Structural Engineering Designs Against Terrorism Construction Essay

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Terrorism in structural engineering is an issue as buildings are a potential target to terrorist attacks. Terrorist attacks can vary from explosions within or outside buildings to large impacts from vehicles such as planes or trucks. Attacks involving explosions generally take the form of explosions within or just outside buildings. This is an issue for structural engineers because the safety of the users of buildings is a major responsibility. It is a responsibility of the structural engineer to ensure that buildings are safe for users and also to allow for the buildings to be used for their intended purposes. The main aim of the project is to develop a method for analysing a structure to determine the requirements for reinforcing the structure and to determine the optimal protective systems for the structure. A literature review is included in this report to demonstrate research that has been conducted in the area of terrorism in structural engineering which then led into the aims and objectives of this project.

Review of Reported Work

Introduction to the Literature Review

This literature review is included in this report as it demonstrates what research and work has been completed in the area of terrorism in structural engineering. This review of previously reported work will allow the determination of the objectives and scope of my project based on what has not been done. The literature review has been split into different sections that all relate to the project of terrorism in structural engineering. Each section has been reviewed to narrow down the broad topic of terrorism in order to define a set of objectives that can build upon what has already been done.

Structural Responses to Explosions, Blasts and Impact Loads

Structural responses are the way in which the structure reacts to loading. This is important as it is necessary to know how structures are going to respond to explosions. Explosion loads involve large impact pressure which is why impact loads have also been included in this section.

Progressive collapse is a very common way for structures to fail after being subjected to explosion loads or large blast or impact loads. Progressive collapse is where one or more load bearing members is lost causing other members to carry the additional load, which these members may not have been designed to carry, causing the failure of these members which can then continue until the building has completely failed (Mohamed 2006). This was also agreed upon by the research conducted by Jayasooriya et al. (2011) who found that the failure of columns near the explosion can then lead to the collapse of the structure. The collapse of the World Trade Centre was a very famous example of progressive collapse.

As with any form of loading the yield point of the structural material has to be reached before any permanent damage is sustained. As explosion loads are very rapid loading conditions, strain hardening is often an issue of structural responses as some structural steels are strain rate sensitive (D. Pritchard, 1989). The duration of blast loading is also a key point in how structures respond to explosions and if the blast wave is long compared to the natural frequency of the structure, the loading can almost be considered static (D Pritchard, 1989). These points were also agreed upon by Hao (Hao et al. 2010).

Explosions can often cause structural collapse due to gravitational mechanisms (Luccioni, Ambrosini, and Danesi 2004). This is due to explosions on lower levels of structures where the columns near the explosion are eroded and columns further away from the explosion can fail due to a combination of shear and flexure. Columns can also fail by losing contact with upper and lower beams from the uplift pressure acting on the floor slabs, therefore removing the lateral support from the columns. This uplift failure was agreed upon by Winget (Winget, Marchand, and Williamson 2005) who said that bridge decks could fail if the uplift pressure can separate the bridge deck from girders. Girum (Girum 2011) also said that beam to column connections play a major role in the structural response to explosions. This shows that the connections between beams and columns are important for steel structures as well as concrete structures.

Localised failure can occur in members by causing craters from large compressive forces from the blast or for concrete members, shockwaves travelling through the member can cause tension forces that lead to spalling of the concrete (Winget, Marchand, and Williamson 2005). Brett (Brett and Yiannakopolous 2008) also said that there could be very significant plastic deformations of members subject to explosions, which can affect the geometric properties of the member and the overall structure.

The location of the explosive device is also a critical aspect of the structural response and degree of damage to structures (Hao et al. 2010). The location of the explosive will make a significant impact on the response to explosions as some elements of the structure will handle different loading conditions differently. This is also agreed by Remennikov (Remennikov 2002).

The review of this section shows that the structural responses to explosions are known for most explosion loading conditions. Most of the documents also show that the structural responses able to be modelled which will be discussed in the next section.

Modelling of Explosions on Structures

Modelling of explosions on structures is now an important stage in the design of many buildings with the possibility of terrorist attacks being taken into account in the design loads of a structure. Modelling is also an important process in determining why and how buildings fail when exposed to explosions.

