erecting long, vertical precast concrete members crane and two ... · pci journal | winter 2010 121...

Winter 2010 | PCI Journal118

Editor’s quick points

n Erectors have reported incidents in which precast concrete members have rolled uncontrollably during erection.

n PCI issued an alert regarding this phenomenon in December 2008.

n This article provides reliable methods to determine the cause of panel instability and discusses ways to anticipate and avoid panel instability.

Erecting long, vertical precast concrete members with one crane and two operating linesDonald R. Logan

The most common procedure to erect long vertical precast concrete members is to rotate the members in midair from their as-delivered flat position to their final vertical position in the structure. To keep member stresses during lifting within acceptable limits, this procedure normally involves supporting the member at three points. One point is at or near the top of the member, and the two lower reactions are supported by slings threaded through rolling blocks, one end attached near the bottom of the member and the other end close to its midheight (Fig. 1).

Erectors have reported incidents that occurred while using this common erection procedure in which precast concrete members suddenly rolled from an apparently stable tilt angle to the vertical position. These incidents often cause damage to the member and erection equipment and can endanger the safety of erection personnel (Fig. 2). Erectors have expressed concern that they have no reliable method to determine the cause of this instability, how to anticipate it, and how to avoid it.

While the concepts described herein apply to all long vertical precast concrete members, the discussion and figures address long nonprestressed wall panels. These are potentially subject to detrimental visible cracking in critical architectural finished surfaces that may result from using handling techniques that address erection safety only. Thus, tensile stresses at an impact factor of 1.2W are

119PCI Journal | Winter 2010

W = weight of panel

RR = right lifting reaction

RL = left lifting reaction

checked for each lifting-layout configuration against the recommended maximum of

5 f

c

' = 354 psi (2.44 MPa)

where

fc' = concrete compressive strength at erection

= assume 5000 psi (35 MPa)

Figure 1. A panel can be erected with one crane using two operating lines. Note: c.g.= center of gravity; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W(c.g.) = panel weight acting at c.g. of panel.

W (c.g.)

R L

W (c.g.)

R R

R L

R R

c. Crane operator tilts panel by raising top end allowing lower sling to rotate through rolling block.

b. Crane operator lifts panel in horizontal position until truck pulls away.

a. Panel arrives on truck in horizontal position.


Figure 2. Panels can experience instability during tilting toward vertical. Note: c.g. = center of gravity; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W(c.g.) = panel weight acting at c.g. of panel.

R RR L

R L= W

(c.g.)

(c.g.)

(c.g.)W

W

W

R L R R

c. Panel suddenly becomes unstable. Top line goes slack. Panel rotates rapidly to vertical position, swinging uncontrollably.

b. Panel feels “light” to crane operator. Top line appears to be going slack.

a. Crane operator tilts panel by raising top line. Panel feels stable to crane operator.


Figure 3. The current PCI stability equation shown with Fig. 5.6.2 on page 5-24 of the sixth edition of the PCI Design Handbook: Precast and Prestressed Concrete is incor-rect. It is necessary for the c.g. of the precast concrete component to be between the two main crane lines for all angles during the rotation. Note: a = distance from up face of panel to center of gravity of the panel; b = distance between lower lifting devices; c.g. = center of gravity; eo = distance from the center of gravity of panel to centerline of rolling-block reaction with panel in horizontal position; L = length of panel; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W(c.g.) = panel weight acting at c.g. of panel.

bL

RL

sRR

a (to

c.g

.)

(c.g.)W eo

Formula in PCI Erector’s Manual and Design Handbook. Purpose is to protect against erection instability.

e = distance from c.g. to RL centerline with panel in horizontal position.

Panel will be stable during erection if:

CAUTION: DO NOT USE THE ABOVE FORMULA IN ITS PRESENT FORM. (Refer to article pages 125−128)

e >L2−

L2 −b2

2−a

tion of Precast Concrete Products2 have addressed this issue, providing a formula intended to protect against sud-den rollover of long wall panels (Fig. 3).

Since the mid-1980s, the third edition of the PCI Design Handbook: Precast and Prestressed Concrete1 and PCI’s Erector’s Manual: Standards and Guidelines for the Erec-


However, erectors have indicated that even if the panel lift-ing points conform to the dimensions derived from the PCI formula, there have been instances in which unexpected sudden rollover has occurred.

The author encountered this problem in 1995 during the erection of a long wall panel produced by Stresscon Corp. for the Soundtrack project in Denver, Colo. In an analysis of the incident, the basic concept of safe handling of long wall panels was recognized and Stresscon adopted a pro-cedure for three-point lifting, imposing restrictions on the location of the lower sets of lifting devices.

Basic instability concept

As illustrated in Fig. 4, with the panel in its horizontal position, the rolling-block lifting reaction RL is centered between the two lower sets of lifting devices. Its verti-cal alignment is located at a distance eo from the center of gravity c.g. of the panel toward the lower end of the panel. As the panel is tilted to its vertical position, RL shifts toward the c.g. of the panel and its distance eΔ decreases.

As the tilt angle increases further, eΔ may decrease to zero so that RL aligns with the c.g. of the panel. At that point, RL carries the entire weight of the panel, the top reaction car-ries no weight, and the panel becomes unstable, swinging rapidly to the vertical position as illustrated in Fig. 2 and 4.

Figure 4. These diagrams show the fundamental issues as the tilt angle increases. Note: b = distance between lower lifting devices; c.g. = center of gravity; CGL = distance from left end of panel to c.g. of panel; CGR = distance from right end of panel to c.g. of panel; eo = distance from the center of gravity of panel to centerline of rolling-block reaction with panel in horizontal position; eΔ = horizontal projection of distance from centerline of center of gravity to centerline of rolling-block reaction with panel in tilted position; L = length of panel; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W = weight of panel; W(c.g.) = panel weight acting at c.g. of panel; α = angle between sling leg and surface of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position).

