The DMG (pages 246-247) has specific information about how objects can be damaged including ACs for different substances and suggested hit points based on size and resilience. These rules can easily be applied to damaging spells too.
I'm not going to reproduce the whole entry, but your pile of wood would probably have an AC of about 15 and hit points of about 3d8 (if it's loose branches) or 4d8 (if its cut lumber).
Please bear in mind that many spells specifically state that the target is a creature. I would probably disallow such spells from being used against objects (unless they've been animated and are attacking the party!).
There are a couple of entries which are quite important though.
Given enough time and the right tools, characters can destroy any
destructible object. Use common sense when determining a character's
success at damaging an object.
Objects are immune to poison and psychic damage. You might decide that
some damage types are more effective against a particular object or
substance than others. For example, bludgeoning damage works well for
smashing things but not for cutting through rope or leather. Paper or
cloth objects might be vulnerable to fire and lighting damage. A pick
can chip away stone but can't effectively cut down a tree.