Skylineplazacollapse.pptx

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

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

Literature Review
• Formwork
• Shoring
• Reshoring
• Flat Plate
• Punching Shear Failure
• Progressive collapse
3

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

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

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

Fig 3 Beam form and supports
Fig 2 Column forms
7

Shoring
• Vertical or inclined support members designed to carry the
weight of the formwork, concrete, and construction loads
above.
8

Shoring
Fig 4
9

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

Reshoring
Fig 5 PHASE 1: PLACEMENT OF CONCRETE Fig 6 PHASE 2: REMOVAL OF SHORES
11

Reshoring
Fig 7 PHASE 3: REMOVAL OF LOWER LEVEL OF SHORES Fig 8 PHASE 4: INSTALLATION OF RESHORES
12

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

• Of equal importance, the forming for a ﬂat plate is generally
cheaper than that for ﬂat slabs with drop panels .
Fig 9
14

Fig 10 Reinforced concrete flat plate system, Eugene, USA
15

Fig 11 A reinforced concrete building with flat plate system under construction, Kenya
16

Punching Shear
• Punching shear failure is caused by the vertical 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

• 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

Laboratory Studies
Fig 14 Exterior connection Fig 15 Interior connection
Photo: Hwang and Moehle, ACI SJ, March-April 2000 19

Fig 16 Building Collapsed due to punching failure
20

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

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

Historical Perspective to Progressive
Collapse
Fig 17 A time line of cases associated with progressive collapse.
23

Progressive collapse ,
Skyline Plaza at Bailey's Crossroads,
National Archives
Fig 18
24

Skyline Plaza Collapse in
Fairfax County, Virginia
Date of Collapse : March 2, 1973
25

Background
• The Skyline center complex located near Bailey’s Crossroad, Fairfax
County, Virginia.
• It was planned to contain
1. Eight apartment buildings (namely A-1 to A-8),
2. Six office buildings,
3. A hotel, and
4. 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

Fig 19 Appearance of completed apartment
27

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

• 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 22nd storey and is typical for the 1st through 26th 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

Fig 20
30

N
Fig 21 Aerial View of Construction at 11 am, March 2, 1973 (Before Collapse)
31

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

The Collapse
• The collapse occurred at around 2:30 pm on March 2, 1973,
while under construction while the 24th 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 23rd
floor slab and the freshly placed 24th floor slab.
• The freshly placed section of the 24th floor slab then fell onto
the 23rd 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

General appearance of
building A-4 as
viewed from the southeast.
Fig 22
34

•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

Closeup view of the top of the east end
of the failure zone, looking south.
Fig 24
36

Closeup view near the midheight
of the east end of the failure zone,
looking south
Fig 25 37

General view of building A-4 as seen
looking southwest
Fig 26
38

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

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

Formwork
Fig 27 General appearance of building A-4 as viewed from the southeast.
41

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

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

The formwork on the 24th story is shown in figure. Several columns
have been identified for reference.
Fig 28
44

Fig 29 A schematic of formwork is shown in figure as derived from OSHA case records
45

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

• 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

•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

Fig 32 The method of attaching the brace to the floor is shown in figure
49

• As in the case of the 24th story. the 23rd story lumber which
was apparently damaged was reused.
• As in the case of the 24th 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

•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

• Photographs taken right after the collapse showed that while
full shoring remained on the 23rd and 24th stories, it had
nearly all been removed from the 22nd 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

Fig 35 Estimated location of forms and reshores at the time of the collapse.
53

• 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

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

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

• 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

• 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 22nd or the 23rd floor slab was not flexural.
58

Investigation to Shear Failure
Fig 36 Summary of shear stresses in the 23rd story floor slab.
59

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 23rd story floor slab resulting from partial or complete
removal of shoring prior to the incident was a major
contributing factor to the collapse.
60

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

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 24th 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

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

Non-Compliance with OSHA
Regulation
• Shoring in section 3 of 22nd 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 22nd story form:
The forms removed on the 22nd 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

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

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

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

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

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