178 REINFORCED CONCRETE CONSTRUCTION 



a building is liable to be greatly weakened by a fire, its strengthen- 

 ing and stiffening value should be neglected in such design." 



In the experiments above referred to, it was also found that 

 the yield point of columns reinforced with both spirals and 

 longitudinal rods is closely given by the formula 



and the ultimate strength approximately by the formula 



In these formulas, Pj=load at yield point, P = maximum load, 

 A = area of column inside of spiral, p = percentage of lateral steel, 

 f c = ultimate compressive strength of concrete, f s = yield point of 

 longitudinal steel, and /'= yield point of lateral steel. The 

 spiral steel is not included in the first formula as its effect on 

 the yield point is very small. 



From the above and other tests made at The University of 

 Wisconsin during the year 1909, Prof. Withey draws the follow- 

 ing conclusions: 



"1. A small amount, 0.5 to 1 per cent of closely spaced lateral 

 reinforcement, such as the spirals used, will greatly increase 

 the toughness and ultimate strength of a concrete column, but 

 does not materially affect the yield point. More than 1 per cent 

 of lateral reinforcement does not appear to be necessary. The 

 use of lateral reinforcement alone does not seem advisable. 



"2. Vertical steel in combination with such lateral reinforce- 

 ment raises the yield point and ultimate strength of the column 

 and increases its stiffness. Columns reinforced with vertical 

 steel only, are brittle, and fail suddenly when the yield point of 

 the steel is reached, but are considerably stronger than plain 

 columns made from the same grade of concrete. 



"3. Increasing the amount of cement in a spirally reinforced 

 column increases the strength and stiffness of the column. A 

 column made of rich concrete or mortar and containing small 

 percentages of longitudinal and lateral reinforcement, is without 

 doubt fully as stiff and strong and more economical than one 

 made from a leaner mix reinforced with considerably more steel. 

 In these texts, doubling the amount of cement increased the yield 

 point and ultimate strength of the columns without vertical 

 steel about 100 per cent, and added about 50 per cent to the 

 strength of those reinforced with 6.1 per cent vertical steel. 



