' 55,000 If the load is assumed uniformly distributed over the upper tier the greatest moment outside of the column load will be at the end having the greatest free length. The maximum moment there fore in this case will be at the edge of the base plate of the column at the left-hand end or 10.9 feet from this end. Call these moments AI and respectively.
If the allowable fibre strain is taken at 18,000 pounds per 3,487,000 square inch, the required moment of resistance = 18,000 194.
The offsets in masonry footings can be determined by the formula for a beam fixed at one end and loaded uniformly. A general practice and one in fairly close accord with the results of the above formula is to draw lines at 60 degrees with the horizon tal from the edges of the column bases and where these cross the joint lines (the thickness of the courses having been assumed) will be the vertical face of the course. When the structure is of such a character that wind load affects the foundations, this must be considered in addition to the other live loads. Such cases would be narrow and very high buildings, chimneys, monu ments, etc.
While the concrete and imbedded steel beams in a footing are undoubtedly much stronger than the simple beams, it is not customary to figure the beams in such cases by the theory apply ing to steel imbedded simply in the tension side of concrete. Foot ings of this character are employed sometimes and their design will be taken up later.
girder is a riveted girder, as is often the case, other features must be considered in its design, as will be explained later.
In case the cantilever is in the floor, as it sometimes is, as shown by Fig. 156, and in addition to the wall column, carries a floor load, then the position of maximum moment must be deter mined in a manner similar to that explained for combined footings. The connection of the cantilever at the interior column must be designed to resist this upward tendency and in case the reaction from the dead-wall load is greater than the dead load carried by this column the cantilever arm should be extended to the next; column so as to decrease this reaction ; or the column must be an chored and all connections designed to resist this upward reaction.
Fig. 156 shows also a steel concrete retaining wall to hold up the earth under an adjoining building which foots some dis. tance above the new foundations.
Fig. 157 illustrates the ease of a party wall foundation designed to early a future wall column for the adjoining building and the column of the present building. The eccentricity of bearing is shown in this case, and this and the necessity of spread ing in the direction of the wall rather than across it are the important features.
The matter of design of foundations is one always requiring accurate knowledge of the special conditions incident to the prob lem and the nature of the soil, and is largely influenced by prae. tical considerations and the judgment of the designer. It is not safe to lay down any fixed values to be followed in all cases. Foundations in soil which are at all questionable, should never be designed except by an expert, who is capable of judging the extent to which the ordinary methods of procedure must be modified.
Retaining Walls are walls built to resist the thrust of earth pressure. These walls may also be bearing walls for loads above. The pressure of earth tends to cause failure of the wall in the following ways : (1). To slide on its base.
(2). To slide on some horizontal joint.
(3). To overturn bodily.
(4). To fail by buckling.