Case Studies on Failure Investigations in Structural and Geotechnical Engineering

Failures of structures occur in all parts of the world as the result of design errors, construction defects, abuse or misuse, ageing and deterioration of the structure, lack of maintenance, as well as environmental effects such as wind, flood, snow, earthquake and, of course, human errors. They can result in catastrophic human costs as well as h

• Language: English
• Publisher: International Association for Bridge and Structural Engineering (IABSE)
• Release date: Sep 20, 2023
• ISBN: 9783857481956

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Chapter 1

Introduction

Fabrizio Palmisano¹,² & Laurent Rus³

1 Politecnico di Torino, Turin, Italy, 2 PPV Consulting, Bari, Italy, 3 Singular Structures Engineering, Madrid, Spain

This chapter gives a brief overview on forensic structural engineering.

1.1 Structural Failure

The failure of a structure is defined as either the non-conformity of the structure to withstand the design expectations leading to loss of structural integrity (Ultimate Limit State ULS failure), i.e., the loss of load-carrying capacity, or the inadequate difference between the intended behaviour and actual performance of the structure (Serviceability Limit State SLS failure). Thus, it ranges from hardly visible minor defects to catastrophic collapses. It can extend from total or partial collapse of the structure (ULS) to extensive or serious damage without collapse (ULS), to signs of distress (SLS), to excessive deformation (SLS), deterioration (SLS), to unacceptable aesthetic appearance (SLS), to unreasonable maintenance need (ULS or SLS), or to excessive soil settlement (ULS or SLS), to name a few structural failures.

There also are cases with no readily visible signs. These can be suspected design errors, construction defects, and hidden deterioration that present risks of potential failures but may not be recognized without structural analysis or testing.

Causes of structural failures can be related to any stage of a construction:

Planning: system concept.

Design: approach, calculations, drafting.

Design-construction interface: shop drawing detailing, drafting, review and approval.

Construction/renovation: erection, inspection, accident.

Use, misuse, abuse, alteration, overload in service.

Lack of maintenance.

Structural failures are often the result of human failings such as:

Negligence: deliberate failure to properly analyse or detail the design and deliberate disregard of codes and standards.

Incompetence: not understanding engineering principles or technical limitations of materials or systems.

Oversight/carelessness: failure to follow design documents and safe construction practices.

Greed: short-cuts, intentional disregard of industry requirements and safe construction practices to save money.

Disorganisation: failure to establish a clear organisation and define responsibilities of parties.

Miscommunication: failure to establish and maintain lines of communication among the parties.

Misuse, abuse, neglect: using the facility for purposes beyond its design intent; foregoing preventive maintenance.

Corruption and bribery: money or favour to influence the decision-making of a person involved in a project.

Structural failures may occur during the so-called ‘Acts of God’ (i.e., extreme environmental events) but they are often the result of human action/inaction/error.

Over the years, forensic engineering experience has demonstrated that often there is no single cause for a structural failure, but instead several causes. These causes shall be categorized as primary and secondary causes due to the ‘domino’ effect generated by the primary causes. Additionally, the primary causes shall each represent an unsafe condition that can jeopardize the structural integrity or serviceability but combining them all would increase the risk of failure.

While there will be several causes leading to the failure of the structure, often only one would be the trigger. Indeed, without the presence of the trigger, the failure of the structure would not have occurred.

1.2 Forensic Structural Engineering Practice

The practice of forensic structural engineering involves engineering investigations of structural failures, determination of the causes and triggering effect, as well as rendering opinions in the share of responsibilities across the different stakeholders (project members) and, should it be required, giving testimony in judicial proceedings.

The ability to locate and identify the cause(s) of a structural failure allows us to improve the performance of the structure and to increase its resilience against external shocks/stresses (‘shock’ in the sense of a sudden and intense demand on the structure compared to ‘stress’, which would rather refer to a cyclical long-term demand over the life cycle of the structure; both external demands will place at risk the performance of the structure).

This ability to locate the cause(s) of the failure allows us to intervene and to implement appropriate measures at design stages in the design life of the structure for a more resilient infrastructure.

Disruption of the planning, design, construction, or operational stages during the design life of the structure is inevitably a reduction in both the efficiency of the stakeholders and financial mechanism set-up in place, which lead ultimately into a reduced country’s growth capacity.

1.3 Forensic Structural Engineering Investigation

A forensic structural engineering investigation generally has the benefit of the evidence, which, correctly analysed and supported by facts and documentation, can explain how and why the failure of a structure occurred.

