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PCI 6 th Edition Fabrication Design. Presentation Outline Planning Discussion Stripping Process Design and Analysis Prestress / Post Tension Effects Handling.

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Presentation on theme: "PCI 6 th Edition Fabrication Design. Presentation Outline Planning Discussion Stripping Process Design and Analysis Prestress / Post Tension Effects Handling."— Presentation transcript:

1 PCI 6 th Edition Fabrication Design

2 Presentation Outline Planning Discussion Stripping Process Design and Analysis Prestress / Post Tension Effects Handling Devices Stripping Stress Examples Storage Discussion Transportation Discussion Erection Discussion

3 Introduction The loads and forces on precast and prestressed concrete members during production, transportation or erection will frequently require a separate analysis Concrete strengths are lower Support points and orientation are usually different from members in their final position

4 Pre-Planning Piece Size The most economical piece size for a project is usually the largest, considering the following factors: Stability and stresses on the element during handling Transportation size and weight regulations and equipment restrictions

5 Pre-Planning Piece Size Available crane capacity at both the plant and the project site. Position of the crane must be considered, since capacity is a function of reach Storage space, truck turning radius, and other site restrictions

6 Planning and Setup Once a piece has been fabricated, it is necessary to remove it from the mold without being damaged. Positive drafts or breakaway forms should be used to allow a member to lift away from the casting bed without becoming wedged within the form Adequate draft also serves to reduce trapped air bubbles.

7 Planning and Setup Lifting points must be located to keep member stresses within limits and to ensure proper alignment of the piece as it is being lifted Members with unsymmetrical geometry or projecting sections may require supplemental lifting points and auxiliary lifting lines to achieve even support during handling “Come-alongs” or “chain-falls” are frequently used for these auxiliary lines

8 Planning and Setup When the member has areas of small cross section or large cantilevers, it may be necessary to add a structural steel “strongback” to the piece to provide added strength

9 Planning and Setup Members that require a secondary process prior to shipment, such as sandblasting or attachment of haunches, may need to be rotated at the production facility. In these cases, it may be necessary to cast in extra lifting devices to facilitate these maneuvers

10 Planning and Setup When developing member shapes, the designer should consider the extra costs associated with special rigging or forming, and pieces requiring multiple handling

11 Stripping: General Orientation of members during storage, shipping and final in-place position is critical in determining stripping requirements They can be horizontal, vertical or some angle in between The number and location of lifting devices are chosen to keep stresses within the allowable limits, which depends on whether the “no cracking” or “controlled cracking” criteria is to be used

12 Stripping: General It is desirable to use the same lifting devices for both stripping and erection; however, additional devices may be required to rotate the member to its final position

13 Stripping: General Panels that are stripped by rotating about one edge with lifting devices at the opposite edge will develop moments as shown

14 Stripping: General When panels are stripped this way, care should be taken to prevent spalling of the edge along which the rotation occurs A compressible material or sand bed will help protect this edge

15 Stripping: General Members that are stripped flat from the mold will develop the moments shown

16 Stripping: General In some plants, tilt tables or turning rigs are used to reduce stripping stresses

17 Stripping: General Since the section modulus with respect to the top and bottom faces may not be the same, the designer must select the controlling design limitation: Tensile stresses on both faces to be less than that which would cause cracking Tensile stress on one face to be less than that which would cause cracking, with controlled cracking permitted on the unexposed face Controlled cracking permitted on both faces

18 Stripping: General If only one of the faces is exposed to view, the exposed face will generally control the stripping method

19 Rigging Configurations Stresses and forces occurring during handling are also influenced by the type of rigging used to hook up to the member

20 Rigging Configurations Lift line forces for a two-point lift using inclined lines are shown

21 Rigging Configurations When the sling angle is small, the components of force parallel to the longitudinal axis of the member may generate a significant moment due to secondary effects

22 Rigging Configurations While this effect can and should be accounted for, it is not recommended that it be allowed to dominate design moments

23 Rigging Configurations Consideration should be given to using spreader beams, two cranes or other mechanisms to increase the sling angle Any such special handling required by the design should be clearly shown on drawings

