CAMBRIA STEEL. 



337 



GRILLAGE BEAMS FOR FOUNDATIONS. 



In designing foundations for walls or columns carrying heavy loads resting upon 

 the soil, it is necessary to distribute the weight over a suitable area, and this is 

 readily accomplished, in a small depth, by using a grillage composed of steel beams 

 imbedded in concrete, thus obviating the necessity of large masses of masonry and 

 deep excavations. For heavy loads on soil of small bearing power three tiers of 

 beams may be necessary, while for lighter loads and soil of greater bearing power 

 two tiers of beams will ordinarily suffice. 



The grillage beams which are to be surrounded by concrete should be spaced not 

 less than 3" apart in the clear between the flanges, so that the concrete may be thor- 

 oughly rammed between them, and gas-pipe, or standard cast-iron separators should 

 be used to maintain the beams in proper position. 



Knowing the total weight to be carried and the allowable intensity of loading per 

 square foot of the supporting soil, the area of the footing required can be readily 

 found, which, taken into consideration with any other conditions limiting the form or 

 proportions of the footing, will determine the external dimensions of the foundation. 

 The beams may be considered as subjected to a uniform load extending over a por- 

 tion of their upper surfaces, the center of which is at the center of length of the 

 beams, and as being uniformly supported from below throughout their length. 



Under these circumstances, the maximum bending moment will occur at the 

 center of the beam and, using the notation given for the upper tier in the sketch 

 below, this bending moment for one beam will be as follows : 



Bending moment in inch pounds = (c b) 



o 



in which c and b are expressed in inches and W is the total weight in pounds on 

 one beam, obtained by dividing the total load by the number of beams composing 

 the tier in question. 



This formula for the bending moment is the same as that for a beam of the length 

 (c b) supported at the ends and uniformly loaded with the total weight W, so 

 that the proper sizes of beams, bending considered, may be obtained directly from 

 the tables of safe loads uniformly distributed for Cambria I-Beams,on pages 106 toll? 

 inclusive, or for cases in which the lengths are shorter than those given in these 

 tables, the sizes may be calculated from the coefficients of strength or the section 

 moduli given in the tables of properties of I-Beams, pages!82 to 185 inclusive, tak- 

 ing care, however, to use as the length, the distance (c b), for the upper tier, and 

 the corresponding figures for the other tiers. 



After determining the size of beam required based upon bending, as stated above, 

 an examination should also be made of the capacity of the beam web to resist buck- 

 ling. This may be done by considering the web as a column of height equal to the 

 clear distance between the fillets and calculating the safe load therefor by the use of 

 the tables of strength for steel columns or struts, on pages 218 to 221, using the 

 proper safety factor. 



If the beam web is found insufficient as a column when calculated in this manner, 

 a beam with a web of greater thickness should be tried until one is found that will 

 meet this requirement and the conditions for bending ; or it might be more economi- 

 cal, in some cases, to use the beam with the thinner web and provide it with suffi- 

 cient separators, fitting between the beam flanges, or stiffeners secured to the web to 

 assist it in resisting as a column. 



