Do my stairs need to be replaced?

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Hi, another newbie question. Our outdoor backyard stairs seem to be splintering. You can fee them sink when you step on them. No cover over them so always exposed to the wind & rain. Is there anything I can do fix the stairs or do they need to be replaced completely? 

 

 

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Thank you @Noyade & @JacobZ. 

@Noyade, we recently bought this house so not sure how the section was built & placed. It is an old house which wasn’t maintained very well by the previous owners.