Failures That Changed Perception of Our Designs-InG IABSE December 14

The Bridge and Structural Engineer Volume 45 Number 4 December 2014 23

SummaryWith the dwindling natural resources, it is very important for engineers to avoid failures of any nature and build structures which are safe, stable, economic, aesthetic, durable, and environment friendly. However, building and other structural failures are occurring at regular intervals in India. These failures, though well reported in newspapers, the reasons for these collapses are not well documented, even though some committees are appointed in some cases to study the failures. If the causes of failures are well publicized, designers and builders can learn from the mistakes done by others and will not repeat them in their practice. Hence an attempt is made in this paper to describe some important failures that resulted in code clause changes or in changes to professional practice. It is high time that legislation is passed in India, similar to those available in Western Countries, such that only qualified and experienced engineers are allowed to practice, so that failures are minimized.

Keywords: Bridge failures, Buckling, Building Collapse, Connections, Cooling towers, Deflection, Foundation failures, Post-tensioned Steel, Progressive collapse, Wind, Earthquakes, Anchor bolt failures, Corrosion, Shear wall, Space frame, Structural failures, Terrorist Attacks

1.0 IntroductionBuilding collapses are common in India, where

ThE FaIlurES ThaT ChangEd ThE PErCEPTIon oF our dESIgnS

dr. Subramanian naraYanan Consulting EngineerGaithersburg, MD [email protected]

Dr. N. Subramanian earned his PhD from IIT, Madras in 1978 and has 40 years of professional experience which includes teaching, research, and consultancy in India and abroad. Dr. Subramanian has authored 25 books and more than 200 technical papers, published in international and Indian journals and conferences. He has won the Tamil Nadu Scientist Award, the Lifetime Achievement Award from the Indian Concrete Institute (ICI) and the ACCE(I)-Nagadi best book award for three of his books. He also served as the past vice-president of ICI and ACCE(I).

high demand for housing and lax regulations have encouraged some builders to cut corners, use substandard materials or add unauthorized extra floors. Let us consider some recent examples: in April 2013, 74 people were killed and 60 people wounded when an eight-story building which was built illegally in the Mumbai suburb of Thane in western Maharashtra state caved in. It was the worst building collapse in the country in decades. The probable cause of the collapse was that an eight-story building was built instead of a sanctioned four-story building. Some sources cite that 80% of constructions in Thane are illegal!

Another major collapse took place on 27 September 2013, at 6 a.m. IST, when a five-story building collapsed in the Mazagaon area of Mumbai city in Maharashtra killing 61 people and injuring 32 others.

In a recent incident on 28th June 2014 in New Delhi, 10 people died after a dilapidated four-story building collapsed to the ground. The New Delhi collapse was probably triggered by digging of the ground in an adjacent plot for foundation work. Hours later, on the same day, one of the twin 11-story buildings under construction collapsed on the outskirts of Chennai, Tamil Nadu, as heavy rains pounded the area. This resulted in a massive rescue effort by more than 300 persons, which found that 61 people were killed and 27 injured. The probable causes were

24 Volume 45 Number 4 December 2014 The Bridge and Structural Engineer

that 11 stories were built using a structural design which was indented for 6 stories only and that a few columns were removed in the ground floor indiscriminately! The official report of the failure may be available only after a few months.

Fig.1 One of the twin 11-storey towers at Mugalivakkam near Porur, Chennai collapsed on

28th June, 2014 killing 61 people

Such failures are common not only in structures built by private builders but also found in prestigious government projects. For example, a footbridge that was built near the main stadium for the Commonwealth Games collapsed in Sept. 2010, just 12 days before the opening ceremony, injuring 23 construction workers. When 50m section of the overhead Andheri-Ghatkopar Metro bridge, at Mumbai came crashing down in Sept. 2012, one person died and eight were injured. This was a joint venture between Reliance Infrastructure, Veolia Transport and the Mumbai Metropolitan Regional Development Authority (MMRDA). Similarly six persons, including an engineer, were killed and 13 others injured

when an under-construction over-bridge of the Delhi Metro collapsed in July 2009. A chimney collapsed in September 2009 at the Balco Plant in Chhattisgarhs Korba district, claiming 41 lives and another 210 m tall newly constructed concrete chimney at Parichha Thermal Power Project in UPs Jhansi collapsed in May 2010.

The majority of structural failures all over the world (whether it is collapse of the structure or functional failure) are generally attributed to some engineering problem, such as poor quality of construction, weak ground conditions, unauthorized extensions, structural alterations and no maintenance, and rarely to design and detailing errors (Brown and Yin 1988 and Prabhakar, 1998). Table 1 shows the comparison of principal causes of building failures in USA during 1977-2000 (Wardhana and Hadipriono, 2003), which also shows that construction deficiencies are the most frequent cause of collapse.

Table 1 Comparison of principal causes of building failures in USA during 1977-2000a

(Wardhana and Hadipriono, 2003)

Principal causes

Collapse distress

1977-1981

number (%)

1982-1988

number (%)

1989-2000

number (%)

1977-1981

number (%)

1982-1988

number (%)

1989-2000

number (%)

design 14 (23) 5 (14) 7 (3) 12 (40) 1 (11) 1 (6)detailing 6 (10) 5 (14) 2 (1) 5 (17) 1 (11) -Con- struction

22 (37) 12 (32) 52 (25) 6 (20) 2 (22) 11 (65)

Maint- enance

1(2) - 22 (11) 1 (3) 4 (44) 1 (6)

Material 1(2) - 3 (1) 2 (7) - -External 16 (27) 11(30) 60 (29) 4 (13) - 1(6)others (na)

- 4(11) 61 (29) - 1 (11) 3 (18)

Total 60 (100)

37 (100)

207 (100)

30 (100)

9 (100) 17 (100)

aOne case unknown

In order that these failures do not happen in future, we need to learn from these failures. Even though some committees are constituted to study the cause of some import failures in India, most often the committee reports are not made public. Whereas, in Western countries such reports are made available to the public and hence all concerned agencies can learn from these


failures and will not repeat them in their projects. In this paper some of the major failures, which changed our perception of design and detailing of structures and resulted in modification of code clauses are discussed.

Foundation Failures Foundations are important to any structure as the entire load acting on the structure is transmitted to the soil below through the foundation only. Due to the complex nature of soils and their behaviour, a hybrid approach is usually adopted in foundation design in which soil bearing pressures are checked based on the working stress method and members of the foundation are designed using the limit states method (Subramanian, 2013). Foundation failures are difficult to rectify and may endanger the entire building. Hence it is important to design them conservatively. If the footing is not of the required thickness, there is a danger of the column piercing through the foundation. Several failures (both partial and total) in the past have demonstrated the importance of foundation failures, especially in poor soils. We will just look at two fascinating foundation failures here.