R L= W

CGL CGR

eo

b/2 b/2b

L

Tilt Angle = 0°

Tilt Angle ∆

∆

= 45°

b∆/2

e

b∆

∆

α

α

∆

shift

b∆/2

(c.g.)W

(c.g.)W

R L R R

R L

R R

c. As tilt angle increases further, RL can align with c.g. of panel, causing panel to suddenly become unstable.

b. As panel is tilted, e decreases as RL shifts toward c.g. of panel, creating potential instability of panel.

a. At zero degrees, RL centered between lifting devices.


Conservative layout procedure

The safeguard against instability is to locate the lower sets of lifting devices so that throughout the tilting of the long panels, the rolling-block reaction RL is prevented from aligning with the c.g. of the panel.

As indicated in Fig. 5, both of the lower sets of lifting devices are located below the c.g. of the panel. Thus at all tilt angles, RL is prevented from aligning with the c.g. of the panel.

There are other considerations that must be addressed, even with this conservative procedure. As the panel is tilted, the load carried by RL increases and the loads ap-plied to the lifting devices increase accordingly, changing from primarily tension to primarily shear. These loads are applied to the lower lifting devices at steep angles, poten-tially exceeding their allowable shear capacity.

As shown in Fig. 6, the conservative layout of the lower lifting inserts causes the moment in the upper portion of the panel to dominate. The consequent tensile stresses in the bottom face of the panel (usually the architectural finished face) may result in visible cracking of this critical surface. Concentric prestressing is often required for long panels erected using the three-point procedure.

Figure 5. A conservative layout prevents instability at all tilt angles. Note: c.g. = center of gravity; eo = distance from the center of gravity of panel to centerline of rolling-block reaction with panel in horizontal position; eΔ = horizontal projection of distance from centerline of center of gravity to centerline of rolling-block reaction with panel in tilted position; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W(c.g.) = panel weight acting at c.g. of panel; α = angle between sling leg and surface of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m.

R R

R L

e =1'-7"

e =9'-01/2

22'-21/2"

4'-71/2"

8'-61/2"

22'-81/2"

22'-81/2"

"

8'-6 "

17'-1"

44'-5"

(c.g.)W

(c.g.)W

Tilt Angle ∆ = 75°

o

Tilt Angle ∆ = 0° R R

R L

∆

Figure 6. This conservative layout shows shear, moment, and tensile stresses. Note: c.g. = center of gravity; e = minimum distance from the center of gravity of the panel, with the panel in its horizontal position, to protect the erection-crew members against the unexpected sudden rollover of the panel; fc

' = concrete com-pressive strength at erection; ft = actual tensile stress in panel during lifting; ft

allow

= allowable tensile stress in panel during lifting; k = K = kip; LF = linear foot; M = moment; R = reaction; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; V = shear; W = weight of panel; W ’ = weight of panel modified by impact factor (1.2W in examples); W(c.g.) = panel weight acting at c.g. of panel. 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN.

0'-6"

e=9'-01/2"

8'-61/2" 8'-61/2"

17'-1" 22'-81/2"4'-71/2"

4.62' 11.16' 5.93' 9.85' 12.85'

16.06

=16.06RR19.72

7.815.77

64.51

103.23-12.31-13.91

42.56

R = 39.44K L

(c.g.) = 55.5KW '

(kips)R

(kip-ft)M

(kips)V

-13.33

19.72

R = 16.06KR

12"

Max. Tensile Stress

6 × 103.23 × 12,0006.94 × 12 × (12) 2

620psi Tension (Bottom)> 354psi allowable

High tensile stresses in bottom face of panel will cause cracking. 4-point lifting or

prestressing required.

44'-5"

f =t

f =t

Allowable Tensile Stress

fc' = 5000psi at erection

5 fc' = 354psif =t

allow

= 620psi


Figure 7. This figure shows the lifting-device layout for the illustrated panel in this article and the consequent balanced positive and negative moments during loading on trucks for delivery to jobsites. Note: K = kip; M = moment; R = reaction; W’ = weight of panel modified by impact factor (1.2W in examples). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN; 1 kip-ft = 1.356 kN-m.

W ' = 55.5K

22'-21/2"22'-21/2"

4'-11/2"12'-111/2"9'-3"12'-111/2"4'-71/2"

44'-5"

13.87

4321

(kip-ft)M -13.33

12.94 12.94

-13.33-13.33-13.33

13.87 13.8713.87(kips)R

Panel 6.94' × 44.42' × 1.0' W ' = 46.25 × 1.2 = 55.5K Max. Tensile Stress = 80psi

4-Point Lift - Stripping & Loading

Figure 8. This figure shows the moments and maximum tensile stress using the conservative three-point lifting-insert layout shown in Fig. 6. Note: K = kip; M = moment; R = reaction; W’ = weight of panel modified by impact factor (1.2W in examples). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN; 1 kip-ft = 1.356 kN-m.

22'-21/2"22'-21/2"

4'-71/2"

44'-5"

19.72

4321

(kip-ft)M -13.33

17'-1" 22'-81/2"

2'

0'-6"

19.72 16.06

64.51

103.23

42.56

W ' = 55.5K

(kips)

Panel is stable at all Tilt Angles

Max. Tensile Stress = 620psi > 354psi

R

Conservative Layout3-Point Lift - Shift Point to Point2 2'

Figure 10. This figure shows the moments and maximum tensile stress resulting from attaching the rolling-block lines to point 3 and a modified location of point 1, farther from the bottom of the panel. Note: K = kip; M = moment; R = reaction; W’ = weight of panel modified by impact factor (1.2W in examples). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN; 1 kip-ft = 1.356 kN-m.

