Some authorities advocate driving piles with the bark on and seine with it off. If the bark is on, the piles should be cut in the fall as otherwise the sap between the bark and wood will ulti mately cause the two to separate and the pile to slip within its bark.
The building laws of some cities require the piles to be capped directly with granite levelers; most authorities, however, prefer a thick bed of concrete encasing the heads of the piles and capping them at the same time.
The factor of safety should be from 2 to 1 2, varying with the accuracy of the knowledge of the loads to be carried and with the closeness with which the formuke used fits the conditions of the special case. Fig. 152 shows a footing supported by piles.
Fundamental Principles. The essential points in the design of foundations is not to overload the soil so as to cause excessive settlement, and to so arrange and distribute the loads as to cause the settlement to be uniform. Some settlement is practi cally sure to occur in almost all cases, but unequal settlement causes strains in the structure and cracks in the masonry.
If the supporting power of the soil is nearly uniform over the whole area of the building, the first problem is to determine the amount of load on each footing. This is not as simple as would at first appear. Not only is it uncertain just how much live load will be carried, but also what proportion of the whole building will be loaded with this live load.
Furthermore the dead load carried by the columns support ing the walls forms a much larger proportion of the total load on these columns than does the dead load carried by the interior columns. The different proportion of loading on the columns must, therefore, be brought to a common basis by some assumption. In the case of office buildings, the actual live load which reaches the founda tions is probably a small proportion of the total live load calculated over the whole area of all the floors. Moreover, the building has considerable time to settle from its dead load before any live load comes upon it. In order, therefore, to harmonize the settlement be tween wall and interior columns it is better to use as a basis the dead loads and a certain percentage of the live loads— say 25 per cent.
A table should be made of the dead load and 25 per cent of the live load of each column footing. The areas should then be made such that these loads on the soil would be the same per square foot in each case. Care must be exercised that in so doing, the total load of dead and live, or if the building laws under which the work is done permit of a reduction in live load, that this percent age of live and dead does not bring the load per square foot above the specified amount. In general, this will not be the case if the
column footing, in which the proportion of dead plus 25 per cent live to the total load is the least, is first proportioned for total load and the others then made proportional to it. The following example will illustrate this point.
No. 1 Dead+25% live=407,000. Total load=629,000 No. 2 Dead+25% live=190,000. Total load=215,000 No. 3 Dead+25% live=275,000. Total load=-195,000 Maximum allowable bearing on soil from total load to be 5,000 pounds per square foot.
In No. I the dead+25% live is 64.5% of the total load on this column. In No. 2 the dead+25% live is 80 % of the total load on this column. In No. 3 the dead+25% live is 59.2% of the total load on this column.
If then, we take column No. 3 as the basis we have the required area equal to 465,000 divided by 5,000 or 93 square feet. This gives 2,960 pounds per square foot from the dead + 25% live load.
For No.1 in order to have the pressure from the dead + 25% live the same as in No. 3 we shall require 407,000 divided by 2,960 or 137.5 square feet. This area gives 4,560 pounds per square foot pressure from the total load.
In column No. 2 we have 196,000 divided by 2,960 or 66 square feet required, and the from the total load is 3,700 pounds per square foot.
A further provision which must be made is to bring the center of gravity of the resisting area, or loaded area, coincident with the axis of the load. The same principle of a strut eccentric ally loaded applies to a footing in which eccentricity of loading exists. In such a case equal distribution on the soil is impossible as the side on which eccentricity exists will always be loaded the most. Furthermore, a bending moment, as in a strut similarly loaded, will occur in the foundations, and even a slight eccentric ity, if the load is considerable, will cause heavy strains in the footing. This latter point is sometimes difficult to accomplish because of the restricted area available for the footings. In some cases the loading and bearing capacity make it necessary to com bine the footings of several columns, or the necessity of combin ing the foundations under an old wall with new footings, or of providing for a future wall or column on the same foot ing, or of keeping the footing for a column in a party mall entirely within the party line,— any or all of these conditions may make it impossible to fulfil exactly the conditions previously men tioned. Departure from these principles should be as slight as possible, and when necessary direct provision should be made for the additional strains consequent thereon.