Successful foundation failure The Tower of Pisa is a freestanding bell tower of the cathedral of the Italian city of Pisa. The tower

Fig. 2 Tower of Pisa, considered as the first foundation failure (Photo: Er. S. Srinivas)

is a 56.4 m tall, circular, eight-story structure made of white marble. Although intended to stand vertically, the tower began leaning to the southeast soon after the onset of construction in 1173 due to a poorly laid 3 m deep foundation and weak, unstable subsoil. Prior to restoration work performed between 1990 and 2001, the tower leaned at an angle of 5.5 degrees, but the tower now leans at about 3.99 degrees. This means that the top of the tower is 3.9 m away from the vertical plane through the tower (see Fig. 2).

Several attempts have been made to stabilize the foundation movement- details of these may be found in Subramanian and Muthukumar (1998) and Burland et al. (2009). After a decade of corrective reconstruction and stabilization efforts, the tower was declared stable in 2008 and is expected to stand for at least another 200 years.

It may be of interest to note that in June 2010, the Capital Gate building in Abu Dhabi, UAE was certified as the World's Furthest Leaning Man-made Tower; it has a 18-degree slope, almost five times as that of the Leaning Tower of Pisa; however this tower is deliberately engineered to slant.

2.2 Rare Foundation Failure in China

On June 27, 2009, an unoccupied 13-storey block of flat building, still under construction, at Lianhuanan Road in the Minhang district of Shanghai city, China toppled over and ended up lying on its side in a muddy construction field (see Fig. 3). One worker was killed.

Fig. 3 Toppling of a complete structure in China [Source: Basulto , David. "Building collapse in Shanghai" 30 Jun 2009. ArchDaily. Accessed 24th July 2014. http://www.archdaily.com/27245]


The cause of this building collapse was due to a pressure difference on two sides of the structure, according to an investigation report. The report said the collapse was caused by earth, excavated along the building on one side with a depth of 4.6 m, for an underground car park, and piled up to a depth of up to 10 m on the other side of the structure. The weight of overburden earth created a pressure differential, which led to a shift in the soil structure, eventually weakening the pile foundation and causing it to fail. This situation might have been aggravated by several days of heavy rain leading up to the collapse, but investigators did not site this as a crucial factor. The sequence of failure of this building is shown in Fig. 4. More details about this failure may be found in Subramanian (2009). This failure underlined the importance of not disturbing the soil near a construction, even if the building is supported on piles.

Fig. 4 Sequence of foundation failure of a tall building in Shanghai, China

Failure of ColumnsNext to foundations, the most important elements of any structure are its columns, as the failure of a column may result in a catastrophic failure of the whole structure. Thus, the designer should carefully design and detail columns and their footings.

3.1 Savar building collapse

On 24 April 2013, Rana Plaza, an eight-story commercial building, collapsed in Savar, a sub-district in the Greater Dhaka Area, the capital

of Bangladesh killing 1,129 and injuring 2,515 people (see Fig. 5). It is considered to be the deadliest accidental structural failure in modern human history.

Fig. 5 Dhaka Savar Building Collapse (Source: http://en.wikipedia.org/)

The building contained clothing factories, a bank, apartments, and several other shops. The shops and the bank on the lower floors immediately closed after cracks were discovered in the building. Warnings to avoid using the building after cracks appeared, the day before, had been ignored. Garment workers were ordered to return the following day and the building collapsed during the morning rush-hour. The main cause of failure is probably because the upper four floors had been built without a permit. It may also be due to the fact the building was designed only for shops and offices, but contained factories and hence the structure was potentially not strong enough to bear the weight and vibration of heavy machinery. The Savar building collapse led to widespread discussions about the corporate social responsibility across global supply chains.

3.2 Failures of Columns During Earthquakes

Observations in several past earthquakes indicated very poor performance of buildings due to several reasons such as shear-critical columns, weak columns and strong beams, sudden change in stiffness, plan and vertical irregularity, unconfined beam-column joints, weak stories (see Fig. 5). Column stiffness is inversely proportional to the cube of column height. Hence, columns with significantly less height than other columns in the same storey will have much higher lateral stiffness, and


consequently will attract much greater seismic shear force. Brittle shear failures have been observed in the unsupported zones of such short captive-columns during several earthquakes in the past. A mezzanine floor or a loft also results in the stiffening of some of the columns while leaving other columns of the same storey unbraced over their full height. Based on these observations, codes now specify that special confining reinforcement be provided over the full height in such columns to give them adequate confinement and shear strength. Such confining reinforcement is also stipulated when shear walls terminate over open storey columns in the ground floor.

The soft first story and weak story irregularities, that are widely used by architects in their designs, were the cause of failure of columns in such stories during earthquakes as demonstrated in Northridge, California (1994); Chichi, Taiwan, and Izmit, Turqua, in 1999; and Bhuj, India in 2001 (see Fig. 6 & 7). Unfortunately buildings with such soft first storey/weak storey are still being constructed in several parts of India. It is because the area enclosed by a soft first story is rewarding to the developer since it is neither considered as part of the maximum allowable built-up area, nor for tax control, but is salable as car parking area.

Deficiency A: Shear-critical columns Deficiency F: Overall weak frames Shs in a moment frame or gravity

frame system. Overall deficint

system strength and stiffness, leding to inadeuacy

of an otherwise r e a s o n a b l y configured building

Deficiency B: Unconfined beam-column Joints Deficiency G: Overturning mechanisms Shear and axial failure of

unconfined beam-column joints, particularly corner joints

Columns prone to crushing from overturning of discontinuous concrete or masonry infill wall.

Deficiency C: Slab-column connections Deficiency H: Severe plan irregularity Punching of slab-column

connections under imposed lateral drifts.

Conditions (including some corner buildings) leading to large t o r s i o n a l - i n d u c e d demands.

Deficiency D: Splice and connectivity weakness

Deficiency I: Severe vertical irregularity

Inadequate splices in plastic hinge regions and weak connectivity between members.

Setbacks causing concentration of damage and collapse where stiffness

and strength changes. Can also be caused by change in material or seismic-force-resisting-system


Fig. 7 The columns on one edge of the open first storey of this building in Bhuj collapsed bringing the

building down on its side.

Several failures of columns during earthquakes were also due to the following reasons: (a) buckling of column reinforcement (due to insufficient lateral ties), (b) insufficient transverse reinforcement in beam-column joints, (c) poor anchorage of beam bars in beam-column joints, (d) provision of strong beams and weak columns, and (e) improper detailing in plastic hinge zones (see also Fig. 6). Based on these observations, several clauses in the codes have been changed. For example, in the draft IS 13920 the following changes have been introduced:

The minimum dimension of a column shall not be less than 20db, where db is diameter of the largest diameter longitudinal reinforcement bar in the beam passing through or anchoring into the column at the joint, or 300 mm

At each beam-column joint of a moment-resisting frame, the sum of nominal design strength of columns meeting at that joint (with

nominal strength calculated for the factored axial load in the direction of the lateral force under consideration so as to give least column nominal design strength) along each principal plane shall be at least 1.4 times the sum of nominal design strength of beams meeting at that joint in the same plane. In the event of a beam-column joint not conforming to above, the columns at the joint shall be considered to be gravity columns only and shall not be considered as part of the lateral load resisting system.

