Shield Bash Attacks: You can bash an opponent with a heavy shield,
using it as an off-hand weapon. See Table 7–5: Weapons for the
damage dealt by a shield bash. Used this way, a heavy shield is a martial
bludgeoning weapon. For the purpose of penalties on attack
rolls, treat a heavy shield as a one-handed weapon. If you use your
shield as a weapon, you lose its AC bonus until your next action
(usually until the next round). An enhancement bonus on a shield
does not improve the effectiveness of a shield bash made with it, but
the shield can be made into a magic weapon in its own right.
phb p.125
顺便楼上是p.122的内容,在武器那栏,而不是防具