The typical process of forensic investigation of a structural failure may be broadly outlined as follows:

first-response;

initial field observation;

fact-gathering and document review;

preliminary evaluation;

development of investigation strategy;

collection of physical evidence and data;

surveys, measurements, testing;

structural analysis;

reporting;

recommendations on how to avoid repeating the same mistakes.

The fundamental answers that should be given at the end of the investigations are the following:

the status of the construction at the moment of the failure;

external loads acting at the moment of the failure;

internal stresses or forces acting at the moment of the failure;

capacity of the structure;

place where the failure was originated;

primary cause of the failure;

contributing factors to the collapse;

trigger and evolution of the failure;

people responsible for the failure.

1.4 Forensic Structural Engineering in Continuous Development that Combines Design & Construction Practice and Research

Forensic structural engineering is a fascinating topic that requires knowledge, experience, judgement, intuition, and passion. The practice in this field is in continuous development due to the proliferation of failure modes, mechanisms, and observed pathologies linked to the development of new construction materials, as well as the complexity of the design and construction techniques of structures, in addition to the scrutiny by insurance companies and readiness of people to sue engineers. Forensic structural engineering provides the necessary technical support in arbitration and judicial proceedings. Moreover, it is recognised that future failures can be mitigated if the lessons learnt from properly investigated past failures are used.

Additionally, the field of research on forensic structural engineering allows us to delve, to learn from mistakes, and to improve the design, construction, and maintenance of new and existing structures following any structural failure, as this later provides a unique insight into real-scale failure that opens our ability:

to better understand the structural behaviour;

to better understand interactions between different materials, technologies, and products;

to validate and to calibrate new parametric formulations;

to investigate and to validate non-standard tests, procedures or analyses.

Section 1: Building Collapses

Chapter 2

‘Palazzo Edilizia’ in Salerno

Fabrizio Palmisano¹,² & Amedeo Vitone³

1 Politecnico di Torino, Turin, Italy, 2 PPV Consulting, Bari, Italy, 3 Studio Vitone, Bari, Italy

This chapter presents the forensic investigations relevant to the partial collapse of a historical masonry building in Salerno (the so called ‘Palazzo Edilizia’), which occurred in 2007 in Salerno (Italy). The investigations revealed that hidden structural defects were the main causes of this apparently unpredictable collapse.

2.1Introduction

On the night of the 15th of June 2007, a corner of one of the most important historical buildings in Salerno, the so called ‘Palazzo Edilizia’, collapsed 80 years after its construction. Such a ruinous failure did not cause any casualties only because the collapsed side was that of the living rooms.

On the 29th of June 2007 the Judge for the preliminary criminal investigations nominated the authors as technical consultants (i.e., expert witnesses). After that, the authors carried out investigations, surveys, and tests in order to obtain useful information to understand the causes and dynamics of such a ruinous and apparently unexpected collapse [1].

The purpose of the investigations was not only to determine the causes of such a collapse but also to provide the Judge for the preliminary investigations with sufficient data to identify the parties responsible.

2.2The investigation Process

In Italy the partial or total collapse of a construction (even if without injuries and/or fatalities) is always a crime. This means that the crime trial and the relevant investigations always start before the civil ones and, in general, their conclusions are adopted also in the civil trial.

The criminal proceeding is always initiated by the Public Prosecutor who starts to carry out the pretrial investigations. In the case of a collapse, the Public Prosecutor appoints experts (i.e., consultants) as expert witnesses to carry out the technical investigations. As, in most cases, these investigations, for many aspects, cannot be repeated (i.e., some operations of the investigations imply the modification of things and places), the suspects and the other parties involved (e.g., the victims’ relatives) can appoint their own consultants to ensure their right of defence. The investigations are chaired by the technical consultants to the Public Prosecutor who plan the surveys, make measurements, conduct tests, etc. Other parties can be present during surveys and tests, can ask consultants to the Public Prosecutor to make specific measurements and tests but can make their own measurements and tests on samples taken on site only if they have the relevant authorization by the consultants to the Public Prosecutor or by the Public Prosecutor himself. The purpose of this rule is to avoid the modification of things and places without the authorization by the Public Prosecutor’s Office. In crime trials, the report of the consultant to the Public Prosecutor is considered as the report of one of the parties involved (i.e., at the same level of the report of all consultants).