24 Rigging Configurations Using a spreader beam can also eliminate the use of rolling blocks Note that the spreader beam must be sufficiently stiffer than the concrete panel to limit panel deflections and cracking Lifting hook locations, hook heights, and sling lengths are critical to ensure even lifting of the member For analysis, the panel acts as a continuous beam over multiple supports

25 Stripping Design To account for the forces on the member caused by form suction and impact, it is common practice to apply a multiplier to the member weight and treat the resulting force as an equivalent static service load. The multipliers cannot be quantitatively derived, so they are based on experience

26 Stripping Design PCI provides a table of typical values

27 Factor of Safety When designing for stripping and handling, the following safety factors are recommended: –Use embedded inserts and erection devices with a pullout strength at least equal to four (4) times the calculated load on the device. –For members designed “without cracking,” the modulus of rupture (MOR), is divided by a safety factor of 1.5.

28 Stress Limits & Crack Control Stress limits for prestressed members during production are discussed in Section of the the PCI Handbook ACI does not restrict stresses on non- prestressed members, but does specify minimum reinforcement spacing, as discussed in Section (PCI chapter 4 member design)

29 Stress Limits & Crack Control Members which are exposed to view will generally be designed for the “no discernible cracking criteria” (see Eq ), which limits the stress to. In the case of stripping stresses, f′ ci should be substituted for f′ c Whether or not the members are exposed to view, the strength design and crack control requirements of ACI , as discussed in Chapter 4 of this Handbook, must be followed.

30 Benefits of Prestressing Panels can be prestressed, using either pretensioning or post-tensioning. Design is based on Chapter 18 of ACI , as described in Chapter 4 of this Handbook. Further, tensile stresses should be restricted to less than, must be followed.

31 Benefits of Prestressing It is recommended that the average stress due to prestressing, after losses, be within a range of 125 to 800 psi The prestressing force should be concentric with the effective cross section in order to minimize camber, although some manufacturers prefer to have a slight inward bow in the in- place position to counteract thermal bow It should be noted that concentrically prestressed members do not camber, hence the form adhesion may be larger than with members that do camber

32 Strand Recomendation In order to minimize the possibility of splitting cracks in thin pretensioned members, the strand diameter should not exceed that shown in the table below Additional light transverse reinforcement may be required to control longitudinal cracking

33 Strand Recommendations When wall panels are post-tensioned, care must be taken to ensure proper transfer of force at the anchorage and protection of anchors and tendons against corrosion Straight strands or bars may be used, or, to reduce the number of anchors, the method shown may be used

34 Strand Recommendation It should be noted that if an unbonded tendon is cut, the prestress is lost. This can sometimes happen if an unplanned opening is cut in at a later date

35 Handling Devices Since lifting devices are subject to dynamic loads, ductility of the material is a requirement Deformed reinforcing bars should not be used as the deformations result in stress concentrations from the shackle pin Also, reinforcing bars may be hard grade or re-rolled rail steel with little ductility and low impact strength at cold temperatures

36 Handling Devices Strain hardening from bending may cause embrittlement Smooth bars of a known steel grade may be used if adequate embedment or mechanical anchorage is provided The diameter must be such that localized failure will not occur by bearing on the shackle pin

37 Aircraft Cable Loops For smaller precast members, aircraft cable can be used for stripping and erection purposes Aircraft cable comes in several sizes with different capacities The flexible cable is easier to handle and will not leave rust stains on precast concrete

38 Aircraft Cable Loops For some small precast members such as coping, the flexible loops can be cast in ends of members and tucked back in the joints after erection Aircraft cable loops should not be used as multiple loops in a single location, as even pull on multiple cables in a single hook is extremely difficult to achieve User should ensure that the cable is clean and that each leg of the loop is embedded a minimum of 48 in.

39 Prestressing Strand Loops Prestressing strand, both new and used, may be used for lifting loops The capacity of a lifting loop embedded in concrete is dependent upon the strength of the strand, length of embedment, the condition of the strand, the diameter of the loop, and the strength of the concrete

40 Prestressing Strand Loops As a result of observations of lift loop behavior during the past few years, it is important that certain procedures be followed to prevent both strand slippage and strand failure Precast producers’ tests and/or experience offer the best guidelines for the load capacity to use A safety factor of 4 against slippage or breakage should be used

41 Strand Loops Recommendations In lieu of test data, the recommendations listed below should be considered when using strand as lifting loops. –Minimum embedment for each leg of the loop should be 24 in. –The strand surface must be free of contaminants, such as form oil, grease, mud, or loose rust, which could reduce the bond of the strand to the concrete

42 Strand Loops Recommendations Continued: –The diameter of the hook or fitting around which the strand lifting eye will be placed should be at least four times the diameter of the strand being used –Do not use heavily corroded strand or strand of unknown size and strength.

