Bridge Falsework Manual
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Bridge Falsework Manual
Discover everything Scribd has to offer, including books and audiobooks from major publishers. Start Free Trial Cancel anytime. Report this Document Download Now save Save Bridge Construction Manual - Falsework and Forms For Later 0 ratings 0 found this document useful (0 votes) 426 views 81 pages Bridge Construction Manual - Falsework and Forms Uploaded by Rocky Cho Description: Full description save Save Bridge Construction Manual - Falsework and Forms For Later 0 0 found this document useful, Mark this document as useful 0 0 found this document not useful, Mark this document as not useful Embed Share Print Download Now Jump to Page You are on page 1 of 81 Search inside document Browse Books Site Directory Site Language: English Change Language English Change Language. The principals of falsework This course includes a True-False quiz at the end, which is designed to enhance Once the reader understands Falsework failure includes We will discuss Other than The only way They are not a substitute Application of this information to a specific Anyone making. These structures have a significant impact on the cost, construction rate and construction safety of the supported permanent structures. However, the relevant stakeholders often do not consider them to be as important as permanent structures, and in recent years a high number of accidents involving bridge falsework systems have been reported, particularly in the developing world. In order to increase the safety and the efficiency of these systems it is certainly beneficial to learn from past failures. In this paper, a survey of the failures of bridge falsework systems since 1970 is presented. Download full-text PDF In order to increase the safety and the efficien cy of these systems it is certainly benefic ial to learn from past failures. In this paper, a survey of the failu res of bridge falsework systems since 1970 is presented.
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The data will provide useful statistics for risk assessments when considering the potential consequences associated with failures of bridge falsework systems. Keywords: temporary works, bridge s, failures 2 Figure 1 illustrates some examples of bridge falsework systems. Figure 1: Examples of bridge falsework sy stems. There are several stakeholders directly or indirectly co ncerned with bridge falsework systems: researchers, designers, producers, clients, consul tants, insurers, contractors, sub - contractors and workers. In this context, the assemblage, use and dis mantling of bridge falsework systems is usually done by a specialise d sub - contractor, in accordance with a standard project or with a spe cial de veloped project depending on the work comple xity. Through time the role of bridge falsew ork in the cost, construction rate, safety, quality, durability, efficiency, utility and aesth etics of any bridge project has increased in a con sistent fashion (fib 2009). Therefore, it is not surprising that a corre ct choice, good planning, design and operation of bridge falsework are keys for the success of every bridge project. In particular, it is vital that synchroni se d planning and continuous knowledge exchange exists betw een the bridge designer, the bridge contractor, the bridge falsework designer, the bridge falsework contractor and others. Unfortunately this is not always a reality. As pointed out in (fib 2009) the framework of bridge construction consists of complex i nteractions between all the abovementione d stakeholders who have different backgrounds and c an have different p riorities, perceptions and goals, some of which can even be contradictory. Consequently, the design and use of bridge falsework sy stems are not usually t reated as carefully as in the case of perman ent structures and do not receive the same le vel of research attention and research funding.
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I n practice the design of bridge falsework is usually an overs implified process, based in a comparison of the design forces with reference resista nce values given by falsework system producers, 3 The general framework outlined above contributes strongly to the high num ber of incidents and accidents involving the use of bridg e falsework systems, whic h frequently cause human casualties and severe injuries, work inefficiency and partial (or total) structural damage o f the infrastructure. Since 1970, several falsework collap ses have been reported worldwide, with a growing trend i n developing countries like China, India and Dubai where a boom in construction work has taken pla ce. According to (Xie and G. Wang 2009) in China, 27 collapses of bridge falsework systems occu rred d uring 2005 - 2009 period, killing 100 workers and causing a higher, although unspecif ied, number of injuries. Beyond human losses and i njuries, these accident s may cause considerabl e economic, financial, environmental and poli tical costs as well as d amage to reputations and increased insuran ce premiums. Yet, despite their impo rtance and extensive p ractical use, the ex isting research concern ing bridge falsework systems is v ery limited, see (Beale 2007). To contribute to a better knowledge about the struct ural behaviour, reliability and rob ustness of bridge falsework systems a joint research p rogramme between Oxford Brookes Univers ity (OBU) and the Portuguese National Laboratory for Civil Engineering (LNEC) was initiat ed in 2010. The information was collected from failure d atabases, forensic engineering literature and from data available on the World Wide Web. From this data the most relevant procedural causes, enabli ng and triggering events of reported failures are identified. Estimates of the individual and of the structural risk involved with the use of bridge falsework systems are also given.
