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Allowble Shear Stress on Fillet Weld for Seismic Loads

Allowble Shear Stress on Fillet Weld for Seismic Loads

Allowble Shear Stress on Fillet Weld for Seismic Loads

(OP)
Hi,

How much increase in allowable shear stress (Weld Strength 0.3Sut or Base Metal Strength 0.4*Sy) is allowed in the presence of Seismic Load per AISC ASD Manual and AISC N690 Manual.

Thanks,
K Nandwana

RE: Allowble Shear Stress on Fillet Weld for Seismic Loads

None.  Current code provisions do not allow increases in allowable stress for seismic or wind loads. In fact, many connections have to be designed to resist an amplified seismic load.

RE: Allowble Shear Stress on Fillet Weld for Seismic Loads

(OP)
I could'nt find it either, but isn't it strange!!

If I can increase the allowable shear stress in Base Metal by 1.4 per AISC N690, then why can't the allowable Stress on the base metal mating surface(0.4*Sy) can be increased while sizing for welds, as long as it is less than 0.3*Sut.

Specifically, for my case I am using ER308L filler material (Sut = 75 ksi) to weld SA312 TP304L Pipe (Sy = 25ksi)

Now under normal load condition allowable Shear Stress on weld throat :

0.3*Sut = 0.3*75       = 22.5 ksi
0.4*Sy  = 0.4*25/0.707 = 14.14 ksi

Allowable Stress on Fillet weld throat : 14.14 ksi

I am of the view, that in the presence of Seismic Load my allowable stress should be =

0.3*Sut = 0.3*75           = 22.5 ksi
0.4*Sy  = 1.4*0.4*25/0.707 = 19.796 ksi

Allowable Stress on Fillet weld throat : 19.8 ksi

Any comments ?

Thanks
K Nandwana

RE: Allowble Shear Stress on Fillet Weld for Seismic Loads

Welds aren't ductile failure modes right?  With seismic, you are attempting to force a ductile failure mode in a brace member, or through bending in a beam.

The connections are required to be sized to a much HIGHER load than the ductile portions of your structure.

Agree with blackmaddog.

 

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