43 Strand Loops Recommendations In the absence of test or experience, it is recommended that the safe load on a single 1/2 in. diameter 270 ksi strand loop satisfying the above recommendations not exceed 8 kips The safe working load of multiple loops may be conservatively obtained by multiplying the safe load for one loop by 1.7 for double loops and 2.2 for triple loops

44 Strand Loops Recommendations To avoid overstress in one loop when using multiple loops, care should be taken in the fabrication to ensure that all strands are bent the same Thin wall conduit over the strands in the region of the bend has been used to reduce the potential for overstress

45 Strand Loops Recommendations When using double or triple loops, the embedded ends may need to be spread apart for concrete consolidation around embedded ends without voids being formed by bundled strand

46 Threaded Inserts Threaded inserts can have NC (National Course) or coil threads Anchorage is provided by loop, strut or reinforcing bar Inserts must be placed accurately because their safe working load decreases sharply if they are not perpendicular to the bearing surface, or if they are not in a straight line with the applied force

47 Threaded Inserts Embedment of inserts close to an edge will greatly reduce the effective area of the resisting concrete shear cone and thus reduce the tension safe working load of the embedded insert When properly designed for both insert and concrete capacities, threaded inserts have many advantages However, correct usage is sometimes difficult to inspect during handling operations

48 Threaded Inserts In order to ensure that an embedded insert acts primarily in tension, a swivel plate as indicated in should be used It is extremely important that sufficient threads be engaged to develop the strength of the bolt

49 Threaded Inserts For straight tension loads only, eye bolts or wire rope loops provide a fast method for handling precast members. Do not use either device if shear loading conditions exist.

50 Proprietary Devices A variety of castings or stock steel devices, machined to accept specialized lifting assemblies are used in the precast industry

51 Proprietary Devices These proprietary devices are usually recessed (using a “pocket former”) to provide access to the lifting unit. The recess allows one panel to be placed against another without cutting off the lifting device, and also helps prevent spalling around the device Longer devices are used for edge lifting or deep precast concrete members Shallow devices are available for thin precast concrete members.

52 Proprietary Devices The longer devices usually engage a reinforcing bar to provide greater pullout capacity, and often have holes for the bar to pass through as shown to the left

53 Proprietary Devices These units have a rated capacity as high as 22 tons, with reductions for thin panels or close edge distances Supplemental reinforcement may be required to achieve these values Shallow units usually have a spread foot or base to increase pullout capacity

54 Proprietary Devices Reinforcing bars are required in two directions over the base to fully develop the lifting unit, as shown in Figure below These inserts are rated up to 8 tons

55 Proprietary Devices Some lifting eyes do not swivel, so rotation may be a concern In all cases manufacturer recommendations should be rigorously followed when using any of these devices

56 Wall Panel Example This example and others in Chapter 5 illustrate the use of many of the recommendations in this chapter They are intended to be illustrative and general only Each manufacturer will have its own preferred methods of handling

57 Wall Panel Example Given: A flat panel used as a loadbearing wall on a two-story structure, as shown on next slide Section properties (nominal dimensions are used for design): Solid panel Panel with openings A = 960 in 2 A = 480 in 2 S b = S t = 1280 in 3 S b = S t = 640 in 3 I x = 5120 in 4 I x = in 4 Unit 150 pcf = 100 psf = ksf Total weight = 35.2 kips (solid panel) = 29.2 kips (panel w/ openings)

58 Wall Panel Example

59 Stripping method: –Inside crane height prevents panel from being turned on edge directly in mold, therefore, strip flat Handling multipliers: –Exposed flat surface has a smooth form finish with false joints. Side rails are removable. Use multiplier of 1.4

60 Wall Panel Example f′ ci at stripping = 3000 psi Allowable tensile stresses at stripping and lifting: Problem: Check critical stresses involved with stripping. Limit stresses to ksi. Compare Simple Solution to Mechanics Solution

