KEYNOTE ADDRESS

ON

“Collapse of Engineering Structures: Causes, Implications and Prevention”

BY

ENGR DR EDET J. AMANANA, FNIStructE, FAEng, OON

(President of the Nigerian Academy of Engineering)

AT THE 23RD ANNUAL CONFERENCE OF

THE NIGERIAN INSTITUTION OF STRUCTURAL ENGINEERS (NIStructE)

The Chairman of this great occasion, Engr Mustapha Bulama, FNIStructE, FNSE

President of the Council for the Regulation of Engineering in Nigeria (COREN),

The President of Nigeria Society of Engineers (NSE),

The President Nigerian Institution of Structural Engineers, Engr Victor Oyenuga,

FNIStructE, FNSE,

President, Nigerian Institute of Civil Engineers (NICE)

Presidents of other Professional bodies here present,

Chairmen of branches of NSE here present,

Distinguished Fellows and Members of NIStructE,

Distinguished Invitees,

Gentlemen of the Press,

Distinguished Ladies and Gentlemen.

Introduction

I thank the President of NIStructE and the organising committee of this conference

for the honour they have done me by inviting me to give this keynote address. I must

confess this invitation came at a time when I had a bunching of activities in my

calendar and there was a great temptation for me to request that I be considered for

another conference maybe next or so. But I must confess that I could not bring

myself to say no to Engr Victor Oyenuga, our able president Victor, I thank you so

much for your call and your soft persuasive voice.

Talking about Collapse of Engineering Structures, structure is talking about the very

life and death of our profession; life in the sense that the greatest joy of the

structural engineer is in seeing an edifice he/she has created standing out

beautifully, majestically and fulfilling the function for which it was designed; death

because many a structural engineer has been known to wish themselves dead when

a structure which they gave their all to design has suddenly collapsed and brought

about the death of so many people.

The fact that the collapse of structures always results in death of so many including

the reputation of the structural engineer underscores the need to carefully study,

document and review each incident of collapse in order to learn new lessons, avoid

future collapses and progress the science and art of structural engineering.

Definitions

But before we go any further we would like to take some definitions.

First: What is Structural Engineering?

Second: What is Structural Collapse as different from Structural Failure – another

term often used when a structure cannot perform its designed function?

For the definition of Structural Engineering I shall adapt a definition credited in

the literature to the man who taught me structural engineering at the Imperial

College, Dr. Eric Brown. I define Structural engineering as “the art and science of

modelling materials we do not wholly understand into shapes and forms we cannot

precisely analyse to withstand forces we cannot properly assess in such a way that

the structure stands up and the public at large has no reason to suspect the extent

of our ignorance.” This definition is instructive and we shall see its relevance as we

analyse specific cases of collapse.

A

Structural Collapse

Before we turn our searchlight on structural collapse, let us look at the

complimentary issue of structural failure which may be defined as “an unacceptable

difference between expected and observed performance of the structure”. I was in

Abuja recently and I saw an almost completed eight storey structure built into the

slope of a hill in Maitama. The building is cracked all over but it is still standing; it

has not “collapsed”. But I bet not even a mad man will try to live there. This is a

situation in which the building structure has failed and cannot fulfil the function it was

designed for. That building will eventually be brought down. But there are many

situations in which failure is localised and the structural system has so much

redundancy that there is a redistribution of forces and structural collapse does not

occur.

Structural collapse may be defined as that state of failure of the structure in which

the whole structure or a part of it is incapable of sustaining any load and therefore

becomes unstable and falls down which result from inability of the structure to resist

static loads (static instability) or inability to withstand dynamic loads (dynamic

instability). It is pertinent to observe that every structural collapse originates from a

failure of a structural member or component. The study of structural collapse will

therefore necessarily entail careful analysis of the issues which bring about

structural failures. The study of these issues is so important to our profession that an

entirely new discipline “Forensic Engineering” has been created to deal with the

investigation of failures. Most of the landmark cases of structural collapse have been

the result of catastrophic failures of structural components which resulted in total

instability of the structure. In the discussion which follows, the words “failure” and

“collapse” shall be used interchangeably. I have done this because from the

engineer’s point of view, a structure which fails to perform its designed function

needs to be brought down – demolished.

Collapse of structures could manifest in three forms:

1) Partial Collapse is that state of failure when only part of the structure is

affected. This would typically happen when a structural element has failed but

the structure system has enough redundancy for the loads to be redistributed

in such a way that the structural system does not become unstable but is still

able to sustain applied load.

