Forensic Engineering (2003)

The Pentagon, the headquarters of the United States Department of Defense, was constructed between September 1941 and January 1943. A major renovation of the entire 6.6 million sq ft facility began in 1999 and is scheduled for completion in 2010. On September 11, 2001, a hijacked commercial airliner was intentionally crashed into the building in an act of terrorism. One hundred eighty-nine persons were killed and a portion of the building was damaged by the associated impact, deflagration, and fire.

On September 11, 2001, a large commercial aircraft traveling at high speed crashed into the Pentagon. The aircraft struck essentially at the second story slab, damaging columns on the first and second story and the slab of the second story in the outer ring of the structure. Then the aircraft debris slid under the second floor, 94.5 m (310 ft) into the building, inducing significant damage to the three outer rings of the west side of the Pentagon. Numerous columns were removed or significantly damaged by the aircraft debris as it moved through the building. As a result, a portion of the outer ring collapsed approximately 20 minutes after the crash. This paper summarizes the condition of the building after the attack.

On September 11, 2001, a large commercial aircraft traveling at high speed crashed into the Pentagon. The aircraft and building debris pushed through the first story a distance of 310 feet into the building, inducing significant damage to the three outer rings of the west side of the Pentagon. The debris destroyed nearly 30 concrete columns and significantly damaged about 25 others. The cast-in-place structure remained standing despite this loss, although a portion of the outer ring collapsed approximately one-half hour after the crash. This paper summarizes the structural analysis of the columns and floor system performed by the team to better explain the building's performance.

An overview of fire damage sustained by the Pentagon structural elements in the September 11 terrorist attack is provided. The fire intensity in some compartments of the affected areas inside Pentagon was approximated to be between those of the two standard fire exposures ASTM E119 and E1529, based the observed fire damage and estimated fuel load. Thermal analyses of the structural columns and beams were performed using the standard fire exposures to demonstrate the increased vulnerability of these structural elements once the concrete cover was lost.

The impact of the aircraft destroyed or significantly impaired approximately 50 structural columns. The ensuing fire weakened a number of other structural elements. However, only a relatively small segment of the affected structure collapsed, approximately 20 minutes after impact. The collapse, fatalities, and damage were mitigated by the Pentagon's resilient structural system. It is recommended that the features of the Pentagon's design that contributed to its resiliency in the crash—that is, continuity, redundancy, and energy-absorbing capacity—be incorporated in the future into the designs of buildings and other structures in which resistance to progressive collapse is deemed important. It is further advocated that additional research and development be conducted in the practical implementation of measures to mitigate progressive collapse and in the deformation capacity of spirally reinforced columns subjected to lateral loads applied over the height of the column.

Public fears surrounding indoor environments contaminated with mold are rapidly driving up the costs of investigation and remediation of even minor incidents. This paper presents an overview of the process of evaluation of mold contamination in buildings. That process is illustrated with a case study of minor contamination in a residence that ultimately cost $1.5 million to remediate. Recognizing that costs are driven more by fear and perceived risks, rather than by objective assessment, a methodology for cost-benefit analysis that can be used by the decision makers, preferably early on in the dispute, to identify the most effective option to resolve the conflict.

Mold growth within buildings is not a new problem, but mold is attracting increased attention from media, litigators, and owners. Because of susceptible building materials and increasing dependence on artificial ventilation, we can expect risk of mold infestation on building interiors to increase. Mold requires four elements: warmth, moisture, nutrients, and spores. Problems can start with moisture trapped in wall or floor assemblies during building construction or by improperly installed or placed vapor retarders. Other sources include moisture allowed through ineffective wall cladding systems, improperly designed or constructed ventilation and humidification systems, and mold-susceptible materials. The prevention of mold growth during the design and construction phases does not involve new or unproven methods. Current means for mold prevention are remarkably similar to recommended construction practices of the past. Today, however, the effects of aberrations from good design and construction practices can be extraordinary in terms of money and schedule. Introduction of a small quantity of moisture can create large amounts of mold growth in modern construction. Designers can reduce the risk of mold growth by specifying suitable materials, proper wall and roof configurations, and the construction sequence when appropriate. Designers should also encourage monitoring of construction and interior humidity. Building owners and developers can help by encouraging a single contractor for construction of the building envelope and by allowing a schedule to permit good construction practices. General contractors must coordinate trades and construction sequences to protect the building at all times. All parties can reduce the likelihood of mold growth by understanding the conditions necessary for mold growth and avoiding situations that encourage that growth. This paper will review mold problems based on experience with a building that developed mold growth during the construction phase. Mold growth resulted from mishaps with contractor solvency, mold-susceptible materials, and improper temporary protection. Each problem that contributed to the mold growth was preventable. The paper will address the specific pitfalls that plagued this construction project and will outline steps designers, owners and contractors can undertake to prevent similar conditions.

Fungal infestation of buildings has become a significant factor in how the integrity of structures is assessed. Fungal growth as a result of building envelope failures in structures is examined in four case studies. Two of the studies are in commercial buildings and two involve residential buildings. Illustrated are the effects of applying EIFS over a failed stucco finish, the lack of weep holes in a veneer cavity wall, lack of roof flashing, plumbing, leaks, high maintenance details, lack of proper maintenance and poor scheduling during construction. Current industry standards were used in the determination of extent of fungal growth at the sites. Types of samples collected included surface, dust and air.

