332 Description of the Stub Girder Floor System

The main element of the system is a special girder, fabricated from standard hot-rolled wide-flange shapes, that serves as the primary framing element of the floor. Hot-rolled wide-flange shapes are also used as transverse floor beams, running in a direction perpendicular to the main girder. The girder and the beams are usually designed for composite action, although the system does not rely on having composite floor beams, and the latter are normally analyzed as continuous beams. The transverse floor beams normally use a smaller size member as a drop-in span within the positive moment region.

Load and resistance factor design (LRFD) is preferred for stub girders, since it gives lower steel weights and simple connections. The costs of an LRFD-designed stub girder are therefore lower than those associated with allowable stress design (ASD).

Figure 33.1 shows the elevation of a typical stub girder. The girder that is shown uses four stubs, oriented symmetrically with respect to the midspan of the member. The locations of the transverse floor beams are assumed to be the quarter points of the span, and the supports are simple. In practice many variations of this layout are used, to the extent that the girders may utilize any number of stubs. However, three to five stubs is the most common choice. The locations of the stubs may differ significantly from the symmetrical case and the exterior stubs may have been placed at the very ends of the bottom chord. However, this is not difficult to address in the modeling of the girder, and the essential requirements are that the forces that develop as a result of the choice of girder geometry be accounted for in the design of the girder components and the adjacent structure. These actual forces are used in the design of the various elements, as distinguished from the simplified models that are currently used for many structural components.

The choices of elements, etc., are at the discretion of the designer and depend on the service requirements of the building as seen from the architectural, structural, mechanical, and electrical viewpoints. Unique design considerations must be made by the structural engineer, for example, if it is decided to eliminate the exterior openings and connect the stubs to the columns in addition to the chord and the slab.

Figure 33.1 shows the main components of the stub girder, as follows:

1. Bottom chord.

2. Exterior and interior stubs.

Stub

Stub

FIGURE 33.1 Elevation of a typical stub girder.

3. Transverse floor beams.

4. Formed steel deck.

5. Concrete slab with longitudinal and transverse reinforcement.

6. Stud shear connectors.

7. Stub stiffeners.

8. Beam-to-column connection.

9. The bottom chord should preferably be a hot-rolled wide-flange shape of column-type proportions, most often in the W12 to W14 series of wide-flange shapes. Other chord cross-sections have been considered; for example, tee shapes and rectangular tubes have certain advantages as far as welded attachments and fire protection are concerned, respectively. However, these other shapes also have significant drawbacks. The rolled tube, for example, cannot accommodate the shear stresses that develop in certain regions of the bottom chord. Rather than using a tee or a tube, therefore, a smaller W-shape (in the W10 series, for example) is most likely the better choice under these conditions.

The steel grade for the bottom chord, in particular, is important, since several of the governing regions of the girder are located within this member, and tension is the primary stress resultant. It is therefore possible to take advantage of higher strength steels, and 50 ksi yield stress steel has typically been the choice.

The floor beams and the stubs are mostly of the same size W-shape and are normally selected from the W16 and W18 series of shapes. This is directly influenced by the size(s) of the heating, ventilating, and air conditioning (HVAC) ducts that are to be used. Although it is not strictly necessary that the floor beams and the stubs use identical shapes, it avoids a number of problems if such a choice is made. These two components of the floor system at least should have the same height.

The concrete slab and the steel deck constitute the top chord of the stub girder. It is either made from lightweight or normal weight concrete, although if the former is available, even at a modest cost premium, it is preferred. The reason is the lower dead load of the floor, especially since the shores that will be used are strongly influenced by the concrete weight. Further, the shores must support several stories before they can be removed. In other words, the stub girders must be designed for shored construction, since the girder requires the slab to complete the system. In addition, the bending rigidity of the girder is substantial and a major fraction is contributed by the bottom chord. The reduction in slab stiffness that is prompted by the lower value of the modulus of elasticity for the lightweight concrete is therefore not as important as it may be for other types of composite bending members.

Concrete strengths of 3000 to 4000 psi are most common, although the choice also depends on the limit state of the stud shear connectors. The concrete does not control the strength of the stub girder, apart from in certain long-span girders, some local regions in the slab, and the desired mode of behavior of the slab-to-stub connection (which limits the maximum fc-value that can be used). Consequently, there is little to gain by using high-strength concrete.

The steel deck should be of the composite type, and a number of manufacturers produce suitable types. Normal deck heights are 2 and 3 in., but most floors are designed for the 3 in. deck. The deck ribs are run parallel to the longitudinal axis of the girder, since this gives better deck support on the transverse floor beams. It also increases the top chord area, which lends additional stiffness to a member that can span substantial distances. Finally, the parallel orientation provides a continuous rib trough directly above the girder centerline, improving the composite interaction of the slab and the girder.

