I would say: Are you sure this is sturdy? Are you sure this is safe?

"Are you sure this is solid/sturdy?" would be the best for a bridge in my opinion. :)

Are you sure this is sound? I take this to mean the bridge is structurally secure. Are you sure this is firm? I would not use this in context of the bridge. It sounds like you are asking if the materials are hard, e.g., the wood. Are you sure this is solid? Again this sounds weird to me, but I do not know what you are asking. Solid could be used here, but "sound" seems the best choice if you are talking about a bridge that you think might collapse/fall.