Overview+of+Temporary+Structures+Failures+in+Construction

Overview of Temporary Structures Failures in Construction //XIAO YUAN, Ph. D. Candidate, Architectural Engineering Department, The Penn State University 2013. // //Key words: temporary structure, safety, Cyber Physical Systems (CPS). // toc
 * Case studies from 1955 till Present **

Introduction


Temporary structure is a broad term for systems and assemblies used for temporary support or bracing of permanent work during construction, and structures built for temporary use. The former are defined as the elements of civil engineering work which are required to either support or enable the permanent works and are found in all areas of construction (Grant and Pallett 2012, pp 15). Included are temporary supporting systems such as earthwork sheeting & shoring, temporary bracing, soil backfill for underground walls, formwork systems, scaffolding, and underpinning of foundations. The second category includes temporary or emergency shelters, public art projects, lateral earth retaining structures in construction zones, construction access barriers, temporary grandstands and bleachers, sound system and lighting support structures for parades and public events, and indoor and outdoor theatrical stages (Parfitt 2009, pp 1-2).

Past decades have seen numerous significant collapses related to improper erection and monitoring of temporary structures. In 1973, the improper removal of forms triggered a progressive collapse of the Skyline Plaza (Bailey’s Crossroads, VA), killing 14 construction workers and injuring 34 others. Another example was the collapse of a section of the University of Washington football stadium expansion in 1987 due to premature removal of temporary guy wires. A major scaffold system on a 49-story building on 43rd street in New York’s Time Square collapsed in 1998 as a result of bracing removal, resulting in the death of one individual, several injuries and hundreds displaced from their residences.

1. Earthwork Shoring/ Sheeting System
Sheeting & shoring using systems such as steel soldier piles, sheet piles, and slurry walls, are used to prevent soil movement and cave-ins during the excavation of earth. These systems help minimize the excavation area and protect nearby buildings or structures. An sheeting and shoring system can be categorized into spaced sheeting or close sheeting. The former method involves inserting spaced timber shores, bracing, trench jacks, piles or other material to resist the pressure from surrounding earth. The close sheeting requires continuous solid sheeting along the entire length of excavation (Berry 2009, pp 15).
 * **Overview**



Inappropriate design and installation of earthwork shoring & sheeting systems result in numerous accidents each year making earthworks a substantial risk for workers. With increased concerns, the government has made an effort to reduce the amount of cave-in accidents throughout the past decades. However, recent reports from OSHA show that cave-in accidents have kept occurring at the rate of around 20 accidents each year from 2006 to 2012. The figure 3 below shows the trends of the fatal injuries in cave-in accidents.

**Figure 3: Fatal Occupational Injuries Due to Cave-in, From 1992 to 2010 (BLS 2007, pp 3; BLS 2012, pp113 & 162)**
 * = year ||= 1992 ||= 1993 ||= 1994 ||= 1995 ||= 1996 ||= 1997 ||= 1998 ||= 1999 ||= 2000 ||= 2001 ||= 2002 ||= 2003 ||= 2004 ||= 2005 ||= 2006 ||= 2007 ||= 2008 ||= 2009 ||= 2010 ||= 2011 ||= 2012 ||
 * = amount of fatal injuries ||= 42 ||= 38 ||= 49 ||= 33 ||= 55 ||= 35 ||= 46 ||= 44 ||= 40 ||= 36 ||= 34 ||= 48 ||= 41 ||= 44 ||= 28 ||= 30 ||= 27 ||= 22 ||= 28 ||= 21 ||= 17 ||

(1) Lack of shoring/ sheeting system. The lack of a shoring/sheeting system causes the majority of trench collapses. Sometimes, excavations are performed to provide access to pipelines or for other small underground projects. In these instances, the excavations are regarded as “easy” and without potential safety hazards so contractors prefer to not install a shoring/sheeting system. In other cases, construction proceeds ahead of schedule and employees work in the trench before the shorting/sheeting system has been fully installed.
 * ** Causes **

(2) Inadequate shoring/ sheeting system. Inadequate shoring/ sheeting system means that a system is improperly installed and fails to meet design expectation. It stands there without protecting workers, and may even make things worse when it collapses with soil towards workers.