Mold is a term used to describe a wide-range of mostly multicellular organisms belonging to the Fungi Kingdom. Molds are saprotrophic organisms, meaning they obtain nutrients and energy from other organic matter and do not use photosynthesis. The use of the term "mold" is not based on any scientific classification. Mold is simply visible fungal growth. The term mildew has been used to describe fungal growth on bathroom tile, fabrics or windowsills. Technically, the scientific description for mildew pertains to certain fungi that cause plant diseases. Molds and mildews are very common in nature, with over 20,000 known species. Molds perform vital functions in our ecosystem. They have the ecological role of being the major decomposers of organic matter on Earth.

This paper presents the background, development, purpose and scope, and table of contents of the recently completed standard, SEI/ASCE 37-02 Design Loads on Structures During Construction. It is the first in the sequence of four papers on SEI/ASCE 37-02. The other three, authored by others, in this sequence are: Loads and Load Combinations in SEI/ASCE 37-02, Construction Loads in SEI/ASCE 37-02, and Environmental Loads in SEI/ASCE 37-02.

Section 2 of the SEI/ASCE 37-02 Design Loads on Structures During Construction Standard deals with the definition of the loads, and the specification of load factors and load combinations to be used in design of structures during their construction phases. Provisions are made for both ultimate strength and allowable stress design. The construction loads, load combinations and load factors were developed to account for the relatively short duration of load, variability of loading, variation in material strength, and the recognition that many elements of the completed structure that are relied upon implicitly to provide strength, stiffness, stability or continuity are not present during construction. The load factors are based on a combination of probabilistic analysis and expert opinion [Rosowsky]. The concept of using maximum and arbitrary point-in-time (APT) loads corresponding load factors is adopted to be consistent with ASCE 7 [ASCE 7].

Section 4 of the SEI/ASCE 37-02 Design Loads on Structures During Construction Standard deals with the construction loads that need to be accounted for in the design of structures during their various phases of construction. Loads on a structure during its construction may be greater or smaller than after its completion, and their combinations may be changing during construction by the construction sequence. The provisions of the Construction Loads section of the Standard are intended to be used to define the construction loads for the design of both temporary structures and permanent structures at various phases of their completion. The provisions cover material loads, personnel and equipment loads, horizontal construction loads, erection and fitting forces, equipment reactions, and concrete form pressures to be used in design. They also address the application, combination, and reduction of the loads when designing for different phases of the construction. Wherever possible, standard construction loads or specific methods are presented to quantify those loads. The complexity of modern construction, including the evolution of new materials and techniques, required the addition of extensive commentaries alongside the standards in order to provide guidance to the determination of the loads.

The duration of the construction phase normally is small compared to the assumed in-service design life for a building. Since the likelihood of severe wind storms, earthquakes, and other loadings are a function of the duration of the construction phase and, sometimes, the time of the year, interim conditions and temporary structures that exist during construction are not exposed to the same environmental load risks as are finished, "permanent" structures. For these reasons, environmental loads that might be used for structures during construction can be substantially lower than those used for finished structures, and still achieve risk that is comparable during the interim condition to the risk associated with the finished structure over its life. This paper summarizes the philosophy for the development of environmental loads for the standard Design Loads on Structures During Construction (ASCE 37-02) (American Society of Civil Engineers, 2002). Discussions address the application of the underlying standard Minimum Design Loads for Buildings and Other Structures (ASCE 7-95) (American Society of Civil Engineers, 1996) and adjustment factors to account for construction duration and season. The paper also covers procedures to address special conditions that exist only during construction, such as wind loads on open frames, ice loads, and thermal loads on components that will be enclosed upon completion of construction.

This paper reviews the research and testing carried out on the Bolted Flange Plate (BFP) steel moment frame connection for the SAC Joint Venture. The connection is classified as a "Prequalified Bolted Fully Restrained Connection" in FEMA 350. Based upon the review carried out by the authors, utilizing the criteria for Prequalified connections stated in FEMA 350, the authors show that, in their opinion, the connection should not be classified as Prequalified. The authors give recommendations for the limited use of the connections and modifications to design procedures.

This paper reviews the research and testing carried out on the Welded Flange Plate (WFP) steel moment frame connection for the SAC Joint Venture. The connection is classified as a "Prequalified Welded Fully Restrained Connection" in FEMA 350. Based upon the review carried out by the authors, utilizing the criteria for Prequalified connections stated in FEMA 350, the authors show that, in their opinion, the connection should not be classified as Prequalified. The authors give recommendations for the limited use of the connections and modifications to design procedures.

This paper reviews the research and testing carried out on the Welded Unreinforced Flange Welded Web (WUFW) Steel Moment Frame Connections for the SAC Joint Venture. The connection is classified as a "Prequalified Welded Fully Restrained Connection" in FEMA 350. Based upon the review carried out by the authors, utilizing the criteria for Prequalified Connections stated in FEMA 350, the authors show that, in their opinion, the connection should not be classified as Prequalified. The authors give recommendations for the limited use of the connections and modifications to design procedures.

