This a 3 year old (lightly used) weather exposed swing set.
Should I be concerned about the cracks?
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This is typical of wood movement. The wood will eventually start to rot because of the "pockets" created by the cracks. Those will hold water, expediting the rot that will occur over the years.
Cedar is rot resistant, but with these cracks it will quicken the inevitable. If a well formed metal cap can be added to the length of the top which would be preferred so it can be fastened on the sides, or covered with a peel and stick waterproofing, so the movement of the wood will not affect the covering. This is another preferred fix, since a metal cap is a nice harbor for insects, depending on how close it is set to the wood.
It happens to all species of wood and he cracking was caused by sun drying not structural issues such as overloading.
Wood expands and contracts on the sun dried side and the sun also has an aging affect on the wood making it more hard and brittle (less able to tolerate expansion and contraction cycles). This causes the wood to tear apart in the weakest places which is the softer spring growth layers that are in between the strong fall growth rings (lines on the edges of the boards).
No and yes.
No, in that it does not yet need to be replaced. It is not a structural issue right now (I suspect that the cracks don't come anywhere close to going through the entire beam). It likely won't become a structural issue ASSUMING that it doesn't also get further weakened by wood decay (rot) that is likely to occur in the cracks.
Yes, it that it is necessary to fix the situation by protecting the wood from further damage. Do this by protecting it from both moisture and the sun's rays. This can be done in many ways including such things as metal flashing, brush on sealant, or a rubberized roofing member.
I suggest cleaning the debris out of the cracks and then covering the top of the wood beam with a rubberized roofing material to prevent moisture from rain and the drying rays of the sun from getting to the top surface of the wood. This material should be available in most home improvement stores
Weathering of the member certainly was the initial cause of this cracking. As the wood expands and contracts with the seasons, the countersunk washers remained (relatively) the same size and thus the wood split. The fact that the member is splitting along the bolt line leads me to the suggestion that this should be replaced.