Yes, you are correct, the actual design would look at the natural frequency of the trailer, and the frequency of the road generates in the trailer due to the control joints in the the road. I have a specialist who will do this loading.
I am looking for a printed standard that I can reference for safety factors, as well as loading combinations.
First off, what type of semi trailer? Platform, box, tank, end dump, belly dump? Should I go on?
All those have different types of frames and most every type mentioned above has a frameless design.
I have not seen anything published although I have worked with several different trailer manufacturers and I know that they have their own in house propriatary designs. THe frameless tankers while slightly thicker in the tank wall than the frame types are still said by their designers to be lighter than the frame designs.
Some boxes use very little frame and use the box itself as part of the load carrying structure. YOu can watch those going down the highway and watch the stress wrinkles in the side walls every time the thing hits a bump.
Frameless end dumps are lighter, but much more suseptable to tipping over on uneven ground.
All designs have trade offs.
THe DOT regulates how you load the trailers, that is to say how the loads are transmitted to the ground and more importantly to bridges, but they don't care if the trailer is substantial enough to carry the load or not. There are bridge law formulas that give axle spacings and the like for brige law requirements.
My tip is to go to the Society of Automotive Engineers (SAE) website and see if they have any publications that look like what you are looking for.
Are you trying to reinvent the trailer? Why not just go to several trailer vendors and have them bid on your requirements and justify their designs to you?
A client of ours has come up with a new design for a belly dump, that he thinks will solve several issues with the current models on the market. It is a frame integral design with a single integrated hopper.
If you have done similar designs, are you just analyzing the frame system for a Safety factor of 3-5 (?). Or do you design the frame fro the actual loads and then run a stress analysis for fatigue, and then increase the connections as required (using engineering judgment)?
When you say "analyzing the frame system for a safety factor of 3-5" I have to ask relative to what?
- If you say relative to service loads then it is not really any different than your second option.
- if you are using steel then the "easy" answer is to limit our stress to half of yield.
-The devil is in the details with fatigue. Small changes can make a huge difference, and usually the more expensive way is better
90% of the time the biggest issue is that you really don't know the maximum service loads. You have to take an educated guess - and then people usually try to put a little cushion on top of that.
If you design it for off highway use it will be loaded to the limit of the tires, or 120,000 lbs or the fill weight with the heaviest material. Many of these trailers do both on highway and off highway hauling and may see both light material and very heavy material. Choosing the intended use / market will determine duty cycle and how it is used and how stout it needs to be. Operating on rough roads may affect fatigue life.
I have seen some steel bottom dumps with some interesting frame designs but what blows my mind is the aluminium bottom dumps that carry the same loads but have no frame; the skin is the frame.