There was extensive damage to steel beam-to-column moment connections in several hundred buildings caused by the M6.8 earthquake in Northridge, California on 17 January 1994. Subsequent inspection of identical connections in San Francisco Bay Area steel structures indicated similar connection damage caused by the 1989 Loma Prieta earthquake. The damaged Moment Resisting Frames (MRF) were fabricated with the beam flanges attached to the column flanges by full penetration welds and with the beam webs bolted to single plate shear tabs. This design is based upon the concept "the flanges resist the moment and the web resists the shear." As a result of the observed connection fracture modes, it was concluded that the field-welded, field-bolted connection, which has become known as the "pre-Northridge" connection, is fundamentally flawed and should not be used in new seismic moment frames." This conclusion, which is stated in the Structural Engineers Association of California Seismic Structural Design Blue Book (1996), was made after post-earthquake surveys of connection damage and their modes of fracture and a review of literature of the historic laboratory tests that led to the design rationale of the "pre-Northridge" connection. The majority of these laboratory tests were made at the University of California at Berkeley during the time period of late 1960 through 1990. Additional testing of this connection was made at Lehigh University during the 1980s and at the University of Texas during the 1990s. This literature review indicated that unexplained sudden and premature connection failures occurred in a significant numbers of these tests. This observation was consistent with the fractures observed in recently (1994–1998) performed full-scale. ATC-24 Multiple-Specimen Multiple Step (1992) tests performed at the University of Texas and at the University of Michigan that failed prematurely. Presented herein are ATC-24 Multiple-Specimen Cumulative Damage tests performed on cantilever beam/column pre- and post Northridge connections. This paper relates the state of stress in these connections to the dominant failure mechanism, which is low-cycle fatigue.

The Schoharie Creek Bridge collapsed on the morning of April 5, 1987 after three decades of service, during a near record flood. The collapse of pier three caused two spans to fall into the flooded creek. Five vehicles fell into the river, and ten occupants died. Over time, the backfill material around pier three had gradually eroded and been replaced by waterborne deposits. In the flood, scour undermined and cracked the pier. Bridges across waterways must be designed structurally not only to carry their own weight and traffic loads, but also to resist the hydraulic forces imposed by rivers and other bodies of water. Moreover, the construction of the bridge abutments and piers alters the river's flow, and may lead to new patterns of erosion and deposition. The collapse of the Schoharie Creek Bridge illustrates the importance of designing bridge piers to resist scour. The case also suggested some important changes to bridge inspection, maintenance, and management practices.

This paper summarizes what was arguably the greatest American civil engineering failure of the last century, the catastrophic collapse of a two hundred foot high curved concrete gravity dam situated in a canyon some forty miles north west of the center of Los Angeles, resulting in several hundred fatalities. The learning from failure process is still an effective, if preferably avoidable, tool of benefit to all branches of engineering. Notwithstanding the fact that the failure of the St. Francis Dam occurred more than seventy-five years ago, the circumstances of its design, construction and demise, coupled with the many efforts made to investigate the event, continue to fascinate forensic engineers.

The Teton Dam in Idaho failed on June 5, 1976, flooding towns and farmland downstream. Fourteen people were killed and property damage estimates ranged from 400 million to one billion dollars. The failure occurred during the initial filling of the reservoir. An independent review panel convened by the Governor of Idaho and the Secretary of the Interior found that the most probable cause was piping of the dam fill material. The fill material was highly erodible fine wind-blown silt. Excavation of the failed dam under the supervision of the review panel found that the grout curtain had gaps or "windows," and the underlying rhyolite formation had fractures and joints that could have allowed enough water to pass to initiate the piping. The weak materials were identified before construction. The lessons learned from this case led to safety improvements for U.S. Bureau of Reclamation design procedures.

In the early morning hours of May 16, 1968, the occupant of apartment 90 on the 18^th floor of the Ronan Point apartment tower lit a match to brew her morning cup of tea. The resulting gas explosion initiated a partial collapse of the structure that killed four people and injured seventeen. On investigation, the apartment tower was found to be deeply flawed in both design and construction. The existing building codes were found to be inadequate for ensuring the safety and integrity of high-rise precast concrete apartment buildings. The Larsen-Nielson building system, intended for buildings with only six stories, had been extended past the point of safety. The tower consisted of precast panels joined together without a structural frame. The apartment tower lacked alternate load paths to redistribute forces in the event of a partial collapse. When to structure was dismantled, investigators found appallingly poor workmanship of the critical connections between the panels.

The national Forensic Engineering Award, given by the ASCE Technical Council on Forensic Engineering (TCFE), recognizes individuals for outstanding contributions to the field of forensic engineering. The award has been given eight times since its establishment in 1989. Past recipients of the award are John P. Bachner (1990), Jack R. Janney (1991), Joseph S. Ward (1992), Lev Zetlin (1993), George F. Sowers (1994), Kenneth L. Carper (1997), Narbey Khachaturian (1998), and John M. Hanson (1999).

