An Internet reflector (mailing list) called TowerTalk devoted to tower and HF antenna topics is now on-the-air! This is a tremendous resource for first-time builders as well as experienced station owners. To subscribe, send a message with the word subscribe in the SUBJECT LINE to firstname.lastname@example.org. You can also read the Frequently Asked Questions at http://www.contesting.com/FAQ/towertalk. Thanks to Bill, W4AN (SK), for his assistance in setting this up.
This study is of guyed tower failures and collapses to investigate what a typical tower fall radius would be. He determined that all of the debris would impact the ground within a circle with a radius which would not exceed 29% of the tower height. This is a draft of the study from 1985 by Gunnar Olsen, Jr., P.E. and is many generations of copies from the original so I apologize for the poor quality.