WELDING DESIGN CALCULATION FOR PRE ENGINEERING I BEAM

well as far as i know you cannot calculate it in staad . 

if you talking of fillet weld between  flange and web , you need to claculate the stress in shear ( consider resultant of Fy and F z shear force) at critical section taken in to consideration , and compare it wiht allowable stress of weld ( 0.3 x Fu of weld ) 

beam flange is 80*500mm and the web is 20*940mm .combined stresses at mid of the beam is 88N/mm2 .so what will be the leg size of the weld?

as mentioned earlier , you need to only consider stress due to shear . combined stress  should  not be used  for weld check . 

choose a weld leg size  and calculate tau 

stress due to shear  - tau = F Ay / (Izz  x b) 

F = shear force on the section   =   sqrt ( Fy2 + Fz 2) 

Fy & Fz = shear stress on the crossection along axis parallel to web and perpendicular to web ) 

A y = (topFlangeWidth*topFlangeThickness*(webDepth/2+topFlangeThickness/2))

b = 0.707 x weld leg size 

this tau < =  0.3 Fu                                                                                          (  as per AISC)

Fu =  ultimate stregnth of weld electrode .

hope it is clear .

as mentioned earlier , you need to only consider stress due to shear . combined stress  should  not be used  for weld check . 

choose a weld leg size  and calculate tau 

stress due to shear  - tau = F Ay / (Izz  x b) 

F = shear force on the section   =   sqrt ( Fy2 + Fz 2) 

Fy & Fz = shear stress on the crossection along axis parallel to web and perpendicular to web ) 

A y = (topFlangeWidth*topFlangeThickness*(webDepth/2+topFlangeThickness/2))

b = 0.707 x weld leg size 

this tau < =  0.3 Fu                                                                                          (  as per AISC)

Fu =  ultimate stregnth of weld electrode .

hope it is clear .