question about steel grate and impact loading
Hi everyone, I'm a engineering student working for the summer in a job that is titled "Engineering Student", but unfortunately there are no professional engineers working here. This hasn't really been a big deal since I haven't had to perform any real tasks where I've needed to consult a real engineer. I've mostly just been doing AutoCAD drawings, and general problem solving; no real calculations. However recently, I have been asked to design a small (about 0.4mx0.5x) steel grate to cover a hatch. The grate will be installed about 10cm into the hatch. My boss prefers that the grate be built in our own shop, so I can't just take the design load specifications from grating catalogues and find one that works. In order to ensure the grate will not fail, I decided to design the grate to withstand the force of a 100 kg man falling from the 10cm height. I have experience in school using strain energy to solve a problem like this, but the numbers I've been coming up with for stress have been seeming unreasonably high (in the ballpark of 350MPa-1.2GPa, depending what I assume), and I have a couple questions. Hopefully someone here can give me a hand, or point me towards some useful resources to help.
My question are:
strain energy calculations assume a rigid impact, where all energy is transferred to the structure. Obviously a falling human isn't very rigid, even in the worst case scenario. Is this strain energy even worth using, or should I focus on a different method for this design? Perhaps something like impulse calculations (which I have less experience with, but still some) would be better? I don't forsee anything other than a person to ever even be near the hatch, so I'm really only concerned with it being able to support people stepping/falling on it, not anything more rigid.
How do I determine how many beams the load is spread onto. For example, if I assume that the width of a regular foot would be in contact with 3 beams, should I only consider those 3 beams, or would it be safe to assume the load is spread to an additional 1-2 bars on either side due to the rigidity of the steel material? I'm trying to avoid having to add any bracing between the beams (other than at the ends), but I'm not sure if that's possible at this point. I can't find much online about this, but from the various papers and design calculations I've found, they seem to assume that the load is spread to at least 5 beams, which seems arbitrary since the various sources I found had difference spacing between the beams.
I've noticed that when most companies who sell grate perform their load calculations, there is no reference given to impact loading. Is this because the safety factor will account for whatever impact loading created due to a normal human walking? I assume that due a regular human walking will do a pretty good job of minimizing loading impacts naturally, but I'm not sure about that either.
Thanks in advance to anyone who can provide me with any help/resources for my problems, I appreciate the help.
Submitted June 22, 2017 at 10:31AM by Willy126
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