A review of a number of peer reviewed journal articles related to the modelling of explosions on structures has shown that many authors that believe that the modelling tools available can accurately predict the failure modes of structures exposed to explosions. Luccioni et al. (Luccioni, Ambrosini, and Danesi 2004) modelled an explosion on a building that had been hit by an explosion in a terrorist attack and compared it to the actual damage. This method of comparing real explosions to modelled explosions was also used by Brett (Brett and Yiannakopolous 2008)

Cullis (Cullis, Schofield, and Whitby 2010) stated that the size of the explosive charge was not always the controlling factor in explosions intensity. The amount of oxygen available for the explosion to consume is a factor that needs to be taken into account when modelling the explosions. Hoiset (Høiset et al. 1997) and Luccioni (Luccioni, Ambrosini, and Danesi 2004) also agreed that some of the previous modelling techniques did not take this into consideration but it had been considered in the newer models.

There were no conflicting opinions between the authors in regards to modelling but Cullis (Cullis, Schofield, and Whitby 2010) also stated that vulnerability models should be included in the modelling process. This allows for faster likelihood of damage assessments to be carried out for structures. This is also related to risk and prevention and mitigation of terrorist attacks so will be discussed later.

From the review of these articles on modelling explosions on structures, it can be seen that the majority of authors agree that modelling techniques can accurately predict the structural responses to explosions. Hoiset (Høiset et al. 1997) also said that one of the issues with modelling of explosions is due to the size of explosion and explosion pressure to use in the design stage, which is more related to risk and probability of attacks, which will be discussed in a later section.

Design Standards for Explosion Loads

Design standards are important in the design of structures as they give the loads that a building has to be designed for to allow for an acceptable level of safety. In Australia the code for structural design loads is AS/NZS 1170. This is important in relation to the project as it explosion loads can vary significantly in magnitude so the criteria for design against explosions do not tend to be easy to estimate. As these explosion loads are difficult to estimate, guidance is needed to aid the design of structural systems to withstand acts of terrorism (Remennikov 2002).

There are no clauses in AS/NZS 1170 that directly relate to explosion loads on structures (Australia 2002). This means that there are no exact rules for design loads to be used for explosions which lead to large differences as to whether buildings are designed for explosions or not. This means that there is no guidance as to the level of safety for explosion loads to protect personnel, the environment and facilities in the building.

From looking at articles relating to offshore oil and gas processing structures, explosion loads have been considered as design accidental load situations (Haaverstad 1994). The design cases used for these oil and gas plants usually start from a worst-case scenario where the mixture of gas at a location on the structure (Tam and Corr 2000). This can be done as the explosive material is known and the largest possible volume of gas can be calculated. These principles can be transferred to other civil structures to aid in the design against explosions but the type of explosive and the strength of the explosive pressures are more difficult to estimate.

Remennikov (Remennikov and Carolan 2006) states that there is a very limited amount of design documentation to give engineers the technical data necessary to design structures for explosion loads. The point is that the design is often a compromise between the unlikely design for an explosion and accessibility and aesthetics (Remennikov and Carolan 2006).

As there are no exact clauses relating to design loads from explosions or blasts in the Australian Structural codes, there is large variation within design loads for explosions. Industries where explosions are likely incorporate explosion loads in the structural design but these principles are sometimes difficult to transfer to other structures.

Risk for Mitigation and Remediation of Damage due to Terrorist Attacks

The risk for terrorist attacks is an important element in designing structures for explosions. The structure could be designed to withstand large-scale explosions but would likely be very costly and have very large structural members. This could result in buildings being to withstand small-scale blasts but not large attacks. In this case the design might have to include remediation strategies for clean-up and processes for making the building safe after attacks.

