Large industrial buildings, auditoriums, gymnasiums, theaters, hangars, and exposition buildings require much greater clear distance between supports than can be supplied by beam and column long span steel framing . When the clear distance is greater than can be spanned with rolled beams, several alternatives are available. These may be classified as girders, simple trusses, arches, rigid frames, cantilever-suspension spans, and various types of space frames, such as folded plates, curvilinear grids, thin-shell domes, two-way trusses, and cable networks.
Girders are the usualchoice where depths are limited, as over large unobstructed areas in the lower floors of tall buildings, where column loads from floors above must be carried across the clear area. Sometimes, when greater strength is required than is available in rolled beams, cover plates are added to the flanges to provide the additional strength.
When long span steel framing depths exceed the limit for rolled beams, i.e., for spans exceeding about 67 ft (based on the assumption of a depth-span ratio of 1:22 with 36-in-deep Ws), the girder must be built up from plates and shapes. Welded girders are used instead of the old-type conventional riveted girds , composed of web plate, angles, and cover plates.
Welded girders generally are composed of three plateslong span steel framing. This type offersthe mostopportunityforsimple fabrication, efficient use of material, and least weight. Top and bottom flange plates may be of different size long span steel framing, an arrangement advantageous in composite construction, which integrates a concrete floor slab with the girder flange, to function together.
Heavy girders may use cover-plated tee sections . Where lateral loads are a factor, as in the case of girders supporting cranes, a channel may be fastened to the top flange . In exceptionally heavy construction,it is not unusual to use a pair of girders diaphragmed together to share the load .
The availability of high-strength long span steel framing , weldable steels resulted in development of hybrid girders. For example, a highstrength steel, say A572 Grade 50, whose yield stress is 50 ksi, may be used in a girder for the most highly stressed flanges, and the lower-priced A36 steel, whose yield stress is 36 ksi, may be used for lightly stressed flanges and web plate and detail material. The AISC specification for allowable-stress design requires that the top and bottom flanges at any cross section have the same cross-sectional area, and that the steel in these flanges be of the same grade. The allowable bending stress may be slightly less than that for conventional homogeneous girders of the highstrength steel, to compensate for possible overstress in the web at the junction with the flanges. Hybrid girders are efficient and economical for heavy loading and long spans and, consequently, are frequently employed in bridgework.
Trusses. When depth limits permit, a more economical way of spanning long distances is with trusses, for both floor and roof construction. Because of their greater depth, trusses usually provide greater stiffness against deflection when compared pound for pound with the corresponding rolled beam or plate girder that otherwise would be required. Six general types of trusses frequently used in building frames are shown in below Figure together with modifications that can be made to suit particular conditions.
Trusses in above Figure to d and k may be used as the principal supporting members in floor and roof framing. Types e to j serve a similar function in the framing of symmetrical roofs having a pronounced pitch. As shown, types a to d have a top chord that is not quite parallel to the bottom chord. Such an arrangement is used to provide for drainage of flat roofs. Most of the connections of the roof beams (purlins), which these trusses support, can be identical, which would not be the case if the top chord were dead level and the elevation of the purlins varied. When used in floors, truss types a to d have parallel chords.
Properly proportioned, bow string trusses have the unique characteristic that the stress in their web members is relatively small. The top chord, which usually is formed in the arc of a circle, is stressed in compression, and the bottom chord is stressed in tension. In spite of the relatively expensive operation of forming the top chord, this type of truss has proved very popular in roof framing on spans of moderate lengths up to about 100 ft.
References:
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts