stress contour

huangyhg

stress contour
this is a very basic theory question that i am hoping someone would clarify for me.
the image below will lead you to an image from my staad model displaying the plate contour stresses.
clansman,
needs a bit of clarification, what is this that you are modelling?
csd72,
it's a hopper for a crushing plant. here's a picture of it below where you can clearly see the steel plates and the vertical and horizontal stiffeners:
right, maybe its your bad luck today because i'm in a bad mood by another post, so dont get upset, but listen and digest my comments.
looking at your contour plot:
your element definition is cr*p, you have too many triangular elements which are unecessary. and also a finer mesh would do you better.
your peak stress is shown to be a single node, which is either a constraint or load application point, your post does not make clear. but it is probably spurious.
now onto your education, you say that your a recent graduate, but i must ask the question that did you take some sort of solid mechanics/stress analysis/any other similar course when you did your time??
if you want to become an engineer, then i suggest you do some serious reading about the basics about the subject. your question by the fact of you posting it gives grave reservations about your abilities and i would sack you on the spot even it you were an intern of mine.
i wont even answer the last part of your question regarding the mesh sizing and quality issue because if you cant stress by hand how the hell do you expect the pc to do it for you?
if you want to explore how meshing sizes affect the stress levels in fe create a simple quad cantilever beam with a point load, and alter the mesh density around the constrained end, watch as the stress goes to infinity and beyond. then think about why this happens, then come back to this post with your answer, only then will i believe you have the curiosity/determination to become a decent engineer.
please, take up the simple challenge and start to develop, you made it to eng-tips in the first place, so you must have some common sense!!
you have finished your education in the field, now you must start to learn.
pull out your strength of materials text and review the strength theories for metals.  if you never had a strength of materials class you had better take one soon.  your analysis (hand calc, fe model) will predict stresses based on the geometry of the structure, loads, material properties, etc.  then you check the stresses against a failure criteria to determine whether the structure will fail.  a typical metal failure criteria is that failure occurs when the von_mises stress equals the material yield strength.
yes the mesh size affects the stress results.  go get out your fe text and review it.  also see the many posts on the finite element analysis engineering fora on this site.  if you don't understand the effects of the mesh on the model results you have no business running a fe analysis.  if you never had an fe class you should take one of those soon also.
yes, the size of the mesh will definitely affect the stress especially where the stress contours are close together.
the particular code that you are designing to will specify the level of allowable stress or the load factors and phi factors for lrfd. i generally ignore small hotspots as yielding will generally redistribute those stresses. don't forget to look at buckling and overall stability if these are important considerations.
40818,
i think he understands the subject matter better than you give him credit for. someone who didnt understand it would accept that the yield stress is the limit and thats that.
the question may be phrased poorly but i think the op is requesting guidance on how to take the buckling of the plate into account when checking the stress.
clansman,
keep at it and dont be discouraged, this is a hard one for straight out of college.
csd
csd72, i disagree with you there i'm afraid, i dont believe the op has the basic knowledge you give credit for. fair play for sticking up for him though.
i belive however that the op has no usuable knowledge of buckling though, just looking at the fe mesh gives me that indication. if the fe was serious about buckling then it would be very non-linear, the plot shown doesnt even show the stiffening   
let me clarify,
1. my question was about the criteria that we use to define failure of the plates. why do we consider yield only and not something like deflection for example... will deflection never govern if plate is below yield point? (deflection is what i meant when i said support spacing in the original post)
2. the geometry of the structure is extremely complex. i have panels at many different beta angles and horizontal, vertical and diagonal stiffeners all over and in order for me to avoid beam plate connectivity issues in staad, i had to model the elements the way they are shown in the screenie. but if you think that is not necessary, perhaps you can give me a few words of advice for an alternative path?
3. where you see the hot-spot in the screenie, there is a support leg on the other side of that panel (not visible in the screenie,) which is why i think that hot-spot is present there and not because of erroneous results.
4. i acknowledged that finer meshing will improve results, my question was whether or not making my meshing finer would significantly alter the stresses that i already have. i'm pretty sure it will and i will do just that, but i just thought i might as well ask.
again, this is my very first fem task and i am learning.
thanks.
clansman,
you should always check the deflection of your structure to ensure it is within allowable limits. you can have quite large deflections in reality and still be linear, it depends upon the radius of curvature of the item. if your new to fe, then i would keep with a linear model, rather than try to understand non-linear large displacement fe. just be carefull if you wish to show stability with a linear fe (i.e have very good margins) because linear fe buckling is pretty much nonsence. yield point is a handy failure point for the basis of an analysis, but not the only one. ultimate strength, max deflections are also used, some structures have high fatigue requirements and need to have very low working stresses. depending upon the application, its governing criteria and the engineer in charge, the failure criteria will be defined.
without knowing what your actual structure looks like it is hard to say what the mesh should look like, though try your best to not have triangular plates as they are overly stiff.
the peak stress is developed very locally around a single node and a better element definition around the area would smooth it out. i would still tend to ignore the peak as being nonsense here though.
if you have a very fine mesh your model is closer to reality. however, just by throwing fine meshes at it wont make it a good model. you can end up just sending your stresses up and up by simply increasing the mesh density. you need to understand what is truth and what is lies with fe. its been said many times about nonsense in = nonsense out and this is so true with fe, always think about what you want to achieve with the model and what you expect the answers to be. to this extent do hand calcs so you know your in the right ball park.
no matter how long somebody is in the business they will never know anywhere near all their is to know. thats one of the beauties about places like this forum, where you can ask advice from the field experts. what should be carried out before posting of a question is trying to determine the answers themselves. you can do this by reading books, searching the web etc. this helps the person develop their own understanding rather than just being blindly told what to do. many young engineers fall initially into the trap of believing they know all they need to know because they are still on the high of graduation. and because of this its hard to get them to "learn". i dont think you fall into this catagory however. dont take my comments as any personal dig, firstly your college should teach you about things like this, and also point you in the right direction to decent books. your boss should also give better guidance and explanation to help you understand. your part would be to take these pointers from your college and boss and then research them. places like this help that research, but do not complete it. keep plugging away.
did you use the mesh generator? (probably yes...)
here's how i would approach the problem (and how i approach a project that i know will require a lot of fem - using staad, 'cos that's what i've got - iterations based on a lot of loading cases): i draw the framing first and make sure i understand my coordinates and such. with staad, you can either draw it directly in staad using the structurewizard or not, or you can draw it in a cad program and import the model, etc. then i set up the framing, in staad, giving all the framing