2 Presentation Outline Planning Discussion Stripping Process Design and AnalysisPrestress / Post Tension EffectsHandling DevicesStripping Stress ExamplesStorage DiscussionTransportation DiscussionErection Discussion
3 IntroductionThe loads and forces on precast and prestressed concrete members during production, transportation or erection will frequently require a separate analysisConcrete strengths are lowerSupport 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 handlingTransportation 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 reachStorage space, truck turning radius, and other site restrictions
6 Planning and SetupOnce 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 formAdequate draft also serves to reduce trapped air bubbles.
7 Planning and SetupLifting points must be located to keep member stresses within limits and to ensure proper alignment of the piece as it is being liftedMembers 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 SetupWhen 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 SetupMembers 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: GeneralOrientation of members during storage, shipping and final in-place position is critical in determining stripping requirementsThey can be horizontal, vertical or some angle in betweenThe 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: GeneralIt 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: GeneralPanels that are stripped by rotating about one edge with lifting devices at the opposite edge will develop moments as shown
14 Stripping: GeneralWhen panels are stripped this way, care should be taken to prevent spalling of the edge along which the rotation occursA compressible material or sand bed will help protect this edge
15 Stripping: GeneralMembers that are stripped flat from the mold will develop the moments shown
16 Stripping: GeneralIn some plants, tilt tables or turning rigs are used to reduce stripping stresses
17 Stripping: GeneralSince 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 crackingTensile stress on one face to be less than that which would cause cracking, with controlled cracking permitted on the unexposed faceControlled cracking permitted on both faces
18 Stripping: GeneralIf 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 angleAny 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 blocksNote that the spreader beam must be sufficiently stiffer than the concrete panel to limit panel deflections and crackingLifting hook locations, hook heights, and sling lengths are critical to ensure even lifting of the memberFor analysis, the panel acts as a continuous beam over multiple supports
25 Stripping DesignTo 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 DesignPCI provides a table of typical values
27 Factor of SafetyWhen 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 HandbookACI 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 toIn the case of stripping stresses, f′ci should be substituted for f′cWhether 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 psiThe 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 bowIt 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 RecomendationIn order to minimize the possibility of splitting cracks in thin pretensioned members, the strand diameter should not exceed that shown in the table belowAdditional 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 corrosionStraight 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 DevicesSince lifting devices are subject to dynamic loads, ductility of the material is a requirementDeformed reinforcing bars should not be used as the deformations result in stress concentrations from the shackle pinAlso, 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 providedThe diameter must be such that localized failure will not occur by bearing on the shackle pin
37 Aircraft Cable LoopsFor smaller precast members, aircraft cable can be used for stripping and erection purposesAircraft cable comes in several sizes with different capacitiesThe flexible cable is easier to handle and will not leave rust stains on precast concrete
38 Aircraft Cable LoopsFor some small precast members such as coping, the flexible loops can be cast in ends of members and tucked back in the joints after erectionAircraft 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 achieveUser 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 loopsThe 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 failurePrecast producers’ tests and/or experience offer the best guidelines for the load capacity to useA 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 usedDo 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 kipsThe 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 sameThin 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 InsertsThreaded inserts can have NC (National Course) or coil threadsAnchorage is provided by loop, strut or reinforcing barInserts 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 InsertsEmbedment 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 insertWhen properly designed for both insert and concrete capacities, threaded inserts have many advantagesHowever, correct usage is sometimes difficult to inspect during handling operations
48 Threaded InsertsIn order to ensure that an embedded insert acts primarily in tension, a swivel plate as indicated in should be usedIt is extremely important that sufficient threads be engaged to develop the strength of the bolt
49 Threaded InsertsFor 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 DevicesA variety of castings or stock steel devices, machined to accept specialized lifting assemblies are used in the precast industry
51 Proprietary DevicesThese 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 deviceLonger devices are used for edge lifting or deep precast concrete membersShallow devices are available for thin precast concrete members.
52 Proprietary DevicesThe 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 DevicesThese units have a rated capacity as high as 22 tons, with reductions for thin panels or close edge distancesSupplemental reinforcement may be required to achieve these valuesShallow units usually have a spread foot or base to increase pullout capacity
54 Proprietary DevicesReinforcing bars are required in two directions over the base to fully develop the lifting unit, as shown in Figure belowThese inserts arerated up to 8 tons
55 Proprietary DevicesSome lifting eyes do not swivel, so rotation may be a concernIn all cases manufacturer recommendations should be rigorously followed when using any of these devices
56 Wall Panel ExampleThis example and others in Chapter 5 illustrate the use of many of the recommendations in this chapterThey are intended to be illustrative and general onlyEach 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 slideSection properties (nominal dimensions are used for design):Solid panel Panel with openingsA = 960 in A = 480 in2Sb = St = 1280 in Sb = St = 640 in3Ix = 5120 in Ix = in4Unit 150 pcf = 100 psf = ksfTotal weight = 35.2 kips (solid panel)= 29.2 kips (panel w/ openings)
59 Wall Panel Example Stripping method: Handling multipliers: Inside crane height prevents panel from being turned on edge directly in mold, therefore, strip flatHandling 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 stressStep 3 – Determine stress from mechanic approachStep 4 – Check panel with openingStep 5 – Check rolling block solutionStep 6 – Check transverse bendingStep 7 – Check secondary effects
62 Step 1 – Determine Section Properties Solid panel dimensionsa = 10 ft, b = 35.2 ft, a/2 = 5 ft = 60 in.S for resisting section (half of panel width)
69 Step 5 – Rolling BlocksIf using a rolling block for handling as shown below, the panel cannot be analyzed with the previous methodEach leg of continuous cable over a rolling block must carries equal load
71 Step 6 – Transverse Bending Consider lower portion of panel with openingsNote 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 angleUsing recessed proprietary lifting anchore = 3.5 in