22'-21/2"22'-21/2"

44'-5"

22.25

4321

M (kip-ft)

-27.38

22.25 12.54

50.84 48.25

0.0

17'-7"

1'

6'-71/2"

4'-71/2"

20'-21/2"

W ' = 55.5K

(kips)R

At Tilt Angle = 60 Panel unstable w/sling line = 30ft. Panel stable w/sling line = 40ft. Max. Tensile Stress = 305psi < 354psi

o

3-Point Lift - Shift Point to Point1 1'

Figure 9. This figure shows the moments and maximum tensile stress resulting from three-point lifting using a single line from the top of the panel and the slings and rolling blocks attached to points 1 and 3 of the four-point lifting configuration. Note: K = kip; M = moment; R = reaction; W’ = weight of panel modified by impact factor (1.2W in examples). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN; 1 kip-ft = 1.356 kN-m.

22'-21/2"22'-21/2"

4'-71/2"

44'-5"

21.48

4321

(kip-ft)M -13.33

21.48 12.54

85.3863.96

27.34

22'-21/2" 17'-7"4'-71/2"

W ' = 55.5K

(kips)R

At Tilt Angle = 60

Panel barely stable w/sling line = 40ft. Panel unstable w/sling line = 30ft.

Max. Tensile Stress = 513psi > 354psi

o

33-Point lift at points , & top1


the rolling block is a critical variable in these analytical methods, and that using a shorter sling than indicated may cause sudden instability of the panel, even though the lifting-device locations are unchanged.

This raises an important caveat that must be considered by designers who use this less-conservative layout. It is essential that the designer be in a position to ensure that the erection crew is properly notified and understands the critical importance of using the minimum specified sling length. If not, this less-conservative layout method may potentially put members of the erection crew at serious risk and should not be used.

The conservative layout procedure for three-point lifting (Fig. 5) is not subject to this risk and is recommended for use in any situation in which close supervision of the erec-tion crew is not available.

Four-point lifting with two sets of rolling blocks

Another method to reduce tensile stresses in the critical face of the panel is to use a four-point layout of lifting devices and two sets of rolling blocks. As illustrated in Fig. 11, both the lower and upper reactions are carried by rolling blocks with upper-block slings engaging the two upper lifting devices and the lower-block slings engaging the two lower devices in the conservative locations shown in Fig. 8.

Figure 11 also shows the moments and maximum ten-sile stress for the same panel for comparison with those indicated from Fig. 7 through 10. Although this method significantly reduces tensile stresses in both faces of the panel, it is also more expensive and requires additional erection equipment and extra erection labor to install and then disengage for each panel lifted.

However, as indicated in Fig. 11, the panel remains stable through all tilt angles from its horizontal position to its vertical position. The demonstration photograph sequence in Fig. 12 further illustrates the stability and clearly shows the shifting of the rolling-block reactions as the panel is rotated through all tilt angles.

Check validity of the PCI Design Handbook formula

As discussed in the “Background” section, four editions of the PCI Design Handbook1,3,4,5 and the Erector’s Manual2 present a formula that is intended to protect against sudden rollover of panels being tilted to their vertical position. Erectors have reported to the author that some long panels that have lifting-device locations conforming to this for-mula suddenly became unstable during erection.

Analytical methods to reduce tensile stresses

Figures 7 through 10 study the effects on panel moments and stresses resulting from varying the lifting-device locations. The layout of lifting devices for long panels is dictated by the condition at stripping of the panels when the concrete strength is low. The PCI Design Handbook5 shows the recommended layout of lifting devices using four-point handling of long panels in the horizontal posi-tion for stripping and loading on trucks. (This is referred to as eight-point lifting in the PCI Design Handbook because there are two devices at each lift point location along the length of the panel.) For the panel in this article, Fig. 7 shows this lifting-device layout and the consequent bal-anced positive and negative moments during loading onto trucks for delivery to jobsites.

Figure 8 repeats the moments and maximum tensile stress using the conservative three-point lifting-insert layout shown on Fig. 6 for comparison with the modified layouts shown in Fig. 9 and 10. Figure 9 shows the moments and maximum tensile stress resulting from three-point lifting using a single line from the top of the panel and the slings and rolling blocks attached to points 1 and 3 of the four-point lifting configuration. This results in a reduction in tensile stress from 620 psi (4.3 MPa) to 513 psi (3.5 MPa), but the analytical stability calculation presented in this article indicates that the panel becomes unstable at a tilt angle of about 50 deg with a sling length of 30 ft (9 m). Even with a sling length of 40 ft (12 m), there is insufficient margin of safety against instability at the critical tilt angle.

In this article, sling length S is defined as the length of the cable attached to a lifting device running diagonally through the rolling block and diagonally back to the com-panion lifting device that shares the rolling-block reaction. It includes the lengths of hooks and any shackles within that length of line.

Figure 10 shows the moments and maximum tensile stress resulting from attaching the rolling-block lines to point 3 and a modified location of point 1, farther from the bottom of the panel. The tensile stress reduces further to 305 psi (2.10 MPa), but the analytical stability calcula-tion shows that the panel is only marginally stable at the most critical tilt angle, using a sling length of 40 ft (12 m). Increasing the sling length to 50 ft (15 m) in both of the previously described cases would ensure stability at all tilt angles.

