buit-up wide flange

huangyhg

buit-up wide flange
hi all,
         i'm doing the design work for the curved roof.  firstly, i decided to use the tubular section to bend to form the curve but unfortunately the contractor cannot bend the tube and they asked me to redesign for the built-up wide flange section (the preliminary size is around w10) which they can fabricate from plate gr.a36. i'm not familiar to the built-up section. anyone has any suggestion what's point i should concern in design the built-up section.  can i design it as hot rolled section or i have to design it as plate girder?
thanks in advance
oap
the built-up section is designed no differntly than any other steel shape, including rolled shapes, except that you have to calculate all the section properties and allowable stresses yourself or have a program that does it. section property calculation can be found in any textbook on mechanics of solids. allowable stresses are calculated using the aisc manual, either asd or lrfd.
the only real "special" provision for built-up shapes is to assure horizontal shear transfer between the web and the flanges. horizontal shear v = vq/it (ksi)where v = shear at section under consideration, q = first moment of inertia of flange about the centroid of the shape, i = moment of inertia of the entire section (ix if bending about x axis), and t is the thickness of the web.
to calculate a weld, simply use v = vq/i, which will tell you the horizontal shear in kips per linear inch. a fillet weld made with e70 electrodes is good for 0.928 kips/in, per sixteenth inch of leg. therefore a 3/16" continuous fillet is good for 3 x 0.928 = 2.784 kips/in. in consideration of unbalanced loads on the flange i would suggest welding both sides, or develop better penetration with a different type of weld (partial penetration)and/or welding procedure (such as submerged arc).
you do need to be carful with built-up shapes in that flanges and webs are not always "compact", which lowers design stresses. it is also more important to check for web crippling and buckling at bearing points.
as stated earlier, there are programs that do a lot of this work for you. general analysis and design programs such as staad allow you to input plate sections directly, or allow you to build tables.
if you have a curved beam, be aware that you will most likely have torsional forces developed and a wide flange-type shape isn't very stiff in torsional rotation.
the tube is much better - more stiff in torsion.