Long Span Roof Construction

Long Span crownwork straying1.0 INTRODUCTIONA crown accomplishment, which is the one of the most essential parts of a facial expression, is the c all overing on the uppermost part of the expression that protects the building and its content from the exploit of weather i.e. rainfall, heat, sunlight, cold and wind dep mop uping on the nature and think normal of the building (Wiki n.d Foster and Greeno 2007). The duad of a capital is a major reflexion amongst other factors including fermental requirements and considerations of speed and economy of erecting. This merchant ship be classified in relative terms as minuscule (up to 7.5m), medium (7.5 m 25m) and broad- deny (over 25m) according to (Foster and Greeno 2007). The focus of this report impart be on long-span crown structures.The idea of utilizing long-span ceilinging establishments in structures was probably demonstrable based on a need to satisfy aestheticalal as intumesce as functional requirements of p articular buildings much(prenominal) that a oddment is reached. Buttressed by Indianetzone Constructions (n.d) opinion, a span is considered to be long-span when as a consequence of its size technical considerations be placed so steep on the list of architectural priorities such(prenominal) that they significantly affect the aesthetic treatment of the building. Long-span buildings create unobstructed, column-free spaces greater than 30 metres (100 feet) for a variety of functions. These involve activities where visibility is important for bear-sized audiences (auditoriums and stadia), where flexibility is important (exhibition halls and certain types of manufacturing facility) and places where conveyable objects argon ho utilise (Indianetzone Construction n.d).Pushing the boundaries of long span structures has always been a field of interest to the public as well as to professed(prenominal) engineers. Of course lightweight and long-span be relative terms and greatly stoo pd by the physicals engrossd and the technology of the times. Westminster Hall was a huge feat of plan in the 14th century and in the 19th century St Pancras postal service hood was the largest span in the UK for m any years. These spans seem very humiliated now with hoods spanning 200 or 300 m and bridges arriver several kilometers(Liddell 2007).An sample of a novel long-span detonator be aftered by the architect Edward Durell Stone in the 1950s based on the sword cables utilize in suspension bridges was the U.S. Pavilion at the 1958 capital of Belgium Worlds Fair (Encyclopdia Britannica 2010).2.0 FUNCTIONAL REQUIREMENTS OF ROOFING SYSTEMSIt is known that a hood chiefly yields a covering over an enclosure, protecting it from the external environmental influence and action by wind, sunlight, play false, temperature, rainfall and other harsh climatic tacks. In order to adaptedly run on the actions of these natural disturbances compel on it by the prevailing envir onmental controls including the homogeneously futuristic effect of climate change, the pileus has to be efficiently contriveed to satisfy certain functional requirements as adumbrate in the work by (Foster and Greeno 2007 Harrison et al. 2009). These include the following stance and stability, which is vital to the per take inance of the structure as a whole.Weather guard including prevention and discharge of rain, snow and condensation.Thermal enemy involving regulating intimate environments by solar heat expiration balance, air temperatures, energy saving and ventilation.Fire resistance including fire preventive measures and/or precautions to keep statistical distribution of fire from source at a minimum and provision of adequate lighting. fail insulation involving maintaining adequate noise levels.2.1 Strength and stabilityThe roof frame functions to provide a great deal of structural primed(p)ity and stiffness in buildings and other disciplines where they may be applied. A naive case is the tying effect the roof gives to simple buildings with short intelligibly spans where the roof tends to hold the despatch-bearing walls together such that they do non tear apart. The plaza is seemingly different and much difficult to cargo bea when the ara of space to be covered by the roof attachs in dimensions. According to (Foster and Greeno 2007), the main factor affecting the excerpt of materials employed in the chassis of a particular roof scheme chosen from a wide range of roof types is the span.Principles of modern building (1961) as cited in Harrison et al. (2009) states that there are three basic structural dodges that can be use over an opening the chain, the arch and the beam, of which the chain is the best form for carry oning stacks over long spans. According to them, roofs can be make out of southwardondary arrangings derived by a careful mess up of these three basic musical arrangements. However, every roof needs to be sufficiently strong to carry the self-weight of the structure together with the intermittent thin for example those payable to environmental effect (e.g snow or wind) or nutriment and it must do this without undue distortion or damage to the building, whether sensible or imperceptible to its occupants. (Harrison et al. 2009). These expectations are codified in edible contained in various national building regulations including the twisting Regulations 2000 as cited in the work by (Harrison et al. 2009), which is specifically for exertion in England and Wales.A cursory witness at the history of roof performance in existing buildings (Harrison et al. 2009) dating back to the eighteenth century, considering the effect of cargoing reveals that prehistoric dwellings recorded a relatively low performance with respect to the general cuting compared to more recent roof brasss ( shelve 1). This is probably due to advancement in explore and technology in this area. Data f