Trusses, offering relatively large depth, open-web construction, and members subjected primarily to axial stress, provide large carrying capacity for comparatively small amounts of steel. For maximum economy in truss design, the area of metal furnished for members should be varied as often as required by the loads. To accomplish this, designers usually have to specify built-up sections that require considerable fabrication, which tend to offset some of the savings in steel.
Truss Spans. Truss bridges are generally comparatively easy to erect, because light equipment often can be used. Assembly of mechanically fastened joints in the field is relatively labor-intensive, which may also offset some of the savings in steel. Consequently, trusses seldom can be economical for highway bridges with spans less than about 450 ft. Railroad bridges, however, involve different factors, because of the heavier loading.
Trusses generally are economical for railroad bridges with spans greater than 150 ft. The current practical limit for simple-span trusses is about 800 ft for highway bridges and about 750 ft for railroad bridges. Some extension of these limits should be possible with improvements in materials and analysis, but as span requirements increase, cantilever or continuous trusses are more efficient. The North American span record for cantilever construction is 1,600 ft for highway bridges and 1,800 ft for railroad bridges.
For a bridge with several truss spans, the most economical pier spacing can be determined after preliminary designs have been completed for both substructure and superstructure. One guideline provides that the cost of one pier should equal the cost of one superstructure span, excluding the floor system. In trial calculations, the number of piers initially assumed may be increased or decreased by one, decreasing or increasing the truss spans. Cost of truss spans rises rapidly with increase in span. A few trial calculations should yield a satisfactory picture of the economics of the bridge layout. Such an analysis, however, is more suitable for approach spans than for main spans. In most cases, the navigation or hydraulic requirement is apt to unbalance costs in the direction of increased superstructure cost. Furthermore, girder construction is currently used for span lengths that would have required approach trusses in the past.
Panel Dimensions. To start economic studies, it is necessary to arrive at economic proportions of trusses so that fair comparisons can be made among alternatives. Panel lengths will be influenced by type of truss being designed. They should permit slope of the diagonals between 40 and 60 with the horizontal for economic design. If panels become too long, the cost of the floor system substantially increases and heavier dead loads are transmitted to the trusses. A subdivided truss becomes more economical under these conditions.
For simple-span trusses, experience has shown that a depth-span ratio of 1:5 to 1:8 yields economical designs. Some design specifications limit this ratio, with 1:10 a common historical limit. For continuous trusses with reasonable balance of spans, a depth-span ratio of 1:12 should be satisfactory. Because of the lighter live loads for highways, somewhat shallower depths of trusses may be used for highway bridges than for railway bridges.
Designers, however, do not have complete freedom in selection of truss depth. Certain physical limitations may dictate the depth to be used. For through-truss highway bridges, for example, it is impractical to provide a depth of less than 24 ft, because of the necessity of including suitable sway frames. Similarly, for through railway trusses, a depth of at least 30 ft is required. The trend toward double-stack cars encourages even greater minimum depths.
Once a starting depth and panel spacing have been determined, permutation of primary geometric variables can be studied efficiently by computer-aided design methods. In fact, preliminary studies have been carried out in which every primary truss member is designed for each choice of depth and panel spacing, resulting in a very accurate choice of those parameters.
Bridge Cross Sections. Selection of a proper bridge cross section is an important determination by designers. In spite of the large number of varying cross sections observed in truss bridges, actual selection of a cross section for a given site is not a large task. For instance, if a through highway truss were to be designed, the roadway width would determine the transverse spacing of trusses. The span and consequent economical depth of trusses would determine the floorbeam spacing, because the floorbeams are located at the panel points.
Selection of the number of stringers and decisions as to whether to make the stringers simple spans between floorbeams or continuous over the floorbeams, and whether the stringers and floorbeams should be composite with the deck, complete the determination of the cross section.
Good design of framing of floor system members requires attention to details. In the past, many points of stress relief were provided in floor systems. Due to corrosion and wear resulting from use of these points of movement, however, experience with them has not always been good. Additionally, the relative movement that tends to occur between the deck and the trusses may lead to out-of-plane bending of floor system members and possible fatigue damage. Hence, modern detailing practice strives to eliminate small unconnected gaps between stiffeners and plates, rapid change in stiffness due to excessive flange coping, and other distortion fatigue sites. Ideally, the whole structure is made to act as a unit, thus eliminating distortion fatigue.
Deck trusses for highway bridges present a few more variables in selection of cross section. Decisions have to be made regarding the transverse spacing of trusses and whether the top chords of the trusses should provide direct support for the deck. Transverse spacing of the trusses has to be large enough to provide lateral stability for the structure. Narrower truss spacings, however, permit smaller piers, which will help the overall economy of the bridge.
Cross sections of railway bridges are similarly determined by physical requirements of the bridge site. Deck trusses are less common for railway bridges because of the extra length of approach grades often needed to reach the elevation of the deck. Also, use of through trusses offers an advantage if open-deck construction is to be used. With through-trusses, only the lower chords are vulnerable to corrosion caused by salt and debris passing through the deck.
After preliminary selection of truss type, depth, panel lengths, member sizes, lateral systems, and other bracing, designers should review the appearance of the entire bridge. Esthetics can often be improved with little economic penalty.