Great Smoky Mountains National Park Road Bridges

Bridges include foot, bridle trail, and vehicle bridges of widely varying widths, spans, heights, and types of construction. In the interest of limiting the classifications within this compilation, the less frequent underpass and the minor culvert are embraced within this section. 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. In gesture to the lay concept of structural sufficiency, it is pardonable park practice to venture well beyond sheer engineering perfection in the scaling of materials to stresses and strains. 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. After the attainment of a sufficiency in material pleasing to the eye, the next demand to be made upon bridges would be for variety, 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—among other variety-making possibilities—promise endless combinations and cross-combinations that could make for such individuality among bridges that none need ever appear the close counterpart of another. This presentation seeks merely to focus on the characteristics that bring to bridges 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 field of source material that concerns itself with bridges that, by reason of appropriateness to natural environment, truly deserve to endure. There are far too many bridges which, after breaking every commandment for beauty and fitness, seem to have sought to wash away all sins through the awful virtue of permanence. Such penitent bridges should have no place in our parks. The quality of permanence cannot be considered a virtue in itself. Unless every other desirable virtue, big or little, is present, permanence is only a vicious attribute. 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 is certain to depreciate the value of these materials for park bridges. Rugged and informal simplicity in use is indisputably the specification for their proper employment in bridges. In no park structure more than bridges is it of such importance to steer clear of the common errors in masonry. Shapeless stones laid up in the manner of mosaic are abhorrent in the extreme. In bridges particularly is there merit in 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. The curve of the arch, the size of the pier, the height of the masonry above the crown of the arch are all of great importance to the success of the masonry bridge. Timber bridges may utilize round or squared members to agreeable results. Squared timbers gain mightily in park-like characteristic when hand-hewn. A common fault in bridges is the too abrupt termination of the parapet, railing, or wing wall. These should carry well beyond the abutments. In general disfavor for park use are bridges of the open wood truss type. There seem to be no arguments to their advantage, while many are raised against them. 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. The culvert is too often handled as a conspicuous bridge, when in reality it is merely a retaining wall pierced by a drain. The facing of the culvert, like the treatment of almost every other facility in natural parks, should be first and always informal and inconspicuous. Facing and culvert proper should be adequate in materials and in workmanship so that once constructed both can be forgotten and make no demands upon maintenance appropriations. The culvert proper is sometimes of local stone when this is abundant and workable, but if, as is more frequently the case, it is of concrete or of galvanized iron, reasonable concealment of the fact is to be striven for. The retaining wall that is the end wall or facing of the culvert should avoid disclosing that it is a mere veneer by extending well into the culvert opening. Natural rock is certainly the preferred material for the end walls. It may be laid either in mortar, or dry, but the latter method of laying to be lasting should be undertaken only when the available stone is of suitably large size. If stone is not available locally or from within a reasonable distance, concrete or wood must be resorted to in constructing the retaining wall. Either is an unsatisfactory substitute for the stone wall—concrete because of its harsh surface, and lack of permanence if inexpertly mixed, and wood because of its tendency to deteriorate rapidly under conditions of moisture. As much care should be given to the design and execution of culvert end walls as to other park structures. Usual mistakes are insufficient care in the handling of mortar, resulting in sloppy joints, and lack of variety in stone sizes, leading to monotony and formality of surface pattern. These faults are common to much contemporary stone work, not limited to park construction only.

Tags: buildings-structures, facilities, great-smoky-mountains-national-park, grsm, roads, transportation
Organization: Department of the Interior