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2 Bridge Falsework Systems Invest igations With the industrial revolution came new challenges for civil engineers: to keep up w ith industrial development new bridges, in larger scales, carrying heavier loads had to be built at a fast pace. Bridge falsework also experienced this new complex ity. However, little attention was drawn to this subject and as a result a series of falsework collapses o f major significance occurred in the industriali s ed countries throughout the XX cent ury. In 1970, as a response to the public outcry following a colla pse with severe consequences, the UK construction industry establishe d a committee under the chairmanship of S.L. B ragg to investigate the use of falsework. The result was the Brag g report (Bragg 1975), a pioneer document wh ich established the basis for the subsequent publication of th e first UK standards concerning falsework (BSI 1982). This report showed that failures often occurred because known rules were n ot applied such as an insufficient allowance for horizontal loads and the possibil ity of stability failures, insufficient consideration of ac tual loads, inadequa te foundations, erection eccentricity and inadequate procedures for dismantling. All these causes still o ccur. Since the Bragg report, there have been a number of fundamental change s to the construction industry which had a profound effect upon the m anner falsework is dealt with by all stakehold ers, see (SCOSS 2002) for details. Accidents involving bridge fals ework often have vast and various negative impacts: on project profitability, on the credib ility and competence of the inv olved companies, on the increa se of the insurance premiums and on the economical, financial and political costs due t o postponed benefits.
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4 These results can easily be extrapolated to the failure of bridges during construction, b y reclassifying traffic delay and traffic management costs as po stponed benefits (loss of service and associated loss of rev enue) due to the delayed bridge opening dat e. The collapse of bridge falsework systems caused by an unaccounted or under - evaluated event cannot be acceptable. According to (Steven 2010), the “mind set within the construction indus try over the decades from the 1900s has change d from accepting (.) 13 deaths for the construction of the major viaduct, to a mindset unacceptable of any level of injury”. The various concern s outlined in the above paragraphs illustrate the ne ed for a holistic approach applied to bridge falsework, e.g. a risk management framework, and adequate competency to undertake the task. It is essential that those involved with bridge falsework realise t he importance of this ch ange, the relevant statutory need to consider whole working li fe risks to structural sa fety, and the commercial benefits that will accrue b y doing so (SCOSS 2005). It is clear that there is a need for scientifi c progress in the field of bridge falsewor k. A measure of the accomplishment of this task is given by our a bility to redu ce the uncertainties associated with bridge falsework. Contributes to a chieve this goal can be derived from investigating past failures. Failures of bridge fals ework structures, w ith very few exceptions that involve a consider able number of fatalities do not fill th e media headlines as a collapse of a bui lding or a bridge do es. They usually happen away from the public eye, at an isolated construction site and the knowledge of their occurrence is often kept limited to very few people.
One should note that e ven when a disaster of a bridge during its construction is covered by the media, reported in books, or in other technical publications, it is nearly always the permanent structu re that is described, with little or no discussion of the details of the tempo rary structure even i f it was the cause of th e collapse (Ratay 2009). Additionally, i t is often difficult or impossib le to determine the precise cause o f a falsework failure. A usual ground zero scenario of a falsework co llapse comprises a pile of wreckage of bent tubes, in which the initial failure is proba bly obscured. Furthermore, the r eports of accident investigations are generally una vailable to those not directl y involved, but who wish to under stand causes in order to avoid repetiti ons. However difficult it may be, it is extremely important to carry out failure investigations so as to understand the conditions giving rise to past failures and ways to avoi d such failures so that loss of life and property can be minimi s ed. 3. Individual and Structural Risk Estima tes In this section, estimates of risks to individuals, and of structural r isks, are presented, specifically in terms of the Indiv idual Risk Per Annum (IRPA), i.e. the annual probability of a fatal accident, a nd the annual probability of structural failure. These variables ar e calculated based on the results of a survey of the co llapses involving bridge falsework s tructures since 1970 in 19 coun tries worldwide, which will be presented in section 4 of this paper. N ote that the two collapses recorded in the s urvey for the UAE occurred in the Emirate of Dubai. 5 Since data relative to the number of bridges built each year in each cou nt ry analysed is not available, an average value of IRP A in the last 42 years was de termined for each country considering the total number of reported fata lities and the total number of persons exposed to the risk of collapse of bridge falsework structure.