(3) Material storage. The slope of excavation is unstable due to the instability of soil. The shoring/ sheeting system is designed to support the soil around the trench. Too much external load, such as heavy trucks, material storage, etc., will excessively impact the shoring/ sheeting system. It gets worse when there is no shoring/ sheeting system at all. The slope collapses when it cannot hold the external load from material storage near the trench.

(1) Trench collapse due to lack of shoring system Accidents occur frequently when workers are working inside of the trench with no shoring system, when the trench suddenly collapses and buries the workers in it.
 * ** Case Study **

A trench collapsed in 1996 on the Wang Lee street in Hongkong, killing one worker when he was laying a pipe line in the trench. The trench is 1.8 m wide and 2.2 m deep. Investigation revealed that the trench was shored improperly, and there was a presence of water inside of the trench.

In 2009, a worker was adjusting a water pipe in the trench with a size of 0.6 m wide and 3.0 m deep. One side of the trench collapsed suddenly, and buried the worker to the level of his chest. The worker passed away the next day. It was investigated and found that there was no shoring system inside the trench, and there was even no proper and egress (Cheong 2013).

(2) Trench collapse before shoring system has been installed in time In 1999, a trench was dug for laying pipelines on the road of Sam Mun Tsai Road, in Hongkong. And the trench suddenly collapsed when the workers were getting ready to install shoring systems. This accident killed one worker, with one other worker injured.

(3) Falling objects due to lack of shoring system Another accident occurred in 2012, when the worker was working in excavation. Part of the excavation collapsed due to the lack of shoring, which caused the falling of a RC pile. The pile fell towards the worker, and hit him on the back of head. Soil came together with the pile to get him stuck (Cheong 2013).

2. Temporary Bracing System
Temporary bracing systems are used to keep a structure or other building systems stable before the permanent bracing is installed, or the element becomes self-supporting. It is commonly used in construction of masonry walls, tilt-up precast concrete panels, steel frames, large timber framed walls and wood trusses. During the whole construction of wood frame, temporary cross-bracing adds lateral stability and help prevent collapse of building structures. During the excavation, there are two main types of temporary bracing systems, namely internal bracing and tie backs. The internal bracing system will hinder movement of equipment and materials and shall not be used for deep excavation. One type of internal bracing is rakers, which rest on foundation mat or rock to support the wall. Another one is cross lot bracing, which extends from one side of the excavation to the other side to retain earth wall. As for the tie backs, it is most effective in firm ground. Tie backs provide a clear working space within the excavation, yet it is more expensive than internal bracing systems, and it might extend beyond the property lines of the building site.
 * ** Overview **

A temporary bracing system is important to construction safety, yet it is often neglected. Insufficient bracing is cited as one of the four most common causes of failures in steel structures under construction (Kaminetzky 1991, pp 204-211). As is pointed out by Feld and Carper (1996, pp 429), perhaps the most dramatic structural failures during construction resulted from a lack of stability. In most of the structural collapses, it is due to the insufficient support of loads that applied at the time of failure (Rens, etc. 2000, pp 653). There is a time during construction before permanent bracing systems have been installed, and the project relied heavily upon the temporary bracing system. The structural load is usually analyzed by conceiving the whole structure as a completed entity, and there is frequently a lack of design or proper implementation of these systems. Often, the specific provisions and requirements of temporary bracing systems are left to the workers on the job site that may not have the qualifications or expertise for proper execution. (Feld and Carper 1996, pp 429).

Three causes are summarized based on past structural failures related to temporary bracing systems. (1) Unexpected natural hazards. According to Rens. et al. (2000, pp 653), the forces that temporary bracing systems are intended to resist mainly come from the wind. When collapse occurred, contractors often explained the failure as "We just got some unexpected wind gusts". Thus a design that accommodates predictable natural hazards was suggested (Feld and Carper 1996, pp 430). However, except for the natural hazards, the so called unexpected circumstance acts only as catalyzer, not the root cause, to a structural failure. In 1986, a concrete wall collapsed in a windstorm in Atlanta, Georgia, killing two construction workers. And it turned out that the concrete block wall was not braced at all.
 * ** Causes **

(2) Insufficient or nonexistent bracing system. As is pointed out by Feld and Carper (1996, pp 435), minor structural failures occur every day due to insufficient or nonexistent temporary bracing. Actually, insufficient bracing is one of the four most common causes of failures in steel structures under construction (Kaminetzky 1991). In Toronto, Canada, a welded steel frame collapsed due to inadequate bracing in 1958. In 1984, a high masonry wall under construction collapsed in downtown Edmonds, Washington, for it was not braced. In 1987, a steel stadium project of the University of Washington football stadium in Seattle collapsed during construction due to inadequate temporary bracing. The insufficient or nonexistent bracing mainly results from human negligence or miscalculation of the load analysis. Contractors often have the attitude that "if we work fast enough, we won't have to brace it, and nothing is likely to happen"(Feld and Carper 1997, pp 430).

