PCI 6 th Edition

1. PCI 6th 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 4.2.2.2 of the the PCI Handbook • ACI 318-02 does not restrict stresses on non-prestressed members, but does specify minimum reinforcement spacing, as discussed in Section 4.2.2.1. (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. 4.2.2.2), 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 318-02, 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 318-02, 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. 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 Threaded Inserts

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