welding a built-up section

[tolind]

I'm trying to design a builtup section to solve an existing problem. I need to increase the moment of inertia for a roof beam for a monorail to be added. I've determined that if I add a 3X1/4" plate to the bottom of a W12X19 I get the properties I'm looking for.

I know that I need a lap joint fillet weld to attach but I don't know the size and the places I'm looking aren't spelling it out clear enough for me. I figured I'd use a 1/4" fillet weld. My max. shear is 8.5 Kips. A 3" weld both sides every 12" should work. Can anyone explain the design process to me?

Thanks.

 

[dik]

You only need to weld to develop the shear at the section under consideration... then I usually weld at the end to develop the full capacity of the added plate. Can you weld the monorail section to accomplish the increase in section property?

Dik

 

[henri2]

"Steel Structure" by Salmon and Johnson, and "Design of Welded Structures" by Omar Blodgett are good resources. I am sure there are a host of other user friendly references out there that will explain the design process.

BTW, the maximum size fillet weld on the edge of the 1/4 inch thick place must not exceed 3/16 inch unless the weld
designated on the drawings to be built out to obtain full-throat thickness: AWS D1.1 Sec 2.3.2.9 (2) & 2005 AISC Spec for Steel Bldgs Sec J2.2.

Since this is for a monorail, will fatigue be considered? ref: D1.1 Table 2.4, item 3.4

 

[UcfSE]

The equation you want is the shear flow equation: f=VQ/I with which your results will give you force per unit length along the plane between your plate and the wide flange. You can look up the definitions of V, Q and I in any mechanics of materials book and most likely online as well. With the shear force per length, you can determine your weld spacing for a given weld size. You should also develop the full tension force at each end of the plate with a weld as dik mentions.