2) Progressive Collapse is the state of affairs in which the failure of a structural

element brings about the transfer of forces to other parts of the structural

system where there is inadequate resistance and so the whole system

becomes unstable and collapse progresses from one part of the structures to

other part until the complete structure falls down.

3) Total/Sudden Collapse happens without any prior indication or warning.

Structural Design

In the design of structures, the limit state of performance can be classified into two

categories i.e. the Ultimate Limit State (ULS) and the Serviceability Limit State

(SLS). A limit state is a set of performance criteria (e.g. vibration levels, deflection,

strength, stability, buckling, twisting) that should not be exceeded when the structure

is subjected to loads. Any design process involves a number of assumptions and

actions. To satisfy the Ultimate Limit State, the structure must not fail when

subjected to the peak design loads. To satisfy the Serviceability Limit State Criteria,

a structure must remain functional for its intended use when subjected to

routine/everyday loading, the structure must not cause users or occupants

discomfort under routine service conditions.

Therefore, in carrying out the design of a structure, it is very important to have:

1) A complete and accurate assessment of all possible loads and different

combinations of their applications throughout the life cycle of the structure

from construction through service.

2) An accurate assessment of the quality, strength and variability of the

materials used in forming the structural components/members. A good

example of how quality, strength and variability play out in our environment in

the wide variability of the strength of concrete used in our construction sites.

What many of our builders called concrete is often a “slurry” which cannot

stand the standard test of concrete mixes with regard to slump and other tests

which would give an indication of what strength to expect.

3) A proper understanding of structural behaviour cases of the structure in which

consideration must be given to all loading under stages of construction and

operation with due regard to possible scenarios of serviceability and ultimate

limit states.

The different codes of practice give guidance on these matters and the use of

appropriate softwares whose limitations must be properly understood will help is

performing the complex calculations involved in structural analysis and design.

B

Study of Landmark Engineering Failures/Collapse

Exposure to landmark engineering failures and the subsequent investigation reports

gives structural engineers a better theoretical understanding of the nature of

materials they work with. They become aware of the manner in which engineering

design strategies evolve and how those design theories approximate to service

conditions. They also gain a greater appreciation for the inherent professional

responsibilities of their chosen profession. In addition to technical factors, the study

of engineering failure case histories provides opportunities for discussions of

important nontechnical topics: ethics, professional liability, human factors, and the

critical interpersonal skills and interdisciplinary relationships and regulatory

framework required for the delivery of a successful project.

We shall now look at a few typical failures which have been documented in the

literature to examine their causes, and implications and draw lessons on how they

could have been prevented. We believe that for engineers, a single sketch

communicates more information than pages of writing. We shall therefore use

sketches to illustrate our points here.

Hyatt Regency Walkway Collapse, Kansas City, Missouri, USA, July 17, 1981

A 40-storey tower comprising of an atrium and a function block with three walkways

spanning 37m, the Hyatt Regency Crown Center in Kansas City, Missouri was

opened in July 1980 after four years of design and construction. The walkways were

suspended from the atrium’s ceiling by 32mm diameter hanger rods. The second-

floor walkway directly below the fourth-floor walkway, was suspended from the

beams of the fourth-floor walkway, and third and fourth-floor walkways hung from

the ceiling.

On July 17, 1981, about 2000 people were on the atrium floor and on the suspended

walkways to see a local radio stations dance competition when a loud crack echoed

throughout the building and the second and fourth-floor walkways crashed to the

ground, killing 114 people and injuring more than 200 others.

Causes

From the investigation carried out by the National Institute of Standards and

Technology (NIST), it was discovered that the hanger rod pulled through the box

beam causing the connection supporting the fourth-floor walkway to fail. Due to lack

of redundancy and alternate load paths, the other rods could not handle the

increased load once the adjacent rod failed and this failure caused the collapse of

both walkways.

Originally, the second and fourth-floor walkways were to be suspended from the

same rod and held in place by nuts. The preliminary design sketches contained a

note which specified strength of 413MPa for the hanger rods which was omitted on

the final structural drawings. Following the general notes in the absence of a

specification on the drawing, the contractor used hanger rods with only 248MPa of

strength. This original design however was impractical because it called for a nut

6.1m up the hanger rod and did not use sleeve nuts. The contractor modified this

detail to use two hanger rods instead of one and the engineer approved the design

change without checking it. This design change doubled the stress exerted on the

nut under the fourth floor beam.