Lap splices shall be provided only in the central half of clear column height, and not within a joint, or within a distance of 2d (where d is the effective depth of column) from face of the beam.

Not more than 50% of area of steel bars should be spliced at any one section.

The parameters such as ratio of concrete strength to tie strength, axial load level, unconfined cover concrete thickness, and longitudinal reinforcement and their spacing affect the effectiveness of confining reinforcement in columns. Hence, Subramanian (2012) suggested that the following equation, based on the work of Elwood et al. 2009, for determining the confining reinforcement in rectangular columns

yt

c

c

gpnsh f

fAA

kkA'

3.0= (1) Where

+

+=

20124.06.0 x

ln

hnnk and

ckg

up fA

Pk 8.0= 0.2 (1a)

With the following conditions: fyt 689 MPa, 3.1c

g

AA and 0.1

2012

+xh

Deficiency E: Weak-story mechanism Deficiency J: Pounding Weak-column, strong-beam

moment frame or similar system prone to story collapse from failure of weak columns subjected to large lateral deformation demands.

Collapse caused by pounding of adjacent buildings with different story heights and non-coincident floors.

Fig.6 Component and system-level seismic deficiencies found in pre-1980 concrete buildings (NIST GCR 10-917-7, 2010)


Where, Ac = area of concrete core within perimeter transverse reinforcement, Ag= gross area of column, Ash = total cross-sectional area of transverse reinforcement (including cross hoops) with spacing s and perpendicular to dimension bc, bc is the cross-sectional dimension of column core measured to the outside edges of transverse reinforcement composing area Ac, = specified cylinder compressive strength of concrete, fck= specified cube compressive strength of concrete, fyl = specified yield strength of longitudinal reinforcement, fyt = specified yield strength of transverse reinforcement, hx = center-to-center horizontal spacing of cross ties or hoop legs, n= total number of longitudinal bars, nl = number of longitudinal bars laterally supported by corner of hoops or by seismic hooks of crossties that are 135 degrees; Pu = factored load on column, and s = centre-to-centre spacing of transverse reinforcement along column height.

Similarly, the following confinement equation is proposed for circular columns (Subramanian, 2011):

=

c

g

yt

ckps A

Affk44.0

(2)

Where s is the volumetric ratio of transverse reinforcement, kp is as defined earlier. Note that the term kn is not required for circular columns, as spirals provide effective confinement than rectangular hoops.

Failure of SlabsSince two-way slabs are three-dimensional elements, they may redistribute the overloads and hence failures in slabs are not frequent, provided they are detailed properly. However, punching failures in flat plates may be catastrophic and may not give enough warning. Hence such flat slabs should not be used in severe earthquake zones. When used in moderate zones, flat slabs should be checked for punching stresses and detailed properly near the vicinity of columns (Subramanian, 2013 & 2014).

4.1 Collapse of Skyline Plaza, Virginia, U.S.A.

Skyline Plaza apartment building in Bailey's Crossroads, Virginia, U.S.A. is an example of a

catastrophic collapse of a 30 story cast-in-place reinforced concrete structure. This flat-plate structure collapsed while under construction, due to punching shear on the 23rd floor and resulted in a progressive collapse (see Fig. 8).

Fig. 8 Progressive collapse of Skyline Plaza building in Virginia, USA (Photo by Nick Carino of NIST)

Source: Ellingwood et al. (2007)

In the midst of construction on March 2, 1973, one apartment building and the parking garage adjoining it collapsed. Fig. 6 shows the damage following the collapse. The incident occurred at around 2:30 in the afternoon and resulted in the death of 14 construction workers and the injury of 34 others. It was designed as a 26 story apartment complex with a four-story basement and a penthouse level. All floor slabs were 200 mm thick and the floor-to-floor height was 2.75 m.

The Center for Building Technology of the National Bureau of Standards (now the National Institute of Standards and Technology, or NIST) investigated this collapse. A three-dimensional finite element analysis was conducted on the 22nd and 23rd floors to determine the magnitude of forces exerted on the floor slabs and whether the slabs could properly handle those forces. Upon completion of the analysis, it was determined that moments in the column strips of the slab were not great enough to cause failure. On the other hand, the analysis showed that the slab around few columns experienced shear stress greater than the shear capacity of the concrete slab. The improper and early removal of forms supporting the 23rd floor resulted in increased shear force around the columns. The recently poured concrete had strength less than


the design strength of 20 MPa at the time of the collapse and was unable to withstand these increased forces. Hence it triggered a punching shear collapse mechanism around a number of columns on the 23rd story. Without the support of these columns, other columns on that story were overstressed which ultimately led to the collapse of the entire 23rd floor slab onto the floor below. The increased loading on the 22nd floor from the weight of the collapsed floors above was sufficient to trigger a progressive collapse all the way to the ground level. (Leyendecker and Fattal, 1977; Schellhammer et al., 2013).

The important lessons learnt from the partial collapse of Skyline Plaza are (Leyendecker and Fattal 1977):

1. Redundancy within structural design is essential to prevent progressive collapse.

2. Construction loads, which will govern the design, must always be estimated and considered in the design.

3. Preconstruction plans of concrete casting, formwork plans, removal of formwork schedules, or reshoring program should be decided in consultation with the contractor.

4. Before the removal of shoring, the concrete strength should be ascertained.

5. Proper shoring of the currently executed floor and the floors below should be verified, especially in flat plate systems.

Following this failure, the Portland Cement Association (PCA) and the Prestressed Concrete Institute both issued new design guides with provisions included to prevent progressive collapse (PCA-IS 184, 2006). The importance of designing for construction loads as well as normal design loads were emphasized in ACI journals (Agarwal and Gardner, 1974).

ACI 318 code included a provision to place rebar continuously through the slab-column intersection at the top and bottom of the slab. If the slab fails in punching shear, the bottom bars act as a catenary and prevent the collapse of the slab onto the structure below.

Several other failures of flat slab structures have been reported in the literature, which

include 2000 Commonwealth Avenue, January 5, 1971: Five storey Harbour Cay Condominium collapse at Cocoa Beach, Florida, March 27, 1981(11 workers killed and 23 injured- The most probable cause of collapse was a combination of design and construction errors: the design did not even consider the possibility of punching shear failure); Four storey warehouse at Ontario, Canada, January 4, 1978; five story Sampoong Department store, Seoul, Korea, June 29th 1995 (The collapse is the largest peacetime disaster in South Korean history - 502 people died, 6 missing, and 937 sustained injuries); Pipers Row Car Park, Wolverhampton, UK, 1997; Geneva, Switzerland,1976; Bluche, Switzerland, 1981; Cagliari, Italy, 2004; and parking garage flat slab at Gretzenbach, Switzerland, 2004 (Subramanian, 2014). In addition several flat plate systems failed during earthquakes.

