As with all rules-of thumb, there are a million exceptions, but the one you are probably looking for is that a double fillet weld, (of matching material strength) will be as strong as the base metal when the throat thickness is 1/2 the thickness of the thinner member.  This does not take into account stress risers, fatigue, etc, only strength of materials.

A reference is contained in Lincolns "Design Of Weldments" on Page 6.2-7. the paragraph states

"(a) for full strength welds, the leg of the fillet weld must be about 75% of the plate thickness."

Here is my take for double fillet welded joints

The effective throat of an equal leg fillet weld is .707 x leg size. .707 x .75t = about .53t. 2 equal leg fillet welds with leg sizes of 75% the thickness of the base metal would result in a fillet weld with an effective throat of 1.06t.

Good day

Thanks all.

I'll look through Blodgett's (Lincoln Press') Design of Welded Structures again to verify this.  I figured this information was in there, but couldn't find it.

--Scott

When a full strength welded joint isn't required, I use the following guideline.  These values are (mostly) from Bilichniansky's The Designers Manual.  It has been my experience that these size recommendations produce weldments that "look right" to most welders, engineers and managers.

Thinnest                     Fillet
Member                  Weld Size
----------------------
 1/8                            1/8
 3/16                          3/16
 1/4                            1/4
 3/8                            1/4
 1/2                            3/8
 5/8                            3/8
 3/4                            3/8
 7/8                            1/2
1                                1/2
1-1/4                          1/2
1-1/2                          3/4
2                                 3/4

According D1.1 the following minimum sizes apply:
                         Table 5.8

   Base Metal Thickness                     Min. fillet size
       T= <1/4"                                 1/8"
       1/4"~1/2"                                3/16"
       <1/2"~3/4"                               1/4"
       3/4" plus                                5/16"
*** leg size need not be greater than the thinner of the two parts joined.

refer to 2.4.5 and find maximum fillet weld size...
   (1) for base materials 1/4" or less full plate thickness
   (2) 1/16" less than the base metal thickness, unless the weld is designated to be built out to obtain full throat thickness.

hope this helps....

Richard Schram
Mechanical Integrity Specialist
Pharmacia Global Supply Arecibo-P. Rico
rschram@pharmacia.com