Concrete Construction II Site-cast and precast framing systems

1. Concrete Construction IISite-cast and precast framing systems Chapter 20

2. Elevated concrete floor systems • Beam supported • Beamless

3. Beam-supported floor systems

4. Load-transfer: one-way and two-way concrete slabs

5. One-way and two way slabs in same floor, view from below

6. Beam and girders with slab floors increase spans, decrease economy

7. One-way and two-way slabs • Slabs become thicker as spans increase • Uneconomical when slab exceeds 8 inches • Dimensions maximized • One-way slab, 16 fts wide • Two-way, square slab 24 ft. wide • Beams and girders can be added to slabs to increase spans & reduce slab thickness • Beam and girder sizes vary • Increase cost and complexity of formwork • Not commonly used due to lack of economy

8. Band beam floor

9. Modular formwork creates economy • Constructing formwork with flat deck as base creates economy • Simple, repetitive formwork • Easier working surface • Faster construction time • One-way and two way floors possible

10. One-way joist floor viewed from below

11. Standard module formwork with beam reinforcement laid

12. Standard module pan sizes

13. Tapered pans, widened joist ends

14. Wide-module pans

15. Wide module floors, thicker slabs • Used when thicker slab is appropriate • Increase fire rating (1 hour or 2 hour floors) • Slabs accommodate electrical conduit

16. Thick slab in one-way joist floor

17. Wide module pans

18. Two-way joist floor (waffle slab)

19. Beamless waffle slab

20. Glass fiber reinforced dome for waffle slab

21. Flat plate slab: light occupancies, lower ceiling height, small, square bays

22. Flat slab has round column and drop panels

23. Post-tensioned elevated concrete floors • Reduces slab and beam dimensions • Reduces dead loads • Reduced seismic load • Reduced foundation requirements • Lower floor to floor height • Reduced cost of cladding

24. Draped cables in reinforcement cage for prestressed beams. Cage will be lowered into beam form

25. Beam cage has been lowered and floor reinforcement and bands of prestressing cables placed.

26. Prestressed concrete frame. Ends of tensioned cables have been flame cut on lower floors, still hanging above

27. Most economical site-cast floor systems • Flat plate • One-way joist floor

28. Precast concrete • Prefabricated in plant and transported to site for assembly • Good for climates where site-cast concrete difficult to use • Cost of formwork and shoring reduced • Permanent formwork • Cast at ground level • Greater quality control • Improved strength • Improved finishes • Disadvantages • Cost of transportation • Heavy hoisting equipment • Skilled workforce • Limitations on sculptability • Straight standard profiles

29. Precast types • Architectural precast (not discussed here) • Concrete elements used as non-structural elements • Typically cladding • Structural precast • All elements of a structural frame • Mixed precast construction • Some elements of structure precast • Most common use of structural precast

30. Precast structural floor and roof elements • Hollow-core slabs • Solid planks • Double-tee units • Inverted-tee beams

31. Section through hollow core slab

32. Precast concrete hollow core slabs rest on site-cast concrete columns and beams

33. Precast concrete hollow core slabs rest on steel beams

34. Connecting hollow core slabs to concrete beams

35. Connecting hollow core slabs to steel beams

36. Section through double-tee floor

37. Double-tee units on precast inverted T-beams

38. Double-tee meet at column and beam

39. Double-tee meet at column and beam

40. Casting double tee units

41. Inverted T-beam

42. Fire resistance of concrete members • Thickness of member • Type of course aggregate • Covering of reinforcement and prestressing tendons

43. Building separation joint: Single column, site cast

44. Slip joint assembly

45. Slip joint assembly

46. Slip joint assembly

47.  = FP Coefficient of Friction