4.2 The LAmbiance Plaza Collapse

L'Ambiance Plaza was planned as a sixteen-story building, with thirteen stories of apartments and three levels of parking, at Bridgeport, Connecticut, USA. It consisted of two offset rectangular towers, 19.2 m by 34 m each, connected by an elevator. These towers were being constructed by the lift slab method, patented by Youtz and Slick in 1948. Floor and roof slabs were two-way, unbonded, post-tensioned flat plates. On April 23, 1987, during construction, the entire structure suddenly collapsed, killing 28 workers and injuring many more (see Fig.9). At the time of the collapse, slabs 3, 4, and 5 of the east tower

Fig. 9 Collapse of LAmbiance Plaza, Connecticut, U.S.A. (Source: Ratay, 2011)


had been placed into final position, and slabs 9, 10, and 11 for the west tower had just been lifted. The entire collapse took only 5 seconds! The collapse was one of the worst disasters in USA. This was the first serious failure of a lift-slab structure, a system that had been in use for about 40 years.

An unusually prompt legal settlement prematurely ended all investigations of the collapse. Consequently, the exact cause of the collapse has never been established. The building had a number of deficiencies; any one of which could have triggered the collapse. The report by the National Bureau of Standards (NBS) concluded that an overloaded steel angle welded to a shear-head arm channel deformed, causing the jack rod and lifting nut to slip out and caused the collapse to begin. Failure was possibly due to high concrete stresses on the floor slabs by the placement process, resulting in cracking of the slab concrete and ending in a punching shear failure. Moreover, the ACI code states that a minimum of two tendons shall be provided in each direction through the critical shear section over the columns. This was not followed in the LAmbiance Plaza structure.

While buildings constructed by the lift-slab method are stable once they are completed, they may be unstable during construction, if the following measures are not taken during construction (Martin, http://911research.wtc7.net; and Cuoco, et al, 1992)

Provision of temporary lateral bracing during all stages of construction.

Provision of concrete punching shear and connections redundancies in the structure.

Provision of temporary posts, to support the concrete slab until it is completely attached to the column.

Provision of sway bracing (cables which keep the stack of floors from shifting sideways). Though this is required, it was not used in LAmbiance Plaza

This failure resulted in temporary ban and tighter design and construction requirements in some states of USA on lift-slab construction (Ratay, 2011).

A more recent failure is that of the six-story parking structure at Berkman Plaza in Jacksonville, Florida, under construction on December 6, 2007, where 60% of the structure collapsed suddenly "like a stack of pancakes", killing one and injuring 23 others (see Fig. 10). The structure consisted of cast-in-place simple reinforced concrete columns, cast-in-place reinforced and post-tensioned concrete beams, and cast-in-place post-tensioned concrete slabs. The Occupational Safety and Health Administration (OSHA) concluded that while the collapse was due to errors made by those on the design, construction, and inspection teams, the structural design had numerous deficiencies including one column that was barely able to support the dead loads of the structure. The formwork plans called for the shoring and reshoring to extend all the way to the ground. However, it was learned that the shoring and reshoring below the 3rd level had been removed shortly before the concrete on the 6th floor was placed (www.oshrc.gov/foia/Rpt_SouthernPanServCo.pdf). This failure along with the failures of Harbour Cay Condominium Cocoa Beach, and the Turner Agri-Civil Center Arcadia resulted in the Florida Structural Engineers Association (FSEA) proposing legislation to amend Chapter 471 of the Florida Statutes to recognize the discipline of structural engineering and provide for a corresponding license. An engineer would apply for the SE license only after being licensed as a PE, and after taking and passing the NCEES 16-hour structural examination.

Fig.10 Collapsed Berkman Plaza, Jacksonville, Florida (Source: Ratay, 2011)


5.0 Progressive Collapse and Ronan Point Building

Progressive collapse provisions were introduced in the British code as early as 1970. This was a direct result of the Ronan Point collapse in 1968. This involved a 23-storey tower block in Newham, East London, which suffered a partial collapse when a gas explosion demolished a load bearing wall, causing the collapse of one entire corner of the building (see Fig. 11). Four people were killed in the incident, and seventeen were injured. (Ronan Point was repaired after the explosion, it was demolished in 1986 for a new low-rise housing development project)

Fig. 11 Ronan Point collapse, London, U.K. (Source:

http://en.wikipedia.org/)

Due to the failure of Ronan Point apartment building, many other similar large panel system buildings were demolished. The Building Research Establishment, U.K., published a series of reports in the 1980s to advise Councils and building owners on what they should do to check the structural stability of their blocks. As a result of terrorist attacks on embassies abroad, along with the Murrah Federal Building

in Oklahoma City, abnormal load requirements were introduced in US Codes. Structural integrity requirements are yet to be introduced in IS 456.

6.0 Failure of BeamsThough flexural failure of beams is rare, shear failure of beams and failure due to improper detailing have been reported in the literature. A few such failures are discussed in this section.

6.1 Partial collapse of Wilkins Air Force Depot in Shelby, Ohio

It is interesting to note that the shear provisions of the ACI code were revised after the partial collapse of Wilkins Air Force Depot in Shelby, Ohio, in 1955 (See Fig. 12). At the time of collapse, there were no loads other than the self-weight of the roof. The 914 mm deep beams of this warehouse did not contain stirrups and had 0.45 percent of longitudinal reinforcement (Feld and Carper 1997). The concrete alone was expected to carry the shear forces- and had no shear capacity once cracked. The beams failed at a shear stress of only about 0.5 MPa, whereas the ACI Code (1951 version) at the time permitted an allowable working stress of 0.62 MPa for the M20 concrete used in the structure. Experiments conducted at the Portland Cement Association (PCA) on 305 mm deep model beams indicated that the beams could resist a shear stress of about 1.0 MPa prior to failure (Feld and Carper 1997). However, application of an axial tensile

Fig. 12 Shear failure of 900 mm deep beams in Air Force Warehouse, Shelby, Ohio (Photo: C.P. Seiss)

(Source: Lubell et al 2004).


stress of about 1.4 MPa reduced the shear capacity of the beam by 50 percent. Thus, it was concluded that tensile stresses caused by the restraint of shrinkage and thermal movements caused the beams of Wilkins Air Force Depot to fail at such low thermal shear stresses (Feld and Carper 1997). The expansion joints locked and did not function as intended to relieve stresses. This failure outlines the importance of providing minimum shear reinforcement in beams. It has to be noted that repeated loading will result in failure loads which may be 50 to 70 percent of static failure loads (ACI-ASCE committee 426, 1973).