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The latter is given by the product of the total number of concrete bridges built us ing these systems with the average number of persons exposed to the risk of collapse of the bridge falsework structure. Figure 2 illus trates the 19 IRPA values. However, the obtained fatal accident rate for bridge falsework systems is similar to the one observed in the c onstruction sector in Portugal, S pain, Italy, the Netherlands and in the US (CPWR 2008; Eurostat 2012). 6 Comparing the valu e for the individual risk obtained for bridge falsework systems with the limits specified in specialised literat ure, for example (HSE 2001), for the acceptable and unacceptable risk levels, it can be concluded that in all countries included in the analysis, ex cept Australia and New Zealand where no fatal injury was reported, the individual risk is higher than the broadl y acceptable risk level (taken as 1?10 -6 per year ) and that in all countries, except Vietnam, the individual risk is lower than the unacceptable risk level (taken as 1?10 -3 per year). Therefore, the individual risk for bridge falsework systems is in general within the risk toler ability zone and must be reduced “as low as reasonable practicable”, or ALARP. Table 1: Summary of data used to calculate risk estimates for bridge falsework systems in 19 countries since 1970 Country Accidents Fatalities Injuries Number of bridges a Reference Persons at risk b Andorra 1 5 6 200 c 40 Australia 1 0 15 20368 (Court et al. 2005) 50 Austria 1 2 0 12942 and 40 Brazil 2 32 40 2700 (Mendes 2009) 100 Canada 3 7 16 17280 (Hammad et al. 2007) 40 Czech Republic 1 7 67 6633 and 60 China 8 98 118 43200 (H. Wang et al. 2011) 100 Denmark 2 1 5 6152 and 40 7 Additionally, using the method presented in (McDonald et al. 2005) the acceptable annual probability of failur e increases t o 2? 10 -5, which is also not satisfied.
Comparing the P f, 1 values presented in Figure 3 with the structural risk of other temporary structures, the UK Health and Safety Executive (HSE) investigated 471 reported scaffold collapses during the period between 1986 - 1993, see (Beale and Godley 2003). Considering an estimated 7.5 million scaffold erections it gives a failu re rate of 6.3 collapses per 100 000 erections ( i.e. 6.3 ?10 -5 per year ), which in the UK compares with an est imated probability of 4. 6 collapses per 100 000 bridge projects for bridge falsework systems, a value clo se to the one observed for scaffold systems. Time analysis of risks to individuals and of structural risks are not presented due to t he difficulty in collecting required data, i n particular about the number of br idges built each year in each cou ntry analysed. 8 I t could be concluded that until the year 2000, the reported accidents occ urred mainly in developed countr ies like Germany and USA; after the year 2000 there are an increasing numb er of reported bridge falsework failures in the developing world such as China, India and Dubai. The numbers also indicate a growing trend i n the number of reported collapses, injuries and fatalities since 2000. Figure 4 presents the evolution with tim e of the total number of collaps es in the countries where three or more collapses have been registered. It can be observed that in most of the 19 countries considered in the survey, the tota l number of registered collapses is smaller than or equal to two, which could mean that the risk of using bridge falsework systems is low (al though not acceptable). For example in the UK this can be justifie d due to established good practices and l egal dut ies, whereas in some other countries it could mean that there are a number of unreported collapses as suggested by (Burrows 1989; Melchers et al. 1983; S ikkel 1982). Therefor e, the figures shown b elow may in some cases only represent a lower bound esti mate.
9 All the collapses resulted from human errors, and the main cause of failure was design error (28 ), see Figure 5. However, in 4 9 of the accidents the causes were unknown. It shoul d be mentioned that in a high percentage of reporte d accidents no detailed informa tion was found, especially until the year 2000 (in 60 of the cases). Noneth eless, it is assumed that the results presented below are representative. Looking in detail into t he available information, the reported deficiencies were categori s ed in the following three categories: 1. Procedural causes 2. E nabling events and 3. T riggering events. The procedural causes are related to management issu es and the interrelationship between partie s involved in a project. The enabling e vents are related to the interna l condition or performance of the structure or its components that contribute to failure. The triggering event s are external events that could i nitiate failure of a structure. It is considered that every collapse occ urs due to a series of events that involve deficiencies in mana gement, errors in design, assembly and operation and a hazard which triggers the collapse. The insight achieved by this deeper investiga tion is considered to be e xtremely valuable information for the identificati on of the major hazards and of the critic al paths of events which could lead to the collapse of a bridge falsework structure. Also, it makes it easier to setup effe ctive and efficient barriers to reduce and control the existin g risk levels. 10 However, i n 44 of the accidents the procedural causes were unknown. It can be concluded that in general several procedural causes coin cide in a given accident, meaning that fai lures are rarely caused by one reason only, but rather by the accumulation of the detrimental effects caused by a series of small events, each of which might be not critical, but the total effect exceeds t he falsework safety margin.
