In response to such an assertion, instances may be afforded, no doubt, of the long existence of ancient materials; of old buildings still affording serviceable shelter; of exposed metal which with care and liberal paint has outlasted a half-century.

Such instances will prove of little value, because modern building methods lack not only the general massiveness of the ancient but their simplicity of composition, and massiveness of construction or mere solidity of one part is not in itself a source of longevity to other more perishable material. Modern construction is composite and employs complex materials, and those materials are generally reduced in proportions and subjected to greater strains than in older structures. The wear and tear of parts is vastly increased, and new forms of strains and vibrations due to modern machinery and appliances, to traffic, to local blasting work, and to the introduction of heating and steam-raising apparatus, all enter into the racking or straining of the elements of the structure, and even extend into the disintegration of inert materials, such as concrete.

In ancient construction, beams and rafters were exposed, so that dry-rot was avoidable, and when houses were unheated, warping of lumber and cracking of trim were rare. The humidity as well as the temperature in old buildings followed natural combinations, while lumber was cheap and plentiful and was used with prodigality and corresponding solidity.

The complexity of materials now employed in construction, the durability of many of which is unknown, is another reason for conservative limitations. Much discussion has been devoted to the subject of the permanency of reinforced concrete constructions, which only time will fully decide; but even if the strength and inert character of reinforced concrete be conceded, it is still found to be subject to deterioration by temperature changes, and even in such enormous masses as are used in dam construction this vast and silent force has rent the mass and opened the channel for deterioration.

The exterior face of a steel-frame and brick building may be in part granite, ashlar, marble, limestone, pressed brick, or terra-cotta. All are not equally durable; some are, in modern construction, extremely slender in dimensions; all are exposed to the action and reaction due to extremes of frost and heat, as well as to the interior expansion and contraction of the frame that supports them.

Materials combined in modern constructions are tabulated on the following page, with assignments of probable life.

Table B. Relative Life Of The Component Parts Of Buildings

I. Good Frame Construction Life Of Most Durable Part=45-55 Years Or 100%

II. Brick And Stone Construction Life Of Most Durable Part = 55-66 Years Or 100%

III. Modern Fireproof Construction. Life Of Most Durable Part = 66-75 Years Or 100%

The life of the combination is, therefore, relative to that of the most durable part, in proportion to the share of the total borne by each portion, or, from the monetary point of view, to their share in the cost.

Life Of The Most Durable Part

The life of the less durable materials which are to be combined with the foregoing becomes related thereto, and may be assigned proportionate terms of existence, such as those which appear in Table B.

The foregoing limitations, varying with complexity or simplicity of the construction, may be modified to meet individual experiences or special or local conditions, but the shifting of one or other element higher or lower in the scale will not affect the principle of application, so long as too lengthy periods are not adopted for the most durable parts.

By applying to the cost or quantity of each element in the building its relative life, and taking the average or mean of the whole, a period is found upon which to base the expectancy of loss of entire value. The ratio which each element bears to the whole is most readily expressed in its monetary value, as the quantities do not relate to one another in any common terms except that of money. The method followed is illustrated by the following tabulation of the component parts and respective values of a steel-frame office building.

Fig. 11

Table C Method Of Ascertaining Mean Life Of A Building. Example Of A Steel-Frame Fireproof Office Building

Mean, 7327.6/100 = 73. 276% of 66 years, or a mean life of 48.36 yrs.

It may be observed that such items as supervision, plans, fees, and carrying charges during construction should be spread over the other items, as each derives a proportionate benefit therefrom.

Relative costs are not difficult to procure from those experienced in constructive work, and it would be well if in every building operation such a record was made.

In Fig. 12 the curve A is a brick railroad roundhouse and C a modern brick and metal factory, with which is repeated the curve B of the average of 36 brick buildings from Fig. 10, plotted, for comparison. Relating all to a life of 66 years of the most durable parts, they give respectively: A 48 years, B 44 years, and C 53 years mean life.

The foregoing instances afford a view of the application of the method of apportionment to quite a variety of structures, and cover relative construction costs under very differing local conditions. In all, the process of depreciation is relatively slow during the first part of the existence of the building, but is greatly accelerated during the latter part, indicating its progressive character. Table E gives the rate of annual sinking-funds for any term of years from 10 to 60, at rates of compound interest from 2 1/2 to 6%.