6.2 Sleipner Offshore Platform

Sleipner A is a combined accommodations, production and processing offshore platform at the Sleipner East gas field in the Norwegian sector of the North Sea. It is a Condeep type platform with a concrete gravity base structure consisting of 24 cells and with a total base area of 16 000 m2. Even though it was analyzed and designed using sophisticated finite element software, it resulted in catastrophic failure on 23rd August 1991 (resulting in an economic loss of about $700 million), due to the under-estimation of applied shear in the analysis and over-estimation of shear strength in the design of the tricell walls; about 15 m height of the tricell walls did not contain any stirrups. It may probably be considered as the most expensive shear failure; more details of this failure may be found in Collins et al 1997.

6.3 Failure of Industrial Building at Neyveli

A single span reinforced concrete structure with a span of 19 m, as shown in Fig. 13, was constructed for a urea drying and cooling building at Neyveli, Tamilnadu in January 1962. The roof beams in the end bays were reinforced with seven 38 mm diameter bars at the bottom and seven 22 mm diameter bars plus one 16 mm diameter bar at the top. The beams in the central bay were reinforced with seven 38 mm diameter bars plus one 20 mm bar at the bottom and one 38 mm diameter bars plus six 32 mm diameter bars at the top. The beam had a cross section of 1425 mm x 600 mm and was made of M 15 concrete.

During the removal of shuttering of the last bay on 26th January 1962, a sudden heavy cracking sound was heard and within a few seconds the roof caved in along the ridge. The beams had cracked in the middle, and the main reinforcements were pulled out. The collapsed roof came to rest on the centering props below.

Fig. 13 Urea drying and cooling building at Neyveli

Testing of concrete cubes revealed that the compressive strength of concrete was more than that required according to the design. The materials used, including the steel reinforcement were of high quality, and yet the structure failed. A close examination of the failed area disclosed that most of the reinforcement bars were lapped at the centre of the beam, where failure had occurred. Naturally, a beam with a clear span of 18 m could not be reinforced without resorting to lapping, as the bars were supplied at a standard length of 12 m. To use the bars more economically, and to avoid more than one lap, all the lapping had been done in only one place, that too at the centre. This resulted in heavy congestion of bars and the bond between steel and concrete was poor. The version of IS 456 in vogue at that time did not prohibit the lapping of tensile reinforcement. Even the revised 1964 code suggested only that laps should be avoided in maximum stressed zones as far as possible, and lapped splices in tension should not be used in the case of bars greater than 36 mm diameter; such splices should be welded (Srinivasan 1975).

7.0 Bridge Failures There are many historical examples of major bridge failures, but one of the major collapses that made an impact on our profession is the collapse of the 41-year-old U.S. 35 High way 'Silver Bridge' across the Ohio River at Point Pleasant,


West Virginia. It has to be noted that out of a total of 503 recorded failures, 243 bridge failures were related to flooding and scour (Wardhana and Hadipriono, 2003). In addition to scour, bridge designers must consider lateral loads caused by significant flooding events as per the AASHTO LRFD Bridge Design Specifications. The lateral loads imparted to bridge piers by ships and/or barges or lateral loads resulting from vehicle or train collisions (when bridge piers are located near traffic lanes or a railroad below) must also be considered in the design. It is because 12 percent of the total number of bridge failures studied, resulted from land and marine vehicle collisions (Wardhana and Hadipriono, 2003). Here only the major failures that resulted in code or specification changes are discussed.

7.1 Collapse of the Silver Bridge, West Virginia, U.S.A.

The Silver Bridge was a chain suspension structure that collapsed suddenly on December 15, 1967 in rush-hour traffic, killing 46 people (two of the victims were never found), injuring nine and sending 31 vehicles into the water. The report of the Federal Highway Administration (FHWA), released after 18 months of the failure, attributed the failure to a small crack in the lower limb of an eyebar in the suspension chain (formed through fretting wear at the bearing), which grew because of internal corrosion (a problem known as stress corrosion cracking) leading to the failure (Petroski, 1985; Seim, 2008). As a result of the collapse, an upstream bridge, the St. Marys Bridge, was immediately closed to traffic and was demolished by the state in 1971. In 1968, the U.S. Congress passed the Federal Highway Act to establish National Bridge Inspection Standards (NBIS); and it was introduced on May 1, 1979. This standard stipulated that each highway department should have a bridge inspection organization capable of performing inspections, preparing reports, and determining ratings, in accordance with the provisions of the American Association of State Highway and Transportation Officials (AASHTO) Manual for Maintenance Inspection of Bridges at regular intervals not exceeding two years (Ratay, 2010). Many bridges throughout the

USA were closed or had speed limits and traffic loads imposed on them. The Silver Bridge was replaced within two years by a cantilever design - the type that failed at Minneapolis, recently (see section 7.3).

7.2 Collapse of I-95 Bridge over the Mianus River in Connecticut

On June 28, 1983, a highway bridge carrying Interstate 95 over the Mianus River in Connecticut collapsed due to the cleavage fracture failure of a pin-and-hanger connection, killing three and injuring another three persons (see Fig. 14). The ensuing investigation cited corrosion from water buildup due to inadequate drainage, and inadequate inspection as a cause. The replacement span completed in 1992, eliminated the pin-and-hanger assemblies that caused the collapse of the original bridge. This collapse focused attention on fracture-critical bridges and established national inspection guidelines, additional inspector training and new fatigue research for these types of structures. The FHWA added a new supplement to the Bridge Inspectors Training Manual 70 in 1986: Inspection of Fracture Critical Bridge Members.

Fig. 14 Collapse of I-95 Bridge over the Mianus River in Connecticut (Source: Ratay, 2010)

7.3 Other Notable Bridge failures

One of the notable failures of a riveted steel truss bridge is the collapse of the Quebec Bridge, Canada on August 29, 1907, during construction, killing 75 workers. The main cause of failure was found to be the buckling of latticed compression chords even though the official report attributed the collapse to a number of other reasons (http://


matdl.org/failurecases/Bridge_Collapse_Cases/Quebec_Bridge). Additionally, member stresses were not recalculated and checked when the center span length was increased by 61 m during the design phase, overstressing several members. As the bridge was erected, ironworkers noticed significant mid-point displacements in some of the truss compression members, but this was not reported to the designer. Additionally, even though it was a major bridge design, no one proof checked the original design (The project suffered a second collapse in 1916, when a casting in the lifting apparatus broke, causing the center span to fall into the water, killing thirteen workers).

With the collapse of New Yorks I-90 over the Schoharie Creek Bridge in 1987 and the deaths of 10 people, attention was turned towards underwater inspections. This structure failed due to scouring of the center pier. In 1988, the FHWA issued a technical advisory guide, Scour at Bridges. In October of that same year, the NBIS was modified based upon suggestions made in the 1987 Surface Transportation and Uniform Relocation Assistance Act. The national underwater inspection frequency interval was set at a maximum of 60 months. Scour critical bridge inspections were initiated.

