This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.

The factor of safety should be from 2 to 12, varying with the accuracy of the knowledge of the loads to be carried and with the closeness with which the formulas 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 practically 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 supporting 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 foundations 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 between 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 percentage 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.

CONCRETE AMD PILE FOOTING UNDER COLUMN Fig. 152.

Problem. Suppose columns as follows:

No. 1 | Dead | + | 25% | live | = | 407,000. | Total load | = | 629,000 |

No. 2 | Dead | + | 25% | live | = | 190,000. | Total load | = | 245,000 |

No. 3 | Dead | + | 25% | live | = | :275,000. | Total load | = | 465,000 |

Maximum allowable bearing on soil from total load to be 5,000 pounds per square foot.

In No. 1 | 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% 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 pressure 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 eccentrically 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 eccentricity, 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 combine the footings of several columns, or the necessity of combining the foundations under an old wall with new footings, or of providing for a future wall or column on the same footing, or of keeping the footing for a column in a party wall entirely within the party line, - any or all of these conditions may make it impossible to fulfil exactly the conditions previously mentioned. Departure from these principles should he as slight as possible, and when necessary direct provision should be made for the additional strains consequent thereon.

The necessity of keeping footings inside of party lines, and the desire to make the axis of load conform to the center of gravity of area, sometimes results in the use of cantilever construction. These cantilevers are in some cases laid directly over the beams forming the grillage in the footing. This construction makes the actual point of application of the loads uncertain as any deflection would tend to throw the load on the outer beams. A better construction is the use of a shoe with a pin bearing.

Improvement of Bearing Power. The supporting power of all soils is improved by compacting, by mixing sand or gravel or by driving piles which prevent the spreading of the soil as well as compacting it. Drainage of a wet soil also greatly improves its bearing power.

The following table taken from Baker's " Treatise on Masonry Construction," gives values for general use in determining the bearing power of soils:

Safe Bearing Power Tons per Sq. Foot. | |||

Clay in thick beds, always dry......... | 4 | to | 6 |

Clay in thick beds, moderatley dry......... | 2 | to | 4 |

Clay soft............ | 1 | to | 2 |

Gravel and Coarse Sand, well cemented..... | 8 | to | 10 |

Sand compact and well cemented....... | 4 | to | 6 |

Sand clean and dry......... | 2 | to | 4 |

Quicksand, alluvial soils, etc........ | 1/2 | to | 1 |

The bearing power of clay depends largely upon the degree of moisture.

Foundations on clay, containing much water, and undrained, are liable to settlements from the escape of the water either by adjacent excavations, or by the squeezing out of the water.

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