The section “Analytical Study—Shifting of Rolling-Block Reaction” and the sidebar “Erecting Long Precast Concrete Members” show two analytical methods that establish the safe location of lifting devices. However, it must be understood that the length of the sling running through


Figure 12. This demonstration shows the tilting of a panel with a four-point layout and rolling blocks at both ends of the panel.

Figure 11. This figure shows the four-point lifting layout with rolling blocks at both ends of the panel. Note: K = kip; M = moment; R = reaction; RL = left lifting reac-tion acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position). 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m; 1 psi = 6.895 kPa; 1 kip = 4.448 kN; 1 kip-ft = 1.356 kN-m.

0'-6"

∆ = 60°

Panel is stable at all Tilt Angles

17'-7"

22'-21/2"

5'-11/2"17'-1"4'-71/2"

22'-21/2"

W ' = 55.5K

12.9K

42.6K

16.58 11.1711.1716.58

-13.33

50.0933.40

-11.12

(kips)R

(kips-ft)M

R = 33.16K L R = 22.34K R

W ' = 55.5K


Figure 13. This figure shows the panel dimensions used in the PCI Design Handbook formula. Note: a = distance from up face of panel to center of gravity of the panel; b = distance between lower lifting devices; c.g. = center of gravity; K = kip; L = length of panel; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W(c.g.) = panel weight acting at c.g. of panel. 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m.

W (c.g.) = 52K

R = 9.8KR

0'-6

" to

(c.g

.)

1'-0"

8'-4" 12'-6" 12'-6" 21'-0"

b = 25'-0"

L = 54'-4"

26'-10" 27'-6"

e = 6'-0"

R = 42.2KL

Sling Length = 40'-0"

e > L L - b2 2_ _______2 2

- - a

e > 54.332

_____ _____________2 2

- - 0.52 (54.33) - (25)

e = 2.55'

e = 6.0' > 2.55' OK per formula

CHECK VALIDITY OF FORMULA IN PCI HANDBOOK AND ERECTORS MANUAL. DOES IT PROTECT AGAINST INSTABILITY?

Panel Instability Incident: Soundtrack project, June 1995 (Stresscon)

PCI Formula

Figure 14. This figure compares the formula e with the actual e in an unstable panel. Note: c.g. = center of gravity; e = minimum distance from the center of gravity of the panel, with the panel in its horizontal position, to protect the erection-crew members against the unexpected sudden rollover of the panel; eΔ = horizontal projection of distance from centerline of center of gravity to centerline of rolling-block reaction with panel in tilted position; RL = left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; W = weight of panel; W(c.g.) = panel weight acting at c.g. of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position); # = pound. 1” = 1 in. = 25.4 mm; 1’ = 1 ft = 0.305 m.

∆ = 60°

R = -0.51KR

e = -11/2"∆

R = 52.51K > W = 52KL

∆

W (c.g.) = 52K

CHECK VALIDITY OF FORMULA IN PCI HANDBOOK AND ERECTORS MANUAL. DOES IT PROTECT AGAINST INSTABILITY?

Panel Instability Incident: Soundtrack project, June 1995 (Stresscon)

PCI FormulaIf e > 2.55’ , stability OK Actual e = 6.0’ > 2.55’

Analytical Model At Δ = 60°RL shifts to right of c.g.RL carries entire weight of panel > 52k RR would require downward pull = 510# to keep panel from rolling

Panel actually rolled in June 1995As predicted by Analytical Model

PCI FORMULA DID NOT PREDICT INSTABILITY


To evaluate the validity of this formula, the panel used for the following example is the Soundtrack wall panel, which became unstable during tilting. Figure 13 shows the formula along with the corresponding dimensions of the lifting-de-vice locations, panel thickness, and location of the c.g. of the panel. The length of the rolling-block sling was 40 ft (12 m).

The PCI Design Handbook1 formula calculates the mini-mum e required to protect the erection-crew members against the unexpected sudden rollover of the panel when it is in the horizontal position. In this case, the actual e = 6.0 ft (1.8 m), which comfortably exceeds the minimum e = 2.55 ft (0.78 m) derived from the formula.

However, as indicated in Fig. 14, the analytical method shows that, with the sling length of 40 ft (12 m), the panel becomes unstable at a tilt angle of 60 deg. In fact, the Soundtrack panel became unstable during erection. The bottom end hit the ground abruptly, and the panel was severely damaged. Fortunately, all erection-crew members escaped without injury. With a sling length of 50 ft (15 m), the analytical methods show that the panel would probably have been stable throughout the tilting process.

Erectors have also reported to the author other documented rollover incidents wherein the dimensional information and sling lengths were available. In each case, the PCI Design Handbook formula indicated that the formula-derived lifting-device locations should have protected the panel from sudden rollover, whereas the analytical methods, which include sling-length effects, predicted the instability that was actually encountered.

In its present form, the PCI Design Handbook formula does not include the critical sling-length variable and therefore does not provide its intended protection against instability. Instead of addressing possible modifications to the PCI Design Handbook formula, this article provides the analytical methods described in the section “Analyti-cal Study—Shifting of Rolling-Block Reaction” and the sidebar “Erecting Long Precast Concrete Members,” which directly model the shifting location of the RL reaction and enable the designer to vary the length of the sling for any case studied.

At the time this article was written, PCI had transmitted a technical bulletin to the industry on December 19, 2008, then published the bulletin in the Spring 2009 edition of the PCI Journal,6 indicating that the formula in its present form “is incorrect and may not provide stability for all con-ditions when rotating a precast concrete component in the air.” PCI has further initiated action to remove the formula from future editions of both the PCI Design Handbook1 and the Erector’s Manual2 and to recommend that the conservative layout shown in Fig. 5 be used for three-point lifting of long vertical precast concrete members.

