@kwlf, the highest stressed area in a strut braced wing is typically where the the strut attaches to the wing spar, at which point the wing outboard of that area is cantilevered, although with much lower bending moment than would be case at the wing root, minus the strut, for a cantilevered design. This often means there is a localized wing spar reinforcement near the strut attachment, but due to the stubby length of wing that is outboard it doesn’t take much added material to lower the stresses a lot in that area, and the strut itself and its pinned joints are generally not the issue. People do in-the-field weld repairs on steel wing struts, supported by guidance in AC 41.13. That’s not exactly a precise or well controlled operation, but its rarely an issue. Strut bolts are usually hugely over strength too, relative to the load applied.
At the wing root of a strut braced wing the spar is in compression, not bending, and given that the spar maintains the same overall dimensions relative to e.g. the area around the outboard strut attachment, the stresses on everything in that area are pretty low. Typically a single pin (bolt) in shear per wing spar takes the wing load into the fuselage. Compressive buckling as opposed to stress can be a bigger issue in this area, as with struts in compression (typically meaning negative g wing loading), but its obviously not an issue for fatigue… which is generally the cause of cantilevered wings failing, if they fail. Look at the aluminum carry through structure above the cabin in a strut braced design and you’ll see that despite carrying the entire compressive load between wings its a pretty lightly built structure. This is why corrosion in the carry through area can be a concern – because the material thickness is so small, not because its highly loaded.
Basically, except at the wing spar in the area of the outboard strut attachment, a strut braced wing is not highly stressed and for a light plane with a strut braced wing fatigue is not so much a major issue. That’s why strut braced designs often don’t have life limits, and why its harder to design and operate a cantilevered wing. When fatigue is the dominant design issue as it more often is with a cantilevered wing you have to do more analysis and/or more testing to determine the useful life, you can’t just do a single load to failure test and know the structural margin quickly.
All of the above is why the Cessna Caravan and Sky Courier have strutted wings, even though they were designed after Cessna had produced a lot of planes with cantilevered wings. For a customer that wasn’t so concerned with cosmetics, it made more sense according to a guy I know who did that structural analysis on the Caravan.
My guess is that the highest loads are on bad landings. Wing mounted main gear would load the wing more unless designed to fail before overloading the mainspar.
Silvaire: To summarise, strutted structures are mechanically more efficient than pure cantilever structures, less prone to fatigue, and easier to analyse.
But given that this is the case, why are cantilever structures not built more strongly to compensate; why not profit from the structural efficiency of strutted wings to build them lighter?
I suspect I would be right in predicting that very clean airframes will be more prone to structural failure, because it will be easier to inadvertently overspeed in them.
Writing generally so as to make the point, strut-braced wing structures are built as light as they can be but often its determined that the issue preventing further reduction in weight is something other than static strength or fatigue. Examples would be to withstand ground handling damage or to be adequately stiff, i.e. to withstand buckling. Structural design has multiple requirements and constraints, in flight stress and fatigue are not the only ones. Conversely, cantilevered wing structures tend to operate at higher material stresses, so stress and fatigue become bigger factors. If those stresses are reduced by heavier spars, thicker section wings etc. the payload and/or performance of the aircraft is reduced, so a compromise is found.
There are those who think the pre-1967 Cessna 210 with strut braced wings was a pretty good going places plane, at least structurally. But they have other problems, and fly like a truck. Retracting the landing gear in combination with a strut braced high wing was problematic. Nothing is perfect 
Yes, clean aircraft with relatively highly stressed structures are more easily broken by aerodynamic loads, especially since the Normal, Utility etc. G loads for certification are the same regardless of aircraft performance. Obviously early Bonanzas and their contemporary era of pilots are the poster child for this issue. Literally hundreds had in flight structural failures, the result of loss of control, overloading and static strength failure and not a fatigue cracking issue in normal service like the PA28 that originated this thread.
Thanks.
My guess is that the highest loads are on bad landings. Wing mounted main gear would load the wing more unless designed to fail before overloading the mainspar.
Is the PA 28 gear (which is attached to the main spar) designed to fail before overloading the spar?
I’m not knowledgeable on engineering stuff but I’d guess even if it were designed that way, repeated hard impacts that are below such limit, will cause gradual fatigue on the spar and attachment points.
Some more PA 28 break ups
http://www.kathrynsreport.com/2022/05/aircraft-structural-failure-piper-pa-28.html
And an interesting comparison of Lock Haven Pipers (Wing spar meets at fuselage center) vs Vero Beach Pipers (Spar carry through design) – attached as pdf.