rom a national house condition survey conducted in England as cited by (Harrison et al. 2009) in Tables 2 and 3 several(prenominal)ly shows details of structural problems recorded in dwellings more than a decade before 2006 and within the year 2006.All over the world, engineers and builders are constantly faced with the challenge of establishing represent-effective, adaptable solutions in the build of roof systems to support the bills that come on them. The aim is to seek and give away the optimum, economically-feasible method of transferring effects on the roofs to the supporting super-structure beneath over spans of variable magnitudes (Foster and Greeno 2007). They further argue that, in order to make huge cost savings in materials utilized in the design and gimmick of the roof, a balance has to be reached such that there is an overall reduction in the total all in(p) load to be carried by the roof, which allow for forget in a situation where light weight materials car ry majorly imposed loads over great spans. With the reduction in the total load to be carried by the roof, materials are saved and little, lighter sections can be used to support loads over long spans. This however, will control significant implications on the serviceability requirements of aside, which must be checked during design of the roof structure. As a corollary to this weight effect, (Foster and Greeno 2007) pointed out that one of the entire structural difficulties in the design of long-span roof structures is reducing the dead/ cost load ratio, expressed as load per square metre of area covered by the roof, to a safe level thereby up(p) the efficiency of maximum load carried. Following their argument, increase in spans of roof systems generally exit in significant increase in the dead weight of the roof which will lead to a corresponding increase in the ratio and an overall decrease in the efficiency loads carried by the structure. However, these problems can be so lved by keeping two key factors as discussed by (Foster and Greeno 2007) in sound judgement when making resource of materials to be employed in the design the characteristics of the material to be used including the strength, stiffness and weight and the form or shape of the roof. They argued that if the strength is high, modester volume of material is required to carry loads also if the stiffness is high the depth of section required will be small as the material will deform under small opposition loads finally, a lightweight material will result in an overall reduction in the weight of the structure. These factors, if carefully considered in the selection of materials will help to develop the most efficient load carrying system where the dead/live load ratio is lowerd to a minimum.another(prenominal) important action apart from effectuate of weight which is critical in the design of roof structures is wind effect. Gales, extremely strong winds, pose adverse effects on bui ldings in particular roofs in the UK (Harrison et al. 2009). Records by them show that since the wake up of the early 90s up till now, about 1.1million houses check affected adversely by gales. This resulted in marked modifications in the inscribes of practice to give a more robust code BS 6399 Part 2 as cited in (Harrison et al. 2009) for wind load calculations on roof, which takes into consideration various building parameters necessary for a good design unlike the previous publications. The application of the code in the design of roof ensure that certain factors like velocity of wind, height of building ground level, locality of the building, altitude, gust, wind manner and seasonal factors (Foster and Greeno 2007 Harrison et al. 2009). There is or so(prenominal) evidence (Foster and Greeno 2007) that wind pressure and suck has a harmful effect on roofs supported by buildings especially on the windward end where its effect is greatly felt. As such, for lightweight roofs particularly ones with distinct overhangs, the uplift is extremely undesirable and should be intentional with careful consideration given to the joints and connections to the ties, walls and columns as the case may be to prevent the roof from being thrown off (Foster and Greeno 2007).2.2 Weather resistanceAs may be given in the provisions of the Building Regulations (2000) document H3 for England and Wales as cited in Harrison et al. (2009), a roof should be adequately designed to perform such that there is cypher-tolerance on seepage of rainfall, snow and/or any form of moisture into buildings. In order to reach this, Harrison et al. (2009) suggests that drainpipe systems (gutters) with adequate drain capacities be installed in line with the provisions of the building regulations above by considering factors such as the rainfall intensities (litres/sec/m2), the predilection of the roof and the effective drained surface area. Furthermore, they stressed that the orientation of th e gutters should be such that it slopes to the closest drain outlet to prevent unreasonable loading of the structure in the event of an overspill. They recommend that in cases where overspills are expected, adequate provisions should be made for the design of the drain in accordance with the performance requirements as stated in BS EN12056-3 and design focal point including canvasing, maintenance and commissioning in BS 8490 both cited in (Harrison et al. 2009).2.3 Thermal resistanceThermal resistance of a roof, which could also be expressed as thermal insulation is a key consideration made in the design of roof so as to train a perfect balance between prevention of heat loss and removal of excessive undesirable heat from dwellings when necessary. Thermal performance of any roof is an important requirement for the design of roof against thermal effects (Harrison et al. 2009). These requirements as encapsulated in the new Approved Document (AD) L as cited in (Harrison et al. 2009 ) are to be adopted in a more flexible way in a bid to conserving energy, promoting more energy-efficient buildings and roofs as well as reaching carbon emission targets as stipulated in the relevant standards. This, as stipulated by (Harrison et al. 2009) can be maintained by institution of roof lights and roof windows. For the case of solar radiation on roofs (Harrison et al. 2009) has suggested that the roof materials should be ones with reflective surfaces such that in periods of summer where the extravagance of the sun radiation on the earth is greater consequent upon the effect of global warming, there is an overall reduction in heat acculturation transmitted to the interior parts of the building.2.4 Fire resistanceThe major safety requirement for roofs is to reach an optimum performance that fire attack will not immediately bring buck the roof and will not affect all other parts as in a domino effect (Harrison et al. 2009). The requirement for dealing with roof fires as c ited by (Harrison et al. 2009) is covered by test methods in BS 476-3. This test occasion determines the fire performance in roofs by effects of penetration and administer of flame which is denoted by two letters. In order to prevent fire, (Harrison et al. 2009) have stipulated quick guidance for fire protection including cavity barriers, rat detectors, sprinklers and smoke extraction systems, which help to maintain an acceptable level of fire safety.2.5 Sound insulationUnwanted sound, which could be termed as noise can be undesirable to dwellers especially when it emanates from an external source. Sound level which is described on a logarithmic scale in decibels (dB) vary in loudness, relative frequency and time (Harrison et al. 2009). They opined that noise could arise from various weather generated sources like rain, snow, sun, wind or hail. However, they pointed out that these effects can be controlled by applying some general noise reduction principles like coating the unde rside of the roof with a thicker layer of a weaker material, damping and introduction of PTFE washers between joints.3.0 DESIGN CONSIDERATIONS/ fall ROOF ONSTRUCTION/ERECTION(Griffis 2004) highlights some of the factors which should be taken into account in the design and construction of long-span roofs. He equally outlined strategies, knowledge of which in addition to a pretty good understanding of the structural conduct of long span structures and careful implementation, will reduce the incidence of break down of long span structures as well as eliminate some of the concomitant problems of hard-on of long span structures. These strategies are presented below major thrust personnel and their roles and responsibilities should be identified at the start of the go for in order to determine the correct chain of command and inform hierarchy This will ensure that proper project management procedures are applied to prevent friction amongst parties come to, eliminate budget overrun s and ensure that project delivery timelines are met.It is better(predicate) to involve the teller/erector police squad at the start of the project This will not only be beneficial to the project cost and time schedules but also enable the team adequately familiarize themselves with certain construction requirements, specifications and details which have been alert in line with the codes of practice at design exhibit. These include, but are not positioned to agreement on the grade of brand name, connection type, move size and grade, welding procedures and processes, erection sequence and method, paint type and construction deviation allowances.Huge overall cost savings can be made on the structure from materials used in the construction e.g brand name by employing high strength steel of the best quality such that light weight materials are used. qualified environmental studies should be conducted and results of these should be employed in the estimation of the wind and sno w load on the structure. Accuracy of load estimation has a long-term saving effect in cost of the structure.Whether using reinforced concrete or purely steel work, struts and adhere chord of the roof structure should be border in order to produce light weight structures.It is never advisable to use movement joints in roof structure because of the inherent difficulties it brings along. adaption should always be made in the initial design of the roof system to take into cognizance additional dead loads which may arise from replacement of roof veneer and other materials in the future. wide-awake thought should be given to factors such as material shrinkage, support settlements and temperature effect including erection processes when making initial designs and construction planning procedures.So long as the architectural shape and line of vision of the roof structure is not impaired, much attention should not be pay to deflections and camber effects of long span roofs.Careful treatm ent should be given to point stresses, choice of diaphragm bracing of structural members and diaphragm attachment, which are important for resisting lateral effects of wind and seismic loads by reaching a decision on the system to use based on considerations of economy and risk.Bolted field connections on shop-welded/built steel members are always the best and should be employed in the construction of long span roof systems. This is good practice which can reduce delays and downtime in construction conduct to timely completion of project.In as much as the agent needs to start communicating with the fabricator early enough to incorporate shop practices to support design calculations, he should never allow the fabricator to take on his primary province of designing the roof system. This may result in conflicts on site.For rest of design/details and avoidance of confusion on site, steel