I am helping a client who needs to qualify a procedure for a rebar indirect butt joint procedure. We have settled on welding Fig 6.5 (c), which is rebar in angle iron. But I am having trouble calculating the required weld length, or at least not believing the figure I come up with.
Per the note on Figure 6.5, the length of the weld, L1, is calculated as follows:
L1 = (5.23 * Fu * D) / (Fxx * n)
Where Fu is the rebar tensile strength, Fxx is the filler tensile strength, D is the bar nominal diameter, and finally, to quote the standard,
"n = number of connection welds between developed bar and splicing materials"
This "n" is what I am having trouble with. My client is planning to weld a #8 rebar (1" dia) with 90 ksi tensile strength, using 70 ksi tensile strength filler. On Fig 6.5 (C), the "number of connection welds" appears to me to be 4 - two each on each piece of rebar. So, by my calculation, the length of the weld will be:
L1 = (5.23 * 90 ksi * 1in) / (70 ksi * 4) = 1.68"
But a 1.68" length of weld seems absolutely tiny, and gives only 3.36" of weld per rebar (because their are two welds per piece of rebar, one on each side) to pull for a tensile test? Just looking at the setup, it seems absurdly small. Is this correct? Am I understanding "n" correctly?
Thank you for your time and assistance
You are attempting to develop sufficient weld strength to equal the strength of the reinforcing steel. As you stated, there are two welds per bar, not four. So, you should used 2 for the value of n.
Developing the full strength of the rebar without considering the allowable stress, but using the UTS of the bar -
Area x TS = 0.785 x 90 = 70.7 kip
Develop the strength of two flare bevel welds on 1" rebar
Size of the flare bevel 5/16 X Radius of Rebar = 5/32 inch assuming SMAW is used
5/32 X 70= 10.9 kip per inch of weld
Length of weld required - Capacity of the rebar / Capacity of weld per inch of length
70.7 / 10.9 = 6.5 inches
Two welds per bar, so 6.5 /2 or 3.3 inches of weld per flare bevel groove if 70 ksi electrode is used.
That agrees pretty closely with the calculations provided by the code, i.e., 3.4 inches of weld per flare bevel groove.
If you were using these calculations for actual construction I would warn you that an engineer should be doing the calculations. Since this is for procedure qualification, the worst that can happen is the welds will be too "weak" and they would fail before the rebar did. If that is the case, simply increase the length of the flare bevel groove welds. The calculated values in this case simply matches the strength of the weld versus the strength of the rebar. They are not the size of the welds used for production. The EOR should specify the length and size of the flare bevel groove welds on the drawings. If they are not, then the drawings need to be revised and approved by the EOR.
Best regards - Al
That is incredibly clear and helpful, thank you.
I was trying to do the calculation cross reference with the "60% of bar shear strength" mentioned for indirect butt-joint tensile testing myself, but was unable to figure out how to calculate the area of the weld metal. I see how you did it, and it makes perfect sense, and I agree that the correlation between that value and the 'n' value as 2 for that weld indicates it is correct.
I am not doing any engineering calculations, only length calculation for the PQR. The actual construction has been more than adequately calculated, I'm sure, as it is in a critical aspect of a refinery.
Thank you again.
(and I love your signature)
Topic American Welding Society Services / Technical Standards & Publications / AWS D1.4 Indirect Butt Joint (Rebar) PQR Question
