Skylineplazacollapse.pptx

collapse

1. Construction Failure Case study : Skyline Plaza Collapse Center for Environmental Planning & Technology Prepared By : Janak Shah (PT401914) Guided By : Prof. R. J. Shah

2. Contents Literature Review Detailed Case Study Summary Lessons Learnt 2 JANAK SHAH (PT-401914) Skyline Plaza Failure

3. Literature Review Formwork Shoring Reshoring Flat Plate Punching Shear Failure Progressive collapse 3 JANAK SHAH (PT-401914) Skyline Plaza Failure

4. Formwork Formwork is the temporary structure which moulds concrete into the desired shape, and holds it in the correct position until it is able to support the loads imposed upon it . Formwork and its supports ( falsework ) is a structural system and must be designed and built accordingly . The actions (loads) on it may be temporary but they can be extremely large. It is a temporary structure that supports its own weight and that of freshly placed concrete as well as construction live loads including material, equipment and workmen. 4 JANAK SHAH (PT-401914) Skyline Plaza Failure

5. Requirements for formwork Property Purpose Strength Carry imposed loads. Stiffness Maintain specified shape and avoid distortion of concrete elements. Accuracy Ensure shape and size of concrete elements. Ensure specified cover to reinforcement. Watertightness Avoid grout loss and subsequent honeycombing of the concrete. Robustness Enable re-use. Ease of stripping Avoid damage to concrete surfaces. Standardisation Promote economy. Safety Ensure a safe working environment. 5 JANAK SHAH (PT-401914) Skyline Plaza Failure

6. Shores Mudsill Wedges Bracing Joist Sheathing Stringer Fig 1 Typical elevated flat -slab formwork elements. (From Hurd , M.K., Formwork for Concrete, 7th ed., SP-4, American Concrete Institute, Farmington Hills, MI, 2005.) 6 JANAK SHAH (PT-401914) Skyline Plaza Failure

7. Fig 3 Beam form and supports Fig 2 Column forms 7 JANAK SHAH (PT-401914) Skyline Plaza Failure

8. Shoring Vertical or inclined support members designed to carry the weight of the formwork, concrete, and construction loads above. 8 JANAK SHAH (PT-401914) Skyline Plaza Failure

9. Shoring Fig 4 9 JANAK SHAH (PT-401914) Skyline Plaza Failure

10. Reshoring Reshoring systems involve the removal of a section of the formwork and support structure . With proper control and good supervision, these systems can give acceptable results. Props must be replaced in the original pattern and not overtightened to avoid causing undesirable stresses in the concrete. 10 JANAK SHAH (PT-401914) Skyline Plaza Failure

11. Reshoring Fig 5 PHASE 1: PLACEMENT OF CONCRETE Fig 6 PHASE 2: REMOVAL OF SHORES 11 JANAK SHAH (PT-401914) Skyline Plaza Failure

12. Reshoring Fig 7 PHASE 3: REMOVAL OF LOWER LEVEL OF SHORES Fig 8 PHASE 4: INSTALLATION OF RESHORES 12 JANAK SHAH (PT-401914) Skyline Plaza Failure

13. Flat plates Flat plates are of uniform thickness throughout without drop panels or column capitals. Flatplate floors are widely used in apartments because the underside of the slab is flat and hence can be used as the ceiling of the room below. 13 JANAK SHAH (PT-401914) Skyline Plaza Failure

14. Of equal importance, the forming for a flat plate is generally cheaper than that for flat slabs with drop panels . Fig 9 14 JANAK SHAH (PT-401914) Skyline Plaza Failure

15. Fig 10 Reinforced concrete flat plate system, Eugene, USA 15 JANAK SHAH (PT-401914) Skyline Plaza Failure

16. Fig 11 A reinforced concrete building with flat plate system under construction, Kenya 16 JANAK SHAH (PT-401914) Skyline Plaza Failure

17. Punching Shear Punching shear failure is caused by the v ertical shear and unbalanced moment borne by the slab-column connection , which makes the flat-slab connections a weak link in the whole flat-slab structure, and then leading to serious damage or even collapse. 17 JANAK SHAH (PT-401914) Skyline Plaza Failure

18. In a slab system with a concentrated load or at a slab column connection, the loaded area is not actually pushed through the slab as shown in Fig 12. Punching shear failures arise from the formation of diagonal tension cracks around the loaded area, which result in a conical failure surface as illustrated in Fig 13. Fig 12 Misconception of Punching Shear Failure Fig 13Correct Representation of Punching Shear Failure 18 JANAK SHAH (PT-401914) Skyline Plaza Failure

