designing a combination section

huangyhg

designing a combination section
i am designing a s12x40.8 beam that needs a c10x20 placed on top to form a combination section.  i would like to show a stitch weld to attach the two sections together.  how do i calculate the size and length of the weld to assure the combination section will act as a single beam? i have looked in several books and can not find any answer
what sort of loads are being transferred across the welds ? ... shear, bending, axial ... i could guess but why ? ...
the beam has a trolly on the lower flange carrying 5.5 tons.  
check shear flow at the interface, f = vq/i.
i guess this is a motor driven trolley, in such case, i will use bolted connection rather than weld.
the contractor wants the connection to be welded.  this beam is being added in the field in a tight spot.  i was just uncertain how to design the weld to make a combination section or "built up" section.
calc the shear flow as mentioned above and size the welds accordingly.  try a stitch weld 3"@12" - with the interface being near the top of the section, the shear flow is likely to be small.
wuth the number of holes that would have to be drilled to a 2.75" gage, field welding would be the better option even considering the uphand weld condition.  
and, yes, vq/ib is your equation of choice to find the shear flow.  you will not find any tables in aisc publications for this type of connection, although you will find combination section tables.  the 9th edition has your specific combination on page 1-83, but you have to calculate the interstitial shear for the connection and design it yourself.
mike mccann
mmc engineering
when i go to calculate q do i use the area of the channel and the area of the flange at the connection?  or do i use the area of my combined section?it has been along time since i did mech of material.  
i believe i figured it out
thank you
check the fatigue. stitch welds performed poorly under repeative loading/high stresses reversal, and vibratory environments. you will find bore holes on site is not that difficult and unusual, provides there is adequate space to lay the beam on ground. on the other hand, the quality of field welds is quite of a concern.
i second kslee1000's last comment.  use a continuous weld.