(3) Imbalanced or lateral loading due to construction sequence. Construction sequencing is very important to preserve the stability of incomplete structures. During construction, the load imposing on an incomplete structure is unstable, due to installation of components and construction activities. While these lateral loads are expected to be supported by temporary bracing, great changes in load may result in failures. This is the usual cause of many roof structures failures, for the roof structure often collapsed before the bracing system has been placed (Feld and Carper 1996, pp 431). Feld and Carper (1996, pp431) further talked about the structural failure of seven concrete girders which tumbled over on a highway construction project near Seattle, Washington in 1988. It is found that diaphragms that would have provided stability were not yet in place. Most of the time, it is the contractor who is responsible for determining the bracing and construction sequencing (Delatte and Rens 2002, pp 98-109), which makes it hard to determine the appropriate safety construction sequence. (1) Collapse of a steel stadium project in Seattle, Washington, 1987. In 1987, an addition of a football stadium at University of Washington collapsed. An inadequate temporary support system was regarded as the most probable cause of failure by Feld and Carper (1997, pp 431). According to other investigators, an incomplete system of temporary guying system was the critical deficiency to the collapse. Detailed information related to this case can be found following this link: https://failures.wikispaces.com/University+of+Washington+Football+Stadium+(Manno)
 * **Case Study**

(2) Collapse of a radial dome in Louisiana in 1964. The dome was designed as spanning 240 feet, with 36 timber arches. These arches had been placed for support on a tension ring on columns around the perimeter. However, before the installation of the deck, the temporary pipe shore of the compression ring at the top was removed. And one hour later, half of the cables connecting the tension ring and the compression ring failed, resulting in the rotation of the compression ring. Thus the whole roof collapsed, with no one component being preserved.

(3) The Chicago City Post Office (November 3, 1993). The new building of the Chicago Post Office partially collapsed in 1993 due to the failure of a temporary connection of temporary erection angle pieces, which were used to secure a beam. This tiny piece of failure triggered the collapse of 70 additional components that had been secured. 2 ironworkers were killed, and 5 others got hurt in this accident. More details can be found here: https://failures.wikispaces.com/Chicago+Post+Office

3. Underpinning of Foundations

 * **Overview**

Underpinning of foundation is to install a support to an existing foundation to provide either additional depth or bearing capacity. It is mainly used in the following situations: 1) construction of a new project with a deeper foundation adjacent to an existing building; 2) settlement of an existing structure; 3) change in use of a structure; 4) addition of a basement below an existing structure (Ratay 1996, pp12.4).

Even the most cautiously installed underpinning will come along with some settlement of the structure, and the difference in settlement from one piont to another may cause structural damage (Ratay 1996, pp 12.4). Meanwhile, i t is common that underpinning of foundations often causes damage to existing adjacent structures (Peraza 2007, pp 1-10). The consequences may involve injuries and loss of life, extensive property damage, construction delays, and expensive litigation (Peraza 2008, pp 70).

(1) Lack of underpinning. The contractor often fail to take into consideration the condition of the foundation of the adjacent building, and conducts construction without underpinning it. Settlement, even collapse of the adjacent building happens frequently in this situation. It is noted that, even the pile driven vibration can damage the foundation of the adjacent building without proper protection.
 * ** Causes **

(2) Inadequate underpinning and bracing. Due to the improper design or construction method, the underpinning and bracing system may be inadequate. For example, the poor material of underpinning system can be damaged easily by water penetration or collapsed due to the lack of capacity to hold the load overhead. An insufficiently installed underpinning system cannot stay firmly in the ground, thus can frequently collapse and result in settlement or shifting of the building.

(3) Over excavation. Sometimes, the excavation for underpinning is conducted more than required, and extends toward the adjacent property line. This affects the foundation of the adjoining property.

