526 Tension Splices

Design rules for tension splices are substantially the same as those for hanger connections. In buildings, splices should develop the strength required by the stresses at point of splice. For groove welds, however, the full strength of the smaller spliced member should be developed.

In highway bridges, splices should be designed for the larger of the following: 75% of the strength of the member or the average of the calculated stress at point of splice and the strength of the member there. Where a section changes size at a splice, the strength of the smaller section may be used in sizing the splice. In tension splices, the strength of the member should be calculated for the net section.

In railroad bridges, tension splices in main members should have the same strength as the members. Splices in secondary and bracing members should develop the average of the strength of the members and the calculated stresses at the splices.

When fillers are used, the requirements discussed in Art. 5.13 should be satisfied. In groove-welded tension splices between parts of different widths or thicknesses, a smooth transition should be provided between offset surfaces or edges. The slope with respect to the surface or edge of either part should not exceed 1:2.5 (equivalent to about 5:i2 or 22°). Thickness transition may be accomplished with sloping weld faces, or by chamfering the thicker part, or by a combination of the two methods.

Splices may be made with complete-penetration groove welds, preferably without splice plates. The basic allowable unit stress for such welds is the same as for the base metal joined. For fatigue, however, the allowable stress range Fsr for base metal adjacent to continuous flange-web fillet welds may be used for groove-welded splices only if

1. The parts joined are of equal thickness.

2. The parts joined are of equal widths or, if of unequal widths, the parts are tapered as indicated in Fig. 5.28, or, except for A514 and A5i7 steels, tapered with a uniform slope not exceeding 1:2.5.

3. Weld soundness is established by radiographic or ultrasonic testing.

4. The weld is finished smooth and flush with the base metal on all surfaces by grinding in the direction of applied stress, leaving surfaces free from depressions. Chipping may be used if it is followed by such grinding. The grinding should not reduce the thickness of the base metal by more than V32 in or 5% of the thickness, whichever is smaller.

Groove-welded splices that do not conform to all these conditions must be designed for reduced stress range assigned to base metal adjacent to groove welds.

For bolted flexural members, splices in flange parts between field splices should be avoided. In any one flange, not more than one part should be spliced at the same cross section.

Fatigue need not be considered when calculating bolt stresses but must be taken into account in design of splice plates.

Example—AASHTO ASD. A plate girder in a highway bridge is to be spliced at a location where a 12-in-wide flange is changed to a 16-in-wide flange (Fig. 5.29). Maximum bending moments at the splice are +700 kip-ft (tension) and -200 kip-ft (compression). Steel is A36. The girder is redundant and is subjected to not more than 2 million cycles of stress. The connection is slip-critical; bolts are A325SC, 7/8 in in diameter, in standard holes. The web has nine holes for 7/s-in bolts. The tension-flange splice in Fig. 5.29 is to be checked.

According to AASHTO specifications for highway bridges, members loaded primarily in bending should be designed for stresses computed for the gross section. If, however, the areas of flange holes exceed 15% of the flange area, the excess should be deducted from the gross area. With two bolt holes, net width of the flange is 12 - 2 = 10 in. With four holes, the net width along a zigzag section through the holes is 12 - 4 + 2 X 32/(4 X 3) = 9.5 in, with the addition of s2/4g for two chains, where s = bolt pitch = 3 in and g = gage = 3 in. The four-hole section, with less width, governs. In this case, the ratio of hole area to flange area is (12 - 9.5)/12 = 0.21 > 0.15. The area reduction for the flange is 12(0.21 - 0.15)0.875 = 0.63 in2, and reduced flange area is 12 X 0.875 - 0.63 = 9.87 in2. With moment of inertia Igw of the web equal to 0.3125(48)3/12 = 2880 in4, the effective gross moment inertia of the girder is

48.875 2

AASHTO requires that the design tensile stress on the net section not exceed 0.5Fu = 29 ksi. The net moment of inertia with flange area = 9.5 X 0.875 = 8.3125 in2 and nine web holes is

FIGURE 5.29 Tension-flange splice for highway-bridge plate girder.

48.875 2

In = 2880 + 2 X 8.3125 (—2—) - (2 X 1 X 0.3125(52 + 102 + 152 + 202)

The net moment of inertia of the web is

Bending stresses in the girder are computed as follows:

, 700 X 12 X 24.875 ^ , . Gross section: fb =-UgTO-= 14.2 ksi < 20 ksi—OK

Net section: fb = 14.2 X 9.87/8.3125 = 16.9 ksi < 29 ksi—OK Stress range: fsr = 0-20Q67O X 24 875 = 18.3 ksi « 18 ksi—OK

1. The average of the calculated design stress at the point of splice and the allowable stress of the member at the same point

2. 75% of the allowable stress in the member

From criterion 1, the design stress Fd1 = (14.2 + 20)/2 = 17.1 ksi, and from criterion 2, the design stress Fd2 = 0.75 X 20 = 15 ksi. Therefore, design for Fd = 17.1 ksi on the gross section. 2

The required splice-plate sizes are determined as follows:

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Renewable Energy 101

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