arrangements of steel are generally used in buildings; but roadway slabs, in view of the heavy concentrated-live-loads that they have to carry, and which produce large shears throughout and positive moments over almost the entire span, require carefully and liberally designed reinforcement.

Barring most of those in railway bridges, slabs are usually continuous over panel points, excepting at the expansion joints. There is but little difference in the actual costs of continuous and non-continuous slabs; but continuity is desirable from the standpoints of paving and drainage; also with continuous slabs T-beam construction can be employed-involving the saving of much material in girders. The continuity of slabs and girders complicates construction problems—sometimes very seriously. The various processes of the construction of a proposed design should be studied through completely in order to make certain that no impracticable or unnecessarily expensive work is involved. Frequently important savings can thus be effected; and sometimes it is found cheaper in the long run to use more concrete than the minimum practicable amount. For instance, a sidewalk slab usually rests upon the curb, which in turn rests on the roadway slab; and in the designing it is advisable to provide for a construction joint at the top of the roadway slab, because it would be both difficult and expensive to pour the curb and the sidewalk slab simultaneously with the roadway slab. Such construction joints can be arranged for by the designer without involving much extra expense, provided that he gives the matter proper consideration at the outset.

Girders

Girders are of two main types, single or continuous; and there is no great difference in their costs, there being more concrete but less steel in the simple-span type. The two-span-continuous type is nearly always a little more expensive than the simple-span type. The simple-span girder can be moulded separately and set in place. This is not a paramount feature in highway bridges, but it is often all-important in railway structures.

Comparing simple girders and continuous ones of three or more spans, the following general observations may be made:

If there is no T-beam action, the simple spans will be the more expensive; because the section will be determined by the moment at mid-span in the simple girder, and this is greater than any of the moments of the continuous girders. Again, higher unit stresses than ordinary are allowed over the supports of continuous girders. For the T-section, if the bottoms are straight, the continuous type will be the more expensive, having more concrete and more steel than the simple type. But if the bottoms of all girders are curved, the continuous girders will be the cheaper, there being decidedly less concrete required for them. When the bottoms are straight for simple-span girders and are curved for continuous girders, the curves on page 1323 of "Bridge Engineering" indicate the relationships. It will be