However, design engineers are hereby cautioned that the PCI Design Handbook1 formula, in its present form, is represented in current and past editions of these publica-tions as providing protection against sudden rollover, and precautionary procedures should be instituted to ensure against its use without a verified modification that incorpo-rates the sling-length variable.

Conclusion

Beneficial effect of longer sling lengths

Through studies of various hypothetical conditions and analyses of incidents that resulted in the sudden rollover of members while using the three-point pickup, it became apparent that increasing the length of the slings would often protect against this condition. Based on these studies, it appears that a simple rule may be applicable in cases in which the erector is confronted with having to evaluate the potential for sudden rollover of long vertical precast con-crete members already cast with lifting devices in place.

Erectors’ quick rule

For the three-point lifting procedure, it is suggested that the minimum sling length be greater than twice the dis-tance between the two lower lifting devices to which it is attached. For example, if that distance is 25 ft (7.5 m), a minimum sling length of 50 ft (15 m) plus 10 ft (3 m), or a total minimum sling length of 60 ft (18 m), is suggested. However, to verify the safety of this quick rule, for any specific case it is critical that the resulting sling length be checked and verified by analytical methods, such as those presented in the section “Analytical Study—Shifting of Rolling-Block Reaction” and the sidebar “Erecting Long Precast Concrete Members.”

Recommendations to designers

The analytical methods shown in the section “Analyti-cal Study—Shifting of Rolling-Block Reaction” and the sidebar “Erecting Long Precast Concrete Members” are also recommended to be used to establish lifting-device locations in situations in which the designer elects not to conform to the recommended conservative lifting-device layout in Fig. 5.

However, the following must be emphasized:

The designer must be in a position to guarantee that •the sling length required by the calculation is actually used by the erection crew.

The location of the c.g. of the member must be clearly •defined. A wall panel with openings near the top of the panel or with the top portion thinner than the


this or any other analytical method may be relied on to ensure the safe erection of these long members.

In the example, this method is applied to the long wall panel that rolled suddenly during erection on the 1995 Soundtrack project. The required input data are taken from the panel dimensions with the panel in its horizontal posi-tion and are highlighted in yellow. The sling length used to erect the Soundtrack panel was about 40 ft (12 m).

Figure 17 illustrates the panel at tilt angle Δ. The value of Δ is checked at 60 deg in the example, and the value of θ is varied until tan θ = qΔ/pΔ. The resulting value of θ =18.2 deg, and at that value the vertical alignment of the rolling-block reaction RL has shifted beyond the vertical alignment of the c.g. of the panel. The value of eΔ becomes negative by 1.54 in. (39 mm), and the panel becomes unstable.

The calculations in the example show that at RR, a down-ward pull of 0.51 kip (2.3 kN) would be required to keep the panel stable, and RL carries the entire weight of the panel. The tension in one of the two sling lines would be 13.56 kip (60 kN) applied at the lower lifting device at angle α = 11.8 deg with respect to the panel surface.

This method also checks the validity of the PCI Design Handbook1 formula and calculates the minimum value of e = 2.55 ft (0.78 m) with the panel in the horizontal position, at which point the panel should be stable. However, the actual value of e is 6.0 ft (1.8 m), but the panel has become unstable at a tilt angle Δ = 60 deg, with a sling length S = 40 ft (12 m). Thus the PCI Design Handbook formula did not predict the instability of the panel.

If the sling length S is changed to 50 ft (15 m), the method will show that the panel remains stable at all tilt angles. However, engineering judgment should be exercised with respect to establishing a safety factor to increase the minimum required eccentricity at the critical tilt angle to account for manufacturing tolerances and other variables associated with the erection process.

Acknowledgments

The author appreciates valuable input, detailed review, suggestions, and editing by Helmuth Wilden, P.E., of Wilden Enterprises and Steve Seguirant, P.E., of Con-crete Technology Corp. Also providing comments were John Jacobsen, P.E., Peter Robinson, and Dennis Cates of Stresscon Corp. and Mike Dixon, P.E., of FDG Associ-ates. The demonstration of the four-point lifting sequence was performed by Stresscon Corp., and its photographic sequence was provided by Ralph Brandhorst.

Diagrams were put into digital form by Austin Bryan of CAD Prototypes LLC.

bottom shifts the c.g. toward the bottom, which may result in the panel becoming unstable if the calculation is based on the c.g. being at midheight of the panel.

While the analytical methods are capable of calculat-•ing the eccentricity between the vertical alignments of the c.g. of a member and the rolling-block reaction through all tilt angles, engineering judgment should be exercised in establishing a required minimum value for this eccentricity to ensure that it is increased by an adequate factor of safety to account for manufacturing tolerances and other imperfections.

The analytical methods should also be used to calculate the loads in the slings and the angles to which they apply loads to the lifting devices, particularly in cases in which the rated capacity of a lifting device is less in shear than in tension.

Analytical study—shifting of rolling-block reaction

This method compares the vertical alignment of the rolling-block reaction with the vertical alignment of the c.g. of long precast concrete members at any selected tilt angle. If at any tilt angle the rolling-block reaction aligns with the c.g., that reaction will carry the entire weight of the member and the member will become unstable.

Figure 15 illustrates this method. The input data utilize values given for each of the notations illustrated on Fig. 16 with the panel in the horizontal position. Those input data are shown in the highlighted boxes in the example.

The notations at any tilt angle Δ are shown on Fig. 17. Dur-ing the tilting of a member, the horizontal components of the loads T in the two legs of the rolling-block sling (T sin θ) must be equal and opposite. Thus the angle of incli-nation θ of each leg with respect to the vertical alignment of the rolling-block reaction must also be equal and opposite.