This underscores the need for design engineers to think through their design details

and not leave room for the contractor to improvise. The contractor is not mentally

equipped to see all the design implications.

Fail

Tacoma Narrows Bridge, Tacoma, Washington, USA, November 7, 1940

Cause

It was the 3rd longest bridge in the world when it was opened in July 1940. It was

much narrower, lighter and more flexible than any other bridge of its time. The

bridge designer wanted the bridge to maintain a sleek appearance, that he designed

it without the use of stiffening trusses, replacing them with shallower plate girders.

This modification left Tacoma Narrows Bridge with one-third the stiffness of some of

its contemporaries like Golden Gate Bridge and George Washington Bridge. These

characteristics coupled with low dampening ability caused large vertical oscillations

in the bridge even in the moderate of winds which earned it the nickname “Galloping

Gertie”.

Under the effect of a storm whose wind speed was about 68kph, the cable stays on

the west side and north centre of the bridge broke thus causing the bridge to twist

violently in two parts (with rotation of more than 45o and vertical movement of 8.5m).

This twisting motion caused high stresses throughout the bridge and led to failure of

suspenders and eventually the collapse of the main span.

Implications

The collapse showed engineers and the world the importance of damping, vertical

rigidity and torsional resistance in suspension bridges. It also highlighted the

importance of failure literacy, knowledge of historical failures and their causes. The

imperativeness of studies on the aerodynamic forces that act on suspension bridges

was also highlighted.

Preventive Measures

Once the threat of twisting in the bridge was realized, there are many ways the

disasters could have been averted. Any of the following adjustments could have

prevented the collapse.

Using open stiffening trusses, which would have allowed the wind free

passage through the bridge.

Increasing the width-to-span ratio.

Increasing the weight of the bridge.

Dampening the weight of the bridge.

Using an untuned dynamic damper to limit the motions of the bridge.

Increasing the stiffness and depth of the trusses and girders

Streamlining the deck of the bridge.

Lessons Learnt

The question to be asked when innovations like suspension bridges were conceived

is “How do we strike a balance between public welfare and progress in the face of

new technology?” If the bridge has been designed similar to the ones that had

already proven their stability, it would never have collapsed. On the other hand, if

engineers never tried innovative techniques, suspension bridges might never have

been built at all.

The limits of technology was pushed by trying to create a longer sleeker and less

expensive bridge. Every time engineers push the limits of technology, they risk a

similar loss, sometimes even loss of life. At the conceptual stage of innovative

design, engineers or designers should ponder on and be able to provide answers to

questions like “When is a possible advance worth a risk to public safety?” and “What

can engineering profession do to make the implementation of new technology safer?”

Teton Dam Collapse, Teton River, Idaho, USA, June 5, 1976

Teton Dam, situated on the Teton River, northeast of Newdale, Idaho was designed

to provide recreation, flood control, power generation and irrigation for over 100,000

acres of farmland. The design of the foundation consisted of four basic elements:

1. 21-meter deep, steep-sided key trenches on the abutments above the

elevation of 1,550 meters;

2. A cut off trench to rock below the elevation of rock under the abutments;

3. A continuous grout curtain along the entire foundation;

4. The excavation of rock under the abutments.

These elements for the foundation were important because the types of rock located

in this area, basalt and rhyolite are not generally considered acceptable for structural

foundations.

When the construction of the dam began in February 1972, the embankment was

proposed to have a maximum height of 93 meters above the riverbed and would

form a reservoir of 365 million cubic meters when filled to the top. The dam was

closed and began storing water on October 3, 1975 but the river outlet works tunnel

and auxiliary outlet works tunnel were not opened. Due to these incomplete

sections, the water was rising at a rate of about 1m per day instead of the targeted

rate of 0.3-0.6m per day.

On June 3, 1976, a major leak flowing at about 500-800L/s was noticed which later

increased to 1100-1400L/s an hour later. Seepage at about 40m below the crest of

the dam was also noticed. Two hours later, a whirlpool was observed in the reservoir

directly upstream of the dam due to the expansion of the location of the leak up the

dam and after a while a chunk of the dam fell into the whirlpool leading to the entire

collapse of the dam.

After the failure, an Independent Panel was set up to review the cause of the

collapse. The panel summarised its conclusions as follows:

1. The design of the dam followed well-established USBR practices, but without

sufficient attention to the varied and unusual geological conditions of the site.

