aluminum pipe design

huangyhg

aluminum pipe design
i am trying to size aluminum pipes for spanning between aluminum posts. the connection at post is by welding. it seems like the allowable stresses change when we use weld. according to aa manual, the material affected by welding for this particular alloy, has less than half (5.5 ksi) the allowable stress. i am new to aluminum design. any help or guidance is greatly appreciated.

that is correct.  the allowable stress on aluminum drops dramatically when welded.  
just in the area of the weld i believe.
that is correct, only near the weld.  it's been a while since i designed with aluminum but i believe it is for a certain distance from the weld, not necessarily at the welded section only.
well if you think about it any type of substance is going to have an effect by it's environment. plus aluminum by it's self is not as strong as an alloy with another metal, espically when it's tempered. you auto matically weaken when you het it up again, unless it is tempered.
kxa,
   your pipe probably is something like aluminium 6061-t6.  the code "6061" specifies the alloy, which contains silicon and magnesium, and is heat treatable.  the code "t6" specifies the heat treatment.  the material has been solution heat treated and artificially aged.  i do not understand the process.  i am just copying this information out of an old machinery's handbook.
   most practical aluminium alloys either are heat treated or work hardened.  when you weld them, you weaken them.  this effect is far more significant than it would be if you used cold rolled steel, which as far as i know is just work hardened a bit.
   in the case of any welded aluminium alloy, you need to look up the mechanical characteristics of the -0 version of the alloy.  also, you need to read up on and specify the weld filler.  your assembly sounds too large to heat treat.  unlike steel, aluminium fatigues to destruction.  
   i have designed welded frames out of aluminium angle.  i used large gussets to provide the relatively weak welds as much leverage as possible.  the ends of the angles were machined and matched carefully, and the gussets were installed and welded inside the angles.  i specified intermittent welds to reduce warping.  the results looked good and they were strong enough for the non-critical application.
   i am a technologist mechanical designer, not a structural engineer.  i have never seen a building or bridge with an aluminium structure.  i have always figured i understood why.  
                   jhg
don't weld it unless you have to with aluminum.  the reduced allowable is only for a certain distance from the weld (see aa design manual), but most often this is in the highest stressed location.  move the weld joint, go bolted, or design for the lower allowables (5 ksi or so). otherwise it is lots of gussets or redesign the weld joint to be groove or full pen versus fillets. once you get a feel for how to handle the joints, you can do a lot with aluminum.
zcp
the weakest point in welded 6061 is in the haz, about 0.5 cm outside the fusion zone. in this area, the strengthening mg2si precipitates have been re-solutionized. post weld heat treatment (artificial aging) can recover much of the strength loss, although there is a slight decrease further from the weld due to over-aging.
"the pwa condition represents a weld that is subsequently aged for one hour at approximately 400°f (205°c). post weld aging improves the mechanical properties for both t4 and t6 starting materials. in fact, often times it is better to weld in the t4 condition and post weld age after the welding process."
i believe that the code specifies anything within 1" of the weld is to use the reduced values (probably to allow for splatter e.t.c.).
5.5ksi is very low, if you use 6061-t6 you should get 11 ksi out of welded tubes.
also specify appropriate filler metal to be used, there is a table in the code i think.
csd
thank you all for responding. it is true that within 1" of the weld the allowable stresses are much less. the bad thing is that that is where the stresses are high.
csd72, i don't know where you got the 11 ksi since whereever i look, for transverse welding, i get 5.5 ksi.