ANSWER TO RIGGING QUIZ No. 22

 

Question No. 1:         

a.        H = vertical distance from centerline of

Spreader bar to bearing on the hook                                  =   6.93   ft.

b.    Angle of the left sling with the horizontal                              =  66.59 deg.

c.    L = length of left inclined section of sling                             =   7.55 ft.

d.    R = length of right inclined section of sling                           =    8.00 ft.

e.        Rl = Left reaction  =  200.0 kips*4.0’/7.0’                         = 114.29 kips

f.         R2 = Right reaction =  200.0 kips - 114.29 kips                =  85.71 kips

g.        C = horizontal compressive stress in the pipe)

Left end:  C = 114.29 kips*3.0’/6.93’                          = 49.48 kips

Right end: C =  85.71 kips*4’/6.93’                             = 49.48 kips

h.    TL = 114.29 kips*7.55’/6.93’                                           = 124.52 kips

i.     TR =  85.71 kips*8.0’/6.93’                                              =  98.94 kips

 

Question No. 2:

As the tension in the left sling is the greatest, size the sling for that value and use the same diameter for both the left and right sling.  The SWL (safe working load) for a 2”dia. EIPS wire rope is 79.2 kips.  The end attachment efficiency for a 2”sling is .9.  The bending ratio for a 2”sling bent over the curved end plate of the spreader bar = 2*12”/2”= 12 and the strength efficiency = 100-76/12^.73 = 87.6% (refer to a Macwhyte graph for the formula used for strength efficiency for wire rope).  The bending ratio for a 2”sling around the trunnion = 12”/2”=6 and the efficiency = 100-50/SQR(6) = 79.6 %.  Using the worst case for bending efficiency,  the SWL for a doubled 2”dia, EIPS sling = 2*79.2 kips*.796 = 126.1 kips > 124.52 kips ==θ   Okay

 

Question No. 3:

The total length of the left sling               = 2(7.55’ + 6’) +3.14*1’/2      = 28.67’.

The total length of the right sling             = 2(8.0’ + 6) + 1.57                 = 29.57’

 

Question No. 4:

Yes, the horizontal force “C” does put a bending moment in the pipe. See figure 1 and note that the sling is in contact with the curved end plate for 23.41 degrees and that due to the geometry, the eccentricity “E” equals 1.69”.  The bending moment in the pipe is equal to E*C*IF = 1.69”*49.48 k*1.8 = 150.52 k-in. The bending stress fb = moment/section modulus S = 150.52 k-in./16.81 in^3 = 8.95 ksi < the allowable of 21.6 ksi. It should be noted that there are other factors to consider to complete the design of the spreader bar and it is not as simple as just completing the above step. The 1.8 used above is an impact factor.

 

Question No. 5:

The horizontal compression force “C” is the same through out the pipe spreader bar.  See the calculations for “C” in question No. 1 above.

 

 

Question No. 6:

See figure 1 for the location of vertical force “V”.  It is equal to C*Tan(11.71) = 10.26 kips pushing down on the spreader bar

 

Question No. 7:

See figure 1 for the location of the sloping compression force “N” pushing against the curved end plate.  This force creates friction between the sling and the curved end plate.  The “N” or normal force = C/Cos(11.71) = 50.53 kips. Using .2 for the coefficient of friction, the friction force = .2*50.53 kips = 10.11 kips.  Note from question 6 above that “V” is approximately equal to friction force. 

Therefore, during lifting with this sling configuration, the friction force will almost keep the spreader bar from slipping down on the slings.  To ensure that the spreader does not slip, clamp plates with half round bars that acts the same way as u-bolts on cable clamps, are bolted to the bottom of the end plates. A torque value is not specified for tightening these bolts.  They should be tightened the same way that cable clamps normally are tightened, so they bite into the cable but not so tight that they damage the cable.

It should be noted that for sling angles of 55 degrees or higher, the eccentricity E occurs below the centerline of the pipe so that the resultant moment from the compression force C tends to bow the pipe up at the middle of the spreader and counteracts the downward bowing of the pipe at the middle due to the dead weight of the pipe. For sling angles below 55 degrees, the eccentricity E occurs above the centerline of the pipe and the moment due to force C and the dead weight of the pipe are in the same direction.