Due to fire protection requirements, the thickness of the concrete cover over the top of the deck ribs is either 416 in. (normal weight concrete) or 3j in. (lightweight concrete). This eliminates the need for applying fire protective material to the underside of the steel deck.

Stud shear connectors are distributed uniformly along the length of the exterior and interior stubs, as well as on the floor beams. The number of connectors is determined on the basis of the computed shear forces that are developed between the slab and the stubs. This is in contrast to the current design practice for simple composite beams, which is based on the smaller of the ultimate axial load-carrying capacity of the slab and the steel beam [3]. However, the simplified approach of current specifications is not applicable to members where the cross-section varies significantly along the length (nonprismatic beams). The computed shear force design approach also promotes connector economy, in the sense that a much smaller number of shear connectors is required in the interior shear transfer regions of the girder [4-6].

The stubs are welded to the top flange of the bottom chord with fillet welds. In the original uses of the system, the design called for all-around welds [1,2]; subsequent studies demonstrated that the forces that are developed between the stubs and the bottom chord are concentrated toward the end of the stubs [4,5,7]. The welds should therefore be located in these regions.

The type and locations of the stub stiffeners that are indicated for the exterior stubs in Figure 33.1, as well as the lack of stiffeners for the interior stubs, represent one of the major improvements that were made to the original stub girder designs. Based on extensive research [4,5], it was found that simple endplate stiffeners were as efficient as the traditional fitted ones, and in many cases the stiffeners could be eliminated at no loss in strength and stiffness to the overall girder.

Figure 33.1 shows that a simple (shear) connection is used to attach the bottom chord of the stub girder to the adjacent structure (column, concrete building core, etc.). This is the most common solution, especially when a duct opening needs to be located at the exterior end of the girder. If the support is an exterior column, the slab will rest on an edge member; if it is an interior column, the slab will be continuous past the column and into the adjacent bay. This may or may not present problems in the form of slab cracking, depending on the reinforcing steel placement around the column.

The stub girder has sometimes been used as part of the lateral load-resisting system of steel-framed buildings [8,9]. Although this has certain disadvantages insofar as column moments and the concrete slab reinforcement are concerned, the girder does provide significant lateral stiffness and ductility for the frame. As an example, the maintenance facility for Mexicana Airlines at the Mexico City International Airport, a structure utilizing stub girders in this fashion, survived the 1985 Mexico City Earthquake with no structural damage [9].

Expanding on the details that are shown in Figure 33.1, Figure 33.2 illustrates two cross-sections of a typical stub girder and Figure 33.3 shows a complete girder assembly with lights, ducts, and suspended

Steel deck Stud shear connector

Steel deck Stud shear connector

FIGURE 33.2 Cross-sections of a typical stub girder.

Column

/ Top of concrete slab Steel deck

Column

/ Top of concrete slab Steel deck

Light fixture

Suspended ceiling Transverse floor beam Bottom chord

Chord to column connection (simple)

FIGURE 33.3 Stub girder with suspended ceiling, lights, and ducts.

Light fixture

Suspended ceiling Transverse floor beam Bottom chord

Chord to column connection (simple)

FIGURE 33.3 Stub girder with suspended ceiling, lights, and ducts.

ceiling. Of particular note are the longitudinal reinforcing bars. They add flexural strength as well as ductility and stiffness to the girder. These bars are commonly placed in two layers, with the top one just below the heads of the stud shear connectors. The lower rebars must be raised above the deck proper, using high chairs or other means.

The transverse rebars are important for adding shear strength to the slab, and they also help in the shear transfer from the connectors to the slab. The transverse bars also increase the overall ductility of the stub girder, and placing the bars in a herringbone pattern leads to a small improvement in the effective width of the slab.

Thirty-six or 50 ksi yield stress steel has been the common choice for stub girder floor systems, with a preference for the latter, because of the smaller bottom chord size that can be used. However, all detail materials (stiffeners, connection angles, etc.) are made from 36 ksi steel. Welding is usually done with 70-grade low hydrogen electrodes, using either the Shielded Metal Arc Welding (SMAW), Flux Cored Arc Welding (FCAW), or Gas Metal Arc Welding (GMAW) process, and the stud shear connectors are welded in the normal fashion. All of the work is done in the fabricating shop, except for the shear connectors, which are applied in the field where they are welded directly through the steel deck.

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