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AISC - Minimum Thickness to Match Weld Strength

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RFreund

Structural
Aug 14, 2010
1,881
In chapter 9 of the AISC360 manual there is a fairly well known equation for the minimum required thickness of the base metal. The manual states that you may calculate a thickness of metal that will match the available shear rupture strength of the base metal to the available shear rupture strength of the weld. The equation uses the strength of longitudinally loaded fillet weld as the available shear rupture strength of the fillet weld.
My question:
In cases where the strength of the fillet weld is increased due to the "Effect of Load Angle" (which may increase the fillet weld strength up to 50% for a transverse load), does the minimum required thickness of the base metal need to be increased?
Why? Why not?


EIT
 
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I would think that, at least in theory, the effect of the load angle could be taken into account for the base metal similarly to how it is for the weld. I haven't delved into the provisions you refer to, but presumably the effect of the load angle accounts for the full load not being applied as shear on the weld. If the geometry of the base metal results in a similar effect, then it should be reasonable to account for it.

At the end of the day, both the base metal and the weld have to be strong enough to resist the loading applied to them, or the connection fails. Presumably, the provision is simply meant to ensure that the weld would fail first if the connection is overloaded.
 
IMHO, the short answer is yes, if you want to develop the full strength of double fillet welds.

The long one:
First, you need to make sure there are fillet welds on both sides to make direction faction valid.
Second, I think the prying check in Chapter 9 of the Manual should govern the plate thickness anyway.
Last, that minimum thickness equation is in the Manual, so it's only recommended, not code complied. The reason why this equation is well known, is to save engineers' time to avoid checking the capacity of the connected parts. If you know what number you need, then the minimum is not the real minimum, you can specify whatever you want.
 
Thanks for the responses. Let me add an example to this situation.
AISC page 9-5 gives an example of beam web welds for a double angle connection. They state that "the strength of the beam web" cannot be directly calculated. They then say that is convenient to match the shear rupture strength of the base metal to the weld rupture strength. They then give the equation:
t.min = longitudinal filet weld strength / shear rupture stress of the base metal.
Back to the example:
We have a double angle connection - bolted to girder web, welded to beam web. The beam web is thinner than the minimum then we must reduce the capacity of the connection. Let's say L4x3.5x3/8 x 9" long with 1/2" setback.

Using Instaneous Center of Rotation Method you can find the strength of the weld. The ICoR method considers the weld shape and the strength increase for a fillet weld loaded at an angle. If you use the tables you can find D_min. You could then use this D_min to find your minimum web thickness (t_w_min) based on the chapter 9 equation (eqn 9-3) = t_w_min = 0.6*F.ex*0.707*D_min/(0.6*F_u). The D_min though is being put into the strength equation for a longitudinally loaded fillet weld.
The issue I have with this, is that you found your required minimum weld strength based off an analysis that considers the increased strength of a fillet welded loaded at an angle, but then you find your minimum required thickness based off the equation for a longitudinally loaded fillet weld.
Maybe I am talking my self in circles here, but I'd be curious to know your thoughts on this.

Thanks!


EIT
 
It is a requirement of AISC 360-10 Section J2.4 that welded joint strength be based on "the lower value of the base material strength determined according to the limit states of tensile rupture and shear rupture and the weld metal strength determined according to the limit state of rupture". One method of accomplishing this is to compare ΦRn or Rn/Ω for the base metal determined per the lesser of Equations (J4-3) and (J4-4) against ΦRn or Rn/Ω per Tables 8-4 trhough 8-11 (utilizing the instantaneous center of rotation method), where Rn = CC1DL. Put another way, I believe the OP is correct that the increased weld strength afforded by taking load direction increases into account could necessitate a thicker base metal or a base metal with greater Fy and/or Fu.
 
So the problem I have with my own statement is that all the commercially available connection software and design examples that I have seen do not take this into account. I'm actually not sure how you would take this into account.
Anytime you solve for your "D_min" using ICoR method you are using some sort of strength increase that accounts for the load angle.
Once you substitute this back into your weld strength equation you will find a required web thickness that is less than what you would need if you did not use the strength increase afforded by taking load direction into account.

EIT
 
I think I solved my own problem and saved every double angle connection ever designed from being questioned [upsidedown].

When you account for the angle of the load you get an increase up to 1.5 when the weld is loaded transversely. When the load is transverse to the weld the base metal would be in tension. Your capacity of the base metal would be based on tension rupture (F.u*A.e) which is 1.67 x as strong as your shear rupture strength (which is 0.6*F.u*A). So I think because we back out the weld strength into it’s longitudinal strength and compare this to the shear rupture strength we are OK.

Would still be curious to hear other thoughts though.
Thanks.

EIT
 
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