(4) Impact from rubble foundation. It would be very difficult for contractors to underpin a rubble foundation. This kind of foundation is composed of large stones, and cannot easily be connected and integrated with underpinning pits. The lack of continuity makes it difficult, and sometimes even unsuitable to underpin a rubble foundation ( Peraza 2006, pp 16-20).

(5) Impact from soil and groundwater. The high level of groundwater makes it necessary to underpin the adjacent foundation, which is often neglected by contractors. It works the same with soils that are susceptible to consolidation or vibration settlement. Meanwhile, the site should be dewatered with the existence of high water table. The dewatering of the site can cause the consolidation of soil, resulting in settlement of buildings (SEAoNY 2005, pp 20 ).

(1) Severe weather. A high rise complex was to be built near an old four story building, which was supported by rubble foundation. During the underpinning of the foundations, this old building settled excessively, resulting in wide cracks in the settled walls. It was found that the underpinning was well planned and executed, including well designed plan and qualified engineers. However, the bad rain greatly impacted on the old rubble foundation, which then resulted in significant settlement.
 * **Case Study** (Peraza 2006, pp 16-20)

(2) Improper underpinning In the 1980s, three old building collapsed during renovation in Lexington, KY. Although a proper underpinning plan was required, the excessive excavation undermined the footings, causing the building to collapse. No one was hurt, yet the contractor was required to rebuild the building.

(3) Human negligence During the examination of the required underpinning location, the consultant missed the underpinning of one basement wall. When excavator took out the soil, the footing was damaged and induced significant settlement. Although measurements were taken immediately to stabilize the building, the contractor was sued by the owners for millions of dollars in compensation.

4. Scaffolding System

 * **Overview**

Scaffolding is used to provide temporary safe working platforms for the erection, maintenance, construction, repair, access or inspection, etc. of structures or other building systems. (Grant and Pallett 2012, pp 259). It has been used for 5000 years to provide access areas for building and decorating structures taller than the people who worked on them (Retay 1996,pp 15.2). The basic components of scaffolding are tubes, couplers and boards.

The widespread use of scaffolding is accompanied with an increasing amount of safety issues, and scaffold work has been defined as one of construction’s highest risk jobs (Hsiao and Stanevich 1996, pp 407-415). In the 2010 report, OSHA regulation of scaffolding ranked first among the Top 10 most cited standards in construction industry (OSHA 2010, pp18). Falls from scaffolds account for a huge amount of fall issues in construction industry (Whitaker, et al., 2003). As identified by National Association of Home Builders (2008, pp 14), 15% of fall fatalities were from scaffolds, ranking third of the fall fatalities in home buildings during the years 2003 to 2006. In addition, falling objects and scaffold collapses also serve as big problems for scaffolding safety management. According to the Health & Safety Executive (2004), from 1989 to 1993, there were 1,304 injuries from falling objects and 345 scaffold collapses in UK, along with 3,738 falls from scaffolds. Analysis of accidents related to scaffolds over the past nine years (Figure 9) shows that even with improved management, approximately eight workers are hurt every month in scaffolding collapses throughout the US. The scaffolding system is still dangerous and calls for more safety precautions.
 * ~ Figure 9: Amount of Fatal Occupational Injuries Related to Scaffolding, of Year 2003-2011 ||||||~ (BLS 2012, pp114, ||~ 128,129, ||~ 141, ||~ 142, ||~ 178, ||~ 198, ||~ 217) ||
 * = Characteristic ||= 2003 ||= 2004 ||= 2005 ||= 2006 ||= 2007 ||= 2008 ||= 2009 ||= 2010 ||= 2011 ||
 * = fall from scaffold, staging, or temporary work platform ||= 85 ||= 90 ||= 82 ||= 91 ||= 89 ||= 68 ||= 54 ||= 44 ||= - ||
 * = floor of scaffolds, staging, or temporary platform ||= 3 ||= 5 ||= 8 ||= 7 ||= 8 ||= 3 ||= 3 ||= 3 ||= - ||
 * = scaffolds-staging of structures other than building ||= 12 ||= 7 ||= 7 ||= 8 ||= 9 ||= 8 ||= 5 ||= 4 ||= 5 ||
 * = scaffolds-staging of building ||= 77 ||= 86 ||= 78 ||= 86 ||= 83 ||= 68 ||= 55 ||= 46 ||= 64 ||
 * = climbing, descending scaffolds ||= 5 ||= 10 ||= 7 ||= 7 ||= 6 ||= 3 ||= 3 ||= - ||= 4 ||
 * = subtotal ||= 182 ||= 198 ||= 182 ||= 199 ||= 195 ||= 150 ||= 120 ||= 97 ||= 93 ||

