weld cracking problem

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I am making a channel by welding 2 sides of A36 metal .625 thick to a piece of T-1 plate (ASTM A514) .5” thick.  The A36 side are only 2 inches high and the T-1 top is 7” wide.  The whole assembly is 2 ft long.  This manufactured channel is placed so the legs are down (so it looks like a squatty “n”) and welded to a large, stiff base structure.  The channel assembly is loaded on the top T-1 plate.Weld cracks have developed in the welds between the side plates and the top plate of this manufactured channel, after 2 years in service.  I butt type weld applied on the outside of the channel assembly only was used to originally to join these two members.I am thinking a full .5” penetrating V weld with a 45 degree opening would be much better and not crack due to more welded material.  I am thinking the loaded center top plate is causing the butt weld to want to pull apart in tension as the top plate edges try and lift off the outboard edge of the legs while pivoting on the inboard edge of the legs, (I had to draw a quick sketch of this).  It is not possible to weld on the inside of the channel.  Any suggestions for a better type of weld?  Is full penetration for a V type weld a good idea with the relative sizes of materials?
Reply:Did you pre and post heat?  How did you weld it stick, mig, tig?  What rod or wire did you use?
Reply:Can you weld the inside corners? If mig welded;If you can weld the inside corners then a 3/8" bevel on the A36. If you cannot weld the inside corners then a 1/2" bevel. 200-250 degree pre heat, use a temp stick or contact thermometer, don't guess. Spray transfer weld with 80 series wire. I like Hobart Bro. Quantum Arc ER-80S-D2. GRIND ALL SCALE from weld zones before fit-up.If stick welded;Same as above but use 8018 rod and make sure you thoroughly clean the slag between passes. If you need to, open up the bevels so you can get the weld all the way into the bottom of the V Attached ImagesWill
Reply:Gabzip: Stick welding used, 7018 rod, no preheat.Will: What you sketched is what I was thinking.  ANy problems with grinding both plates to make a "V"? or is that not really necessary?  Thanks for the comments.
Reply:Oh yes, and I can't weld on the inside.
Reply:I would try a 250 deg pre-heat and a slow ramp down post-heat with a deep V on both parts.
Reply:Always preheat T1 ...  It would be wise to bevel the end of the plate so that the weld crosses the section of the plate.  If 4on12's picture is true the bevel on the outside is incorrect and if done that way will risk lamellar tearing of the T1 plate.