The City and County of Denver (CCD) Public Works Department owns, inspects, and maintains 531 bridges in its inventory of which 264 are considered major structures spanning over 6.1 m in length. In this paper a methodology using the CCD major bridge network for the application of nondestructive evaluation (NDE) methods in bridge inspections is explained. The methodology, called Bridge Evaluation using Nondestructive Testing (BENT) helps systematically integrate NDE methods and conventional Bridge Management Systems (BMS) by using a Markovian deterioration model. Although the BENT method can be applied to timber, steel, and concrete bridges, in this paper the application of the method will be restricted to concrete bridges. The BENT system is part of a comprehensive geographic information system whereby database queries can be completed using a map interface. The database contains a wide array of information in the CCD infrastructure inventory including: bridges, pavements, alleys, and street subsystems.

Tubular steel pedestrian bridges, like all bridges, must be inspected and maintained to limit the threat to life and safety. While many types of bridge inspection systems are in use today, visual inspection is the generally accepted method used to measure corrosion. However, the geometry of tubular members makes it difficult to measure their thickness and determine if there has been any section loss. With the use of non-destructive evaluation (NDE) techniques, specifically the use of an ultrasonic flaw detector, the thickness of these members can be accurately measured. Formulas were derived from NDE to predict the safety of this type of structure based on the geometry of the bridge and thickness of the members. These results were then used to develop a management system for tubular steel pedestrian bridges.

This paper presents an overview and results of a strain gauge study conducted on the eastbound 6^th Avenue Viaduct Bridge in Denver, Colorado and a general description of the subsequent repair. The 6^th Avenue Viaduct was modified in a 1997–98 rehabilitation making the entire superstructure continuous. New sliding bearing pads failed to properly accommodate expansion and contraction of the bridge superstructure due to temperature changes. Visible damage due to contraction of the superstructure prompted a study of substructure elements to determine the short-term safety and long-term viability of the bridge. Substructure elements were instrumented with a total of 62 strain gauges and 2 concrete crack gauges. Data were taken for a period of three months. Results of the study suggest that the bridge is safe for short-term use, but that the long-term viability of the structure has been compromised. Interim strengthening of critical substructure elements were conducted and replacement of a substantial portion of the substructure on the west half of the bridge will commence in the summer of 2003. Results of the study were used in design to verify assumptions and target these repairs.

This paper summarizes an investigative effort to determine the extent and severity of fire damage to lightweight precast pre-stressed concrete double-tee members at a parking structure. The study involved non-destructive evaluation techniques such as pulse velocity and radiography, as well as laboratory testing of concrete cores. Laboratory testing included determination of Dynamic Young's Modulus of Elasticity and Air Permeability Index on 25 mm. (1 inch) thick disks taken from cores, and determination of strength. Analyzing concrete specimens in 25 mm. (1 inch) thicknesses provided an insight into fire damage gradients. Based on the results of the first phase of testing, the structural elements were load tested. The load test produced a deflection pattern consistent with the findings of the testing program.

Evaluation of cosmetic distress to light framed buildings, constructed above a concrete parking structure, is often misdiagnosed. This paper describes and discusses the deflection of concrete slabs resulting from shrinkage and long-term creep. The use of a concrete platform above a subterranean or at grade parking structure to serving as the foundation for a light framed bearing wall superstructure is a very popular type of construction in urban environments. Unless care is taken in the design to consider the long term deflection phenomena, significant distress to the superstructure can result.

This paper summarizes the investigative effort to determine the extent of damage to the elevated concrete pedestal and pre-cast concrete piles supporting a liquefied natural gas (LNG) storage tank following a leak of the LNG at cryogenic temperatures typically below-160 °C (–260 ° F) and lasting approximately 48 hours. The study consisted of a non-destructive evaluation phase to determine the extent of damaged concrete and a subsequent laboratory testing phase based on concrete cores. Pulse velocity measurements during the non-destructive testing phase indicated the distressed concrete was localized. Cores removed from both unaffected and distressed locations identified during the non-destructive phase were tested in the laboratory phase to determine the Dynamic Young's Modulus of Elasticity and Air Permeability Index on 25 mm (1 inch) thick disks taken from the cores, and compressive strength. Analyzing concrete disks at 25 mm (1 inch) depths provided insight into damage gradients. The Air Permeability Index and Dynamic Young's Modulus of Elasticity measurements of the disks were effective in determining both depth and damage relative to depth as a result of the cryogenic spill. The laboratory studies confirmed that the distressed zone of the concrete was limited to a near-surface zone as suggested by the pulse velocity testing. The structure was returned to service upon completion of the assessment.

The American Society of Civil Engineers (ASCE) formally established the Technical Council on Forensic Engineering (TCFE) in 1985. This paper reviews the 20-year history of the TCFE, beginning with several catastrophic events of the 1970s and early 1980s that led to its formation. The current committee structure of the Council is presented. Past activities and products of the TCFE are reviewed along with anticipated future directions.