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Bridge falsew ork projects are most of the times performed without interaction, consultation and planning with other relevant stakeholder s, such as the permanen t structures ’ designer, the principal contractor and the supervision team. Frequently, changes in the permanent str uctures’ design or in the construction sequence, which often occur, with a direct impact on the pe rformance of the bridge falsework, are no t properly addressed. Furthermore, sp ecific bridge falsework activities, related to the determination of the foundation gr ound properties for example, ar e not given the correct priority. Additionally, decision criteria rega rding approval of bridge falsework prior to loa ding, elements tolerances and quality requirements are someti mes set without consultation with the designer. I t is not uncommon for bridge falsework proj ects to be made of “standard” solutions taken from the system’ developers guide without e nsuring that they are appropriate and c onsistent with the project specific design requirements. T he same can be said regarding the design specifications or method statements which often are a copy o f the system’ developers guide. Often lack of in formation is found in the design brief regarding site investigation, fo undation testing, assembly tol erances, material requirements (important because there are v arious material grades available), load cases considered, loading sequence, ma intenance and inspection procedures and priorities. 11 I n particular, the client shall appoint a CDM coordina to r as his key adviser wh o will as sist him with his duties during the construction project. Addi tionally, the construction phase cannot s tart until the pri ncipal contractor has prepared a construction phase p lan (HSE 2007b).
A temporary structure must be of such design and so installed and maintained as to withstan d any foreseeable loads which may be imposed on it, and must only be used for the purposes for which it is so designed, ins talled and maintained (HSE 2007b). BS 5975:2008 (BSI 2008), recommends th e appo intment of a Temporary Works Co ordinator (TWC) to co ordinate and supervise the activities of all concerned, to ensure the work s are brought to a safe conclusion. Additionally, Temporary Works S upervisors (TWSs) can be ap pointed to assist the TWC with his duties. Checking and inspe ction by competent TWSs should be a cont inuous process, starting with the materials to be used, the f oundations, and progressive inspection an d checks as the structure is erected. Leaving such check s until the falsework is complete is usel ess. Errors in the materials used, in the foundations and in the assembly procedure will be impossible to correct without dismantling. Possible checklists are presented in ( CIP 2011 ). Ad ditional references are (CIRIA 2007; HSE 2007a; HSE 2010). Additionally, BS 5975:2008 also covers the design of temporary works and in particular provides useful information on loads, capabilities of formwork, etc.This can be justified by the lack of awareness in the design and in the construction stage of the stability requirements of each bridge falsework solution. T he secon d most important enabling event was found to be under - designed components such as jacks, couplers, standards or ledgers, but also support steel gi rders used to span open traffic areas. This in turn can in part be justified due to the reuse of falsework elements which are subjected to heavy loads and improper maintenance and thus can ac cumulate damage leading to a reduced load bearing capacity. Incorrect assembly procedures of the falsework system were report ed to have been involved in only 3 of the collaps es.
However small this percentage is, it must be noted that before or after the collapse of the system, it is not very easy to determine if it was erected as planned, so this number should be read tak ing this into account. Additionally, (Burrows 1989) showed that falsework is not always assembled as in drawings, in particular the adop ted lacing configuration can be incorrect, the geometrical imperfections of the standards often exceed the tolerance limits and frequently there are a number of missing brace elements. Finally, in a great number of accidents (45) the enabling events are still unknown. Additionally, 26 of the accidents were caused by unknown design related erro rs. 45 26 19 15 11 3 0 10 20 30 40 50 60 70 80 90 100 Other or unknown cause s Other or unknown desig n relate d cause s Inadequa te f alsewor k brac ing Inadequa te f alsewor k main eleme nt Inadequa te f alsewor k founda ti on Imprope r assem bly proc edure Figure 7: Enabling events of bridge falsework collapses since 1970. In the following, the most impor tant enabling events will be presented. 4.2.1 System Stiffness Conside rations A very common enabling event is related with the failure to properly consider in the structural analysis the stiffness of the falsework sy stem and the interaction of the falsework syst em with the permanent structure. Stiffness is an important character istic of any structure because it not only con trols its deformation but also force distributi on, in the case of continuous structures. Interaction between 13 These effects may not be determined by the usual analysis hypo thesis and methods of calculation of the load distribution bet ween standards, such as the tributary area method and assuming the formwork and the standards as ri gid elements. In general, the design loa d tables provided by the falsework producers take into account these effects only indirectly by app lying a large safety factor (equal or greater than two).