19. Laboratory Studies Fig 14 Exterior connection Fig 15 Interior connection Photo: Hwang and Moehle , ACI SJ, March-April 2000 19 JANAK SHAH (PT-401914) Skyline Plaza Failure

20. Fig 16 Building Collapsed due to punching failure 20 JANAK SHAH (PT-401914) Skyline Plaza Failure

21. Progressive collapse ‘Progressive collapse denotes an extensive structural failure initiated by local structural damage , or a chain reaction of failures following damage to a relatively small portion of a structure. This can be also characterized by the loss of load-carrying capacity of a relatively small portion of a structure due to an abnormal load which, in turn, triggers a cascade of failures affecting a major portion of the structure.’ 21 JANAK SHAH (PT-401914) Skyline Plaza Failure

22. Chain of Events Progressive collapse occurs when an initiating event leads to damage to part of the structure by which this part looses its load bearing capacity. As a consequence the loading pattern of the structure is changed leading to an overloading of other structural elements which are thereby also damaged. This process continues until the whole structure collapses or a greater part of it. 22 JANAK SHAH (PT-401914) Skyline Plaza Failure

23. Historical Perspective to Progressive Collapse Fig 17 A time line of cases associated with progressive collapse. 23 JANAK SHAH (PT-401914) Skyline Plaza Failure

24. Progressive collapse , Skyline Plaza at Bailey's Crossroads, National Archives Fig 18 24 JANAK SHAH (PT-401914) Skyline Plaza Failure

25. Skyline Plaza Collapse in Fairfax County, Virginia Date of Collapse : March 2, 1973 25 JANAK SHAH (PT-401914) Skyline Plaza Failure

26. Background The Skyline center complex located near Bailey’s Crossroad, Fairfax County, Virginia. It was planned to contain Eight apartment buildings (namely A-1 to A-8), Six office buildings, A hotel, and A shopping center. Two apartment buildings which had been completed are shown in figure 19 . A pair of apartment buildings and adjoining parking and lobby structure were under construction 26 JANAK SHAH (PT-401914) Skyline Plaza Failure

27. Fig 19 Appearance of completed apartment 27 JANAK SHAH (PT-401914) Skyline Plaza Failure

28. Description of the Structure The three structures under construction were designed under the Fairfax County Building Code Ordinance which incorporates by reference the provision of ACI 318-63 . Building A-4 Reinforced concrete flat plat construction Supported on 4-ft thick foundation mat Completed structure was to have 26 stories of apartments, a penthouse and a four story basement(B-1, B-2, B-3, B-4) 28 JANAK SHAH (PT-401914) Skyline Plaza Failure

29. The typical story height from the from the first story up was 9 ft 0 in from top of slab to top of slab. Floor slab were 8 inches thick. Total basement height of about 40 ft as measured from top of the foundation mat(level B-4) to the top of the first floor slab. The plan view without parking garage plan shown in fig. is that of 22 nd storey and is typical for the 1 st through 26 th stories in the column layout. There are eight shear walls in the structure. These are designated as A through H shown in fig 20. A ½ inch expansion joint separated the building into two parts at grid line H. 29 JANAK SHAH (PT-401914) Skyline Plaza Failure

30. Fig 20 30 JANAK SHAH (PT-401914) Skyline Plaza Failure

31. N Fig 21 Aerial View of Construction at 11 am, March 2, 1973 (Before Collapse) 31 JANAK SHAH (PT-401914) Skyline Plaza Failure

32. Design Strength of Concrete Concrete Design Strength (psi ) Floor Base - 7 7 - 17 17 - Top Columns 5000 (34.5MPa) 4000 (27.6MPa) 3000 (20.7 MPa) Slabs 3000 (20.7 MPa) 3000 (20.7 MPa) 3000 (20.7 MPa) Table 1 32 JANAK SHAH (PT-401914) Skyline Plaza Failure

33. The Collapse The collapse occurred at around 2:30 pm on March 2, 1973 , while under construction while the 24 th floor slab was being casted. Shortly after lunch, some workers observed slab deflections of approximately 150–600 mm (6 in. to 2 ft) for both the 23 rd floor slab and the freshly placed 24th floor slab . The freshly placed section of the 24 th floor slab then fell onto the 23 rd floor slab, starting a collapse that continued all the way to the foundation. The building’s collapse removed the edge support from the parking garage, and falling debris triggered the failure of the garage. 33 JANAK SHAH (PT-401914) Skyline Plaza Failure

