NATIONAL PARK SERVICE Park and Recreation Structures

BRIDGES IN PARKS include foot trail, bridle trail, and vehicle bridges. There should be proved necessity for every bridge before it is undertaken to build it. This refers chiefly to foot trail and bridle trail bridges. Many such trails, certainly those crossing a dry ravine or gully at remote distances from intensive use areas, can make the dip rather than be kept to a constant grade by a bridge. The location of the trail with respect to intensive use areas and the extent of the drainage obstacle are determining factors in the justification of bridges.

This presentation seeks merely to focus on the characteristics that bring to park bridges of varying widths, spans, heights, and types of construction the most promise of compatibility with natural environment. There is elsewhere abundant information, including diagrams, rules, and formulae, for the design of structurally enduring bridges. Much more limited is the presentation of source material concerning itself with bridges which, by reason of appropriateness to natural environment, truly deserve to endure. There are far too many bridges which, after having broken every commandment for beauty and fitness, seem to have sought to wash away all sins through the awful virtue of permanence. Such penitent bridges surely have no place in our parks. The quality of permanence cannot be considered a virtue unless every other virtue, big or little, is present. It is otherwise only a vicious attribute.

In outward appearance, the bridge calls most importantly for visible assurance of strength and stability. To be entirely successful, it is not enough for the bridge to be functionally adequate within the exact knowledge of the engineer; it must proclaim itself so to the inexact instincts of the layman. It is pardonable park practice to venture well beyond sheer engineering perfection in the scaling of materials to stresses and strains, not alone in gesture to the lay concept of structural sufficiency, but to satisfy the claims of comparative scale. Overemphasis of the structural elements of the bridge is usually necessary in order to maintain a good scale relationship with the natural elements of the more or less rugged landscape widely prevailing in park areas.

THE ATTAINMENT of "the little more" that is so desired by those who would have an eye-appeal scale brought to the slide rule is all too rare in park bridges. Rather is there a too prevalent flimsiness, ocular rather than structural. Considerably fewer bridges fail to satisfy by seeming too ponderous for their function.

Of perhaps equal importance is the choice of materials for the bridge. Only those which are native to the area and predominate near the bridge site will constitute a convincingly appropriate and harmonious medium of structural interpretation. While this applies, of course, to all structures in parks, it is particularly important to stone bridges, which in their most happily successful expressions seem almost to spring from the stream or river bank when truly related in color, texture, and scale to adjacent rock outcrops.

After wise choice of a native material, used in a sufficiency pleasing to the eye, the next demand to be made upon bridges would be for variety within reason, avoiding the commonplace at one extreme, and the fantastic at the other. The ranges of use, span, and height, and the broad fields of materials, arch and truss forms, local practices (to name a few variety-making possibilities) promise endless combinations and cross-combinations making for much individuality among bridges.

In general, bridges of stone or timber appear more indigenous to our natural parks than spans of steel or concrete, just as the reverse is probably true for bridges in urban locations or in connection with broad main highways. Probably there are few structures so discordant in a wilderness environment as bridges of exposed steel construction. Too great "slickness" of masonry or timber technique, however, is certain to depreciate the merits of these materials for park bridges. Rugged and informal simplicity in use is the indisputable specification for their proper employment.

In no park structure more than in bridges is it of such importance to select a type of stone masonry construction that will reflect the natural formations in the immediate area and steer clear of the common errors in masonry. Shapeless stones laid up in the manner of mosaic are abhorrent in the extreme, having no precedent in Nature or in the traditions of sound masonry. In bridges particularly is there merit in pronounced horizontal coursing, breaking of vertical joints, variety in size of stones—all the principles productive of sound construction and pleasing appearance in any use of masonry. Often the creation of an effect that recalls any natural ledge stone formations in the vicinity is the indicated technique for the masonry of the bridge. The curve of the arch, the scale of the arch stones and masonry generally, the size of the pier, the height of the masonry above the crown of the arch are all factors vital to the success of the masonry bridge.

TIMBER BRIDGES may utilize either round or squared members to agreeable results. Squared timbers gain mightily in park character when hand-hewn. Simplicity of constructional pattern is a paramount consideration. It offers lesser contrast with natural elements, and is less distracting where competition between rigid artificial forms and free natural forms is an acknowledged taboo.

For practical as well as aesthetic reasons, bridges of open wood-truss type are in general disfavor. Arguments to their advantage seem to be lacking, whereas many are raised against them. When trusses are not structurally required, they are to be condemned for complicating the design and obstructing the view. In spite of most careful detailing to prevent water entering and lying in the joints, this is hard to overcome entirely. Shrinking of the timbers, rack under impact and strain, and rot developing in the opening joints speed the deterioration of this type of construction. It is short-lived and soon unsafe. It was practical effort to overcome this inherent weakness of the open wood-truss bridge by sheltering the trusses from exposure to the weather that brought about the development of the old-time covered bridge.

Many an otherwise altogether satisfying bridge fails of complete success through careless disregard in the matter of finishing touches. These, in relation to total effect, are actually far from so trivial as the frequent neglect of them might indicate or the man-days they involve. The parapet, railing, or wing wall should not terminate abruptly as though content with a mere toe-hold on the abutment, but should carry well back from the face of it to a very evidently sufficient bearing. Sometimes there is great merit in splaying the parapet or railing at the point of approach to a bridge, particularly if the bridge is narrow. Often the silhouette is much enhanced by stepping down, "easing" or "merging" parapet or railing into the grade at the terminals. Logical transition, convincing structurally, is desirable to relieve any abrupt change of materials in construction, especially in bridges that combine wood spans and stone piers. A similarly unpleasant and sharp transition sometimes occurs along the grade line of a bridge structure. A disturbing, unfinished, unassimilated appearance, noticeable in a number of the illustrations that follow, might have been dispelled by a mere warping of grade or bank slope, a suitable clean-up, and some cooperation with Nature to encourage the return of natural ground cover for the softening effect that is always its contribution.

• Plate I M—1 — Plate I M—24