Limitation of fillet weld size 1

[CHARLESCOSTA]

What is the rationale behind limiting fillet weld size as .8to .9t,where t is plate thickness?Is this also valid for T joints(double fillet)?One of the reasons that appears to me is that there is no point in making weld stronger than the plate. In addition to this,isit also(or only)related to reduction in properties of plate in HAZ?In
double T joints can the sizes of both fillets be say .9 t?

 

[nutte]

This limit wouldn't apply to a T joint, with fillet welds on each side. It only applies when you're welding along the edge of the plate.

The reason for it is because the weld can melt back the corner of the plate, which would make the weld appear to have a wider leg, and thus larger throat, than it really does. If you hold the weld size to less than the plate thickness, the corner won't melt back and you won't have a potentially deceptive weld size.

For a sketch of what I'm talking about, look in the 13th edition AISC steel manual, page 16.1-331, in the Chapter J Commentary. In the 3rd edition LRFD manual, it's on page 16.1-233.

 

[twinnell]

Fillet welds of a T joint can be any size you want...but why deposit more weld than the plates can take. Refer to page 9-5 of the AISC 13th ed for an equation that equates the weld rupture strength of the base metal to the weld metal. Same page for single sided fillet weld as well.

 

[CHARLESCOSTA]

Thanks nutte and twinnell for your helpful response.I am now clear on it. Unfortunately I don't have access to aisc manual.Mukhonov states on page112"The strength of side welds also depend on their size,the ultimate strengthdecreasing somewhatwith anincrease of the size.The size of the weld is stipulated in standards and is
taken not over 1.2 t(t is the minimum thickness of the elements beig connected)". I believe he is referring to maximum weld size under exceptional cases.Reason given by you about melting of plate top edge has made it clear.