34. General appearance of building A-4 as viewed from the southeast. Fig 22 34 JANAK SHAH (PT-401914) Skyline Plaza Failure

35. A partial view of the north face of building A-4 is shown in figure. A number of columns and floors are identified. The collapse extended between shear wall H and column 33 on the south face a distance of about 65 ft (refer to building plan). On the north face the collapse extended between columns 12 and 17, a distance of about 104 ft. Fig 23 35 JANAK SHAH (PT-401914) Skyline Plaza Failure

36. Closeup view of the top of the east end of the failure zone, looking south. Fig 24 36 JANAK SHAH (PT-401914) Skyline Plaza Failure

37. Closeup view near the midheight of the east end of the failure zone, looking south Fig 25 37 JANAK SHAH (PT-401914) Skyline Plaza Failure

38. General view of building A-4 as seen looking southwest Fig 26 38 JANAK SHAH (PT-401914) Skyline Plaza Failure

39. The Collapse Once the section of the apartment building collapsed, the failure propagated horizontally through the attached parking garage. It resulted in the death of 14 construction workers and the injury to 34 others. Of the workers killed, ten were in the tower and four in the garage. 39 JANAK SHAH (PT-401914) Skyline Plaza Failure

40. Investigation of failure The collapse of the Skyline Plaza apartment building A-4 has been studied by using information contained in case records of the Occupational Safety and Health administration (OSHA) , U.S. Department of Labor and obtained from on-site inspections by investigators from the National Bureau of Standards(NBS) 40 JANAK SHAH (PT-401914) Skyline Plaza Failure

41. Formwork Fig 27 General appearance of building A-4 as viewed from the southeast. 41 JANAK SHAH (PT-401914) Skyline Plaza Failure

42. Formwork Figure 27 shows the locations of the formwork on March 2, shortly after the collapse. Full formwork was in place on the 24 th and 23 rd stories in sections 1 and 2. Some formwork may be seen on the 22 nd story in sections 2 and 3. Full formwork was also in place under section 4 on the 22 nd and 23 rd stories. No reshoring can be seen under section 4 below the 22 nd floor in figure 27 . 42 JANAK SHAH (PT-401914) Skyline Plaza Failure

43. Formwork, Shoring and Reshoring Formwork sheets submitted to Fairfax County contained no mention of lateral bracing for the form system . However, a limited amount of bracing may be seen in figure 28. OSHA, regulations (ANSI-A10.9, Sections 6.3.2 and 8.1.5) require a lateral bracing system capable of resisting a lateral force of at least 2 percent of the dead load of the slab. Stringers were erected with a shore under each end wherein stringer were inserted in a basket like socket connected to the shore. Diagonal lateral bracing was installed after every fourth shore at about 16 ft intervals. Joists were placed over stringers at 16 inch intervals over which plywood sheathing was provided. Stringers and Joists were usually 16 ft long although some were as short as 6 to 8 ft. 43 JANAK SHAH (PT-401914) Skyline Plaza Failure

44. The formwork on the 24th story is shown in figure. Several columns have been identified for reference. Fig 28 44 JANAK SHAH (PT-401914) Skyline Plaza Failure

45. Fig 29 A schematic of formwork is shown in figure as derived from OSHA case records 45 JANAK SHAH (PT-401914) Skyline Plaza Failure

46. Formwork, Shoring and Reshoring During the NBS site inspection. a considerable portion of the lumber use for the remaining forms in the 24th story was found to be in poor condition or out of plumb . The photograph illustrates a shore which is out of plumb and has a vertical crack near the top. OSHA regulations (ANSI-A10.9, sections 8.1.24 and 8.1.25) require correction of this condition prior to placing concrete (which was not yet placed on these forms). Fig 30 46 JANAK SHAH (PT-401914) Skyline Plaza Failure

47. One concrete finisher working near the stair well (around columns 65 and 66) indicated that a large deflection was seen in the middle of section 3. However. the middle of section 3 is about midway between columns 67 and 85. 47 JANAK SHAH (PT-401914) Skyline Plaza Failure

48. The formwork on the 23rd story is shown in figure 2.20. These forms supported the 24th floor which had been cast on February 28 (section 1) and March 1(section 2). Note the number of shores out of plumb. Fig 31 48 JANAK SHAH (PT-401914) Skyline Plaza Failure

