"as Kenneth sat down on the rickety old chair, it abruptly collapsed beneath him" what kind if sentence is this

It is a complex sentence.

Explanation:

There are  four basic types of sentences: simple, compound, complex, and compound-complex.

A complex sentence has one independent clause joined to a dependent clause by a subordinating conjunction.

An independent clause has a subject and a predicate, and can stand alone as a sentence, as it makes complete sense.  A dependent clause cannot stand alone.

In this case, "Kenneth sat down on the rickety old chair" is an example of an independent clause.  "as it abruptly collapsed beneath him" is a dependent clause.

I cant put them in order without the story. sorry