The error know as the comma splice occurs when two independent clauses are joined, with only a comma to separate them. Each of these clauses can stand on its own. Using only a comma between them can cause confusion, forcing the reader to re-read the sentence to make sense of it. To be able to recognize the comma splice, it is neccesary to understand what makes up a clause. A group of words containing a subject and a verb is a clause.
Bridge is the subject.
Collapsed is the verb. There are two kinds of clauses. An independent clause is one that makes complete sense on its own. It can stand alone, as a sentence, as both parts of the first example can. A dependent clause, as its name suggests, cannot stand alone as a sentence, it depends on another clause to make complete sense.