2. The volcanic rocks of the site are “highly permeable and moderately to

intensely jointed”.

3. Considerable effort was used to construct a grout curtain of high quality, but

the rock under the grout cap was not adequately sealed. The curtain was

nevertheless subject to piping – “too much was expected of the grout curtain,

and the design should have provided measures to render the inevitable

leakage harmless”.

4. The dam’s geometry caused arching that reduces stresses in some areas and

increased them in others and favoured the development of cracks that will

open channels through the erodible fills.

5. Finite element calculations suggested that hydraulic fracturing was possible.

6. Piping was identified as the most probable cause of the failure. Two

mechanisms on how the piping started were suspected to be possible. The

first was the flow of water under highly erodible and unprotected fill, through

joints in unsealed rock beneath the grout cap, and development of an erosion

tunnel. The second was cracking caused by differential strains or hydraulic

fracturing of the core material.

7. The fundamental cause of failure may be regarded as a combination of

geological factors and design decisions that, taken together permitted the

failure to develop.

This case may be best summarised in the words of the panel’s report,

1. The dam failed by internal erosion (piping) of the core of the dam deep in the

right foundation key trench, with the eroded soil particles finding exits through

the channels in and along the interface of the dam with the highly pervious

abutment rock

2. The exit avenues were destroyed and removed by the outrush of reservoir

water.

3. Openings existed through inadequately sealed rock joints, and may have

developed through cracks in the core zone of the key trench.

4. Once started, piping progressed rapidly through the main body of the dam

and quickly led to complete failure.

5. The design of the dam did not adequately take into account the foundation

conditions and the characteristics of the soil used for filling the key trench.

Ronan Point Apartment Tower Collapse, London, Britain, May 16, 1968.

Ronan Point Progressive collapse was due to failure of a structural element cause

by gas explosion on 19th floor of the 22 storey block of flats. The blocks of flats were

constructed using precast floors, walls and staircases. Ecah floor slab was

supported by the load bearing walls beneath it and the wall and floor panels were

fitted together in slots and bolted; the joints being packed with dry mortar to secure

the connection.

The force of the explosion knocked out the opposite corner walls of the apartment

which was the only support of the walls above. So there we had a structural system

which had no support resulting in static instability and collapse. The design life

expectancy of the building was 60 years. It was however demolished after 10 years.

Poor workmanship was suspected and so demolition was by dismantling the building

floor so that the joints could be studied. The degree of shoddy workmanship was

shrieking.

A public enquiry was held in Britain after the extensive research into all the factors

associated with the design, construction and failure and collapse of the building

Advisedly, the research focused in the following

1) Construction materials for building and associated facilities – the gas supply

system

2) Adequacy of design loads (dead, live, wind, explosion)

3) Design method modified to introduce progressive collapse for buildings over 5

storeys high. Design checked by analysing the structure after removal of

statical structural member ensuring alternate load path.

This collapse triggered investigation in the design and construction methodology by

government and private organisations in Europe and USA. The lessons learnt

changed building regulations throughout the world.

In many cases the analysis and study of structural failures/collapses have resulted in

the advancement of structural engineering knowledge and improved practices

especially where the original engineer appears to have done everything in

accordance with the state of the profession and acceptable practice yet

failure/collapse still occurred. Such cases are.

1) Tacoma narrows Bridge

2) Ronan Point Collapse

L’Ambiance Plaza Collapse

L’Ambiance Plaza was planned as a 16 storey building with 13 apartment levels on

top of 3 parting levels set between two offset rectangular towers. Post-tensioned

concrete slabs 175mm thick and steel columns made up the structural frame with

two shear walls in each tower to provide lateral stability. Using the lift slab method,

the 16 floor slabs were constructed on the ground one on top of the other. Then

packages of two or three slabs were lifted into temporary position by hydraulic jacks

and held in place by steel wedges. Once the slabs were positioned, they were

permanently attached to the steel columns. Because of the importance of the shear

walls to stability of the system, the drawings specified that they should be within

three floors of the lifted slabs.

At the time of collapse, the 9th, 10th and 11th floor slab package in the lowest tower

were packed under 12th roof slab and shear walls were about 5 levels below the

lifted slabs. Steel wedges were being welded to support the slabs when all of a

sudden the slabs above cracked like a “sheet of ice” and fell on the slab below and

with that high impact loading the whole construction came tumbling down in 5

seconds only. Twenty eight (28) construction workers died in the collapse.

