predicting failure load

huangyhg

predicting failure load
i have the rare opportunity to test a couple of old a-frames to failure.  these are old a-frames that have been inspected and determined to be unfit for refurbishment due to a combination of weld cracks and corrosion.  i have been asked to predict the theoretical failure load of these two a-frames when they were new.  i primarily use visual analysis to check the overall design using aisc code checks combined with some hand calcs and every once in a while ansys for potential problem areas.  how much "extra" is left between a unity of "1" and theoretical failure?  does it vary depending on the particular aisc equation used, or are they fairly consistent?  the cross member is loaded primarily in bending and the legs are a combination of bending and compression.
after the units fail we are going to send some of the material off for testing to get an accurate yield strength of the material that failed and use that info to adjust the model and see how close our models are.
in theory at least, the failure load of the structure should be the resistance of the weakest part of your truss under your test loading conditions, not including any resistance factor.
note that this approach ignores both helpful parameters (over strength, strain-hardening, etc.) as well as factors that may reduce your strength (for example, the cracking in the welds and reduced cross-section you mentioned).
note: to come up with a failure load, you must also come up with failure criteria (i.e. what constitutes a failure? excessive deflections, the first   
from my general knowledge of the structure, i am pretty sure the failure is either going to be yielding of the legs right above the connection for the hydraulic cylinders or it is going to be a catastrophic "boom" of the weld holding the hydraulic cylinder padeye to the leg and the entire structure will come crashing down.  i'm not going to begin to try and predict the failure load of the padeye weld because mpi inspection has shown cracks in the weld.  
my hope is to predict the failure load based on the yielding of the leg and hope the padeye weld will hold on one of the two frames so i can get a correlation between our models and the existing frame.  this is a secondary goal though so i am not allowed to strengthen the connection.  the primary goal is to find out what the failure load is "as is".
i'm thinking that i should combine the bending and shear stress to get the von mises stress and compare that to the theoretical yield stress of the material and after the test compare that the actual yield stress once we complete the material testing.  thoughts?
it depends if you are talking abot lrfd or asd.
the theoretical failure load is when the stress equals fy or fu (depending on how it fails) whereas, for example, the allowable stress for bending is 0.6fy for design.
i use asd.  i guess i am trying to figure out what the factors are that are built into the various aisc asd equations compared to the yield strength.
csd72..
i disagree. the failure load should be independent of the method of analysis. the true resistance of any member is some nominal value. this value is, in theory at least, the expected failure load. asd and lrfd simply are means of determining what load can safely be applied to the   
i wouldn't apply a code... they are by nature empiracle and safety-oriented.  what you want is fundamentals, such as raw bending strength and tensile strength in welds.
you don't need to apply lrfd or asd.  neither should be applied...  what you do need is to include the overstrength which frv mentioned (need to get this from the rolling mill who produced the steel), or in a pinch you could use whatever the standard overstrength factor currently is in the usa.  here in new zealand it's 1.25, so you can get a lot more strength.
i would suggest:
mn = 1.25zxfy (where zx= plastic section capacity)
welds = code weld strength/resistance factors applied
good luck, take lots of phtos, and please please please let us know how it goes!
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
thanks ys.  i think i will look at just the stress values.  question is on how to combine the bending stress (both axis) and the shear stress.  i figure going back to the basics and using these to calculate the von mises stress would be the easiest and then comparing that to the yield strength would give me a reasonable estimate without spending days, if not weeks look at a non-linear anaylsis, plastic deformation, etc.
we haven't made a final decision yet on whether we are going to use water bags or rig up a hydraulic cylinder.  the hydraulic cylinder will be more contolled, but may not give us the catastrophic failure we are after.  water bags will be quite dramatic, but i have a feeling we will damage the bags if we have a sudden catastrophic failure and the frame comes crashing down on top of them.  there would also be a good chance of the water bag splitting open like a giant water balloon when it hits the ground.  water bags are not that cheap.  a hydraulic cylinder could be put at the other end of the wire away from the a-frame so it wouldn't be damaged.
we are going to video tape the entire affair, but i doubt my client will make the results public and publish the video.  i have done some cool stuff in the past, but this will take the cake.  i grew up listening to my dad telling stories of destroying things in the lab (on purpose) while testing down hole oilfield tools.  now i will have a good story of my own!
sounds like a good plan, too bad about the photos though!
cheers,
ys
b.eng (carleton)
working in new zealand, thinking of my snow covered home...
yield in the extreme fiber is rarely the limiting load.  that is why the plastic section modulus was referenced in previous posts.  compression buckling would be my guess as to the failure mechanism.
concur with ys, this is a "strength of materials" situation, not a code issue. anyway, failures almost always occur at the connections, not the