Augusti (Augusti and Ciampoli 2008) said that to consider risks requires designing under uncertainties and should include the probabilities of the events. The probabilities of attacks can be found by doing a risk evaluation based on the location and political scenario surrounding the building (Pedrycz et al. 2011). Loads are related to probabilities in the same way that seismic loads are calculated for earthquakes as in low probability loads will have high consequences (Ellingwood 2006)

Thompson (Thompson and Bank 2007) has stated that in the design of buildings, there will be trade-offs in acceptable risk versus costs. This shows that it is accepted that people are willing to save costs as long as the structure is designed for what is accepted as reasonable explosion loads. Stewart (Stewart 2010) states that the parameters that need to be considered for terrorism protective measures should include threat scenarios and probabilities, the value of human life, the physical damage, indirect damage, and the risk reduction and protection measures costs.

Whicker (Whicker, Janecky, and Doerr 2008) stated that immediate response to terrorist acts is critical in limiting human and environmental harm, effectively restoring the functions of the building and maintaining public confidence. Restoring the building quickly is important as it reduces further safety risks from a site of a partially destroyed building. This is an issue as walls might be standing without supports and could possibly fall which is safety issue. This was based on an adaptive management strategy that is to be set up after an attack takes place.

Most of the information that has been reviewed has stated that the likelihood of attacks and the extra costs of designing structures to withstand large explosion loads are main reasons for structures not being designed to withstand large explosions.

Protective Systems for Structures Against Explosions

There are a number of techniques that have been developed to protect structures against the destructive effects of explosions. These techniques incorporate devices that are designed to absorb or disrupt the blast wave before it reaches the structural elements. This section has been included in the literature review as it is an important aspect of the protection of structures and is therefore directly related to the project scope. This section will also determine what research has already been conducted in the area of protective systems.

The first area within the section of protective systems for structures is the types of protective systems available for structures. Langdon et al. (Langdon, Rossiter, et al. 2010) stated that there are two main types of protective systems available to protect structures. These are classified as either active or passive systems. Active systems have an accelerated buffer that is directed to oppose the blast and is set off by the detection of an explosives detonation (Wadley et al. 2010). This buffer opposes the blast and minimises the pressure that is hitting the structure but is found to be impractical to deploy. Passive systems are further classified into four main sub sections. These are impedance mismatching, sacrificial cladding, geometric arrangements and blast wave disrupters. These are all static systems that do not require a trigger mechanism to protect the structure (Langdon, Rossiter, et al. 2010).

The second area will focus on how the passive systems work. Impedance matching works by covering the main structural element with a protective material with different density properties which may reduce the stress transfer by increasing the stress wave reflection at the interface between the surfaces. Sacrificial cladding works by applying a protective layer that is going to absorb the energy of the blast and minimise the stress transferred by increasing the duration of the transference. Geometrical arrangements are where the structure is designed to redirect the blast wave away from areas of vulnerability. Blast wave disrupters use objects placed between the blast wave and the structural element to disrupt the path of the blast wave (Langdon, Rossiter, et al. 2010). The effectiveness of the passive systems has been reviewed and found that impedance mismatch techniques provide high levels of resistance to shockwaves through the interface reflections and have been found to reduce the damage to structures (Hui and Dutta 2011). Wang et al. (Winget, Marchand, and Williamson 2005) also concluded that including cavities inside concrete walls also reduces the stresses transferred through the wall and therefore reduces the damage to the structure. Ma et al. (Ma and Ye 2007) found that sacrificial foam cladding can reduce the damage of structures to blasts and that the amount of protection depends on the properties of the blast load and the foam cladding. Li et al. (Li 2009) and Langdon et al. (Langdon, Karagiozova, et al. 2010) found that in some circumstances the addition of cladding panels can increase the damage to the structure as the dynamic responses to the structure were found to be more severe. Li et al. (Li 2009) concluded that this was due to the variations in the blast forces which was also agreed by Hanssen et al. (Hanssen, Enstock, and Langseth 2002). Geometric arrangements have found to have reductions in damage to certain areas of structures (Langdon, Rossiter, et al. 2010). Blast wave disrupters such as steel plates with small holes were found to reduce the damage to the structure (Langdon, Rossiter, et al. 2010).

A review of the articles on protection systems for structures found that many authors agreed that active systems requiring detonation triggers were very complicated and impractical to use. It was also seen that there were some disagreements over the ability for some sacrificial cladding systems to reduce damage to the main structure. This was based on some experiments showing that damage to the main structure was more severe with the foam cladding. It was also agreed upon by many authors that blast wave disrupters are able to reduce the damage to structural elements.

