Difficult foundations favor long spans, not only because of the reduction in the number of piers but also because the unit costs for small piers are much higher than those for large ones. On the other hand, if the foundations are very deep, the effect of unbalanced thrusts becomes of great importance; and this favors shorter spans. Poor foundation conditions also militate for shorter spans, as do invariably pile foundations.

If it be decided for the sake of appearance to make the piers heavy and massive, this will tend towards greater span-length; because, in that case, up to a certain limit, an increase of span will augment the size of each individual pier but little, if any. It will rarely pay to reduce the span-length, if such reduction will not decrease the size of the pier or piers.

Light live loads in proportion to the dead loads tend, for economy, towards the adoption of longer spans, especially when the adjoining spans are of the same length. With such light live loads the economic span-length is not greatly affected by the distance from springing to bottom of base. When a pier carries one arch span only, the ratio of live load to dead load is of much smaller importance than it is in the case where there is a succession of spans.

The type of arch ring, whether it be of one solid-barrel or of two or more arch ribs, often affects materially the economic span-length, but to what extent it is difficult to predict in advance of designing and estimating. The piers of the solid-barrel type are generally more expensive than those of the ribbed type, as are also the arches; but in most cases the piers are comparatively the more expensive; and this favors the employment of longer spans. Also, it will frequently be found that increasing the span-length will augment but slightly the quantities in the arch rings of the solid-barrel type-which also favors longer spans. The economic ratio of rise to span-length for solid-barrel arches may generally be taken at 0.25, varying, of course, with the other factors previously discussed. The foregoing conclusion may seem to be in contradiction to the well-known fact that long spans are nearly always of the ribbed type. This, however, is because the solid-barreled arch is generally used for low rises, which necessarily means comparatively short spans, while the open-ribbed arches are employed for nearly all arch structures of high rise. The comparative economics of these-two types will be discussed later in this chapter.

When adjoining spans must be unequal, the inequality, for economy's sake, should be made as small as practicable. This is very important if the springings are far above the base; but is of small consequence if they are not. If the springing of the smaller arch can be located well above that of the larger one, it may be possible so to adjust the spans and rises that there shall be very little eccentricity of pressure on the base, in which case the pier will not be much more expensive than it would have been had the adjacent spans been of equal length. The appearance of the piers