Comparing wood strengths
The ramps have oak in them now, but some needs to be replaced. Would maple work as well, or do I need to stick to white oak?
White oak has quite a bit of natural decay resistance, making it a good choice for outside use. The ash and maple would be able to handle it in strength, but would not last nearly as long. I would recommend sticking with the white oak if at all possible. If not, keep the ash or maple relatively clean and dry and they should last a good while. Ash does have a "limberness" to it, so it will likely bend more than the oak or maple.
I would use ash before I would use oak for loading ramps any day. Ash has a better bending strength than oak does. Did you ever notice that ash is more commonly used for things like hockey sticks, baseball bats, hammer handles, broom handles etc. This is because it gives before it breaks. But if it is available the wood most sought after(at least in my experience) for loading ramps is red elm. That wood just keeps on bending instead of breaking.
Here is the strength of the woods in question. Note that there are several ash species and they are not all the same strength. Likewise, within the white oak grouping, some are stronger than others.
Strength in bending (MOR) of white oak at 12 percent moisture content (MC) is 15,200 psi. (Bur oak in the white oak lumber group is 10,300 psi.) When dealing with lumber we cannot separate these species at all.
White ash is 15,000 psi MOR; black is 12,600; green is 14,100.
Hard maple is 15,800.
The stiffness (MOE) of white oak is 1.78 million psi; white ash, 1.74; black ash, 1.60; green ash, 1.66; hard maple, 1.83. (Larger is stiffer.)
In short, ash is fully acceptable. White oak has natural decay resistance, as mentioned.
A weak piece of any species is one that has steep slope of grain, knots, shake, decay, etc. These factors are more important than species!
As Gene says, growth factors are more important than species. I have sold many replacement boards for trailers, ramps, truck beds and cheater boards, and usually decay isn't the problem, but weak areas of the board because of the defects Gene described. Cracking from the continual shrink and swell cycle from being rained on, then drying in the sun also takes its toll. Selecting good pieces would be the key.
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