During the pretrial investigations, for witnesses or evidence that may not be available at trial, either the Public Prosecutor or the suspects’ defence can request the activation of a so-called special evidence pretrial hearing. This procedure allows for the hearing of testimony from a witness, thus preserving a witness’s testimony for future trial proceedings. The testimony is then included in a file for the trial. If this procedure is activated, the chairmanship of the investigations passes from the Public Prosecutor to the Judge for the preliminary investigations, who appointed experts (i.e., consultants) as expert witnesses to carry out the technical investigations. Also in this phase, the suspects and the other parties involved (Public Prosecutor included) appoint their own consultants to take part in the investigation.

The rules for the technical investigations are the same as those in absence of the special evidence pretrial hearing, with the difference that the chairmanship belongs to the consultants to the Judge for the preliminary investigations, and the consultants to the Public Prosecutor are at the same level as the consultants to the other parties involved (e.g., they can be present during surveys and tests, but cannot make their own measurements and tests on samples taken on site without the authorization by the Judge or the consultants to the Judge). When the pretrial investigations are completed, the preliminary hearing occurs. In this hearing the Judge evaluates all the evidence collected by both the Public Prosecutor and, if applicable, the Judge for the preliminary investigations, decides if the first-grade trial shall be initiated, who shall be committed to stand trial and if the report of the consultants to the Judge for the preliminary investigations can be accepted as a proof for the first-grade trial.

In general, in Italy there are three grades of crime trial and, if necessary, in the first and second grade, the Judge can appoint experts (i.e., consultants) as expert witnesses to carry out further technical investigations.

In the case of the partial collapse of Palazzo Edilizia, during the pretrial investigations the special evidence pretrial hearing was activated and the authors were appointed as consultants to the Judge for the preliminary investigations. The methodology they adopted in the investigations was mainly composed of the following steps:

historical-kinematic reconstruction of the collapse based on questioning, video-photographic documentation, and visual inspections of the ruins;

acquisition of technical data necessary for the numerical analyses: documentation found in public offices, documentation on the on-going restoration works in the Varese Bar (ground and underground floor), on-site geometrical surveys (also during the works of debris removal), visual sample inspections, laboratory and on-site tests, analysis of video-photographic documentation;

numerical analyses;

identification of design and construction errors and analysis of their influence on the collapse kinematics.

In the investigations, a clear distinction between the triggering cause and the causes of the damage evolution was made.

2.3Background

No data about the construction were found in the investigations, but it is certain that the building was already completed in the second half of the twenties, since the first regulation of condominium found in the investigations is dated 1928.

A project of 1951 relating to the modification of openings of the Varese Bar at the ground floor on the corner between Verdi Street and Trieste Promenade (the corner of which collapsed in 2007) was found in the investigations. In 1955 some modifications were made on the opposite façade in order to have a homogeneous ground floor.

At the time of the collapse, some restoration works were in progress in the Varese Bar (i.e., in the area affected by the collapse). This work did not include interventions on structural elements.

The building (Figure 2.1) is one of the most important in Salerno since it was the first built on the new sea promenade.

Figure 2.1Picture of the north façade.

2.4The Collapse

The building’s south-west corner, between Verdi Street and Trieste Promenade, collapsed (Figure 2.2) on the night of the 15th of June 2007, at about 3.30 a.m. In that corner, only the slab of the ground floor, the vertical structures of the underground floor, and the foundations were not involved in the collapse. Just before the collapse, some creaks were heard and plaster detachments were seen. This was why some people in the building called the fire brigade. A team of firemen arrived at the building at 1.30 a.m. and started to evacuate the building and to close the near roadways. During these operations the collapse occurred.

Figure 2.2Picture of the south-west corner (between Verdi Street and Trieste Promenade) before (left) and after (right) the collapse.

As previously mentioned, some restoration works at the ground and underground floors in the collapsed corner were in progress in the Varese Bar. During these works, some cracks in the masonry walls were detected after removing the plaster, mainly near the openings on Verdi Street. The site supervisor stopped the restoration works and sent a report to the condominium manager on May 2nd, 2007, that included a proposal for a retrofitting intervention. It is worth mentioning that, according to the Italian law, the owner of the bearing structures of a building is the whole Condominium; this means that works on the bearing structures must be approved by the Condominium. The Condominium did not answer the report of the site supervisor until the 13th of June 2007, when one of the tenants saw some large cracks around two openings of the Varese Bar on Verdi Street (Figure 2.3). The tenant called the Condominium Manager who immediately went to Palazzo Edilizia for a survey. The morning before the collapse, the Condominium Manager undertook another survey with the site supervisor of the Varese Bar, a consultant engineer to the Condominium and the tenant who had seen the cracks the day before. The consultant to the Condominium suggested only to prop up the slab of the first floor in the area where cracks were detected to reduce the load acting on the cracked masonry pier of the ground floor. Those props were not installed since the corner collapsed during the night following the survey.