Findings from the Literature Review

The literature review has resulted in the findings that there has been substantial research conducted in the area of terrorism in structural engineering. The main findings were that the failure modes and mechanisms of structures are known for most blast loading conditions and that the failure modes can be accurately predicted by software as was documented in section 2.2 and 2.3. It was also found that designing for explosion loads is extremely difficult as there is not much documentation and no clauses in the Australian loading codes for explosion loads. Section 2.4 also showed that designing for explosions is also difficult due to the varying magnitudes of possible explosive attacks. Another major finding from section 2.6 was that there a number of techniques that are available to minimise the damage to structures. This showed that there are effective measures to increase the safety of structures that are exposed to explosion loads.

Development of Objectives & Scope

The literature review from the previous section has led to the development of the objectives for the project. The objectives have been derived based on the main findings of the literature review summarised in section 2.7. The objectives are defined below and are all within the aim of the project to develop a system to analyse a structure to determine the need for reinforcement and to determine the “optimal” reinforcement for the structure. This system will be able to be transferred to all kinds of structures that could be exposed to an explosion or terrorist attack.

Objective 1: Determine the effectiveness of Protective Systems for Structure against Explosions

The first objective is to determine the effectiveness of protective systems

The first objective is to review the Australian design standards relating to explosion and accidental loads on structures. This is to determine if explosion loads could be included in the loading codes. This has been derived from the information in section 2.4 of the review of reported work. This was found to be necessary as the current codes do not give any guidance on explosion loads for structures. This review of the structural loading code will include the possible use of modelling in the design of structures. This will be included because of the information found in section 2.3 related to modelling of explosions on structures. This is because it was found that modelling techniques are able to accurately predict damage and failure mechanisms of structures exposed to an explosion. Risk analyses will also be included in this review of the design loading code. This is because of the variable nature of explosive attacks and probabilities of attacks should be included in the design of the structure as was found in section 2.5 with regards to likelihood of attacks. The scope of this structural code review will be restricted to the Australian codes.

Objective 2: Determine the feasibility of reinforcing existing structures with passive protection systems.

The second objective is to look in to the feasibility of reinforcing existing structures to withstand explosive attacks. This is to help in making existing vulnerable structures safer to use. This is in relation to information found in section 2.2, structural responses to explosions, and section 2.6, protective systems for structures against explosions. It relates to reinforcing areas of structures that are the most common in the failure of structures when subjected to explosion loads using some of the protective systems that were found in section 2.6. This will involve a cost benefit analysis of reinforcing structures. This can also include the use of modelling to determine current explosion tolerances and to determine the level of protection that the users of the building want as found in section 2.5 in relation to trading costs verse additional costs. This will look into reinforcing columns and connections with slabs that are commonly eroded in explosion and strengthening connections that sometimes fail in explosions. This is to reduce the chances of local failure which can lead to progressive collapse which was found in section 2.2.

Objective 3: Determine whether it is practical to reinforce vulnerable structural elements using protective systems.

The third objective is to look into the practicality of reinforcing vulnerable structural elements in existing structures. This will follow on from the cost and effectiveness of the reinforcing methods determined in the second objective. This also relates to the protective systems that were found in section 2.6 of this report. It will look into the construction issues related to the installation of the protective system and the effects that the protective system will have on the users and productivity of the structure. This research is needed as the costing and effectiveness of reinforcement for structures will not be of use if the reinforcement cannot be applied to the structure.

Methodology for Data Collection

The scheduled progress is the breakdown of work that needs to be done in order to complete the project in the time available. This breakdown of work refers to the tasks listed in the table of progress section of the report in section 4 where the scheduled times can be found. The writing of the report has been broken down into small sections and has been scheduled to be undertaken after the analysis of data for each different section.