Student learning can be optimized when higher level cognitive, social, affective and psychomotor learning skills are developed. In this case, a junior-level soil mechanics class conducted a failure investigation and provided repair recommendations for a landslide that was threatening a home. Assessment of learning indicated students learned more and felt better about the work they did, despite rating the class as being a heavy workload as compared to other courses. The instructor noted that while more learning apparently occurred, facilitating a course of this type can be very time consuming, stressful, and a potential source of liability. Further, suitable projects cannot be expected to become available at just the right time on a yearly basis. Even so, the instructor was encouraged by the project and would continue to be receptive to administering similar projects in the future, with the belief that future work would not be as time consuming and could be conducted with less stress and liability.

In the broad sense of the term, the common objective of forensic engineering is to diagnose problems and remedy them, making the structure sound and serviceable. In the literal sense of the term, the primary objective of forensic engineering is to identify the source of the noted problem and assign responsibility for the failure. The practice of forensic engineering, in both the general and literal sense, likely began shortly after the first man-made structures were completed. In the Code written in 1780 B.C.E. (before Common Era) by Hammurabi, the ruler of Babylon, it states "If a builder builds a house for some one, and does not construct it properly, and the house which he built falls in and kills its owner, then that builder shall be put to death." Clearly, the need to determine the causes of building failures is a longstanding one with serious, sometimes dire, consequences. In daily practice, the forensic engineer's understanding of design and detailing practices, material properties, investigative techniques and construction practices dictate his or her ability to comprehend and properly diagnose problems. This paper is directed to engineering education administrators, instructors and those with a general interest in the current state of forensic engineering education. A review of civil engineering, architectural engineering and architecture coursework offered at twenty-one (21) universities, coupled with an evaluation of recent graduates into the practice, has led to a conclusion that forensic engineering education is limited in offering, narrow in focus and concentrates upon the dramatic collapse rather than the routine failures that are encountered in daily practice. An outline is provided for a comprehensive curriculum, intended to introduce the subjects that most often pertain to a forensic engineer's daily practice. This paper presents a new methodology in the instruction of forensic engineering: one that exposes engineers to architectural components and details, and suggests that universities allow architects (and architectural engineering) students to expose themselves to structural details and material sciences.

Structures have been examined with traditional x-ray technology using film and high-energy x-ray sources for decades. However, due to safety concerns and the high costs involved, use of this technology has been quite limited. This paper presents a summary of recent x-ray examinations of connections in buildings and potential applications of digital radioscopy for investigation of joint damage and wood deterioration in structures. The field investigations showed that adoption of digital, real-time x-ray equipment for structure investigations can provide numerous benefits to the engineering community.

Subway tunnel condition assessment presents significant challenges for engineers, and is becoming increasingly important as the systems continue to age. Technologies are needed that can rapidly and accurately assess the condition of subway tunnels, and identify distress such as delamination, moisture-related damage, without interfering with the normal operation of the system. Towards this goal, different nondestructive evaluation methods including SASW, Impact Echo, GPR, and Impulse Response were evaluated to determine their advantages and limitations for tunnel evaluation. Since tunnels are in constant heavy use in an aggressive environment, it is necessary to distinguish between methods that can be used for high speed screening, and those that require interruption of subway traffic. It is also necessary to develop automated procedures to process the vast amounts of data generated during extensive NDE testing.

When testing and evaluating in-service concrete slabs that evidence differential displacements, non-destructive non-invasive diagnostic techniques help to minimize disruption to both the structure and its users. In most cases such techniques are capable of yielding sufficient information for a qualitative evaluation of the slab. Thereby eliminating the need for destructive testing.

As the 2002 season was winding down for the Milwaukee Brewers Professional Baseball Team, the Southeast Wisconsin Professional Baseball Park District (SEWPBPD) expressed the desire to initiate a major off-season repair. After only two years of service, the pivot bearings of all five of the moveable roof panels of Miller Park Stadium supporting as much as 1,090,000 kg of steel and roofing required replacement. Provisions for the bearing replacement were never developed into the original stadium design. Tight quarters, severe time constraints and winter weather conditions needed to be overcome to fulfill the mission. These and other constraints presented a series of daunting challenges addressed by LZA Technology/TTE using a number of 3-D graphical simulation models and finite element assemblages for planning the replacement program. We will share this experience to serve as a case study reminding us of what is possible when we work as a team with other design professionals, contractors, and owners to achieve the ambitious.

The purpose of this study is to analyze the damage to the structure of each of the WTC Twin Towers due to the high speed impacts of the Boeing 767 airplanes and subsequent fires in order to understand why the Twin Towers stood for as long as they did, and why they ultimately collapsed. The Boeing 767 airplane attacks on WTC 1 and WTC 2 caused immediate and significant structural damage to the towers: In each case, exterior columns were severed and the floor system at the point of impact was damaged. The airplanes broke up during the impact and the resulting projectiles and fragments proceeded to inflict further damage to the cores. Much of the impact damage to the exterior walls of the towers was evident. However, damage to the interiors was not visible and cannot be quantified on the basis of the physical evidence. Dynamic nonlinear explicit finite element FLEX simulations coupled with independently validated airplane crash models were leveraged to understand and assess the structural states of damage to the tower interiors that could not be observed; this includes the degradation or loss of the load carrying capacity of columns and floor assemblies as well as the stripping of fireproofing from structural members. The impact damage to the structures was substantial but so were their reserve capacity and redundancy. Iterative analyses of the load redistribution in the impact damaged towers clearly indicate that the outer tube structures were very effective in developing Vierendeel action around the severed exterior columns and that the outrigger hat trusses provided a substantial redundant load path away from the damaged core columns, thus delaying the eventual collapses of the towers. These analyses also indicate that the damage to the corner of the core in WTC 2 left it in a state more vulnerable to subsequent thermal loads compared to WTC 1. This eccentric damage, more than the height of the airplane impact, resulted in a shorter time to collapse for WTC 2, considering that the estimated fire environments in both towers were not meaningfully different.