 

Question No. 8:

The three ways that the pipe spreader bar shown in figure 1 is adjustable are:

a.                   Length:  Pipe inserts are used to attain the correct overall spreader bar length required from centerline to centerline of lifting lugs.

b.                  Sling angle:  For a certain length of sling, the required sling angle can be attained by moving the spreader bar up or down and then tightening the clamp plate bolts. For a specific length of sling, the author specifies a vertical distance on the rigging hook up drawing from bearing on the sling eye at the lug up to the centerline of the spreader bar. Setting the spreader bar at this distance and then clamping it will set the correct sling angle above the spreader bar.

c.                   Pipe wall thickness:  For example, on a 8” dia. adjustable pipe spreader, pipe inserts of any wall thickness can be used as the OD remains constant.  The tolerance between the ID of the 10”dia. XXS end caps and the 8”dia pipe inserts is 1/8 inch. 

 

COMMENTS ON SPREADER BAR DESIGN:

 

There are many ways to design a spreader bar and some times the design is based on personal preferences and not on good engineering parameters.  But no matter how it is designed, it should be fabricated, labeled, proof tested and used according to an approved design. In all cases, the design should conform to ASME B30.20 and AISC.

 

 

 

If possible, a spreader bar should be designed with zero moment due to the sling force.  This will produce the greatest SWL for the spreader bar.

 

See figure 2 for a design that is popular in Europe.  It is similar to the adjustable pipe spreader design shown in figure 1 except that lugs are welded to the top of the pipe at each end.  Support slings attach to these lugs to keep the spreader bar positioned during lifting.

 

See figure 3 for a spreader bar designed for zero moment. This type of spreader bar has the best capacity of all the spreader bars shown, but requires four slings to make it work.

 

See figure 4 for an example where lugs are located on the spreader bar such that at a 60 degree sling angle the line of force along the inclined portion of the sling intersects the line of force from the vertical portion of the sling at the centerline of the spreader bar.  This ensures that when a sling angle of 60 degrees with the horizontal is used, the spreader bar will have zero moment due to the sling force. Using a sling angle greater than or less than 60 degrees will decrease the SWL of the spreader bar.  Also, for added safety, the top and bottom lug at each end should be made as one plate and welded into a slot in the pipe.  This will eliminate using butt welds to attach the lugs to the pipe.

 

Figure 5 shows a spreader bar commonly used in the construction business where the top lug is located directly over the bottom lug. For different sling angles, a constant eccentricity E times the compression force C produces a maximum allowable moment in the pipe. This type of spreader does not have good capacity compared to the other types.  The lug plate should be designed as for Figure 4 above.

 

See the tables below for a comparison of safe working loads between the five types of spreader bars that have been discussed. All spreader bars in the figures are made of 8” dia, std wall pipe, 7 ft long centerline to centerline of slings and with the center of gravity of the load at the center of the bars.  All values are based on AISC & ASME B30.20 allowables.  A doubled 2” diameter sling was assumed in calculating “R” for Figures 1 & 2.

 

After viewing the Safe Working Loads listed in the tables for the various types of spreader bars and sling angles, it becomes apparent that we must be careful when calling out the SWL of a spreader bar on a drawing, etc.  Say the lifting capacity of a spreader bar is listed as 100 ton.  To be absolutely correct and not misleading, detail information should state at what sling angle, design standard, safety factor, etc, that the SWL is based on.

 

 

 

TABLE 1:

 

Sling angle

    SWL

     E

     C       

   Moment

  Degrees

   Kips

    Inches

    Kips

   K-in.

55

236

.28

149

-38

60

226

.90

117

-102

65

234

1.50

98

-143

75

308

2.67

74

-194

 

 

Where:             1.         The sling angle is with respect to the horizontal

2.         E is the vertical eccentricity

3.                  C is the horizontal compressive force due to the effect of the sling

4.                  A negative net bending moment indicates that the spreader is being bowed upward in the middle

 

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TABLE 2:

 

Sling angle

    SWL

     E

      C

Moment

  Degrees

   Kips

  Inches

    Kips

K-in.

55

145

1.68

91

157

60

189

1.43

98

144

65

251

1.18

106

129

75

512

0.70

123

123

 

 

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TABLE 3:

 

Sling angle

    SWL

    E

     C

Moment

 Degrees

    Kips

  Inches

    Kips

   k-in.

55

262

0

165

4

60

320

0

165

4

65

392

0

165

4

75

684

0

165

4

 

 

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TABLE 4:

 

Sling angle

    SWL

     E

    C

Moment

Degrees

   Kips

   Inches

   Kips

K-in.

55

148

8.31

93

148

60

320

0

165

4

65

205

8.31

86

-163

75

125

8.31

30

-286

 

 

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FIGURE 5:

 

Sling angle

     SWL

     E

    C

Moment

Degrees

    Kips

   Inches

   Kips

  K-in.

55

 53

8.31

33

281

60

 64

8.31

33

281

65

79

8.31

33

281

75

138

8.31

33

281

 


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