Whitaker, et al. (2003) examined 186 access related cases, and 2,910 incidents recorded in UK from 1997 to 2000, and summarized the most common root causes to the collapse of scaffold. (1) Improper ties to buildings. Scaffolds that are improperly attached to buildings are dangerous. Several scaffolds collapsed when the ties were removed after fitting. Some incidents occurred due to improper fitting or lack of ties. This kind of situation happens when there is a need to remove ties so that access to key areas can be reached. However, this modification of a scaffolding system is done randomly without qualified inspection and analysis.
 * **Causes**

(2) Insufficient bracing within the structure. In the analysis of the 186 access related cases, 62 incidents are related to scaffold collapse. 35.5% of the scaffold incidents occurred due to insufficient bracing system, which ranked a top cause of scaffolding collapses.

(3) Overloading with building material. Some scaffolding collapse due to instability or overloading of materials. OSHA investigated the 16 structural failures related to scaffolding between 1990 to 2008, and revealed that 4 out of 16 incidents occurred due to the overloading of building materials (Ayub 2010, pp 12-20).

(4) Subsidence of foundations. Foundation provides permanent support of scaffold systems on the place where the system rests. Take the regular scaffolding system for example. The foundation of scaffold may be placed on soils with different capacities. Thus loads from scaffold will cause different settlement of the foundation, which then make the scaffold platform imbalanced, or even collapsed.

(5) Inadequate supervision. Most of the accidents related to scaffolding systems cited the unsafe working system as the general causation. This failure to access or control risk can be caused by deficiencies in the working system, defects of platforms, inadequate supervision, as well as improper work procedures. (1) John Hancock Center - Suspended Scaffold Collapse (March 9, 2002) In 2002, a suspended scaffolding system was used to restore the façade of John Hancock Center in Chicago, Illinois. Two outriggers were installed on the roof to hold the scaffolding platform, yet one outrigger overturned that afternoon and caused the scaffolding platform to swing back and forth along the facade. The façade was disintegrated, and multiple windows and debris of scaffolding systems fell down, killing 4 people, and injuring 8 others. More details can be viewed here: https://failures.wikispaces.com/John+Hancock+Center+%28Chicago%29+Suspended+Scaffold+Collapse
 * **Case Study**

(2) Four Times Square-Scaffold Collapse (July 21, 1998) In 1998, a 49-story scaffold system collapsed on 43rd street in New York. One woman was killed, while dozens of other people were injured by the falling debris. More details can be viewed here: https://failures.wikispaces.com/4+Times+Square+Scaffold+Collapse

**5.** Formwork



 * **Overview**

Formworks are primarily used for standard poured-in-place concrete construction. They are used wherever the concrete is placed, such as a factory setting for precast sections and building sites. Various materials can be used for formworks, such as wood, steel, plastic, aluminum, etc. Formwork construction is associated with a relatively high frequency of disabling injuries and illness (Hallowell and Gambatese, 2009, pp 990-998). With the increasing use of formwork, related safety issues have become serious problems (Shapira, 1999, pp 69-75). In high-clearance concrete buildings, formwork collapse is defined as the failure of all or a substantial part of a structure (Kim 2006,pp 1-14). Kim (2006, pp 1-14) also pointed out that because of the potential collapse of elevated slab formwork during concrete placement, the assessment of the shoring system is essential. As for other formwork related injuries, 5.83% of falls and 21.2% of struck accidents mainly result from the construction of formwork (Huang and Hinze 2003, pp 262-271). The preparation of formwork for concrete structures was defined as the dangerous stage of construction (Jannadi & Assaf 1998, pp 15-24) Hadipiono and Wang (1986) studied 85 cases related to the formwork system collapse in the past 23 years, and found that almost half of the formwork system failures occurred during the pouring of concrete. The second critical stage is during formwork removal and post concrete curing. According to their study, the causes to formwork systems failures are summarized below (Hadipiono and Wang1986).
 * ** Causes **

(1) Improper/premature removal of formwork. Untimely removal of formwork is noted as the second most significant event, which is relative to the weak concrete and inadequate removal sequence of formwork. The premature removal of formwork usually comes from the desire to reuse form quickly either because of the pressure of scheduling or budget, while the concrete at that time might haven't attained the expected strength (Feld and Carper 1996, pp242).