61 Solution Steps Step 1 – Determine section properties Step 2 – Select number of pick points and determine maximum stress Step 3 – Determine stress from mechanic approach Step 4 – Check panel with opening Step 5 – Check rolling block solution Step 6 – Check transverse bending Step 7 – Check secondary effects

62 Step 1 – Determine Section Properties Solid panel dimensions a = 10 ft, b = 35.2 ft, a/2 = 5 ft = 60 in. S for resisting section (half of panel width)

63 Step 2 – 4-point pick Figure (a) (page 5-5)

64 Step 2 – Check Stresses 4 Point Stresses Not Good try 8 point pick

65 Step 2 – 8 Point Pick Figure (b) (Page 5-5)

66 Step 2 – Check Stresses 8 Point Stresses

67 Step 3 – Mechanics of Materials

68 Step 4 – Panel With Openings

69 Step 5 – Rolling Blocks If using a rolling block for handling as shown below, the panel cannot be analyzed with the previous method Each leg of continuous cable over a rolling block must carries equal load

70 Step 5 – Rolling Block

71 Step 6 – Transverse Bending Consider lower portion of panel with openings Note that Figure Without the concrete in the area of the opening, the weight is reduced and unevenly distributed. Also, the resisting section is limited to a width of 4.7 ft.

72 Step 6 – Transverse Bending Section through lifters: From continuous beam analysis, load carried by bottom two anchors is 7.2 kips, therefore:

73 Step 7 – Secondary Effects Check added moment due to sling angle Using recessed proprietary lifting anchor e = 3.5 in

74 Step 7 – Secondary Effects Resisting Section Therefore Section is OK

75 Prestressed Wall Example Given: Same wall panel as previous example

76 Prestressed Wall Example Problem: Determine required number of 1 / 2 in diameter, 270 ksi strands pulled to 28.9 kips to prevent cracking in window panel. Assume 10% loss of prestress. From previous example, tensile stress is ksi. The desired level of tensile stress is or ksi

77 Solution Steps Step 1 – Determine additional compressive Required Step 2 – Determine the number of strands required based on stress Step 3 – Calculate the number of strands

78 Step 1 – Additional Compressive Compressive stress required – = ksi

79 Step 2 – # Of Strands Based On Stress From previous the max moment/stress occurs at lifting points (-M). This results in tensile stresses on the top face.

80 Step 3 – Number of Strands 0.060(no. of strands) – 0.019(no. of strands) = ksi No. of strands = 3.8 Add four strands to panel (two on each side of opening)

81 Storage Wherever possible, a member should be stored on points of support located at or near those used for stripping and handling Where points other than those used for stripping or handling are used for storage, the storage condition must be checked

82 Storage If support is provided at more than two points, and the design is based on more than two supports, precautions must be taken so that the element does not bridge over one of the supports due to differential support settlement

83 Storage Warpage in storage may be caused by –temperature or shrinkage differential between surfaces –creep –storage conditions Warpage can only be minimized by providing Where feasible, the member should be oriented in the yard so that the sun does not overheat one side

84 Storage By superposition, the total instantaneous deflection, y max, at the maximum point can be estimated by: I c, I b = moment of inertia of uncracked section in the respective directions for 1 in. width of panel

85 Storage This instantaneous deflection should be modified by a factor to account for the time dependent effects of creep and shrinkage ACI suggests the total deformation y t, at any time can be estimated as:

86 Storage λ = amplification due to creep and shrinkage as a function of ′ρ (reinforcement ratio for non-prestressed compression reinforcement, A s /b·t)

87 Transportation The method used for transport can affect the structural design because of size and weight limitations and the dynamic Except for long prestressed deck members, most products are transported on either flatbed or low-boy trailers Trailers deform during hauling Size and weight limitations vary from one state to state Loads are further restricted on secondary roads The common payload for standard trailers without special permits is 20 tons.