However, th is may not be enough to compensate the cu mulative effects of the hazard scenario s not explicitly accounted for. 4.2.2 Bracing Bracing is one of the most i mportant aspects in a bridge falsework structure, since their performance depends greatly on the stiffness against lateral movements provided by the bracing elements. The b racing configuration should be determined by proper structural analysis. However, pr oducers of bridge falsework system s often specify, in their design gui dance documents, standard bracing requirements; yet, these are not always fulfilled. For example, in some projects only the exterior bays are braced, leaving the stability of internal ba ys resting with the (low) lateral stiffness provided by the lacing elements and by the formwork (which might be disc ontinuous or not designed to resist the resulting bending a nd axial forces). An additional err or, som etimes found in support towers, is to not include sufficient bracing elements in both directions. This not only reduces the ultimate resistance of the foundation but also its stiffness (Carvalho et al. 2004). I n several projects of falsework structures, the desig n of foundation elements and the safety verification of the foundation soil are ofte n treated lightly, for example by jus t using the heel of a boot of an experienced inspector or enginee r. Design details, control and inspe ction guidelines usually do not appear explicitly. Usually, it is only made reference to a permissible stress required during the construction phase, verified later against a “safe valu e” obtained through some simple soil testing. However, in some cases, see below and section 4.3.2 for examples, a detailed soil investigation should be carried out. Problems with foundations can occur due t o the substructure deficiencies or due to weak ground properties: resistance and stiffne ss.
Substructure deficiencies are found when co ncrete weaker than specified is used in footings, when weak or damaged wood footings are (re)ut ilised, w hen inclined footings are used and resistance ag ainst the resulting horizontal forces is reli ed solely on 14 Permanent loads include the self - weight and the superimposed dead load. Variable loads include construction loads such as the weight of the fresh con crete, the reinforcement and other materials stored in the deck, and the equipm ent used, the load redistribution due to the application of pre stress, accidental dynamic imp act loads, and environmental loads such as the actions of s now and wind, ground settlements, thermal variations and seis mic actions. In general, the most important loads that bridge falsewo rk structures are subjected to are the weight and pressure from concrete (the la t ter in particular whil st the concrete i s still fresh), followed by other types of construction action s. In contrary to permanent s tructures which only receive their full design load in rare cases ( e.g. the design traffic loads on b ridges are rarely reached), usually falsework structures are subjected for a long period of their desig n working life to loads whose values are close to their design values. Thus the actual safety margin of falsework structures is lower than in permanent structures, i.e. the probability of fai lure of temporary structures i s higher than that of perm anent structures (fib 2009). Many researchers have tried to improve the availab le models by moni toring the construction loads during concrete pouring, see (Ika heimonen 1997; Peng et al. 2007; Peng et al. 1997). Additionally, there are no general guidance docu ments or standards addressing the issue o f choosing the loads or safety factors to be used in fal sework design.
It can be seen tha t expected loads during design of the fals ework are responsible for 55 of collapse s by triggering a local failure which then generally d evelops as a progressive collapse of part o f the bridge falsework structure. These loads are mainly due to concretin g operations. 15 Finally, they suggested that, for vertical loadings, a minimum factor of safety of 2,0 should be adopted for bridge fals ework systems. 4.3.2 Set tlements Settlement should be assessed correctly, to avoid unwanted and unusual load distributions within the elements of the bridge falsework syste m, and problems related to the geometr y control of the permanent structure. Settlements are most often relat ed to movements in the foundations, but elastic deformations and initial gaps betwe en elements and within the connections can a lso produce settlements. 16 Furt hermore, the occurrence of these settlements may result in th e overturning of part of the structure, cau sing secondary stresses f or which the falsework st ructure was not designed for. This be haviour, if neglected in the desi gn phase, may lead to the collapse of the structure. 5. Conclusions Bridge falsework systems are the structures most commonly used during the construction of co ncrete bridges. They play a significant role in the safety and profitabilit y of a bridge project. However, based on a survey of the available literature an d other information channels, 73 accident s associated with the use of b ridge falsework systems have b e en identified since 1970 in 1 9 countries worldwide. The most critical stage for temporary works safety occurs during concrete brid ge deck casting. The most common design error is insuf ficient bracing. Based on the information collect ed in the survey, t he average risk to individual s, and structural risk have been estimated and evaluated ag ainst proper risk criteria.
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