49. Fig 32 The method of attaching the brace to the floor is shown in figure 49 JANAK SHAH (PT-401914) Skyline Plaza Failure

50. As in the case of the 24 th story. the 23 rd story lumber which was apparently damaged was reused. As in the case of the 24 th story. OSHA regulations (ANSI-A10.9, sections 8.1.24 and 8.1.25) required correction of these conditions prior to placing concrete. ` Fig 33 50 JANAK SHAH (PT-401914) Skyline Plaza Failure

51. Reshoring in section 2 of the 22nd story as of March 6 is shown in figure. There is conflict in their statements, a number of workmen that escaped the building by way of the stairs in section 4 have indicated that the 22nd story was either partially or entirely stripped in section 3. Fig 34 51 JANAK SHAH (PT-401914) Skyline Plaza Failure

52. Photographs taken right after the collapse showed that while full shoring remained on the 23 rd and 24 th stories, it had nearly all been removed from the 22 nd story in sections 1 and 2 but remained in section 4. According to OSHA regulations, forms are required to be in place for a minimum of 10 days with temperatures greater than 50 degrees F for spans longer than 20'-0“ (ANSI A10.9, Section 6.4.8). The engineer’s structural drawings required two full stories of shoring and one story of reshoring while a concrete slab was being cast. 52 JANAK SHAH (PT-401914) Skyline Plaza Failure

53. Fig 35 Estimated location of forms and reshores at the time of the collapse. 53 JANAK SHAH (PT-401914) Skyline Plaza Failure

54. The statements of one worker gave the investigators a good idea of the state of the forms at the time of the collapse. “One workman indicated that, at the time of the incident, he was placing reshores in section 3 of the 21st story and that some reshores were present when he started working. Prior to the incident, all the reshores fell out (except those in the balcony areas). This is consistent with what could occur if the forms had been removed in the story above.” If the shoring had been removed in the 22nd floor, the 22nd floor slab would have been relieved of its previous loads. With less loading, the deflection would have decreased in the slab causing the reshores on the 21st floor to fall out. 54 JANAK SHAH (PT-401914) Skyline Plaza Failure

55. Concrete Strength Results of standard ASTM C39 concrete cylinder tests showed adequate strength at 7 and 28 days. However, these were laboratory tests, with cylinders stored at a controlled temperature. Particularly in cold weather, laboratory tests do not correspond to the strength of the concrete in place in a structure. OSHA regulations required field cured cylinders, stored at the same temperature as the structure, to verify the strength of the structure before removing shores and formwork 55 JANAK SHAH (PT-401914) Skyline Plaza Failure

56. Structural investigation of Failure The evaluation of the capacity of structural elements was based on the Provisions of the ACI 318-71 Code and on procedures from published analytical and experimental research. The three dimensional elastic analysis was performed using the finite element analysis program known by the acronym SAP. 56 JANAK SHAH (PT-401914) Skyline Plaza Failure

57. Each of the three finite element analyses had a particular purpose: Case 1: Some of the employee statements indicated that the 22nd story forms were entirely removed at the time of collapse. (Complete removal of shoring) Case 2: The concrete to have attained its full 28-day design strength of 3000 psi. Case 3: Some of the employee statements indicated that removal of shoring under the 23rd-floor slab in the central corridor area of the building was in progress on the day of the collapse .(Partial removal of shoring) 57 JANAK SHAH (PT-401914) Skyline Plaza Failure

58. Investigation to flexure failure: Upon Examination of all the probable conditions prior to the collapse, It is concluded that the initial mode of failure of either the 22 nd or the 23 rd floor slab was not flexural. 58 JANAK SHAH (PT-401914) Skyline Plaza Failure

59. Investigation to Shear Failure Fig 36 Summary of shear stresses in the 23rd story floor slab. 59 JANAK SHAH (PT-401914) Skyline Plaza Failure

60. Investigation to Shear Failure Upon examination of all the probable conditions prior to the collapse, it is concluded that the initial shear mode of failure of the 23 rd story floor slab resulting from partial or complete removal of shoring prior to the incident was a major contributing factor to the collapse. 60 JANAK SHAH (PT-401914) Skyline Plaza Failure

61. Probable mode of failure The most likely mode of collapse has been determined to be a shear failure around columns 67, 68, 83, or 84. The premature removal of forms supporting the 23rd story slab when the concrete of that slab had a relatively low strength produced shear stresses that were in excess of the concrete capacity at the time of the incident. The collapse is believed to have started with shear around columns 67,68, 83, or 84. The loss of support from anyone of these columns would lead to overstressing of the remaining columns. The accumulation and impact of debris from the 23rd and 24th floor slab would have overloaded the 22nd floor slab and induced progressive collapse of successive floors to the ground. 61 JANAK SHAH (PT-401914) Skyline Plaza Failure

