Applied Structural Steel Design (4th Edition)
by George F. Spiegel and George F. Limbrunner     
9-2
Open Web Steel Joists, K-Series
The design of the K-Series joist chord is based on a steel minimum yield strength of 50,000 psi. The design of the web members may be based on a steel minimum yield strength of 36,000 psi or 50,000 psi.
An example of the standard designation for K-Series joists is 22K7. The depth of this joist is 22 in. K represents the series, and the number 7 denotes the relative size of the chords of the joist. Chord sizes are designated by the numbers 3 through 12, the size increasing with increasing number. The chord and web members may vary in shape and makeup from manufacturer to manufacturer, but the design and the capacity of the joists must conform to the SJI specifications and to the standardized load tables. The K-Series standard load table
and the economy table (which is used for selection) are applicable where the joists are installed up to a maximum slope of 2 in. per foot.
The use of open web steel joists in any given application must be based on SJI requirements as furnished in its standard specifications. These requirements for the K-Series joists are summarized as follows:
In construction that uses joists, bridging and bridging anchors are required for the primary purpose of furnishing lateral stability for the joists, particularly during the construction phase. The bridging spans between and perpendicular to the steel joists.
It is required that one end of all joists be attached to their supports before allowing the weight of an erector on the joists. When bolted connections are used, the bolts must be snug tightened. All bridging must be completely installed and the joists permanently fastened into place before the application of any construction loads. Even under the weight of an erector, the joists may exhibit some degree of lateral instability until the bridging is installed. The bridging also serves the purpose of holding the steel joists in pos
ition as shown on the plans. The minimum number of rows of bridging is a function of the joist chord size and span length. A table is furnished in the standard specifications that establishes the required number of rows of bridging. Spacing of bridging rows should be approximately equal. Two permissible types of bridging may be observed in Figure 9-2. Horizontal bridging (Figure 9-2a) consists of two continuous horizontal steel members, one attached to the top chord and the other attached to the bottom chord by means of welding or mechanical fasteners. The attachment must be capable of resisting a horizontal force of not less than 700 lb. If the bridging member is a round bar, the diameter must be at least 2 in. The maximum slenderness ratia4V/rj~of-the bridging member cannot exceed 300, where k is the distance between bridging attachments and r is the least radius of gyration of the bridging member. The bridging member shall be designed for a compressive force of 0.24 times the area of the top chord. Diagonal bridging (Figure 9-2b) consists of cross-bracing with a maximum B/r of 200, with Z and r as defined previously. Where the cross-bracing members connect at their intersection, E is the distance between the intersection attachment and chord attachment. The ends of al
l bridging lines terminating at walls or beams must be properly anchored. A typical detail may be observed in Figure 9-2b.
FIGURE 9-2 Typical bridging.
Joist extensions are frequently used with K-Series joists to support a variety of over-hang conditions. Two types are shown in Figure 9-3c and d. The first is the Top Chord Extension (S Type), which has only the top chord angles extended. The second is the Extended End (R Type), in which the standard 2 and 1/2-in. end bearing depth is maintained over the entire length of the extension. The R Type (reinforced) involves reinforcing the top chord. The S Type (simple) is more economical and should be specified whenever possible.
Load tables for K-Series Top Chord Extension and Extended finds axe furnished by the SJI. Specific designs and load tables, however, are generally furnished by the various joist manufacturers and can be used to advantage.
Ceiling extensions (Figure 9-3b) in the form of an extended bottom chord element or a loose unit, whichever is standard with the joist manufacturer, are frequently used to support ceilings that are to be attached directly to the bottom of the joists. They are not fu
rnished for the support of suspended ceilings.
design翻译
FIGURE 9-3 Typical joist details
When joists are used in conjunction with a corrugated metal deck and concrete slab, the cast-in-place slab should not be less than 2 in. thick.
The typical standard K-Series joist is designed for a simple span subjected to uniformly distributed load for its full span length, resulting in a linear shear distribution (maximum at the supports and zero at midspan) and a parabolic moment distribution (zero at the supports and maximum at midspan). The KCS joist is a new type of K-Series joist developed to overcome some of the limitations of the standard K-Series joist. The KCS joist may be used for special design applications requiring a joist capable of supporting nonuniform loads, concentrated loads, or combinations thereof in addition to or independent of the normal uniform load.
The KCS joists are designed in accordance with the SJI Standard Specifications for K-Series joists and range in depth from 10 in. to 30 in. Load tables furnished by the SJI provide the shear and moment capacity of each joist. The designer must calculate the ma
ximum moment and shear imposed and then select the appropriate KCS joist.
9-3
Floor Vibrations
Even when the structural design of the steel joists is accomplished in accordance with design specifications, a floor system may be susceptible to undesirable vibrations. This phenomenon is separate and different from strength and has to do mainly with the psychological and physiological response of humans to motion. Large open floor areas without floor-to-ceiling partitions may be subject to such undesirable vibrations.
The ASDS Commentary recommends a minimum depth-to-span ratio of 1/20 for a steel beam supporting a large open floor area free of partitions. In addition, the SJI requires a minimum depth-to-span ratio of 1/24 for steel joists, although a generally accepted practice for steel joist roofs and floors is to use a minimum depth-to-span ratio of 1/20. Even if these recommendations and requirements are satisfied, a vibration analysis shoul
d be made, particularly when a floor system is composed of steel joists that support a thin-concrete slab placed on steel metal deck. References 2 and 3 contain relatively brief and sufficiently accurate methods that can be used to determine (1) whether disturbing vibrations will be present in a floor system and (2) possible design solutions for the problem. Reference 4 contains insight on vibrations in steel framed floors.

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