I assume that this load is applied for a short period, so a Dynamic Time analysis would be the most appropriate. This means that you need to do a modal analysis first, capturing enough modes to make it valid. Check on the Knowledge Base in the Customer Support area of the PTC web site and you should find an explanation of Dynamic Time analysis and an example. This is available if your company has a current maintenance contract. You will need to figure out how to load and constrain your frame and what the damping should be. Further Tips
1 - If you have run your modal analysis first and then you set up the dynamic time analysis and ask it to refer to the previously run modal for the modal results it may not find them. This is a bug. The answer is to get it to run the modal from within the dynamic time analysis. If you are running separate inputs in the X, Y and Z directions, do that for each one, and put them all in a Design Study and run the design study. then it will run the modal first then the 3 dynamic Time analyses without running the modal 3 times.
2. To get stress plots, you have to run the Dynamic Time analysis, twice - firstly to get a plot of response v. time to see when the peak amplitude occurs and then you tell it to produce full results at that time or times in a second run.
3. To get the initial response, you need to make some measures, e.g. max accel, max disp, max stress whatever, so that it has something to plot. - This step can easily be forgotten.
That deals with Mechanica, but it sounds like you are trying to do a crash analysis. In a motor vehicle this is usually highy non-linear, with the material plastically deforming, large displacement and contact. Unfortunately this is beyond the capability of Mechanica and you will need a specialist code such as Dyna, Pamcrash, Radioss etc, as used regularly in the motor industry.
However by all means give it a go in Mechanica and see what you get. If the yield stress has been exceeded, then you will probably need to consider another method, unless your target is for it not to yield, in which case you can continue with Mechanica and modify the design until that target is reached.
This can be a very involved subject, and it may take more than a 5 minute reply like this to satisfy your requirement, which is why we offer training courses and consultancy, based in the UK, on these kinds of topics.