Howdy all.
I have a question concerning shield use in 5e.
IS there a mechanic directly associated with shield-bashing? Would it be considered no more than an unarmed strike?

This has been discussed before - but the gist from what I understand is that it can be no more than an Improvised Weapon (https://www.dndbeyond.com/sources/basic-rules/equipment#ImprovisedWeapons).

If you want to shield bash (shove) then you need the Shield Master feat.

The better fitting rule is that it is an improvised (melee) weapon that does 1d4+str. The shield master feat also provides some special attacks to be made with a shield.

Quote from Emmber >>

If you want to shield bash then you need the Shield Master feat.

 Well, you can shove with a shield (the shove attack does not require a free hand), though it has no mechanical effects that are different from a normal shove.

Yeah, Shield Master just gives you a way to do it with a bonus action.

I personally rule that shields can't be used as an improvised weapon while they are being worn to boost AC, for mechanical balancing. You can still make an unarmed strike with your shield arm though.

Again, personal ruling to prevent abuse. RAW is not clear either way on wether shields and armor can do the damage of an improvised weapon while still worn. It sounds stupid and impossible, but the rules don't specifically prevent it, so some people try.

Quote from DxJxC >>

I personally rule that shields can't be used as an improvised weapon while they are being worn to boost AC, for mechanical balancing. You can still make an unarmed strike with your shield arm though.

Again, personal ruling to prevent abuse. RAW is not clear either way on wether shields and armor can do the damage of and improvised weapon while still worn. It sounds stupid and impossible, but the rules don't specifically prevent it, so some people try.

Couldn't agree more.

Yeah, one or the other.

so ....
if shields are improvised weapons, and adding their ac modifier if not used, dual wielding shields, while having the "Dual Wielder" feat would give me ac bonus of + 5 to AC .... not to shabby :-)

Nope. Nothing in that feat defeats the sentence describing shields that says you may only benefit from one shield at a time. Also, when a feature asks you to be "wielding a weapon" it is generally accepted that this means something on the weapon tables (just like "wearing armor" has mechanical meaning). If you are holding any shields you don't gain the benefits of dual wielder.

Also, remember that if you try to do anything that requires your hands not to be full, it takes an entire action to doff a shield.

are you for real ?

Certainly with your reply, I imagine you are trolling. Nonetheless, yes. You may benefit from only one shield at a time, and most people agree on the definition of wielding a weapon (with the occasional question about overlap between things like staff and quarterstaff).

The Sage Advice Compendium explicitly states you keep the shield's AC boost when attacking with it and also implies you attack with the shield as an improvised weapon:

Quote from the Sage Advice Compendium

If you attack with a shield — most likely as an improvised weapon — do you keep the +2 bonus to AC?

Attacking with a shield doesn’t deprive you of the shield’s bonus to AC.

The improvised weapon strike also aligns with several monster attacks, such as the hobgoblin warlord.  

Also, I would rule that you keep the AC bonus on a “bash” as the shield is still being held in its typical position.  If you were to improvise it differently, like for example the way Tyrion Lannister uses his in GOT season 1 in the mountain tribes attack (two handed using its pointed end to stab a prone enemy) that might be different, but that’s not what a shield “bash” usually is

plus, i wouldn’t penalize a player for doing something identical to something a monster can do without penalty

Quote from iso101 >>

so ....
if shields are improvised weapons, and adding their ac modifier if not used, dual wielding shields, while having the "Dual Wielder" feat would give me ac bonus of + 5 to AC .... not to shabby :-)

 No, not even close to accurate.

You can benefit from no more than a single shield at any time. A shield is not a weapon, regardless of whether it's being used as one or not. You only benefit from the Dual Wielder feat when you are actually wielding two weapons. 

I think I'd allow some leeway if someone had the Tavern Brawler feat (or some other ability that gives them proficiency with improvised weapons)

There is certainly no general consensus that wielding improvised weapons as improvised weapons doesn't count as wielding weapons, not sure what Bees and Sigred are on about that.

You can dual wield table legs, broken bottles, whatever... improvised weapons are a type of weapon, and nothing in the Two Weapon Fighting fighting style, Dual Wielder feat, or general Two Weapon Fighting section of Chapter 9 in any way limit two weapon fighting or its modifiers to simple or martial weapons. The entire argument against it seems to flow backwards from an attempt to enforce that Dueling fighting style and Two Weapon Fighting style can never benefit someone on the same round of combat, which seems misguided considering that all of the other fighting styles can easily overlap within the same round. The two styles certainly can't apply to the same attack, since at the time of that attack you are either wielding one weapon or not... but if you change your wielded items later that round (such as beginning to wield your shield as an improvised weapon instead of simply wearing it defensively as a shield, or drawing a second weapon that was previously sheathed), there's nothing RAW that suggests this is unintended or alarming. 

Lets not get sucked back into the whole dueling/twf stance dance debate. But suffice to say, there is nothing saying that fighting styles, feats, class features, etc. don't apply to improvised weapons merely because they don't appear on the weapon table in the equipment section. 

I have written two homebrewed feats intended for exactly this: A Pair of Shields and All a Hero Needs....

Quote from Chicken_Champ >>

There is certainly no general consensus that wielding improvised weapons as improvised weapons doesn't count as wielding weapons, not sure what Bees and Sigred are on about that.

 

You are right, it is better referred to as a fact of the game. It is a fact that wielding a weapon has mechanical significance. It is also a fact of the game that anything held in a hand can be used to make an improvised attack. It does not follow that any time you are holding any item you are wielding a weapon (otherwise all sorts of features that require holding a specific number of weapons at a particular time would break), therefore wielding a weapon must be different from holding an object. 

1. An Improvised Weapon is a Weapon
   1. They are literally called "Improvised Weapons"
   2. They appear under the "Weapons" heading in Chapter 5
   3. Bonus: all of the examples of Improvised Weapons that we're provided are improvised melee weapons, which might be the only type of Improvised Weapon? 
      1. "Every weapon is classified as either melee or ranged. A melee weapon is used to attack a target within 5 feet of you, whereas a ranged weapon is used to attack a target at a distance."
      2. The Improvised Weapons listed (broken glass, a table leg, a frying pan, a wagon wheel, dead goblin, ranged weapon used to make a melee attack, thrown melee weapon) are all examples of melee weapons. 
2. An Improvised Weapon is Wielded
   1. "An improvised weapon includes any object you can wield in one or two hands."
   2. "Wield" isn't explicitly defined anywhere as a base term, but I can't find an example of it being used anywhere in a way that would exclude Improvised Weapons or treat them differently than any other "weapons"?
3. Dueling applies to a (1) wielded (2) melee (3) weapon
   1. "When you are wielding a melee weapon in one hand and no other weapons, you gain a +2 bonus to damage rolls with that weapon."
   2. Improvised weapons are weapons, see above
   3. Improvised weapons are wielded, see above

Notwithstanding your sincerely held beliefs, I fail to see any written language that creates a "fact of the game" that contradicts any of the above. There is no "must" to be found that holding objects is different from wielding them. For JC fans, he unfortunately has been characteristically cagey whenever someone tries to get him to define "wield" (can't find the link to the tweet, but he says something like "wield means what it commonly means.")