74 Step 7 – Secondary Effects Resisting SectionTherefore 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 stressStep 3 – Calculate the number of strands
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) = ksiNo. of strands = 3.8Add four strands to panel (two on each side of opening)
81 StorageWherever possible, a member should be stored on points of support located at or near those used for stripping and handlingWhere points other than those used for stripping or handling are used for storage, the storage condition must be checked
82 StorageIf 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 surfacescreepstorage conditionsWarpage can only be minimized by providingWhere feasible, the member should be oriented in the yard so that the sun does not overheat one sidePCI handbook provides detailed information on thermal bowing in chapter 4
84 StorageBy superposition, the total instantaneous deflection, ymax , at the maximum point can be estimated by:Ic , Ib = moment of inertia of uncracked section in the respective directions for 1 in. width of panel
85 StorageThis instantaneous deflection should be modified by a factor to account for the time dependent effects of creep and shrinkageACI suggests the total deformation yt, at any time can be estimated as:
86 Storageλ = amplification due to creep and shrinkage as a function of ′ρ (reinforcement ratio for non-prestressed compression reinforcement, As/b·t)
87 TransportationThe method used for transport can affect the structural design because of size and weight limitations and the dynamicExcept for long prestressed deck members, most products are transported on either flatbed or low-boy trailersTrailers deform during haulingSize and weight limitations vary from one state to stateLoads are further restricted on secondary roadsThe common payload for standard trailers without special permits is 20 tons.. Thus, support at more than two points can be achieved only after considerable modification of the trailer, and even then results may be doubtful
88 TransportationLow-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 TransportationErection is simplified when members are transported in the same orientation they will have in the structureFor example, single-story wall panels can be transported on A-frames with the panels uprightA-frames also provide good lateral support and the desired two points of vertical support
90 TransportationLonger units can be transported on their sides to take advantage of the increased stiffness compared with flat shipment
91 TransportationIn all cases, the panel support locations should be consistent with the panel designPanels with large openings sometimes require strongbacks, braces or ties to keep stresses within the design values
92 TransportationFor 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 yb = distance from the bending axis to the bottom fiberyt = distance from the bending axis to the top fiber
94 Transportation For two ends cantilevered… Where yb = distance from the bending axis to the bottom fiberyt = distance from the bending axis to the top fiber
95 ErectionPrecast concrete members frequently must be reoriented from the position used to transport to its final construction positionThe analysis for this “tripping” (rotating) operation is similar to that used during other handling stagesIn chapter 5 in the PCI handbook, maximum moments for several commonly used tripping techniques are illustrated
97 ErectionWhen 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 rotatedTo ensure that this is avoided, the stability condition shown must be met:
98 ErectionThe capacities of lifting devices must be checked for the forces imposed during the tripping operation, since the directions varyWhen 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 examplesProblem:Determine appropriate procedures for erecting the wall panels with openings, panel will be shipped flat
100 Erecting Wall Panels Example AssumptionsLimit 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.
102 Erecting Wall Panels Example In Horizontal PositionTherefore, 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 usedHowever, this rigging may become unstable at some point during tripping, i.e., continued rotation without tension in Line ATherefore, 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
106 Erection Bracing Introduction This section deals with the temporary bracing which may be necessary to maintain structural stability of a precast structure during constructionWhen 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 erectionSequence of erection must be established early, and the effects accounted for in the bracing analysis and the preparation of shop drawingsThe 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 erectorThe manufacturer is responsible only for supplying the product, F.O.B. plant or jobsiteErection 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 EngineerErection drawings define the procedureon how to assemble the components into the final structureThe 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 sequencesIt is desirable that this design be directed or approved by a Professional EngineerErection drawings define the procedureon 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 connectionsWhen 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 EquipmentThe type of jobsite handling equipment selected may influence the erection sequence, and hence affect the temporary bracing requirementsSeveral types of erection equipment are available, including truck-mounted and crawler mobile cranes, hydraulic cranes, tower cranes, monorail systems, derricks and othersThe PCI Recommended Practice for Erection of Precast Concrete provides more information on the uses of each.
116 Surveying and LayoutBefore 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 unitsThis check should include location, line and grade of bearing surfaces, notches, blockouts, anchor bolts, cast-in hardware, and dimensional deviationsSite conditions such as access ramps, overhead electrical lines, truck access, etc., should also be checked
117 Surveying and LayoutAny discrepancies between actual conditions and those shown on drawings should be addressed before erection is startedSurveys 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 StructureThe 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 constructionIn 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 plumbA 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 rotationThe erection drawings should address these Conditions
121 Temporary Loading Examples Some solutions are:Install wood wedges between flange of tee and top of beamUse connection to columns that prevent rotationErect tees on both sides of beamProp 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 members3Reusable hardware5Lifting 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 consideredWhen the pipe braces are in tension, there may be significant shear and tension loads applied to the deadmenProperly designed deadmen are a requirement for safe bracingCable 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 directionCompression struts, which may be the precast concrete components, are needed to complete truss action of the bracing systemA number of wire rope types are availableNote that capacity of these systems is often governed by the turnbuckle capacity
128 General Considerations Careful planning of the erection sequence is importantThis plan is usually developed by a coordinated effort involving the general contractor, precast erector, precaster production and shipping departments and a structural engineerA properly planned erection sequence can reduce bracing requirementsFor 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 requirementsAll parties should be made aware of the necessity of closely following erection with the welded diaphragm connectionsThis 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 readyThe to-be-erected products must be readyPrecast shipping must be plannedThe erection equipment must be readyBracing equipment and deadmen must be ready
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