At any tilt angle Δ, the method requires the selection of trial values of θ, calculates the consequent values of dimensions qΔ and pΔ until, through successive approxima-tions, tan θ = qΔ/pΔ. At that value of θ, the method calcu-lates the horizontal shift of the vertical alignment of RLΔ and its eccentricity eΔ with respect to the vertical alignment of the c.g. of the member.

If, at any selected tilt angle, eΔ approaches zero value or becomes negative, the panel becomes unstable and will roll suddenly and uncontrollably.

If the value of eΔ remains positive through all tilt angles, the member will remain stable. However, it is emphasized that the selected sling length S is a very important variable and its length must be established and controlled so that


Figure 15. This Microsoft Excel diagram shows the lifting angle’s effect on panel reactions. Note: b = distance between lower lifting devices; L = length of panel; SL = S = sling length, defined as length of cable attached to lifting device, running diagonally through rolling block and diagonally back to companion lifting device that shares rolling-block reaction, includes lengths of hooks and any shackles within that length of line; W = weight of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position); θ = angle of inclination. 1 in. = 25.4 mm; 1 ft = 0.305 m.

L =SL =

b =c =

CG(R) =CG(L) =

f =e =

sin∆ = cos∆ =

b∆ = b*cos∆ =

p∆ = b*sin∆ =

f∆ = f*cos∆ =

f'∆ = f∆ - (a*sin∆) =

CG(R)∆ = [CG(R)*cos∆] – a*sin∆)

j∆ = p∆/cosθ =

k∆ = (SL – j∆)/2 =

m∆ = k∆*sinθ =

q∆ = b∆ – 2m∆ =

n∆ = k∆*cosθ =

e∆ = m∆ – f'∆ =

CHECK q∆/p∆ = tanθ =

SHIFT = b∆/2 – m∆ =

54.33 ft40.00 ft25.00 ft8.33 ft

27.50 ft26.83 ft6.50 ft6.00 ft

0.8660.500

60.00 deg

LIFTING ANGLE EFFECT ON PANEL REACTIONS DON LOGAN v3 09/30/09Notations Illustrated on Figs. 16 and 17

PANEL LENGTHSLING LENGTH

BOTTOM RIGGING

CHECK VALIDITY OF PCI FORMULA

pci e = L/2 – {[(L2 – b2)0.5]/2} – a pci e = 2.547 ftIf e(actual) > pci e, then stablee(actual) = 6.000 ft > pci e, OK?

THICKNESS = 12.00 in.a = 0.500 in.

SOUNDTRACK 1995-STYROCORE PANEL 10 FT × 54.33 FT;3 POINT PICKUP, b(Lower) = 25.00 ft

SELECT ∆ =

12.500 ft

21.651 ft

3.250 ft

2.817 ft

13.317 ft

a*sin∆ = 0.433 ft

22.792 ft

8.604 ft

2.689 ft

7.122 ft

8.173 ft

-0.128 ft

0.329

3.561 ft

; * 12 =

tanθ =

; *12 =

=

=

=

=

-1.538

0.329

42.73

11.79

-0.51

5251

13.82

in.

deg

K downward*

K vertical*

K resultant

= 52 K ; ALPHA = 90 – ∆ – θ

= W*e∆/(CG(L)∆ + e∆)

= [W*(CG(L)∆/[CG(L)∆ + e∆]

Line Tens. (4 components) = RL∆/(4*cosθ)

THE RL∆ REACTION LINE HAS SHIFTED PAST THE CG LINE AND RL CARRIES ENTIRE PANEL

WEIGHT; RR < ZERO KIPS. PANEL IS UNSTABLE.

* NOTE:

UNSTABLE

TRY θ = 18.211 deg

W

RR∆

RL∆


Figure 16. Notations are given for the panel when it is in the horizontal position. Note: a = distance from up face of panel to center of gravity of the panel; b = distance between lower lifting devices; eo = distance from the center of gravity of panel to centerline of rolling-block reaction with panel in horizontal position; L = length of panel; RL

= left lifting reaction acting at centerline of rolling block; RR = right lifting reaction acting at right end of panel; SL = S = sling length, defined as length of cable attached to lifting device, running diagonally through rolling block and diagonally back to companion lifting device that shares rolling-block reaction, including lengths of hooks and any shackles within that length of line; W = weight of panel; α = angle between sling leg and surface of panel; Δ = tilt angle of panel in tilted position (Δ as a subscript identi-fies horizontal projection of dimensions with panel in tilted position).

b

w CG(R)CG(L)

fe

b/2b/2

L

o

c L-(c+b)

SL/2 SL/2

α

θ

RR

RL

oθo

o

∆ = 0Tilt Angle

ο

a

o

Panel in horizontal position showing notationsRefer to Fig. 15 for specific dimensionsof unstable Soundtrack panel − 1995


Figure 17. Panel instability is shown for tilt angle Δ. Note: a = distance from up face of panel to center of gravity of the panel; b = distance between lower lifting devices; eΔ = horizontal projection of distance from centerline of center of gravity to centerline of rolling-block reaction with panel in tilted position; RL = left lifting reaction acting at centerline of rolling block; W = weight of panel. Δ = tilt angle of panel in tilted position (Δ as a subscript identifies horizontal projection of dimensions with panel in tilted position).

b

ca

θ∆

θ∆θ∆

m∆ m∆

p

nΔ

CG(R)Δ

α∆

f

b∆

-e unstable

f

2m

RL Shift

w

qΔ Δ

Δ

Δ

RLΔ

Δ

RR Δ(-)

