276 PRACTICAL STRUCTURAL DESIGN 



This is illustrated in Fig. 178. The direct weight W is made 

 up of the weight carried to the fpundation by the column, plus 

 the overturning moment exerted by the force H acting on the 

 windward column. The force H, acting on the leeward column 

 is an eccentric load on the footing, as already described. The 



student must remember 

 that each column carries 

 part of the total horizontal 

 thrust. 



Owing to the uncertainty 

 of just where the force will 

 act because the attachment 

 of the columns to the base 

 may not be rigid, there is 



some uncertainty as to the 

 Fig. u8 Foundations under Columns 



exact amount of force ex- 

 erted on footings. If the columns are fixed (hinged) in such a 

 way that they bend at the top of the footing, the only force 

 exerted by wind on a footing will be that transmitted by the 

 windward column as a direct vertical load. If the column is 

 rigidly attached then the leeward column adds an eccentric load. 



If the columns are rigidly attached the force H acts at a height 

 equal to one-half the column length. If there is a knee brace 

 some men assume the length I to be the distance from the base 

 to the lower end of the knee brace. To be safe it is best to con- 

 sider the column length to be measured to the bottom chord of 

 the truss. 



If the columns are poorly connected, or hinged, to the footing, 

 the total wind force on the windward column is assumed to act 

 at a height equal to the full column length, plus one-half the 

 height of the roof truss. 



For taller buildings than here illustrated the total force of the 

 wind is assumed to act at half the height of the building, or if 

 only the. upper portion of the building is exposed, at half the 

 height of the exposed portion, measured from the ground, the 

 full amount of wind being figured only on the exposed portion. 

 This moment divided by the width of the building gives the 

 weight which must be opposed to resist overturning. Determine 

 the direct weight coming on the footing and if it is not enough 

 increase the depth of the footing so it will have enough weight. 



