I've read through the whole long thread, thanks all for great contributions. One thing I was specifically looking for was info on what deflection is "strong enough." I saw the question asked a few times, but only answered in general terms, I did not see any quantitative examples.
What I'm looking for is some guidelines/rules of thumb that can be applied by a non-structural engineer. For example, carpenters can frame a wood house without calculating doing structural analysis of each stud or header. There are some basic rules that can be applied. How about for aluminum extrusions in a van? For example:
- Is 0.1" deflection OK or too much? What is a good rule of thumb to use?
- How many Gs should I use as a multiplier on weight? Is 3G vertical enough for road shocks? How about forward (crash) shocks?
- Should I assume "Supported 2 ends" or "Fixed 2 ends" in the deflection calculator if it is attached to vertical structural members? I assume the vertical members will have some flex, but which is closer to correct?
A specific example I have is a 100lb battery mounted on a shelf with a 39" span over the wheel well--it will see a lot of vertical shocks. Here is my current utility cabinet module--initially done in 15 series but looking at changing to 10 series. Note that the inverter will hang on a plywood sheet attached to the back, so the horizontal extrusions in the shelf will not carry that load. And this module will be bolted to the van wall and to the cabinet in front of it.
View attachment 148806
I'd like to minimize the height of the battery shelf above the wheel well. What size should the 39" structural members be? The load would be shared by two 39" extrusions. So if I assume 3Gs then each extrusion would see a 50 lb static load and up to a 150lb dynamic load. Also assuming a point center load (conservative), supported two ends, the deflection calculator gives me
- 1010-S: 0.14" static and 0.4"
- 1020-S: 0.02" static and 0.06" (strong direction), 0.07" static and 0.22" dynamic (weak direction)
- 1515-LS: 0.03" static and 0.1" dynamic
If I assume "Fixed 2 ends" then 1010-S would be 0.03" static and 0.1" dynamic, which seems good enough (?)
It's also worth noting that I will also have a 1/2" plywood shelf on top of the extrusions to support the battery--this would further stiffen the shelf. And in reality, the battery will be on one side not in the center. So in conclusion, my gut feeling (ungrounded in any deep structural engineering expertise) is that the 1010-S with a 1/2" plywood shelf should be strong enough.
Comments invited.