel just one plate, the weld has to be placed in a relatively tight area. Also, the weld does not always "sink" to the bottom of the V, leaving a gap. This is the reason for the "-1/8" in the effective throat size. When both plates are beveled such as in a single V, the opening is larger and the angle is not as tight, allowing more weld to penetrate to the very bottom of the V. In that case, there is no calculated reduction for the effective size. To further illustrate this, consider that the single bevel has to be made at a more severe angle than each plate of the single V due to the tighter gap and need for weld to "sink" down to the bottom of the gap.