306 CONCRETE DESIGN 



and on hooped columns that contain in addition more than 

 1% and less than 4% of longitudinal reinforcement they 

 allow a unit working stress of 650 Ib. This is for the grade 

 of concrete above specified. Some lengineers consider these 

 stresses rather high; nevertheless, in all cases the stress 

 selected should be governed by the laws of the locality and 

 the experience of the designer. 



Eccentrically Loaded Columns. It is difficult to find a 

 column that is entirely concentrically loaded. If, in a build- 

 ing, the live load is transferred from one girder to another, 

 the girders being carried by the same column, their deflection 

 will put a twist in the column itself. Also, the outside 

 columns of buildings are often eccentrically loaded. In the 

 lower floors of buildings the eccentric load due to unequal 

 distribution of load, in panels, may usually be neglected 

 because of the large concentric load on the column, but in 

 the top floors of a building the matter may at least deserve 

 attention. If the load is eccentric, the following method 

 will usually give safe results: 



First, assume a section and find the stress in the concrete 

 and steel due to bending as in a beam with double reinforce- 

 ment. Next, find the stress in the concrete and steel due 

 to direct compression and add the two algebraically. The 

 sums obtained should not exceed the allowable stress for 

 columns. Sometimes it is found advisable to put more 

 steel in the tension side than is used in the other side. As 

 this method is approximate, low unit stresses should be used. 



ARCHES 



In reinforced-concrete arches, the arch ring is nearly always 

 thinner at the crown than near the haunches. The arch in 

 reinforced-concrete work is often either true parabolic or 

 the curve is made up of a series of circular arcs approach- 

 ing the shape of a parabola. The reinforcement for arches is 

 ' usually placed in two layers one near the intrados and 

 the other near the extrados and these two layers of 

 reinforcement are usually laced together with lighter rods 



In constructing bridges of reinforced concrete, it is consid- 

 ered advisable to lay all the concrete at once, but if this is 