(2) Inadequate design of formwork system. Most of the cases related to design flaw are relative to the inadequate consideration of lateral forces and temporary structure's stability. The lack of a bracing system to deal with lateral forces, like wind load and construction load, fails to prevent the formwork system from collapsing when an excessive load is imposed on it. In practice, the formwork components are reused, and the capacity to withhold a load will be reduced. Yet the designer of the formwork often omits the safety factor and calculates the load using the data of the original capacity. From the procedural perspective, the lack of review of the formwork design is also a big issue. Normally, the design of formwork should have been approved by an engineer before installation. Yet in several cases of formwork system failures, it has been identified that this step has been omitted.

(3) Improper shoring of formwork. Several important incidents have occurred due to the improper shoring of formwork. It is found that the improper installation of shores is a significant cause of formwork failure, where impact loads from concrete debris and other effects trigger the collapse of vertical shores during concreting.(Hadipriono and Wang 1986, pp 112-121).

(4) Defective component. Some cases of formwork system failure have been the result of the improper maintenance of formwork components, which then become defective after being reused several times. The capacity of these formwork components has been reduced due to corrosion and damages, yet it is seldom taken into consideration during the erection.

(5) Improper connection. The formwork components are usually connected inadequately so that it is easier for workers to dismantle it. However, this lack of proper connection has induced several progressive collapses. Two types of improper connection have been identified. One is the lack of bolts, nails or splicing. Sometimes, there is no connection at all between two components. The other is poor weld quality and faulty wedges.

(6) Insufficient strong foundation. In the studies of the 85 cases, many foundations of formwork system failed to transfer the load to the ground, and some were laid on weak subsoil. These foundations are often constructed from mudsills, concrete pads, and piles, which can cause differential settlement of formwork and overloading of shores, and finally resultes in collapse. Another problem related to the foundation is insufficient depth of the foundation piles, for it reduces the carrying capacity of the formwork.

(7) Lack of inspection of formwork during concreting. Pouring concrete is easily accompanied by formwork collapses, and many failures occurred when the inspector was absent or he just overlooked the problems. The lack of inspection also involves a situation in which the inspector is inexperienced or unqualified. (1) Bailey's Crossroads - Skyline Plaza (March 2, 1973) On March 2, 1973, the improper removal of forms supporting the 23rd floor of an apartment building in Skyline Plaza triggered a progressive collapse all the way to the ground floor. 14 construction workers were killed, and 34 others were injured. More details can be found here: https://failures.wikispaces.com/Bailey%27s+Crossroads+-+Skyline+Plaza
 * ** Case Study **

(2) Harbour Cay Condominium (March 27, 1981) The Harbor Cay Condominium collapsed in Cocoa Beach, Florida, in 1981. One of the main causes was the premature removal of forms. As is stated by a worker on the jobsite, "twenty-two years I’ve been pouring concrete and they’ve never pulled the forms in two days like they did here. They usually set there for a week or 10 days” (Montgomery 1981). 11 workers paid their lives for this failure. More details can be found here: https://failures.wikispaces.com/Harbour+Cay+Condominiums

(3)Collapse of New York Coliseum (1955) In 1955, in New York Coliseum, an exhibition hall collapsed during construction. It was found that the live load of buggies imposed more load than the formwork could hold. One worker was killed and fifty others were injured in the accident. More information can be read here: https://failures.wikispaces.com/New+York+Coliseum

**6. Temporary Performance Stage**

 * **Overview**

Temporary performance stages are defined as a structural assembly that is used for an outdoor performance for less than 90 days of one year (Wainscott 2011). Collapses of temporary performance stages have occurred frequently in recent years. In 2008, two of the stages for the Rocklahoma music festival collapsed, resulting in ten injuries when severe winds struck northeast Oklahoma. In 2009, the main stage of Big Valley Jamboree in Toronto collapsed, killing one and injuring at least seventy people during another wind storm. Additional collapses occurred in 2011, including the well-publicized Indiana State Fair Grandstand, which resulted in multiple fatalities and over fifty injured people in total. More recently, the Downsview Park in Toronto collapsed in 2012, killing one person and injuring three others, while another stage roof collapsed in North Carolina in 2013 during bad weather. These accidents continue to occur with little warning to the general public.

