FREE BOOKS

Author's List




PREV.   NEXT  
|<   142   143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163   164   165   166  
167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183   184   185   186   187   188   189   190   191   >>   >|  
s of lead about 1 in. thick extending over the middle third of the depth of the voussoir joints, the rest of the joints being left open. As the lead is plastic this construction is virtually an articulation. If the pressure on the lead is uniformly varying, the centre of pressure must be within the middle third of the width of the lead; that is, it cannot deviate from the centre of the voussoir joint by more than one-eighteenth of its depth. In any case the position of the line of pressures is confined at the lead articulations within very narrow limits, and ambiguity as to the stresses is greatly diminished. The restricted area on which the pressure acts at the lead joints involves greater intensity of stress than has been usual in arched bridges. In the Wuerttemberg hinged arches a limit of stress of 110 tons per sq. ft. was allowed, while in the unhinged arches at Cologne and Coblentz the limit was 50 to 60 tons per sq. ft. (_Annales des Fonts et Chaussees_, 1891). At Rechtenstein a bridge of two concrete arches has been constructed, span 751/2 ft., with lead articulations: width of arch 11 ft.; depth of arch at crown and springing 2.1 and 2.96 ft. respectively. The stresses were calculated to be 15, 17 and 12 tons per sq. ft. at crown, joint of rupture, and springing respectively. At Cincinnati a concrete arch of 70 ft. span has been built, with a rise of 10 ft. The concrete is reinforced by eleven 9-in. steel-rolled joists, spaced 3 ft. apart and supported by a cross-channel joist at each springing. The arch is 15 in. thick at the crown and 4 ft. at the abutments. The concrete consisted of 1 cement, 2 sand and 3 to 4 broken stone. An important series of experiments on the strength of masonry, brick and concrete structures will be found in the _Zeitschr. des oesterreichen Ing. und Arch. Vereines_ (1895). The thermal coefficient of expansion of steel and concrete is nearly the same, otherwise changes of temperature would cause shearing stress at the junction of the two materials. If the two materials are disposed symmetrically, the amount of load carried by each would be in direct proportion to the coefficient of elasticity and inversely as the moment of inertia of the cross section. But it is usual in many cases to provide a sufficient section of steel to carry all the tension. For concrete the coefficient of elasticity E varies with the amount of stress and diminishes as the ratio of sand and stone to cement increas
PREV.   NEXT  
|<   142   143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163   164   165   166  
167   168   169   170   171   172   173   174   175   176   177   178   179   180   181   182   183   184   185   186   187   188   189   190   191   >>   >|  



Top keywords:

concrete

 

stress

 

pressure

 

joints

 

springing

 
coefficient
 

arches

 

articulations

 

materials

 
cement

stresses

 
middle
 

centre

 

voussoir

 

section

 

elasticity

 

amount

 

masonry

 

diminishes

 

strength


joists

 

increas

 

eleven

 

spaced

 

rolled

 

experiments

 

abutments

 

channel

 

broken

 

consisted


supported

 
series
 

important

 

direct

 

proportion

 
inversely
 

moment

 

carried

 

disposed

 

symmetrically


inertia

 

tension

 

sufficient

 

provide

 

junction

 

shearing

 
Vereines
 

oesterreichen

 

Zeitschr

 

varies