88 Transportation Low-boy trailers permit the height to be increased to about 10 to 12 ft. However they have a have a shorter bed length. This height may require special routing to avoid low overpasses and overhead wires

89 Transportation Erection is simplified when members are transported in the same orientation they will have in the structure For example, single-story wall panels can be transported on A-frames with the panels upright A-frames also provide good lateral support and the desired two points of vertical support

90 Transportation Longer units can be transported on their sides to take advantage of the increased stiffness compared with flat shipment

91 Transportation In all cases, the panel support locations should be consistent with the panel design Panels with large openings sometimes require strongbacks, braces or ties to keep stresses within the design values

92 Transportation For members not symmetrical with respect to the bending axis, the following expressions can be used for determining the location of supports to give equal tensile stresses for positive and negative bending moments

93 Transportation For one end cantilevered… Where y b = distance from the bending axis to the bottom fiber y t = distance from the bending axis to the top fiber

94 Transportation For two ends cantilevered… Where y b = distance from the bending axis to the bottom fiber y t = distance from the bending axis to the top fiber

95 Erection Precast concrete members frequently must be reoriented from the position used to transport to its final construction position The analysis for this “tripping” (rotating) operation is similar to that used during other handling stages In chapter 5 in the PCI handbook, maximum moments for several commonly used tripping techniques are illustrated

96 Tripping Design Guide

97 Erection When using two crane lines, the center of gravity must be between them in order to prevent a sudden shifting of the load while it is being rotated To ensure that this is avoided, the stability condition shown must be met:

98 Erection The capacities of lifting devices must be checked for the forces imposed during the tripping operation, since the directions vary When rotating a panel with two crane lines, the pick points should be located to prevent the panel from an uncontrolled roll on the roller blocks can be done by slightly offsetting the pick point locations to shift the weight toward the upper crane line lift points, or by using chain drags on the rolling block

99 Erecting Wall Panels Example Given: The wall panels with openings used on previous examples Problem: Determine appropriate procedures for erecting the wall panels with openings, panel will be shipped flat

100 Erecting Wall Panels Example Assumptions Limit stresses to (0.354 ksi). Crane has main and auxiliary lines. A telescoping man lift is available on site. Solution: Try three-point rotation up using stripping inserts and rolling block: To simplify, conservatively use solid panel (no openings) to determine moments.

101 Erecting Wall Panels Example

102 In Horizontal Position Therefore, 3 point pick not adequate

103 Erecting Wall Panels Example Knowing from the stripping analysis that a four-point pick can be used, the configurations shown here may be used However, this rigging may become unstable at some point during tripping, i.e., continued rotation without tension in Line A Therefore, the lower end of the panel must stay within inches of the ground to maintain control.

104 Erecting Wall Panels Example Because the previous configuration requires six rolling blocks and can be cumbersome, the method shown on the following slide may be an alternative

105 Erecting Wall Panels Example

106 Erection Bracing Introduction This section deals with the temporary bracing which may be necessary to maintain structural stability of a precast structure during construction When possible, the final connections should be used to provide at least part of the erection bracing, but additional bracing apparatus is sometimes required to resist all of the temporary loads

107 Erection Bracing Introduction These temporary loads may include wind, seismic, eccentric dead loads including construction loads, unbalanced conditions due to erection sequence and incomplete connections Due to the low probability of design loads occurring during erection, engineering judgment should be used to establish a reasonable design load

108 Erection Bracing Responsibilities Proper planning of the construction process is essential for efficient and safe erection Sequence of erection must be established early, and the effects accounted for in the bracing analysis and the preparation of shop drawings The responsibility for the erection of precast concrete may vary as follows: – (see also ACI Section 10.3)

109 Erection Bracing Responsibilities The precast concrete manufacturer supplies the product erected, either with his own forces, or by an independent erector The manufacturer is responsible only for supplying the product, F.O.B. plant or jobsite Erection is done either by the general contractor or by an independent erector under a separate agreement

110 Erection Bracing Responsibilities The products are purchased by an independent erector who has a contract to furnish the complete precast concrete package. Responsibility for stability during erection must be clearly understood. Design for erection conditions must be in accordance with all local, state and federal regulations. It is desirable that this design be directed or approved by a Professional Engineer

111 Erection Bracing Responsibilities It is desirable that this design be directed or approved by a Professional Engineer Erection drawings define the procedure on how to assemble the components into the final structure The erection drawings should also address the stability of the structure during construction and include temporary connections

