The requirements for strength of splice given in Art. 5.26 for tension splices apply also to compression splices.
Compression members may be spliced with complete-penetration groove welds. As for tension splices, with such welds, it is desirable that splice plates not be used.
Groove-welded compression splices may be designed for the basic allowable stresses for base metal. Fatigue does not control if the splice will always be in compression.
Groove-welded compression splices should be made with a smooth transition when the offset between surfaces at either side of the joint is greater than the thickness of the thinner part connected. The slope relative to the surface or edge of either part should not exceed 1 :21⁄2 (5:12, or 22). For smaller offsets, the face of the weld should be sloped 1:21⁄2 from the surface of the thinner part or sloped to the surface of the thicker part if this requires a lesser slope.
At bolted splices, compression members may transmit the load through the splice plates or through bearing.
Example—AASHTO ASD. Investigate the bolted flange splice in the highway-bridge girder of the example in Art. 5.26 for use with the compression flange.
The computations for the bolts are the same for the compression splice as for the tension splice. Use 12 bolts, for sealing, in a staggered pattern (Fig. 5.29).
The splice-plate net area for the tension flange is OK for the compression flange because it is subject to the smaller moment 200 kip-ft rather than 700 kip-ft. The gross area, however, must carry the force due to the 700-kip-ft moment that causes compression in the compression splice. By calculations in Art. 5.26, the splice plates are OK for the maximum compressive stress of 20 ksi and for stress range. Therefore, the splice plates for the compression splice can be the same as those required for tension.
To avoid buckling of the splice plates, the ends of compression members in bolted splices should be in close contact, whether or not the load is transmitted in bearing, unless the splice plates are checked for buckling. For the compression splice, the unsupported length of plate L is 3.75 in, and the effective length is 0.65L 2.44 in. The slenderness ratio of the plate
Columns in Buildings. Ends of compression members may be milled to ensure full bearing at a splice. When such splices are fabricated and erected under close inspection, the designer may assume that stress transfer is achieved entirely through bearing. In this case, splice material and fasteners serve principally to hold the connected parts in place. But they also must withstand substantial stresses during erection and before floor framing is placed. Consequently, standard specifications generally require that splice material and fasteners not only be arranged to hold all parts in place but also be proportioned for 50% of the computed stress. The AISC specification, however, exempts building columns from this requirement.
But it also requires that all joints with stress transfer through bearing be proportioned to resist any tension that would be developed by specified lateral forces acting in conjunction with 75% of the calculated dead-load stress and no live load.
When fillers are used, the requirements discussed in Art. 5.13 should be satisfied.
In multistory buildings, changes in column sizes divide the framing vertically into tiers.
Joints usually are field bolted or welded as successive lengths are erected. For convenience in connecting beam and girder framing, column splices generally are located 2 to 3 ft above floor level. Also, for convenience in handling and erection, columns usually are erected in two- or three-story lengths.
To simplify splicing details while securing full bearing at a change in column size, wideflange shapes of adjoining tiers should be selected with the distance between the inner faces of the flanges constant, for example, note that the T distances given in the AISC ‘‘Steel Construction Manuals’’ for W14 43 and heavier W14 sections are all 11 or 111⁄4 in. If such sections are not used, or if upper and lower members are not centered, full bearing will not be obtained. In such cases, stress transfer may be obtained with filler plates attached to the flanges of the smaller member and finished to bear on the larger member. Enough fasteners must be used to develop in single shear the load transmitted in bearing. Instead of fillers, however, a butt plate may be interposed in the joint to provide bearing for the upper and lower sections (Fig. 5.30). The plate may be attached to either shaft with tack welds or clip angles. Usually, it is connected to the upper member, because a plate atop the lower one may interfere with beam and girder erection. The butt plate should be thick enough to resist the bending and shear stresses imposed by the eccentric loading. Generally, a 11⁄2-inthick plate can be used when a W8 section is centrally seated over a W10 and a 2-in-thick plate between other sections. Plates of these thicknesses need not be planed as long as they provide satisfactory bearing. Types and sizes of welds are determined by the loads.
For direct load only, on column sections that can be spliced without plates, partialpenetration bevel or J welds may be used. Figure 5.31 illustrates a typical splice with a partial-penetration single-bevel groove weld (AWS prequalified, manual, shielded metal-arc welded joint BTC-P4). Weld sizes used depend on thickness of the column flange. AISC recommends the partial-penetration groove welds in Table 5.17. A similar detail may be used for splices subjected to bending moments. A full moment splice can be made with complete-penetration groove welds (Fig. 5.32). When the bevel is formed, a shoulder of at least 1⁄8 in should remain for landing and plumbing the column.
When flange plates are not used, column alignment and stability may be achieved with temporary field-bolted lugs. These usually are removed after the splice is welded, to meet architectural requirements.
To facilitate column erection, holes often are needed to receive the pin of a lifting hitch.
When splice plates are used, the holes can be provided in those plates (Fig. 5.34). When columns are spliced by groove welding, without splice plates, however, a hole must be provided in the column web, or auxiliary plates, usually temporary, must be attached for lifting.
The lifting-lug detail in Fig. 5.33 takes advantage of the constant distance between inside faces of flanges of W14 sections, and allows for welding the flanges and web of the splice, as shown in Fig. 5.32. Bolted column splices generally are made with flange plates. Fillers are inserted (Fig. 5.34) when the differences in column sizes are greater than can be absorbed by erection clearance.
To provide erection clearance when columns of the same nominal depth are spliced, a 1⁄16-in filler often is inserted under each splice plate on the lower column. Or with the splice plates shop fastened to the lower column, the fastener holes may be left open on the top line below the milled joint until the upper member is connected. This detail permits the erector to spring the splice plates apart enough for placement of the upper shaft.
For the detail in Fig. 5.34, the usual maximum gage for the flanges should be used for G1 and G2 (4, 51⁄2, or 111⁄2 in). The widths of fillers and splice plates are then determined by minimum edge distances (11⁄4 in for 3⁄4-in fasteners, 11⁄2 in for 7⁄8-in fasteners, and
13⁄4-in for 1-in fasteners).
Fill plates not in bearing, as shown in Fig. 5.34, may be connected for shipping, with
two fasteners or with welds, to the upper column. Fillers milled to carry load in bearing or thick fills that will carry load should be attached with sufficient fasteners or welds to transmit that load (Art. 5.13).
When fasteners are used for a column splice, it is good practice to space the holes so that the shafts are pulled into bearing during erection. If this is not done, it is possible for fasteners in the upper shaft to carry the entire load until the fasteners deform and the joint slips into bearing.
Usually, for direct load only, thicknesses used for splice plates are as follows:
Four fasteners are used in each half of a 3⁄8-in splice plate and six fasteners in each half of a 1⁄2-in or heavier splice plate.
If, however, the joint carries tension or bending moments, the plate size and number of
fasteners must be designed to carry the load. An equivalent amount of weld may be used
instead of fasteners.
A combination of shop-welded and field-welded connection material is usually required
for moment connections. Splice plates are shop welded to the top of the lower shaft and
field welded to the bottom of the upper shaft. For field alignment, a web splice plate can be shop bolted or welded to one shaft and field bolted to the other. A single plate nearly full width of the web with two to four fasteners, depending on column size, allows the erector to align the shafts before the flange plates are welded.