All the parties involved in the design and construction of the building hired forensic

engineering firms to investigate possible causes of collapse. Prompt legal

settlements prematurely ended the investigation. $14 million was paid in settlement

to different parties. Investigations revealed the following.

a) Although the buildings constructed by the lift slab method are stable once

they are completed, great care must be taken to ensure lateral stability during

construction.

b) Many design deficiencies were undetected because design was fragmented

among many subcontractors and there was no Engineer of Record”

c) For system buildings such as lift slab construction there is need to establish

and strictly follow a standardised step-by-step procedure with at least 3

experts working in close collaboration to ensure – the structural engineer of

record, lift slab expert, and post-tensioning expert.

Considering overall structural stability of the system;

It is important that the designer’s assumption about structural behaviour are

consistent with detailing mistake

Manufactured Structures – where structural components are bought and

assembled to form a building could lead to a disconnect between the

manufacturer’s structural engineer who would typically use the latest software

to design the structure to the finest limit to make their product competitive and

the contractor’s/erector’s engineer who would typically be unfamiliar with the

software and even all the design assumptions. This situation underscores the

need for “ENGINEER OR RECORD”

Collapse of structures especially buildings is also on the increase in Nigeria these

days. It is instructive to note that most of the buildings in which collapse cases were

recorded are newly-built which are yet to fulfil their intended design life. Recent

researches have shown that small scale structures are more likely to collapse than

high-rise structures due to the fact that adequate care and precautions are taken

into consideration in designing and constructing high-rise structures while a lot of

negligence is employed in designing and building low-rise structures. It is of interest

to say that most of the collapses we have experienced in Nigeria in the past years

are majorly low-rise structures and few high-rise structures if any.

Some Collapse of Building Incidents in Nigeria

1) The collapse of a multi-storey (4 – 5 storey) building in Nigeria’s capital Abuja

on Aug 11 2010 claimed about 23 lives and 10 seriously injured. The building,

condemned as dangerous by the authorities and which was thought to be four

or five floors high, tumbled down at around dawn on Wednesday in Garki

Area 11 neighbourhood of Abuja, south of the Central Business District. The

collapse was linked to substandard materials and disregard for building

regulations. The building has been condemned by the authorities and

developers had continued to add an additional floor despite the warnings from

the regulatory authorities.

2) A similar incidence in Lagos where a 4-storey building in Ebute- Metta, Lagos

collapsed which claimed nearly 30 lives and lot of people serious injured. The

building suddenly gave in on a Tuesday evening. The building was supposed

to be a residential apartment but it also housed restaurants and 18 shops on

the ground floor thus exceeding the intended design usage. The building has

been just three years old, and officials are blaming poor construction for the

collapse. Local media are reporting that city planners only gave permission for

two storeys building on the site, not four. The owner of the construction

company fled after the incidence.

3) The case of the collapse of a building located on a plot adjacent to 109

Western Avenue, Iponri, Lagos State was investigated by the Nigerian

Institution of Structural Engineers. The investigation revealed that:

a) The building originally existed as a single storey block (bungalow)

b) The owner who acted as his own contractor, decided to add many more

storeys (floors to it).

c) There is evidence of a planning permit of only on additional storey.

d) That the Town Planning Authority discovering the contravention, marked

it for demolition, the owner went ahead with construction at nights until

the collapse occurred during the concreting of the fourth floor.

Structural failures/collapses are always caused by the aggregate effect of a number

of factors which come together at a particular time to cause distress.

Failures/Collapses may be systemic – which means the failure is brought about by

the manner in which the organisation/project is organised/managed

Responsibility for failure must be apportioned.

“Corporate Manslaughter and Corporate Homicide Act 2007” concentrates on

corporate management standards and make the organisation – contractor or

designer responsible for collapse to undergo appropriate sanctions”.

Failures/Collapse due to Systemic inadequacies – adoption of foreign codes without

associated recognition of necessary management procedures to ensure that safety

is achieved.

RISK MANAGEMENT PRINCIPLES in which the key stakeholders in a construction

project take adequate precautions to assess, understand and management their

risks and rewards.

Owner/Investor

Designers

Builders

There is need to clarify responsibility between all parties involved in the construction

process and emphasise obligation to manage risk.

Adequate Design

Adequate understanding of structural action

Use of appropriate software

Construction Lapses

Failure to build in line with proper design and specification

Wrong construction methodologies (lack of training and experience)

Operational Lapses

Faulty operation subjects structure to forces not envisaged in design