62. Contribution of Crane There is no indication that the crane was a contributing factor to the beginning of collapse in building A-4. No witness statements indicated that the crane moved prior to or during the initial sagging of the 23rd and 24 th floors. The crane supports on the 14th and 16th floors are far enough away from the initiation of failure to preclude the crane as a cause. However, the crane probably became a driving force in the collapse once its support was lost. 62 JANAK SHAH (PT-401914) Skyline Plaza Failure

63. Summary An analysis of the 23rd-floor slab indicates that its most likely mode of failure was in shear around one or more columns in section 3 of the floor slab. The strength of the 23rd-floor slab on the day of collapse has been found to be of a magnitude that complete or partial removal of shoring underneath the slab would have produced a shear failure in the slab. The weight of debris resulted in failure in the slabs below and carried through the height of the building. 63 JANAK SHAH (PT-401914) Skyline Plaza Failure

64. Non-Compliance with OSHA Regulation Shoring in section 3 of 22 nd story: The architect‘s specification required "In all cases two floors shall be fully shored."The removal of the 23rd story forms left only one story of formwork in place under the recently poured 24th floor. Premature removal of 22 nd story form: The forms removed on the 22 nd story were in an area with spans exceeding 20 ft and therefore, should have been in place for 10 days of temperatures exceeding 50°F. 64 JANAK SHAH (PT-401914) Skyline Plaza Failure

65. Non-Compliance with OSHA Regulation Lateral bracing: Minimum values of 100 pounds per foot of floor edge or 2 percent of the total dead load of the floor, whichever is greater, is required. No evidence has been found which indicates that lateral load was considered in the design of forms. The lateral bracing provided would not provide this resistance. Shoring out of plumb Damaged Shoring Inspection 65 JANAK SHAH (PT-401914) Skyline Plaza Failure

66. Lessons Learnt The contractor should be responsible for preparing formwork drawings, including shores and reshores ; The contractor should prepare a detailed concrete testing plan for stripping forms, including cylinder tests; Inspectors and other quality control agencies should verify that the contractor performs the previous two items; The Engineer of Record (EOR) should make sure he/she provides the contractor with all necessary design load data and other unique project information; Uncontrolled acceleration of formwork removal may cause a total or partial collapse; and Continuous top and bottom slab reinforcement is necessary around the columns. Continuous reinforcement provides overall ductility. 66 JANAK SHAH (PT-401914) Skyline Plaza Failure

67. References Edgar V. L. and S. George Fattal , “Investigation of the Skyline Plaza Collapse in Fairfax County, Virginia”, U.S.Department of Commerce, NBS, February 1977. Jeffrey S., Norbert D. and Paul A. Bosela , “Another Look at the Collapse of Skyline Plaza at Bailey’s Crossroads, Virginia”, Cleveland State University, June 2013. N.subramanian , “Alternative punching Shear reinforcement for RC flat slabs”, The Indian concrete Journal, January 2014. K. Micallef , J. Sagaseta , M. Fernández Ruiz, A. Muttoni , “Assessing punching shear failure in reinforced concrete flat slabs subjected to localised impact loading ”, International Journal of Impact Engineering,27- February 2014. Devin K. Harris , “Characterization of punching shear capacity of thin UHPC PLATES”, December 2004. 67 JANAK SHAH (PT-401914) Skyline Plaza Failure

68. References M. Smith, “Progressive collapse assessment , non-linear behaviour of concrete structures in damaged state”, January 26th, 2007. Alkarani , Ravindra . R, “Evaluation of punching shear in flat slabs”, International Journal of Research in Engineering and Technology, November 2013. Prof. Kamran M. , “Temporary Structures : Shoring, scaffolding, and underpinning”. David W. Johnston, P.E., Ph.D. , “Design and Construction of Concrete Formwork”, Concrete Construction Engineering Handbook. “Guide to Concrete Construction”, Cement & Concrete Association of New Zealand (CCANZ). Basher Alamin , “Analysis of Construction Loads on Concrete Formwork”, August 1999. 68 JANAK SHAH (PT-401914) Skyline Plaza Failure

69. Thank You 69 JANAK SHAH (PT-401914) Skyline Plaza Failure