112 Erection Bracing Responsibilities When necessary, special drawings may be required to include shoring, guying, bracing and specific erection sequences It is desirable that this design be directed or approved by a Professional Engineer Erection drawings define the procedure on how to assemble the components into the final structure

113 Erection Bracing Responsibilities The erection drawings should also address the stability of the structure during construction and include temporary connections When necessary, special drawings may be required to include shoring, guying, bracing and specific erection sequences

114 Erection Bracing Responsibilities For large and/or complex projects, a pre-job conference prior to the preparation of erection drawings may be warranted, in order to discuss erection methods and to coordinate with other trades

115 Handling Equipment The type of jobsite handling equipment selected may influence the erection sequence, and hence affect the temporary bracing requirements Several types of erection equipment are available, including truck-mounted and crawler mobile cranes, hydraulic cranes, tower cranes, monorail systems, derricks and others The PCI Recommended Practice for Erection of Precast Concrete provides more information on the uses of each.

116 Surveying and Layout Before products are shipped to the jobsite, a field check of the project is recommended to ensure that prior construction is suitable to accept the precast units This check should include location, line and grade of bearing surfaces, notches, blockouts, anchor bolts, cast-in hardware, and dimensional deviations Site conditions such as access ramps, overhead electrical lines, truck access, etc., should also be checked

117 Surveying and Layout Any discrepancies between actual conditions and those shown on drawings should be addressed before erection is started Surveys should be required before, during and after erection: –Before, so that the starting point is clearly established and any potential difficulties with the support structure are determined early. –During, to maintain alignment. –After, to ensure that the products have been erected within tolerances.

118 Loads on Structure The publication Design Loads on Structures During Construction (SEI/ASCE 37-02) provides minimum design loads, including wind, earthquake and construction loads and load combinations for partially completed structures and structures used during construction In addition to working stress or strength design using loads from the above publication, the designer must consider the effect of temporary loading on stability and bracing design

119 Temporary Loading Examples Columns with eccentric loads from other framing members produce sidesway which means the columns lean out of plumb A similar condition can exist when cladding panels are erected on one side of a multistory structure

120 Temporary Loading Examples Unbalanced loads due to partially complete erection may result in beam rotation The erection drawings should address these Conditions

121 Temporary Loading Examples Some solutions are: –Install wood wedges between flange of tee and top of beam –Use connection to columns that prevent rotation –Erect tees on both sides of beam –Prop tees to level below

122 Temporary Loading Examples Rotations and deflections of framing members may be caused by cladding panels. This may result in alignment problems and require connections that allow for alignment adjustment after all panels are erected

123 Temporary Loading Examples If construction equipment such as concrete buggies, man-lifts, etc., are to be used, information such as wheel loads and spacing should be conveyed to the designer of the precast members and the designer of the erection bracing

124 Factors of Safety Suggested safety factors are shown Bracing inserts cast into precast members3 Reusable hardware5 Lifting inserts4

125 Bracing Equipment and Materials For most one-story and two-story high components that require bracing, steel pipe braces similar to those shown are used

126 Bracing Equipment and Materials Proper anchoring of the braces to the precast members and deadmen must be considered When the pipe braces are in tension, there may be significant shear and tension loads applied to the deadmen Properly designed deadmen are a requirement for safe bracing Cable guys with turnbuckles are normally used for taller structures

127 Bracing Equipment and Materials Since wire rope used in cable guys can resist only tension, they are usually used in combination with other cable guys in an opposite direction Compression struts, which may be the precast concrete components, are needed to complete truss action of the bracing system A number of wire rope types are available Note that capacity of these systems is often governed by the turnbuckle capacity

128 General Considerations Careful planning of the erection sequence is important This plan is usually developed by a coordinated effort involving the general contractor, precast erector, precaster production and shipping departments and a structural engineer A properly planned erection sequence can reduce bracing requirements For example, with wall panel systems a corner can first be erected so that immediate stability can be achieved

129 General Considerations Similar considerations for shear wall structures can also reduce bracing requirements All parties should be made aware of the necessity of closely following erection with the welded diaphragm connections This includes the diaphragm to shear wall connections

130 General Considerations In order for precast erection to flow smoothly: –The site access and preparation must be ready –The to-be-erected products must be ready –Precast shipping must be planned –The erection equipment must be ready –Bracing equipment and deadmen must be ready

131 Questions?


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