GIRDERS USED IN CIVIL CONSTRUCTION LONG SPAN FRAMING BASICS AND TUTORIALS

GIRDERS IN LONG SPAN FRAMES CONSTRUCTION
What Are Girders In Construction?


Girders are the usual choice 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 (Fig. 7.10a) to provide the additional strength.
When 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 (Fig. 7.10b), composed of web plate, angles, and cover plates.Welded girders generally are composed of three plates (Fig. 7.10c). This
type offers the most opportunity for simple fabrication, efficient use of material, and least weight.

Top and bottom flange plates may be of different size (Fig. 7.10d), 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 (Fig. 7.10e). Where lateral loads are a factor, as in the case of girders supporting cranes, a channel may be fastened to the top flange (Fig. 7.10ƒ).

In exceptionally heavy construction, it is not unusual to use a pair of girders diaphragmed together to share the load (Fig. 7.10g).

The availability of high-strength, weldable steels resulted in development of hybrid girders. For example, a high strength 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 high strength steel, to compensate for possible overs tress 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.

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