(1) Poor capability of components. Take the Sugarland stage collapse for example. It has been revealed that four structural failures (jersey barrier, guy line and ratchet strap, fin plate) lead to the collapse of the main stage.
 * **Causes**

(2) Insufficient structural connection. The connection is often weak and easily damaged, especially under severe weather. The design or installation of connection is often overlooked, which then results in big problems.

(3) The lack of engineering review after the stage is erected. There are few regulations on the responsibilities of engineers during the construction of temporary performance stages. Thus, the engineer is seldom required to inspect the stage after installation. Even if the structure is well designed, there might be a big difference between the actual installation and the requirements of engineers.

(4) Bad weather. Most of the temporary performance stages collapsed during bad weather, such as storm. The rain and wind impose bad damage and a great load onto the temporary stages. Once the load is more than what the structure can withstand, it collapses immediately.

Temporary performance stages collapsed frequently every year. In 1990, a singer, Curtis Mayfield was hit by a scaffold of the stage during a show in Brooklyn. In 2008, two of the stages for Rocklahoma music festival collapsed, resulting in ten minor injuries. In 2009, the main stage of Big Valley Jamboree stage in Toronto collapsed, killing one and at least seventy people got hurt. More collapses occurred in 2011, with seven people passing away and around fifty people injured in total. Even after that, the Downsview Park in Toronto collapsed in 2012, killing one person, with three people injured. Detailed information related to these accidents can be viewed here: https://failures.wikispaces.com/Temporary+Structure+Failure+-+Case+Studies
 * **Case Study**

Pattern of Temporary Structural Failures
(1) Earthwork shoring/ sheeting system According to Andresen (2011), 63% of the fatalities occurred due to lack of the shoring/ sheeting of excavation, 20% of the trench collapsed when workers were working ahead of support, and the inadequate support by the shoring/ sheeting system leads to fatalities, accounting for 14% of the total accidents. It is fairly clear that the excavation without the shoring/ sheeting system imposes great risks on the life of workers.

(2) Temporary bracing system A review of 45 accidents relative to temporary bracing recorded by OSHA reveals that 16 of the 45 collapses occurred due to an inadequate bracing system, ranking as the most common cause to accidents related to a temporary bracing system. Another common problem is the lack of a bracing system, which causes 16 accidents among the 45 cases.

(3) Formwork system The majority of formwork related accidents occurred when the formwork system is inadequately designed or constructed.

(4) Temporary performance stages Most of the temporary performance stages collapsed under severe weather, usually with high-speed wind. However, further investigation reveals that it is the difference of structural capability between as planned and actual stages that results in the collapse. The actual capability of temporary performance stages to withstand force from wind is far less than designed. Meanwhile, the connection of the structure is not good enough to stand during the bad weather.

Prevention of Temporary Structural Failures
(1) Earthwork shoring/ sheeting system Unless excavated entirely in stable rock, OSHA requires that all employees working in an excavation should be protected by supportive system. In OSHA regulations (Standards-29CFR), there are requirements for timber shoring, aluminum hydraulic shoring, pneumatic/ hydraulic shoring, trench jacks and trench shields. It is required that soil type should be examined using specified soil classification methods. For assistant in designing a shoring system, the required minimum dimension of shoring members is presented in forms of charts, so that designers can calculate the minimum size of members under specific conditions. Besides, OSHA requires that shoring/ sheeting systems should be designed by a registered professional engineer when it is to be used in an excavation deeper than 20 feet. For the excavation less than 20 feet in depth, a graphic summary of requirements is presented for easy application.
 * **__ Regulations & Standards __**

(2) Temporary bracing system There are few regulations relative to the safe construction of a temporary bracing system. As for the temporary bracing system for masonry wall, the Code of Federal Regulations (CFR 1926.706(b)) simply specifies it as "adequate bracing", yet no more instructions are provided. The international building code specifies the temporary bracing system as a secondary member. And simply states that inspectors should verify if the temporary bracing is installed as designed if the steel/ wood truss spans no less than 60 feet.

(3) Underpinning of foundations Few regulations have mentioned underpinning of foundations. In the international building codes, the underpinning of a foundation is only specified as the required step before removing lateral support of foundations. Only a few local states hold some brief regulations about the underpinning of foundations. For example, the New York City requires controlled inspections of the underpinning of foundations. Yet no more specifications are provided as for how to control it during the process.

