268 COLLAPSE OF TEXAS TOWER NO. 4 



tural Deficiencies." This summary shows the existence of loose pin connections 

 on the A-B side for all of the pins at elevations —2~) and —76. This looseness 

 permits excessive oscillations of the tower resulting in annoyance to the per- 

 sonnel on the tower and causes concern as to the ultimate safety of the struc- 

 ture. The looseness of the pins will, in all probability, continue to worsen at, 

 perhaps, an accelerating rate since the sloppiness contributes to the magnitude of 

 the impact stresses on the connections and the increased impact in turn aggra- 

 vates the wear in the joints. It was, therefore, concluded by our office that it 

 would be hazardous to permit the conditions to remain uncorrected through 

 any more winter seasons, with the ultimate loss of the tower being the possible 

 result of such neglect. Statements of our conclusions were contained in a letter 

 from our firm to the Air Force dated April 1, 19G0. In that letter we recom- 

 mended the installation of additional braces between the legs above the water on 

 all three sides. 



POSSIBLE CORRECTIVE MEASURES 



The choice of schemes for correcting the deficiencies are limited and, from 

 the point of view of cost and practicability of accomplishment within the 

 summer season of 11)60, the placing of the braces proposed offers the only 

 satisfactory answer. The seemingly obvious solution of repairing the pin 

 connections had to be rejected because of the complexity of the connections 

 and the inaccessibility of the inner pin plates. All of the defective connections 

 are underwater which makes any reliable repair work, especially that involving 

 welding, almost impossible. In addition, there is motion in the joints at all 

 times and no practical means could be suggested which would eliminate this 

 motion while repairs were attempted. All work of assembling and attaching the 

 proposed braces will be above water and the work will be capable of being 

 inspected at all times during installation and afterward. 



DESIGN CRITEKIA 



The original design of the tower was based on the action of a 35-foot breaking 

 wave combined with a 12.5 mile per hour wind. The behavior and strength of 

 the tower with the new proposed braces was analyzed for the same forces. 



BEHAVIOR OF TOWER 



The diver survey indicates that the pin connections are in a satisfactory 

 condition on the B-C and A-C sides so that the discussion which follows, 

 and which is illustrated on attached SK-1, applies to the A-B side only. 

 The forces indicated and discussed are those resulting from the portion of 

 the total applied force taken in the A-B plane. 



The behavior of the A-B system with moments in the legs and reactions 

 at the bracing points is shown on SK-1 for both the present condition and the 

 condition after the new bracing is installed. It is assumed, in each case, that 

 before the loose connections take up, the system acts as a portal frame sup- 

 ported at the lov\'er end at elevation minus 126 feet (the first level at which the 

 pins are tight). Because of the improve<l stiffness of this portal system with 

 the new braces installed, the magnitude of the lateral force taken by the portal 

 before the original bracing system is called into action is much greater than 

 for the existing condition. After the original bracing comes into action the 

 proportion of the total load taken into the damaged pin connections will be 

 much less for any given wind and wave load. As a result of introducing the 

 new braces, moments in the legs and forces on the existing bracing are changed 

 as follows : 



1. The maximum moment in a leg is reduced from 29,600 foot kips applied 

 at elevation plus 67 to 10,640 foot kips applied at elevation plus 12. 



2. The horizontal force on the existing bracing is reduced from 850 kips 

 at elevation minus 25 to 556 kips at the same elevation. It is to be pointed 

 out that these figures are, to a certain extent, qualitative since the computa- 

 tions are based on an estimated 1.6-inch deflection at elevation minus 25 

 before the existing braces come into action. The deflection is derived from 

 the diver's estimate of the play in the pins. If the pins are in reality less 

 sloppy than estimated, this is all to the good but in any case if the wear 

 in the pin connections increases, the more beneficial the new braces become. 



