i recommend splitting your "huge" job up into several major sub assemblies.
when you say "assemble" your job, do you mean that you have already created the parts or that you are trying to figure out how to start the job?
if you are staring out on the new design, then consider that each major sub assy or zone of the platform can look at a dedicated sketch, that will define each level critical beams and cross members etc.
try to define each zone practically for your needs and for the workshop to handle, ie make them in nice bite size chunks and then put those smaller weldments or zones together to form larger sub assemblies and so on.
so, you need to make some easy to manage sketches for example for each level of your platform, but tie them to a master sketch that would for example define the overall boundary or foot print of the platform.
take some time to set these sketches up nicely because if you need to change things half way through the design you do not want to have difficult sketch changes.
the sketch will operate as a skeleton that frame generator can use to position each member.
also, take some time to think about whether you need to give each part your own part number or whether you are happy to use default frame generator names.
i suggest you do a few small practice designs before you embark on your real job so you can learn what will and will not work for you.
there are other ways to position frame member, so maybe go through some of the tutorial help examples as well.
that is the concept i would suggest to start with. again, there are many ways to design things. i am not really sure where you are on all that right now with your original post not overly descriptive.
Inventor 2014 Pro on Windows 7, 64bit
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