All the stress that the bolt is holding has to go thru the first "length" of 1.5 * dia. this is comparable to the weakest link in a chain. If the first length can not do the job, you have the wrong bolt. It really does not matter if the bolt was one mile long.
According to your reasoning then, a one inch bolt embedded one and one half inches into concrete would be capable of holding the full tensile strength of the bolt.
Were you on the Big Dig design team?
The failure was undoubtedly NOT in the bolt, rather in the epoxy attachment to the concrete. The 1.5X rule is valid for a threaded fastener into similar materials (steel into steel), but in this application, it does not apply.