'

k ∆

j ∆

o

θ∆ = θ∆

tan θ∆ = q∆ / p∆

f

a

Δ

f-a

Δ

Δ

Shift in CG(panel thickness effect)

Δ

∴

-e unstable

RL Shift

w

Δ

Δ

∴

'Δf

RL

of

w sinΔasinΔ Δ

w

∆Tilt Angle =

b /2Δ

b∆/2 b∆/2

T

T

Panel ininstability at tilt angle ∆ illustratedRefer to Fig. 15 for specific dimensionsof unstable Soundtrack panel − 1995

b∆/2 b∆/2


RL = left lifting reaction acting at centerline of rolling block

RR = right lifting reaction acting at right end of panel

S = sling length, defined as length of cable attached to lifting device, running diagonally through rolling block and diagonally back to companion lifting device that shares rolling-block reaction, including lengths of hooks and any shackles within that length of line

T = tension in the rolling block sling

W = weight of panel

W’ = weight of panel modified by impact factor (1.2W in examples)

W(c.g.) = panel weight acting at c.g. of panel

α = angle between sling leg and surface of panel

Δ = tilt angle of panel in tilted position (Δ as a subscript identifies angles and dimensions with panel in tilted position)

θ = angle of inclination between sling legs and cen-terline of rolling block reaction

References

Industry Handbook Committee. 1985. 1. PCI Design Handbook: Precast and Prestressed Concrete. MNL-120. 3rd ed. Chicago, IL: PCI.

PCI Erectors Committee. 1999. 2. Erectors Manual: Standards and Guidelines for the Erection of Precast Concrete Products. MNL-127. 2nd ed. Chicago IL: PCI.

Industry Handbook Committee. 1992.3. PCI Design Handbook: Precast and Prestressed Concrete. MNL-120. 4th ed. Chicago, IL: PCI.

Industry Handbook Committee. 1999. 4. PCI Design Handbook: Precast and Prestressed Concrete. MNL-120. 5th ed. Chicago, IL: PCI.

Industry Handbook Committee. 2004. 5. PCI Design Handbook: Precast and Prestressed Concrete. MNL-120. 6th ed. Chicago, IL: PCI.

PCI Journal6. . 2009. Figure on Stability during Han-dling and Erection to Be Revised. In “From PCI Head-quarters.” V. 54, No. 2 (Spring): p. 21.

Notation

a = distance from up face of panel to center of grav-ity of the panel

b = distance between lower lifting devices

c.g. = center of gravity

CGL = distance from left end of panel to c.g. of panel

CGR = distance from right end of panel to c.g. of panel

eo = distance from the center of gravity of panel to centerline of rolling-block reaction with panel in horizontal position

eΔ = horizontal projection of distance from centerline of center of gravity to centerline of rolling-block reaction with panel in tilted position

fc' = concrete compressive strength at erection

ft = actual tensile stress in panel during lifting

ftallow = allowable tensile stress in panel during lifting

L = length of panel


Erecting long precast concrete members

Stephen J. Seguirant, P.E.Vice president and director of engineeringConcrete Technology Corp., Tacoma, Wash.

This sidebar shows the method that Concrete Technology Corp. uses to evaluate the stability of long precast con-crete members lifted using three points during erection. This method is based on the law of cosines and is simple to understand and incorporate into a spreadsheet.

In the horizontal position, the panel hangs as shown in Fig. S1. The only information required as input for the method is:

dcg = distance from up face of panel to c.g. of panel

hcg = distance from bottom of panel to c.g. of panel

h1 = distance from bottom of panel to lowest set of lifting devices

h2 = distance between lower two sets of lifting devices

L = length of panel

S = sling length, defined as total length of lifting sling from lowest set of lifting devices through the rolling blocks to middle set of lifting devices

In the horizontal position, the length of the lower leg of the sling S1 and the upper leg of the sling S2 are both equal to S/2. During all angles of rotation, S1 + S2 = S. When the panel reaches vertical, S1 = S - (S - h2)/2 and S2

= S - S1. By varying S1 between the possible extremes, the stability of the panel during rotation can be investigated.

Figure S2 shows the panel in a tilted position. Because the distance between the lower two sets of lifting devices h2 is fixed, angles A and B can be found by the law of cosines for any assumed value of S1.

A = cos−1

S1

2+ S

2

2− h

2

2

2S1S

2

B = cos−1

h2

2+ S

2

2− S

1

2

2h2S

2

Figure S2. The panel is in a tilted position. Note: A = angle; B = angle; C = angle; e = eccentricity; Rl = left lifting reaction acting at centerline of rolling block; Rr = right lifting reaction acting at right end of panel; S1 = the length of the lower leg of the sling; S2 = the length of the upper leg of the sling; xcg = distance between the bottom up-face corner of the panel and the c.g. of the panel; xlift = distance between the bottom up-face corner of the panel and the vertical crane line reaction Rl; θ = angle of panel rotation from horizontal.

Panel in tilted position.

R rR l

S 2

A

C

S 1

ex lift

x cg

B

∆θ

Equal

Figure S1. The panel hangs like this when it is in the horizontal position. Note: c.g. = center of gravity; dcg = distance from up face of panel to c.g. of panel; eh = dis-tance from c.g. of panel to Rl; hcg = distance from bottom of panel to c.g. of panel; h1 = distance from bottom of panel to lowest set of lifting devices; h2 = distance between lower two sets of lifting devices; h3 = length of panel minus (h1 plus h2); L = length of panel; Rl = left lifting reaction acting at centerline of rolling block; Rr = right lifting reaction acting at right end of panel; S = sling length, defined as total length of lifting sling from lowest set of lifting devices, through the rolling blocks, to middle set of lifting devices; S1 = the length of the lower leg of the sling; S2 = the length of the upper leg of the sling.