The work that needs to be completed before the end of CEP461, semester one, and through the mid-year break is the organisation of industry interviews and questions that will be needed to allow the required information to be found from the industry interviews. The collection of data on design standards, the philosophies behind design standards and the procedures for amending the design standards can also begin before the end of semester one. This information will be obtained from Engineers Australia and through other industry inquiries about the relevance and need for amendments to the loading code to incorporate explosion loads. The task is the collection of further information as to whether the design loading codes should be amended. The next task is the collection of information on risk analysis of terrorist attacks and also data on the correlations between the magnitude and risk analysis of explosions for terrorist attacks. This information can be found from government and tourist risk analyses and from previous research into the evaluations of magnitudes of earthquake loads from probabilities.

Analysis of the collected data can start once the data has been collected for each of the areas. This plan proposes that the analyses of the data on the philosophies behind and the procedures for changing the design standards be analysed concurrently and that this task should take around 8-10 hours. The analysis of this data will involve determining the information and elements that would be needed in a design loading code and how the information could be incorporated in a loading code. This analysis will determine what information would be required to be included in the design standards and the procedures for amending the design standards. This will be followed by the analysis of the data on reasons why the design loading codes should be changed. Once it is known what information will be required for inclusion into the codes that information can start to be collected.

For the start of CEP462, the first task will be the analysis of the data on risk analyses and the correlations between magnitude and probabilistic risk analyses. This analysis will provide the estimation tool that will aid in the design loads that can be used in the design of the structures. This analysis is scheduled to take around 15 hours. This section of the report can then be written up whilst the combination of the analyses for all of these sections occurs. This combination analysis will give the result for objective one. It will determine if the design loading codes could be changed and what information should be included in the possible changed codes. The write-up of the report will be undertaken concurrently throughout the whole process of collecting and analysing data for the possible design loading code amendments.

Once objective one is complete, attention can be turned to objective two. The data for objective two can start whilst collecting information for objective one. The first tasks are to collect the data for the designs of common structures in Australia that may be vulnerable to explosion loads or terrorist attacks. The next set of data needed is information on the effectiveness and costs of using higher strength and larger sections to resist explosion loads. This data will be collected by looking into explosion research on concrete sections and modelling results of explosion loads on structures. This data collection time is scheduled to take 16 hours. Data on the effectiveness and costs of using protective cladding on vulnerable structural elements also needs to be collected. This data will be collected from manufacturers of protective cladding and other independent research into the effectiveness of protective cladding on structures. This data collection time is estimated to be 18 hours. Once enough data has been collected, the analysis of the common designs of vulnerable structures can begin in order to determine the amount and strength of reinforcing is needed. The analyses on the effectiveness and the feasibility of using larger and higher strength sections can then begin after the previous analysis on vulnerable structures. This analysis will determine whether it is feasible to reinforce structures by replacing vulnerable elements or whether other protection measures are needed. The analysis of the data on protective cladding can begin once the information has been collected and can be undertaken concurrently with the analysis of larger or higher strength sections. This analysis will give the feasibility of applying protective cladding to vulnerable elements of existing structures. Once the results of the two different methods of reinforcing the structure have been obtained, comparison can be done to determine the best way or reinforcing vulnerable structures. This analysis will result in the outcomes for objective two. The report will be written whilst conducting the research and analyses of information for objective two.

Work on objective three can begin next. The data for objective three can be collected whilst the data for objective two is being collected. The data needed for objective three will be related to the methods of installing the reinforcement. This will cover the construction methods that would likely be used to replace existing members of structures and also the construction methods that would be used for installing protective cladding to structures. This data will be collected from contacting companies within the construction industry with experience in refitting and refurbishing old structures and also from the suppliers of protective cladding products. Other information on construction and refitting methods will also be collected from research papers and articles. Once sufficient information has been collected, it will then be analysed by looking into the issues and problems that are involved in reinforcing the structure. The writing of the report will be done throughout the whole process of data collection and data analysis.

The next task will be to review and edit the report. This will be followed by creating a summary report for the project. The written and summary report will then be due. After these tasks for the report, the preparation of the oral presentations will begin. The preparation for the oral presentation is scheduled to be finished before the exam period begins but there is some allowance for further preparations to occur during the exam period. The oral presentations will then begin after the exam period. This entire schedule is summarised in the tables of progress shown below.