272 



PRACTICAL STRUCTURAL DESIGN 



shows the load per square foot over the footing, provided the 

 load is applied through the center of gravity. The lower diagram 

 shows the effect of the bending moment to increase the compres- 

 sion on one side and lessen it on the other. 



If the sum of the two does not exceed the safe allowable pres- 

 sure at one toe and there is no uplift (tension) on the other toe 

 the footing will be safe. The rule to as- 

 sure safety is known as the "Middle-third 

 rule" in which the resultant pressure to 

 prevent overturning must be kept within 

 the middle third of the base. The middle- 

 third rule has been considerably over- 

 worked. What it really amounts to is a 

 statement that if the resultant of all pres- 

 sures brought to bear on a footing base is 

 kept within the middle third the average 

 stress will not exceed one-half the maxi- 

 mum and there will be no tension. 

 The condition shown in Fig. 175 is not 



Fig. 175 Eccentric Load always possible to avoid, for foundations 

 on Wall Footing must be kept within lot ^^ The remedy 



apparently is to so construct the footing that a line may be drawn 

 at an angle of thirty degrees, with the vertical, from the edge of 

 the wall to the lower edge of the footing and keep within the 

 footing. When the load brought down by the wall reaches the 

 footing it will spread out and thus the center of effort of the load 

 will not be directly under the center of the wall, but "will be some- 

 what nearer the center of gravity of the footing. This may be 

 the case if the wall is not in excavation. If it is in excavation 

 the load is no doubt partly distributed to the earth on the out- 

 side of the wall, so the center of effort of the load is actually 

 under the center of the wall, provided the load passes as readily 

 through the masonry as it does through the earth on the side. 



The footing may be of solid concrete, with a depth fixed by the 

 sixty degree line, so that it will not distort under load. If this 

 is the case, then as soon as the earth under the heavily pressed 

 edge gives way the entire bottom of the footing will come into 

 bearing and relieve the stress on the soil. The same effect should 

 possibly be secured by using a lighter footing of concrete heavily 

 reinforced so it cannot bend. Something also may be gained 