Even with these guidelines several bridge failures have occurred. Most are small bridges that collapsed due to overloads and many might not have caused great loss of life. A recent exception however was the sudden collapse on August 1, 2007 of the I-35W Bridge over the Mississippi River in Minnesota killing 13 people and injuring 145 (See Fig. 15). The findings by the NTSB (National Transportation Safety Board) indicate under-designed gusset plates were a

Fig.15 Two different views of the I-35W Bridge failure (Source: http://en.wikipedia.org/)

major factor, coupled with weight added to the bridge over the years as well as ongoing construction that had more than 191 tons of construction material piled over the failure area on the bridge on the day of the collapse. More details of this bridge failure may be found in Subramanian (Feb 2008).

Subramanian (June 2008) provides a brief report about the measures taken by engineers to prevent failures similar to that of the I-35W Mississippi river bridge collapse.

The collapse of shoring in the construction of a highway bridge built to carry Maryland Route 198 over the Baltimore-Washington Expressway, in 1989 injured nine workers and five motorists, and killed one. This led to the research and publication of new design specifications and construction practices for temporary works by the FHWA and to the changes in the provisions for temporary works in the AASHTO Standard Specifications for Highway Bridges (Ratay 2010). A few more bridge failures that resulted in changes to code and standard clausess and practices may be found in Ratay (2010).

7.4 Collapse of Koror - Babeldaob Bridge, Republic of Palau, Micronesia

The importance of considering long term deflections in the design was revealed in the 1996 collapse of the Koror-Babeldaob Bridge (see Fig. 16). This bridge was completed in 1977, to connect the two main islands of Koror and Babeldaob in the Republic of Palau. It is a reinforced concrete, balanced cantilever prestressed concrete box girder bridge with a


total length of 385.6 m, with a main span of 241 m. Its two lane single cell box girder superstructure was built using cast-in-place segments and a permanent mid-span hinge. After 18 years, the deflection in the main span was found to be excessive (the total deflection was 1.61 m compared to the calculated final sag of 0.46 m to 0.58 m, measured from the design camber of -0.3 m), and the prestress loss was measured as 50%. Two independent studies were conducted and they concluded that the bridge was safe and the large deflections were due to actual creep and the lower value of modulus of elasticity of the concrete compared to those adopted in design.

Fig. 16 The Koror-Babeldaob Bridge, Republic of Palau, Micronesia, before and after failure (Source:

Bazant et al. 2010 & 2011)

It was decided to install additional prestressing and eliminate the hinge at the mid-span. The retrofit began on 17th October 1995, with the removal of concrete overlay. But, the bridge collapsed suddenly on 26th September 1996, 3 months after the reopening, with 2 fatalities. A new bridge was constructed in its place and was opened on January 11, 2002.

It wasnt until 2008 that the technical data necessary for complete analysis were released.

Bazant et al 2010 showed that the main lessons from this failure are (1) the use of a realistic creep and shrinkage model is important (existing models for creep and shrinkage prediction grossly underestimate the deflections and prestress loss); (2) three-dimensional finite element analysis is required; and (3) the differences in drying rates among slabs of different thicknesses and exposures must be taken into account. They also showed that the Model B3, as per 1995 RILEM recommendation, when modified, could be used to estimate the long-time deflections reliably.

8.0 Wind Induced Failures

It is estimated that about 80 - 85% of economic losses due to natural disasters in the world are caused by extreme wind and its related events (Smith and Katz, 2013). Here only the collapse of Tacoma Narrows Bridge and Ferrybridge Cooling towers are considered.

8.1 The wind-induced collapse of Tacoma Narrows Bridge

The original Tacoma Narrows Bridge opened on July 1, 1940, and dramatically collapsed on November 7 of the same year (See Fig. 17). This suspension bridge spanned the Tacoma Narrows strait between Tacoma and the Kitsap Peninsula, in the United States and had a total length of 1,810.2 m with a central (longest) span of 853.4 m.

Fig. 17 Collapse of the Tacoma Narrows Bridge, Washington state, 1940 (Source: Smithsonian

Institution)


The failure of the bridge occurred due to the twisting of the bridge deck in mild winds of about 64 km/h (See Fig. 17). This failure mode is termed as torsional vibration mode (which is different from the transversal or longitudinal vibration mode). This vibration was caused by aeroelastic fluttering (a phenomenon in which aerodynamic forces on an object is coupled with a structure's natural mode of vibration to produce rapid periodic motion).

In the Tacoma Narrows bridge, instead of the usual deep open trusses, narrow and shallow solid I- beams were used in the decks, which resulted in the build-up of wind loads. This bridge collapse boosted research in the field of bridge aerodynamics which resulted in better designs. After the collapse, two bridges were constructed in the same general location. The first one, now called the Tacoma Westbound bridge is 1822 m long -12 m longer than Galloping Gertie. The second one, the Tacoma Eastbound Bridge, opened in 2007.

8.2 Failure of Ferrybridge Cooling towers

Large cooling towers are susceptible to wind damage, and several spectacular failures have occurred in the past. One such dramatic failures is the three 115 m tall, hyperbolic cooling towers failed by snap-through buckling at Ferrybridge power station, England on 1st Nov. 1965 due to vibrations in 137 km/h winds. The structures were designed to withstand higher wind speeds. But the following two factors caused the collapse: The average wind speed over a one minute period was used in design; whereas, the structures were susceptible to much shorter gusts, which were not, considered in the original design. The designers used wind loading based on experiments using a single isolated tower. But, in reality, the shape and arrangement of these cooling towers, created turbulence and vortex, on the leeward towers that collapsed. An eyewitness said that the towers where moving like belly dancers. Three out of the original eight cooling towers were destroyed and the remaining five were severely damaged, as shown in Fig. 18. The failed towers were rebuilt and the others strengthened. Occurrences of failure of cooling towers have also been reported in Ardeer, U.K.

in 1973, Bouchain, France in 1979, Fiddler's Ferry, U.K. in 1984, and in Willow Island, West Virginia, USA and Port Gibson, Mississippi, USA in the 1980s. These failures resulted in the revision of building codes all over the world, to include provisions regarding improved structural support, and necessity of doing wind tunnel tests for complicated configurations and arrangements.

Fig. 18 Three collapsed cooling towers at

Ferrybridge, UK (Source: http://www.knottingley.org/history/tales_and_events.htm)

In addition to the above failures, the pedestrian steel suspension London Millennium Footbridge over the River Thames in London, England, resulted in a serviceability failure due to excessive vibration. To improve the view, the bridge's suspension design had the supporting cables below the deck level, giving a very shallow profile. Construction of the bridge began in 1998, and it was opened on 10th June 2000. Londoners nicknamed the bridge the Wobbly Bridge after they felt an unexpected and uncomfortable swaying motion on the first two days after the bridge opened. After extensive analysis by the engineers, the problem was fixed by the retrofitting 37 fluid-viscous dampers (energy dissipating) to control horizontal movement and 52 tuned mass dampers (inertial) to control vertical movement. After a period of testing, the bridge was successfully re-opened


on 22 February 2002. The bridge has not been subject to significant vibration since then. This bridge outlined the importance of considering serviceability limit state due to vibration in the design of bridges.

