WORLD'S LONGEST BRIDGE SPAN— PAINE 5g7 



been asked all kinds of questions about this unusual structure and is 

 prepared with all the answers. 



Under the efiect of a broadside wind at 100 miles per hour, the 

 bridge floor at midspan might swing 21 feet out to one side. Since 

 winds in San Francisco rarely reach half this velocity and never have 

 exceeded 60 miles per hour, it is quite probable that it wUl never be 

 called upon to withstand such a strain. Yes, the floor moves up and 

 down too with every change in temperature and load. A rise in 

 temperature causes the cable to lengthen and consequently the sag 

 increases. Obviously, the sag in the center span will be decreased by 

 loading the side spans and increased by loading the center span. The 

 most severe combinations of loading and temperature will cause the 

 floor of the bridge at midspan to rise 10 feet above or fall 10 feet below 

 its normal elevation. In order to accommodate these movements 

 as well as the lengthening or shortening of the suspended steel structure, 

 itself, special joints are provided where the floor of the suspended 

 span joins the floor that is carried directly by the tower. These joints 

 permit angular motion in horizontal and vertical planes combined 

 with a longitudinal movement of 5 feet from one extreme to the other 

 and they are so designed as to give a smooth riding surface across the 

 joints under all conditions of load and temperature. 



Does the cable sUde over the top of the tower? No! The tower 

 tops are pulled back and forth by the cables under the varying combi- 

 nations of temperature and loading. Under normal conditions, that 

 is, with temperature at 70° F., and no load on the span (other than 

 its own dead weight), each tower is bent shoreward 6 inches. Now, 

 if the temperature rises 40° and full live load is applied to the center 

 span and the far side span only, the tower will be deflected channel- 

 ward 18 inches. If the temperature drops 40° below the normal 

 temperature of 70° F. and full live load is applied to the near side 

 span only, the tower will be deflected shoreward 22 inches. In other 

 words, the towers may be bent to and fro so that their tops move 

 through a range of 40 inches in the longitudinal direction of the bridge. 

 In a direction transverse to the bridge, the towers would deflect 

 12K inches under the same wind load which would cause a 27-foot 

 deflection in the center span. 



Each tower receives from the two cables a total load of 123 miUion 

 pounds. The concrete piers upon which the steel towers rest must 

 carry this load plus the weight of the steel tower, which is 44 million 

 pounds, or a total applied load of 167 million pounds. The San 

 Francisco pier, allowing for buoyancy, weighs 560 million pounds, 

 and therefore the total load on the foundation which supports the 

 San Francisco pier and tower is 727 mfllion pounds. 



In design and construction the engineers were confronted with 

 some unique problems. From the outset the construction of the 