Panel in horizontal position.

R rR l

h cg

S 2 = S/2

e h

h 2 h 3h 1

L

d cg

S 1 = S/2

60 o min


e = xcg – xlif t ≥ 0

The results of this method on the example illustrated in the section “Analytical Study – Shifting of Rolling-Block Reaction” are tabulated in Table S1. The values desig-nated Δ are used to modify S1 for the entire range of rota-tion. The mesh of Δ values decrease as the panel rotation angle increases because instability is more likely to occur at higher tilt angles. At S1 = 31.40 ft (9.6 m), Δ has been manually adjusted to result in a panel tilt angle θ of 60 deg. The resulting e = -0.13 ft = -1.54 in. (39 mm) agrees with the results of the method shown in “Analytical Study—Shifting of Rolling-Block Reaction.” However, actual instability will begin at roughly θ = 50 deg.

The mathematical method presented herein is straight-forward. However, engineering judgment should be exercised with respect to a minimum required eccen-tricity (greater than zero) to account for manufacturing tolerances and other imperfections associated with the erection process.

Due to the free rotation of the rolling blocks, angle A is bisected by the crane line resisting Rl at all angles of rota-tion. Consequently, the angle of panel rotation θ can be found using

θ = 90− 180−A

2− B

⎛⎝⎜

⎞⎠⎟

The distance between the bottom up-face corner of the panel and the vertical crane line reaction Rl can be found using

x

lift= cosθ h

1+ h

2( )− sinA

2S

2( )

The distance between that same bottom up-face corner and the c.g. of the panel, adjusted for the depth of the c.g. below the up face, is

xcg = cosθ(hcg + dcg tanθ)

The panel will remain stable as long as

Table S1. Stability calculation results for example panel from “Analytical Study – Shifting of Rolling-Block Reaction”

S1, ft S2, ft A, deg B, deg θ, deg xcg, ft xLift, ft e, ft Δ, ft

20.00 20.00 77.36 51.32 0.00 26.83 20.83 6.00 n.a.

21.56 18.44 76.92 57.15 5.62 26.75 21.71 5.05 1.563

23.13 16.88 75.57 63.61 11.40 26.40 22.34 4.07 1.563

24.69 15.31 73.17 70.94 17.52 25.74 22.66 3.08 1.563

26.25 13.75 69.47 79.52 24.26 24.67 22.55 2.11 1.563

27.03 12.97 67.01 84.47 27.97 23.93 22.28 1.65 0.781

27.81 12.19 64.01 90.00 32.01 23.02 21.81 1.21 0.781

28.59 11.41 60.35 96.29 36.47 21.88 21.07 0.80 0.781

29.38 10.63 55.81 103.61 41.51 20.42 19.99 0.44 0.781

29.77 10.23 53.11 107.78 44.33 19.54 19.27 0.28 0.391

30.16 9.84 50.04 112.39 47.41 18.53 18.39 0.13 0.391

30.55 9.45 46.51 117.57 50.82 17.34 17.32 0.01 0.391

30.94 9.06 42.37 123.49 54.67 15.92 16.00 -0.08 0.391

31.40 8.60 36.42 131.79 60.00 13.85 13.98 -0.13 0.459

31.79 8.21 29.85 140.74 65.66 11.51 11.62 -0.11 0.391

32.18 7.82 20.48 153.23 73.47 8.11 8.09 0.02 0.391

32.50 7.50 0.00 180.00 90.00 0.50 0.00 0.50 0.322

Note: A = angle; B = angle; e = eccentricity; S1 = the length of the lower leg of the sling; S2 = the length of the upper leg of the sling; xcg = distance between the bottom up-face corner of the panel and the c.g. of the panel; xlift = distance between the bottom up-face corner of the panel and the verti-cal crane line reaction Rl; Δ = dimensional increment used to modify S1 during the rotation of the panel to vertical; θ = angle of panel rotation from horizontal.


About the author

Donald R. Logan, P.E., is principal of Logan Structural Research Foundation in Colorado Springs, Colo. He was founder and CEO of Stresscon Corp. from 1967 to 2007. He is a PCI Fellow and was named a Titan of the

Industry in 2004. Logan received the Martin P. Korn Award and the American Society of Civil Engineers’ T. Y. Lin Award for his 1997 paper on strand-bond acceptance criteria and the Korn Award again for his 2007 paper on eccentrically loaded L-spandrels. He serves on the PCI Research and Development Com-mittee.

Synopsis

Erection of long, vertical precast concrete members from their as-delivered horizontal position to their vertical position in the structure involves devising handling procedures that protect the safety of the erec-tion personnel while preventing the precast concrete member from cracking during the tilting procedure. The author investigates the most commonly used procedures, three-point and four-point rolling-block lifting systems, and identifies a common instability

condition that causes sudden, uncontrolled rolling of the members as they are being tilted toward their vertical position. Methods are devised to detect in any member the potential for such instability, to establish safe location of lifting devices and minimum lengths of crane slings to prevent this condition from occur-ring, and to analyze and control tensile stresses from causing cracking of long precast concrete wall panels as they are lifted from their horizontal position, rigged for the tilting operation.

Keywords

Erection, long members, rolling block, safety, stabil-ity, tilting, wall panel.

Review policy

This paper was reviewed in accordance with the Precast/Prestressed Concrete Institute’s peer-review process.

Reader comments

Please address any reader comments to PCI Journal editor-in-chief Emily Lorenz at [email protected] or Precast/Prestressed Concrete Institute, c/o PCI Journal, 200 W. Adams., Suite 2100, Chicago, IL 60606. J

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