Evening Republican, Volume 16, Number 300, Rensselaer, Jasper County, 18 December 1912 — Home Course In Road Making [ARTICLE+ILLUSTRATION]

Home Course In Road Making

Vn. —Highway Culverts and Bridges.

By LOGAN WALLER PAGE,

Director Office of Public Roads, United States Department of Agriculture Copyright by American Press Association, 1912.

CULVERTS and bridges are constructed for a twofold purpose. In the first place, they are required to provide the necessary drainage for the road and. In the second place, to furnish a suitable crossing, fortrerffie over Waterways. A large percentage of the highway culverts arid bridges in this country were bnitt of wood in the first Instance, and In later years many of the smaller culverts have been rebuilt with some kind of pipe, either of terracotta, cast iron or more recently of corrugated metal. It is Impracticable in a short paper to discuss tbe various forms of pipe culverts. They are purchased in the open market, and the road official uses his judgment about the size of pipe that should be laid to serve the requirements of the location in question. The following principles should, however, be borne in mind: All pipe culverts should be laid deep enough so that the pipe will not be injured by the

traffic passing over It, and head walls should In all cases be built at each end of the culverts to prevent them from being washed out The maximum. fill to be allowed over a clay pipe’ Culvert should be at least three ’feet'. The objection to pipe culverts is that they become easily clogged and are thus inade useless. Clay pipe culverts are easily broken unless they are well laid and well protected. The most simple and natural form of bridge consists of timbers laid across the stream or'opening which is to he passed over and covered with planks to form the roadway. Walls should be built to support each end of the timbers, and these are called' abutments. Tbe width of tbe opening which they cross is termed the span. The timbers themselves are called stringers, and the planks are usually referred to as the flooring. The size of the stringers required Increases with the span and the distance apart, center to center, that they are laid. For example, a 2 inch by 6 Inch stringer will do for a two foot span, while a G inch by 10 inch or 6 inch by 12 inch stringer is required for a twenty foot span. The distance apart that the stringers are required to be laid varies With the thickness of the plank flooring and the amount of traffic. The weight of the materials in the bridge is commonly referred to as the dead load. The additional load which the bridge is designed to carry is known as the live load and consists of animals, wagons or motor vehicles or pedestrians. A crowd of people standing close together on a bridge is usually estimated at about 100 pounds per square foot of floor space. Th^. strength'of the bridge depends upon the kind of timber used, the dimensions of the timber, tbe amount used and its location, and also very largely upon the span of the bridge. For example, assuming a loaded wagon carrying 500 pounds per wheel, a yellow pine board one inch thick and eight inches wide would require stringers to support ft about every thirteen inches apart to carry the load safely, while a plank of the same width and two inches thick would require stringers three feet apart, and a three inch plank would require stringers about every four feet apart. These figures are based upon the assumption that yellow pinev Douglas fir or a good quality of oak would be, used. If such timbers as white pine, hemlock or spruce are used, theft” stringers would be required about every eight inches for a one inch board, every two and one-half feet for a two inch plank and every three feet for a three inch plank. The following table gives the approximate sizes of stringers required for the different spans:

’ B * sill si|| i« Ig&l ssll sits 4) # C J VK £ 2 2x6 3x6 9 3x6 4xß a 2x6 Bx 6 10 3x6 6x 10 A 2x6 3x6 12 4x 8 6xlo 5 3x6 4*B 14 4x 8 6xlo « 3x6 4xß 16 4xlo 6x 12 7 3x6 4xß 18 4xlo 6x12 . g 3x 6 4x 8 20 4x 10 6x12

•*ln the above table round timbers or logs may be substituted for tbe stringers, in which case tbe diameter of the log should be about one and one-eighth times tbe largest dimension of the stringer as given. Fig. 2 Illustrates two methods by Vkfch the simple beam or stringer may

be streEgt&eneJi' and states the "corresponding loads that will be carried safely. That Is, the simple beam 12 inches by 12 inches square and 24 feet between points of support will carry safely a concentrated load of one and one-half tons at its center, while if tbe same beam be made into a King post I beam by passing underneath the beam ' a'one Inch steel rod, which is made fast at either end of the beam, and inserting a single post under the load at the center of the beam, then such a beam will carry about two and threequarter times as. much, or a load of three and three-quarter tons, safely, while If tbe same beam be built into s King truss beam the load concentrated at the center may then be increased to nine tons. The best culverts and smaller bridges are built of re-enforced concrete. The cost is greater than for wooden or pipe constructions in the first Instance, but If well built there should be no farther cost for repairs. That is, the first cost is the last cost, while durability and safety are secured from the oatset. \ / .Good materials, Consisting of crashed stone ,or gravel, wand and Portland .cement and required for concrete. Deforojed steel rods imbedded in the concrete are used for strengthening the cover spans. The mixing of these materials into concrete and placing it In the'forms are extremely simple matters after they are once well understood, but nevertheless should not be undertaken by one who is unfamiliar with the use of concrete. There are three general kinds of concrete culverts, which are known as the concrete box type. They are built for the smaller sizes up to such as have an openiiig about four feet by six feet wide. For sizes above that the floor is usually left out where It is not needed to protect the foundation, or paving may be substituted for it The floor and side walls are constructed of concrete with or without metal re-enforce-ment, usually without such re-enforce-ments. The proportions of concrete used for the floor and side walls are usually, 1,3, G—that is, one part by measure of Portland cement, three parts by measure of sand and six parts by measure of crushed stone or gravel. For spans above ten or twelve feet the cover nheds to be strengthened with concrete beams. This type is known as tbe concrete T beam from its resemblance to tbe capital letter. They are placed adjacent to each other, the distance from center to center depending upon tbe load which the bridge is expected to carry. The best type of concrete culverts for spans from ten feet up to thirty feet Is the steel I beam incased in concrete, as shown in Fig. 1. Here tbe concrete floor is designed to carry the load' across the span from one I beam to another, while the steel I beams carry the load from one abutment to the other. It often happens that the culverts are built on yieldlug foundations and that the abutments sometimes settle, causing cracks that would be dangerous in some types of culverts, but it is the ability of tbe steel I beam type to withstand such conditions as these just mentioned that makes it the best type to buUd. The I beams are incased in concrete to protect them from rusting. Sometimes they are simply-painted or more often not painted at all, and what would be a permanent bridge is allowed to rust out for lack of proper care. For details in regard to the methods of designing and building concrete bridges attention is called to bulletin No. 39, “Highway Bridges and Culverts,” issued by the office of public roads, United States department of agriculture. v No road can be called a good road that is dotted with broken, wornout and unsafe wooden culverts and bridges, such as are encountered on

Kino Tauas 1 3 mfm Load mt Cnrtttr 9 Tin* FIG. It—COMPARATIVE TYPES AND LOADS FOB WOODEN BEAMS. t many of oar highways at the present time. Such bridges are a menace to onr traveling public and are expensive *to maintain. Tbe price of timber is advancing, and the increasing traffic demands safer bridges and culverts. Re-enforced concrete for this class of work appears to form the best solution of this problem. Bridge construction is eminently tbe work for the engineer, and his sejyiees should in all be secured.

FIG. I. CONCRETE CULVERT STEEL I BEAMS INCASED IN CONCRETE.