Figure 2.3Cracks around openings of the Varese Bar (Verdi Street, 13th of June 2007).

2.5Structural Pre-existent Symptoms

The minutes of the Condominium General Assemblies from 1993 to 2007 were found during the investigations. These minutes reveal that since 1993 many cracks in the masonry walls were detected, and retrofitting and monitoring interventions were planned. Neither detailed reports on the cracks nor project or construction records of the interventions were found in the investigations.

Moreover, the authors observed many symptoms of compressive ‘over-stressing’, such as vertical cracks, since the first survey, also in areas very far from the collapsed corner and, hence, not attributable to the collapse. It is worth noting that after the collapse, the whole building was evacuated and declared unusable. During the investigations, the Condominium General Assembly decided to make a lot of cement grout injections in masonry piers of the whole building (i.e., not only near the collapsed side) in order to guarantee the temporary safety of the building at least during the inspections.

2.6Structural Characteristics of the Building

The vertical bearing structures of Palazzo Edilizia are mainly tuff masonry walls made of irregular-shaped units. In some internal areas of the building some reinforced concrete (R.C.) columns were found. It is unknown whether these columns are of the original structure of the building or if they were built during some later modifications. All the floors are made with a composite R.C. beam – masonry block structure.

Regarding the collapse, the most important critical aspects of the structural layout are the following: very few vertical diaphragms, weakening of the collapsed corner due to the presence of large openings, lack of tying systems, eccentric width reduction of the masonry walls from the lower to the upper floor, ground floor masonry piers narrower than those of the other floors (because of the larger openings at the ground floor). Moreover, the following critical aspects relevant to the geometry and the arrangement of the masonry units have been found: irregular shape and dimension of the masonry units, large thickness of mortar joints, absence of transversal bondstones.

2.7Tests Performed During the Investigations

During the investigations, visual inspections, terrestrial laser-scanning surveys before and after the removal of the rubble, measurements, sonic-pulse velocity tests on masonry walls, as well as laboratory tests on samples of reinforcement and tuff units were made by the technical consultant to the Judge for the preliminary investigations. Moreover, the technical consultant to the Public Prosecutor’s Office made some in-situ flat-jack tests on masonry to assess both the acting compressive stress and the compressive strength.

With reference to the results of the investigations, the most important tests were those on the tuff units. 12 tuff units were taken from walls not involved in the collapse and 24 from the ruins. In total, 108 tuff samples were tested according to EN 772-1 [2] in order to obtain the compressive strength in natural conditions and in dry conditions as well as the Young’s Modulus.

In the case under study the mortar seemed to be, to the naked eye, of very bad quality. It was not possible to take mortar samples because the mortar immediately tended to crumble even at the simple contact with hands. This is why the value suggested by the Italian National Annex to EN 1996-1-1 has been assumed. The characteristic compressive strength of masonry fk, evaluated according to EN 1996-1-1 [3], by using the results of the compressive tests on tuff samples and the assumed compressive strength for mortar, is equal to 1.26 MPa. This value is consistent with the results of the flat-jack tests performed by the technical consultants to the Public Prosecutor’s Office.

2.8Numerical Analyses

Numerical analyses were performed with reference to the element that, according also to depositions, triggered the collapse: the second masonry pier from the right on the Verdi Street façade (Figure 2.4). These analyses were performed according to the approaches proposed by EN 1996-1-1. As discussed in the following paragraphs, four different approaches have been used: (a) simplified analysis, (b) linear finite element analysis, (c) nonlinear finite element analysis, and (d) simplified probabilistic analysis.

Figure 2.4CAD reconstruction of the Verdi Street façade with the indication of the masonry pier used for the numerical analyses.

Numerical analyses according to approaches (a), (b), and (c) were performed according to the limit state design method. The aim of these different analyses is both to catch detailed aspects of structural behaviour and to highlight that each improvement in the analysis implies a reduction of the safety factors. The simplified probabilistic analysis was performed in order to immediately understand and quantify the risk level of the collapsed side.

2.8.1 Simplified Analysis

In this kind of analysis, the mean stress on