Allowble Shear Stress on Fillet Weld for Seismic Loads

[kapilengtips]

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

 

[blackmaddog]

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.

 

[kapilengtips]

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

 

[JAE]

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.