9.0 Failures due to Terrorist Attacks

The collapse of Alfred P. Murrah Federal Building in Oklahoma in 1995 and the terrorist attack on World Trade Center Towers in New York and the Pentagon in Washington D.C. during 2001 resulted in renewed research in fire resistant design and blast resistant design.

9.1 Failure of Alfred P. Murrah Federal Building in Oklahoma

On 19 April 1995, the nine-story concrete framed Alfred P. Murrah Federal Building in Oklahoma was struck by a huge car bomb causing partial collapse, resulting in the deaths of 168 people. The bomb, though large, caused a significantly disproportionate collapse of the structure. The bomb blew all the glass off the front of the building and completely shattered a ground floor reinforced concrete column. At second story level wider column spacing existed, and loads from upper story columns were transferred into fewer columns below by girders at second floor level. The removal of one of the lower story columns caused neighbouring columns to fail due to the extra load, eventually leading to the complete collapse of the central portion of the building (See Fig. 19). The bombing was one of the first to highlight the extreme forces that blast loading

Fig.19 Collapsed Alfred P. Murrah Federal Building in Oklahoma (Source: http://eyeofthefish.org)

from terrorism can exert on buildings, and led to increased consideration of terrorism in structural design of buildings. The Federal Emergency Management Agency (FEMA) developed a number of design guidelines for limiting or mitigating the effects of terrorist attacks, focusing primarily on explosions, but also addressing chemical, biological, and radiological attacks (FEMA 426, FEMA 427, FEMA 428, FEMA 430, and FEMA 439 A &B).

9.2 Collapse of World Trade Center Towers, New York

The twin 110 storey towers of the World Trade Center in New York City, USA collapsed on September 11, 2001, as a result of two commercial airliners, hijacked by terrorists, deliberately crashed into them. The impact and resulting fires caused both towers to collapse within two hours. Later that day, WTC Building 7 also collapsed from fires that had started when the North Tower collapsed. As a result of the attacks to the towers, 2,752 people died, including all 157 passengers (including the hijackers) and crew aboard the two airplanes (Subramanian, 2002).

The following changes were included in International Codes as a result of 9/11 attack (http://www.iccsafe.org.): Elevators must be provided in high-rise

buildings more than 36.5 m tall so that firefighters can get into and fight fires, without having to walk up the stairs with heavy equipment;

An additional stairway has to be provided for high-rises that are more than 128 m tall;

In lieu of the additional stairway, extra elevators may be provided that can be used to evacuate building occupants without waiting for assistance from emergency personnel;

A higher standard for fire resistance has to be adopted in high-rise buildings more than 128 m tall;

More robust fire proofing has to be provided for buildings more than 23 m tall, so that they will not be dislodged by impacts or explosions;

Shafts enclosing elevators and exit stairways


should have impact resistant walls; Self-luminous exit pathway markings should

be provided in all exit stairways; and

Radio coverage systems should be available within the building to allow emergency personnel to better communicate with the people inside the building and with emergency staff outside the building.

10.0 Failure of anchor bolts On July 10, 2006, about 26 tons of concrete and associated suspension hardware fell on a passenger car when it was passing the Interstate 90 connector tunnel in Boston, USA (This tunnel is often referred to as the Big Dig), killing a passenger and injuring the driver. A later investigation found hundreds of dangerous adhesive anchors were holding together the tiles on the tunnel ceilings, which had to be removed.

The National Transportation Safety Boards (NTSB) investigation of that accident determined that the ceiling collapse was due to the use of an epoxy anchor adhesive with poor creep resistance, that is, an epoxy formulation that was not capable of sustaining long-term loads. Over time, the epoxy deformed and fractured until several ceiling support anchors pulled out and allowed a portion of the ceiling to collapse. Selection of a better adhesive could have prevented the accident. Powers Fasteners has increased the safety factor on its fast-setting materials by a factor of four since the Big Dig collapse. NTSB recommended federal and state highway authorities develop standards and protocols for the testing of adhesive anchors used in sustained tensile load overhead highway applications, and consider the creep characteristics of polymers. A mandatory tunnel inspection was also suggested. More information about this failure and recommendations by NTSB may be found at NTSB/HAR-07/02 (2007). This led ACI committee 318 to work diligently on design requirements for adhesive anchors and include them in Appendix D of ACI 318-11.

In this connection, it is important to note that ACI 503.5R-92, Guide for the Selection of Polymer Adhesives with Concrete, which was first

published in 1992 and reapproved in 1997 and 2003 cautions about creep failure of adhesive anchors and suggests pre-testing of such anchors. ACI Committee 355 also developed ACI 355.2-07, Qualification of Post-Installed Mechanical Anchors in Concrete.

New Steel Erection Final Rule by the US Department of Labor, Office of Safety and Health Administration (OSHA), issued on January 18, 2001, effective January 18, 2002, mandated the use of four, rather than two, anchor bolts in structural steel column base plates, as well as a minimum design load and eccentricity in Section 1926.755(a) General requirements for erection stability of the Construction Industry Standards. This rule was "negotiated" as a result of numerous construction accidents caused by the toppling of unbraced steel columns during erection (Ratay, 2011)

11.0 other FailuresIn this section we will consider other failures which prompted revision of code provisions.

11.1 Collapse Due to Corrosion of Post-tensioned Steel

The Benjamin Franklin Hall, (also called Der Kongresshalle or the 'pregnant oyster'), built in 1957 collapsed on May 21, 1980, without any visible deterioration prior to failure, killing one and injuring numerous people. The 76 mm thick reinforced concrete shell roof resembles an open human eye with a tension ring as the pupil and the two arches at the edges representing the upper and lower lids. The two arch support points represent the corners of the 'eye'. The thin shell roof had post-tensioning bars in ducts which had corroded partly due to poor quality of grouting, and led to the collapse. The hall was rebuilt in its original style and reopened again in 1987 at the 750 years jubilee of Berlin. More details of the failure may be found in Subramanian, 1982.

It has to be noted that the internally post-tensioned Ynys-y-Gwas Bridge in Wales collapsed in 1985. In 1986 the bridge over the Mandovi River in Goa, India collapsed after less than 20 years in service due to corrosion of the post-tensioning cables and the Malle Bridge over


the river Schelde in Belgium collapsed in 1992 for similar reasons. Based on these failures, the UK Department of Transport banned ducted grouted post-tensioning in bridges in 1992.