(4) Scaffolding system OSHA requires that scaffolds should be designed by a qualified person, and constructed following this design. Several items are pointed out by OSHA. In terms of capacity, it states that the scaffold should be able to support not only its weight, but a specified maximum load applied or transmitted to it. For the scaffold platform construction, it requires the working platform on a scaffold to be fully planked or decked, except for the one used as walkways or to perform scaffold erection or dismantling. Besides, there are several limitations on the width of space between adjacent units and the space between the platform and the uprights. Other items include criteria for supported and suspension scaffold, scaffold access, use of scaffold, fall protection, and falling object protection

(5) Formwork system It is required by OSHA that the formwork should be designed, fabricated, and maintained to be able to support all external loads that may be reasonably placed on the formwork. T he design of shoring should be performed by a qualified designer. All shoring and reshoring equipments should be immediately inspected prior to, during and after concrete placement, by a qualified engineer. Once the shoring equipment is found to be weak or damaged, it should be reinforced immediately. Other regulations include the requirements of sills, base plate, shore head, and extension devices. In addition, the formwork and shoring should not be removed until the concrete reaches enough capacity.

(6) Temporary performance stages Although structural engineers analyzed the structural load for the permanent structures, there are few standards or guidelines on the design of temporary performance stages. In addition, while there are a few local requirements, such as New York and Chicago, no national regulations have about the safe construction and maintenance of temporary performance stages. Severe accidents of temporary performance stages have focused spotlight on the call for regulations of such a structure, and wind-load standards for temporary performance stages are also in high demand. To ensure the life safety of workers, the Mason Contractors Association of America (MCAA) published the Standard Practice for Bracing Masonry Walls under Construction. It provides procedures for the design of temporary bracing systems for masonry walls. In addition, a restricted zone is specified, so that workers are forbidden to work in that zone once the wind reaches prescribed speed. This standard practice helps to reduce risk to the life of workers, yet it pays little attention on the prevention of structural failures.
 * **__ Recommended Practices __**

There is a voluntary guideline for the design, manufacturing and maintenance of temporary outdoor stage roofs, yet this guideline fails to provide regulations relative to the entire structure of temporary performance stages. In addition, this guideline is not promoted and followed nationwide. Details can be viewed here: [] According to OSHA regulation, employers are responsible to provide safety training and education programs to their employees. Meanwhile, OSHA and local governments provide lots of safety training resources online, including excavation safety training, fall protection, scaffolding, concrete, and masonry. However, there are few training resources related to underpinning of foundations, temporary bracing, and temporary performance stages.
 * **__ Education __**

Future Solution to Prevent Temporary Structural Failures-Cyber Physical Systems (CPS)
The problems of temporary structures identified above calls for accurate and real time inspection of temporary structures, for delayed detection of initial damage can result in critical failure. This requirement can be met by the application of CPS.

In the general sense, CPS are defined as the integration of computation with physical processes. Embedded computers and networks monitor and control the physical processes, usually with feedback loops, where physical processes affect computations and vice versa (Derler et al. 2012). With the implementation of CPS, a virtual model of temporary structure can be developed and updated according to the performance of temporary structures on the job site. Image comparison, 3- dimensional analysis, as well as load analysis can be conducted through computing system. Once there is a potential danger, an alarm will be sent to workers and engineers for further measurements.

Conclusions and Discussion
This overview of temporary structures examines the importance of temporary structures in the construction industry. Six typical types of temporary structures are selected for discussion, including earthwork shoring/ sheeting system, temporary bracing system, underpinning of foundations, scaffolding system, formwork, and temporary performance stages. Each of these temporary structures is analyzed for safety hazard problems, causes, case studies, patterns of accidents, and available safety regulations. In addition, future solutions to prevent such incidents are suggested. From this overview, it can be concluded that:

(1) Temporary structures hold great risks to structure performance, and have potential safety hazards to workers on the job site. (2) Inappropriate monitoring of temporary structures turns out to be the major problem of temporary structural failures. (3) There is a call for more safety regulations regarding temporary structures, especially the underpinning of foundations, temporary bracing systems, and temporary performance stages. (4) More efforts and safety precautions should be taken for better monitoring of temporary structures. (5) Informatic technologies, such as CPS, can help improve the performance monitoring of temporary structures.