The collapse of each tower was caused by extensive damage resulting from the aircraft impact to each tower and subsequent weakening of the steel at the impact zone by heating of unprotected steel by fire. While the fire was massive in spatial extent, the thermal environment created by the fire was significantly less severe than normally occurs in fully developed compartment fires. The spray applied fireproofing was removed from structural members directly exposed to the impact debris field. The heating of the unprotected columns resulted in the loss of structural capacity and ultimately resulted in the collapse of the towers. The collapse mechanisms for the two towers were different as a result of the differing impact dynamics rather than differences in fire dynamics.

LZA Technology (LZA) was retained on behalf of Silverstein Properties, a lease-holder of the World Trade Center (WTC) site, to perform structural investigative services in connection with the World Trade Center attacks. Our work included a review of all relevant available photographic evidence (still photos and video) and utilization of the knowledge LZA gleaned through our continual site observations from September 11, 2001 through July 2002, as the New York City Department of Design and Construction's lead engineering consultant. This paper summarizes LZA's evaluation of the following: 1) Structural damage sustained by the exterior frames of WTC 1 and WTC 2 due to the aircraft impacts, 2) Debris distribution resulting from the collapses of WTC 1 and WTC 2. This includes an assessment of which tower's debris damaged the remaining World Trade Center buildings and other nearby buildings, and 3) Structural damage sustained by WTC 1 due to the collapse of WTC 2.

Recent events have brought considerable attention to the role of the engineer in emergency response. While the tragic attacks on our nation have placed engineers in the spotlight, the interaction of engineers and first responders is not a necessarily new trend. One such initiative that has utilized the expertise of structural engineers for many years is the Federal Emergency Management Agency (FEMA) National Urban Search and Rescue (US&R) Response System. This paper shall review the history and operational methodology of the National US&R Response System and provide an overview of the role of the engineer as a Structures Specialist on an US&R Task Force. In addition, the authors shall describe the specific responsibilities of their respective Urban Search and Rescue Task Forces' Structures Specialists during the response to the terrorist attacks of September 11, 2001.

This paper discusses the requirements of the "Existing Structures" provisions of various model building codes and how the requirements relate to repairs of existing buildings. Some of the common misconceptions with regard to interpretation of these provisions will be discussed. Theoretical and actual examples from past projects are provided to demonstrate varied interpretations of this portion of the code by practicing engineers.

This paper presents an overview of bridge failures in Vietnam, drawn from an analytic study of 53 failed large bridges over the last twenty years. It includes main failure types and causes for bridges in Vietnam, along with lessons of bridge failure drawn from the analytic study. The main failure types of bridges are classified in two groups: failure of abutments and piers, and failure of superstructures. The main failure causes of bridges are classified in six groups: failure causes related to environmental conditions, structural engineering, construction on site, repair projects, operation and management, and material used. The lessons of bridge failures drawn from the study can be of help to avoid similar mistakes in bridge construction in future.

The use of electrochemical rehabilitation on chloride contaminated reinforced concrete structures has increased in recent years. One of these electrochemical methods is electrochemical chloride extraction, in which an electrical field is applied between the reinforcement and an external electrode. The method is applied to chloride-contaminated concrete where corrosion has not yet caused delamination or spalling of the concrete cover. The chloride is removed from the vicinity of the reinforcement within a short period of time (3 to 5 weeks) which makes it a cost effective alternative compared to traditional repair methods. This paper presents the latest developments in the field of chloride extraction. It explains its benefits and how to assess these, which is of particular importance when the efficiency of the method is to be evaluated. Questions have been raised about the occurrence of side effects such as spalling of concrete cover during chloride extraction, softening of the cement matrix and hydrogen embrittlement. This paper is aimed at the practitioner and, therefore, explains how to recognise and mitigate side effects through correct application of electrochemical chloride extraction.

A number of events occurred in the UK in and around the year 2002 that are aimed at improving in-service safety of structures. These events will be introduced in this paper together with the idea of a life-care plan for helping with the management of building structures as a way of improving safety — and with a beneficial spin-off. The idea of using life-care plans is gaining credibility in the UK, as it should help owners, operators and managers of facilities to discharge regulatory duties and also contribute to optimising the resources required to do so. In the same year a new regulation for safety of buildings was introduced in the UK, and the regulation together with its background, is described in this paper as well as the potential relevance of using a life-care plan. These two events followed the spontaneous collapse of part of an in-service structure some years earlier. An outline description of the collapse is given in this paper. The principles described could be used internationally.