11.2 Hyatt Regency walkway collapse

The lobby of the 40-story Hyatt Regency hotel in Kansa City, Missouri, USA featured a multistory atrium, which had suspended concrete walkways on the second, third, and fourth levels. The Second and fourth level walkways were suspended from a set of steel tension rods of size 32 mm, with the second floor walkway hanging directly underneath the fourth floor walkway. The walkway platform was supported on 3 cross-beams suspended by steel rods retained by nuts. The cross-beams were box beams made from C-channels welded toe-to-toe. The original design called for three pairs of rods running from the second floor all the way to the ceiling. On July 17th 1981, when a party was going on, the 4th floor walkway failed and fell on the lower walkway, both walkways crashing into the floor three stories below killing 114 people and injuring 185. The separate third floor walkway was not involved in the collapse.

Fig. 20 Difference between the design and construction of the walkway support system

The cause of the failure is found to be that the contractor replaced the single vertical suspension rod specified by the original designer, by two shorter rods; one from the upper support to the first walkway, and another from the bottom beam of the first walkway down to the second walkway (see Fig. 20). Now the nut and washer under the upper rod is subjected to double the design load (in addition the resulting eccentricity created a local bending moment), which lead to the failure. Failure to communicate this detail properly to the original designers and failure to check the details were cited as the main problems for the failure;

the faulty connection detail that failed was never shown on any drawings, and it was not even designed. The high number of fatalities resulting from the walkway's collapse raised the question of whether the factor of safety required for a building should be proportional to the possible consequences of its collapse (Kaminetzky, 1991).

11.3 Hartford Civil Centre roof collapse

The Hartford Civic Centre Coliseum, Connecticut, USA, was completed in 1973. The space frame roof structure was 7.6 m high and covered 110 m by 91 m, with clear spans of 64 to 82 m. On January 17, 1978, at 4:15 a.m. the roof crashed down 25.2 m into the floor, due to a large snow storm. Luckily it was empty by the time of the collapse, and no one was hurt. Though there were several causes for the collapse, the main cause was the relatively minor changes in the connections between steel components, i.e., the fabrication deviating from design. A few centimeters shift of the fabricated connection, cut down the axial force capacity to less than tenth of the design value! Some angle sections found at the wreckage were found to have failed in block shear. Epstein and Thacker, in 1991 used finite element analysis and found that block shear was the mode of failure for these angles. This study also established the difference in behaviour of coped beams (where the load is applied to the connection in the plane of the web, which also is the block shear plane) and angles (where the load is applied eccentric to the failure plane).

In addition, the Hartford Civic Centre Coliseum roof design was extremely susceptible to torsional buckling of compression members which, as a mode of failure, was not considered by the computer analysis used by the designers. Had the designers chosen tubular or even I sections, instead of the cruciform section adopted in the roof members, the failure might have been averted (the four steel angles forming the cruciform cross-section has much smaller radius of gyration than tubes or I-sections, and hence not efficient in resisting compressive loads). This failure also showed that computer software should be used only as a software tool, and not as a substitute for sound engineering experience


and judgment (Smith and Epstein, 1980).

11.4 Failure of Slender Shear Walls

Observed wall damage in recent M 8.8 earthquakes in Chile (2010) and New Zealand (2011), where modern building codes exist, exceeded expectations. In these earthquakes, structural wall damage included boundary crushing, reinforcement fracture, and global wall buckling. IN ACI 318-11, A single curtain of web reinforcement is allowed if wall shear stress is less than 0.17f_c^' MPa, where f_c^' is the cylinder compressive strength of concrete. This provision is acceptable for squat walls with low shear stress (e.g., walls with aspect ratio less than 1.5); however, for slender walls where buckling of boundary vertical reinforcement and lateral instability are more likely due to significant tensile yielding of reinforcement under cyclic loading, two curtains should always be used. This recommendation applies to both Special Structural Walls (high ductility) and Ordinary Structural Walls (moderate ductility). Based on laboratory tests it was suggested to change the value of the denominator in Eqn. 21.8 of ACI 318-11 from 600 to 1200 (Wallace, 2012). To ensure spread of plasticity consistent with the derivation of Eqn 21.8 of ACI 318-11, walls should be designed and detailed as tension-controlled.

11.5 Failure of Welded Beam-column Connections

Subsequent to the January 1994 Northridge earthquake in California and Kobe earthquake in Japan in 1995, it was determined that some damage to momentresisting frames occurred at the beam-column connections. Failures included fractures of bottom beam flange-to-column flange complete-joint-penetration groove welds, which propagated into the adjacent column flange and web and into the beam bottom flange. This failure was accompanied in some instances by secondary cracking of the beam web shear plate and failure of the beam top flange weld. The factors that contributed to the damage included the following (FEMA 2000):

Stress concentration at the bottom flange weld, due to the notch effect produced by backing strips left in place;

The use of low toughness weld metal at the beam-column connection;

Uncontrolled deposition rates; The use of larger members than those

previously tested; Lack of control of basic material properties

(large variation of member strength from the prescribed values);

Inadequate quality control during construction; and

The tri-axial restraint existing at the center of beam flanges and at the beam-column interface, which inhibits yielding.

A multi-billion dollar research conducted over 10 years resulted in the development of design provisions for moment resistant frames, and prescribed in AISC 341-05 (Seismic provisions for structural steel buildings, American Institute of Steel Construction), which was again revised in 2010. In addition, AISC developed another standard, AISC 358-05 (Pre-qualified connections for special and intermediate steel moment frames for seismic applications including Supplement No.1), which was revised in 2010. This Standard presents materials, design and detailing, fabrication, and inspection requirements for a series of pre-qualified moment connections. The AISC 358-2010 contains a number of pre-qualified connections and these are discussed in Subramanian, 2010.

12.0 Summary and ConclusionsThe earliest building code is The Code of Hammurabi circa 1760 BC. Unlike todays codes, the Code of

Hammurabi dealt more with the consequences of building failure rather than how to safely construct a building. For instance, Law #229 stated If a builder builds a house for someone, and does not construct it properly, and the house which he built falls and kills its owner then the builder shall be put to death. Modern building codes shifted from outlining the punishment for poor construction to mandating requirements that would make building safe and better. Todays building codes are built on the experience of


the past. Hence, each new earthquake, fire, tornado, hurricane or other natural/man-made disaster results in improved codes for building construction. Therefore, when a structural failure occurs, investigators review the adherence of the failed structure to the governing codes, standards, regulations and industry practices. If it is found that some of the clauses in these Standards contributed to or, indeed, created the cause of the failure, attempts are made to review and revise those provisions. In this paper, some of the important collapses that resulted in revision of code clauses are described. Such failures if documented properly will be useful to the practicing engineers who can learn from these failures and will not repeat the mistakes in their designs. In addition, in Western Countries the concept of conducting professional examinations has been introduced to qualify Engineers for professional practice. Such a practice will reduce the number of failures, as the practicing Engineers will be required to constantly upgrade their knowledge, as more and more materials, design and construction techniques and methods are being introduced rapidly due to research and development conducted all over the world.

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Failures That Changed Perception of Our Designs-InG IABSE December 14