This paper reviews the role of the expert witness within the American legal system, and defines the bases for ethical obligations imposed upon the expert. This includes those obligations imposed by the court, by the state licensing organization, and the engineering community. Many of those obligations are voluntary, or are rarely enforced, so it is up to the individual expert to maintain his own high standards of ethical behavior. This paper addresses those standards.

Forensic technical experts often describe themselves as objective and impartial, professing to arrive at the same opinion in a dispute regardless of whether their client is a plaintiff or a defendant. Why is it then that so often there are impartial, objective technical experts for both sides of a dispute, maintaining contradictory and mutually exclusive positions concerning a given situation? If all experts in a dispute are impartial and objective, and if, because of the discovery process, they all have access to the same set of facts, how can they hold different opinions? This paper examines this paradox of forensic technical testimony, and addresses several other ethical dilemmas of forensic engineering practice. For instance, can an expert ethically serve as an advocate for one side in a dispute? What ethical entanglements does a forensic engineer face when the client limits the involvement of the expert in the investigation of the facts? How does an expert witness fulfill the obligation of the oath to tell "the whole truth," after being given the contradictory instruction to "only answer the question posed?" This paper describes the process involved in providing technical forensic expert services, the role the technical expert plays in the execution of a lawsuit, and some of the ethical dilemmas the expert faces in that role.

The work of an engineer as expert witness is not much different from investigating and reporting on any building problem. The legal requirements for the investigation are the same as any other professional work, requiring exactitude, knowledge, and sound reasoning. Once the engineer is confronted with the rules of the legal profession, he/she must be aware of the additional rules of forensic behavior. This does not change the general procedures for engineering work, but makes it important to understand the conduct and performance of an expert witness. This paper uses experience derived from forensic work to list some of the lessons learned over the years, such as: 1) Full documentation, understandable to laymen, 2) Investigation in the field to be used for incontrovertible evidence, 3) Combining field, laboratory, and literature work into a well balanced report, 4) Awareness of the function of attorneys, judges, and juries to make a truthful and effective presentation, 5) Professional performance and effective communication with the trier(s) of fact, and 6) Ability to marshal facts and not to be disturbed by aggressive interrogation.

Open web steel joists are most always specified by the joist manufacturer to be erected in accordance with Technical Digest No. 9 of the Steel Joist Institute. During the investigation of a collapse of deep longspan steel joists, the issue of whether the contractor and erector followed the requirements of Technical Digest No. 9 came up, and whether not following them contributed to the collapse. This paper examines some of the requirements of Technical Digest No. 9, their relationship to the collapse, and their implications on erection practices.

This paper discusses Redundant Design, a systematic design method based on procedural redundancy. This method is not to be confused with structural redundancy. The method is intended to effectively prevent failures in innovative or high-risk construction projects. The paper presents a case study from which the generalized concept of Redundant Design is derived.

Mediation of construction defects for a town-house community can produce meaningful, long-term and cost-effective results when both sides (plaintiff and defendant attorneys along with their experts) work together under the guidance of a seasoned mediator. A hillside town-house community was experiencing distress from deep-seated differential ground deformation. Some units had differentially settled. over seven inches prior to remediation. The key for success was the cooperation of the experts in implementing a joint subsurface investigation for the purpose of establishing the source of the ground movement, and formulating a cost-effective solution comprised of pressure grouting, mechanical lifting and slab-on-grade replacement of selected units. The distressed units were situated at the top of a large fill slope that added both complexity and constraints to normal pressure grouting procedures. After mediation, the plaintiff expert was retained by the homeowners association to review, observe, monitor, document and provide assistance throughout the year-long remediation work. The community has now enjoyed nearly a decade free of distress, as if it had never suffered a major deep-seated settlement problem.

Two 9' diameter by 56' tall steel storage silos were moved 1,000 miles from an existing location to a manufacturing plant in 1985. The steel storage silos were filled pneumatically with small plastic pellets. A review of photographs taken after installation and prior repainting operations indicated the distress was not present at that time. During the fall of 2002, a "buckled" portion of the eastern silo was discovered requiring it to be temporarily supported and unloaded. It was theorized by the plant manager that the distress might be due to the nearby railroad trains approximately 80' away. Based upon our observations and later mathematical analysis, it was discovered that the shell plates of these silos were at or near the plastic buckling limit when this silo was fully loaded with pellets.

"When you eliminate the impossible, whatever is left, however improbable, must be the truth." These words come from Arthur Conan Doyle, creator of the world's most famous detective. While forensic engineers rarely demonstrate the deductive powers associated with the great Sherlock Holmes, our investigations may sometimes resemble detective work. If we're methodical, observant, thorough, and perhaps most-especially lucky, we may unearth the truth. Once our clients' needs have been met, in a case where the circumstances were unusual or the results perhaps surprised us, we may be left with interesting stories. This paper presents a particularly challenging investigation; one with enough twists to test the consultant's knowledge of engineering and his powers of reasoning. In this case, electrical transformers that were buried beneath a sidewalk exploded and burst into flames. Happening within a block from the State Capitol, the local news trucks found it easy to cover this event and so it even made the evening news. The media coverage showed flames coming out of the sidewalk and the fire department hosing down the adjacent house. Measurements made later show that the steel grates covering these transformers were 40 and 50 inches away from the basement wall of the adjacent house. There was no immediate evidence of cracking or other vibration damage and the owner didn't notice any damage for a few weeks. That's when he called his insurance company. The insurance adjuster called me. In this and other investigations, the facts at first seemed confused and even contradictory. The owner noted that he had felt the explosions but he did not observe any damage. Weeks later, however, he noticed bulging siding, nails popping, and significant movement in the cladding on a timber-framed wall. Although the wall appeared damaged, this wall is supported by a centuries-old stone foundation that has no visible cracks. Immediately behind the damaged exterior, the owner noticed water stains and paint peeling from the plaster wall that follows a curving staircase. Throughout our investigation into this case, the actual causes of the visible damage emerged only after the "obvious" and expected causes were eliminated. In the end, this investigation tested imagination as much as knowledge, casting doubt on our initial reactions, constantly challenging assumptions, and forcing me to question myself at every turn. Many investigations are puzzles, and for me, success — when and if it comes — always owes a debt to instinct, persistence, and deliberation, but especially to a healthy dose of luck.

On January 21, 2002 the bottom chord of a timber roof truss at Nettelhorst School in Chicago suddenly fractured. This fracture caused the partial collapse of the roof structure and fourth floor suspended ceiling at the school. The roof truss that failed was one of four timber trusses that support the roof over the fourth floor gymnasium at the school. The school was built in 1893. Eyewitness accounts, the available physical evidence, and structural analysis indicated that the failure occurred at midspan of one of the interior trusses at the bottom chord panel point. The failure of an individual truss did not result in the catastrophic collapse of the entire roof structure because of the inherent redundancy in the hip roof system. There was no extraordinary superimposed load such as snow on the roof at the time of the truss failure. The truss appears to have failed under its own self-weight, and the weight of the roofing, sheathing, purlins, rafters and plaster finishes it supported. The estimated stress in the bottom chord member at the time of failure was greater than the current code-prescribed allowable, but less than what we would anticipate to be the ultimate (failure) stress. The failed truss was subject to a relatively high dead (permanent) load. Based upon the absence of any extraordinary superimposed load and an estimate of the level of stress in the failed chord member, the failure was determined to result from a combination of high permanent load and long-term time-dependent loss of strength of the chord member. The reduction in the strength of a wood member when subjected to sustained loads for long time periods is related to a process generally known as damage accumulation or creep rupture. As a result, fracture occurs at a stress level that is lower than the ultimate strength of a member. The transfer of the load in the bottom chord at splice locations was also determined to be inadequate by current design standards. The relatively high bolt forces at the bottom chord splices contributed to the fracture of the chord member.

The exterior walls of the subject three-story office building constructed in the 1920s are composed of multi-wythe clay masonry with a limestone facade and a decorative entablature at the roof level. A roof parapet sits atop a limestone water table that extends approximately 1 m out from the building facade. After multiple cracks, open joints, and spalls near the building corner drew attention from municipal officials and the local press, an investigation was undertaken to determine the extent and causes of the observed distress. This investigation revealed that expansive forces from the roof parapet, coupled with structural separation at a roof construction joint, likely led to the facade distress at the building corner. In addition, spalls and cracks on the underside of the limestone water table were attributable to improper roofing details. Repairs were designed to stabilize and remediate affected portions of the facade while minimizing the aesthetic impact to this historic structure. Lessons learned from the investigation of this uncommon failure and the subsequent repair designs are presented herein.

Long-term problems with masonry veneer often go undetected until a sudden failure occurs — a portion of the masonry falls off the building, sometimes with tragic consequences. This type of failure is of particular concern due to the frequency at which it occurs. Research by Grimm indicates that "masonry falls off a building façade somewhere in the United States about every three weeks," and that "the failures occur under normal loading conditions." This paper presents a case study of a masonry veneer failure, which occurred under normal loading conditions. The forensic methodology included on-site examination, review of structural and architectural plans, review of maintenance, repair, and renovation history, destructive testing, review of meteorological data, and research of related events to investigate the primary cause of failure. The investigation concluded that wall tie corrosion was the cause of failure, allowing the veneer to pull away from the building at wind loads significantly lower than the design pressure. Examples of corroded masonry ties and other reinforcement are shown to illustrate the extent of corrosion at the time of failure.

A 45-story all-concrete high-rise building located in Chicago, Illinois was recently investigated in an attempt to qualify and quantify building shortening, and its apparent effects, in the approximately 12-year old structure. The building movement appeared to manifest itself in four distinct and measurable areas. The first of these areas was located at the 45^th floor mechanical room, where the condenser water riser pipes penetrate the floor slab; the second was at the condenser water piping supports, also located within the 45^th floor mechanical room; and the third was at the top floor of each elevator run, where the supporting bracket for the elevator guide rail had produced a downward "scrape" on the rail. The fourth area included the individual tenant floors, where the horizontal branch pipes are tied into the condenser riser piping. In this paper, the physically measured shortening at three of the four areas identified above are compared to predicted values obtained from creep and shrinkage calculations, resulting in a rarely available comparison against theoretical values for high-rise buildings. A checklist is provided at the conclusion of the paper such that future designs may properly consider building shortening as a significant serviceability component.