sections should be selected such that one size fits all This will reduce overall cost of materi als and facilitate fabrication.Where possible a detailed documented erection method should be outlined to ensure clarity to all parties concerned and uniformity of installation procedure.The structural engineer should bear in mind that any structure designed should be analyzed and that built should be designed. Also he must ensure that careful supervision of the erection process on site is carried out properly to confirm that results of the design are reflected on site.4.0 PROBLEMS WITH LONG SPAN ERECTION/CONSTRUCTION.The design of long span structures for erection with constructability in mind often poses challenges on the designers which are related to both technological and aesthetical aspects (Kawaguchi 1991). Some of the key questions a designer should find answers to in order to overcome these challenges as outlined by Ruby (2007) areWhat is the loading trajectory for the structural system to be developed?How can the productive use of the structural members in terms of span, s ize, quantity of shop pieces and constructability be optimized?How can the bracing system determined from a structural perspective be efficiently integrate into the initial architectural layout?How can shop fabrication be efficiently utilized to reduce haulage cost, if it will be shipped and not field-built?What will be most effective construction draw order?At what strategic locations would ephemeral bracings be placed duration construction and erection is still in progress?How will the determined construction flow order be applied to minimize the use of temporary sustain for truss during erection?All these questions, carefully evaluated will guide the designer in preparing functional designs which can easily be integrated in the construction and erection process to achieve the best results at decreased overall costs with prompt project delivery.A look at the typical problems associated with long span roof construction will be presented below using a case study of a large sing le storey building with long span roof as presented by Khup (2009).4.1 Description of the entire structureThis case study illustrates the construction of a large single-storey, long-span industrial building with external dimensions 200m x 60m. The 10.8m high roof which is sustained by rc beams and columns is a 59m span structure with 29 individual steel components at 10.8m maximum height.Main members were double tip steel sections connected back to back.4.2 Erection of the trussThe truss as shown in Figure 4 below was erected by lifting truss units, 3 at a time, to the required height starting from the centre of the building and effectively supporting adjacent truss units against each other while providing temporary shoring towers for props at the bottom chords of the truss assembly.4.3 outline of the sorrowShortly after the first two trusses were erected, they failed and all came down Figure 5 shows the details. The immediate cause of the catastrophic collapse of the lithesome truss was the removal of the temporary shoring towers soon after installation of the truss in position.Some of the remote causes includecommencing installation at the centre of the building rather than at the firm gable end wall,omission of a number of tie beams and purlins close to the shoring towers in order to create allowance for the great lift,non-utilization of temporary diagonal bracings to provide sufficient lateral support and torsional rigidity considering the slender nature of the truss,no continuity in the web angle cleats at the knee-joint support due to obstruction from the holding-down bolts at that point which made the support behave as a pin-joint,eccentric loading and non-uniform distribution of stresses and forces at the joints due to the irregular order of construction,angle cleats which connects the purlins to the truss as well as all key truss members were not provided as a continuous strip along the its distance to hold the double angles in position andomissi on of a diagonal strut which made the truss collapse/fail in flexure.4.3 Lessons learnedKhup (2009) has drawn out culture points for further action which could be noted for correction and application in future jobs. These areThe effect of overall dimensions and section properties of the truss must be considered when dealing with trusses to avoid issues linked with torsion and lateralAdequate site monitoring and effective supervision should be the ultimate tariff of the engineer as has been highlighted as one of the design considerations given rather in this report by (Griffis 2004) to ensure erection is done to design specification.Members with slender forms e.g. purlins with angle sections should be properly battened along its entire length to provide sufficient stiffness and braced for lateral stability.Temporary props, if used for erection of the truss should be supported on relatively rigid members like concrete cores within the building frame.All shoring towers should be des igned against accidental lateral or gravity loads that may occur during erection of the truss.Details of connections at joints should be clearly provided such that there are no eccentric moments arising from induced forces as result of misinterpretation of details by the fabricators.5.0 DESIGN GUIDANCE FOR LONG-SPAN ROOF SYSTEMS5.1 geomorphological design rulesFor the design of roof systems, The Corus (2010) has recommended BS 5950-6 (1995) for full design rules and test procedures used by various manufacturers of roof systems, the basis on which the respective load/span tables are generated. The design rules for metal roof cladding systems have not yet been included in the Eurocode 3 promulgated earlier in the year, April, 2010. As a guide for assisting engineers and practitioners especially in the UK to make quick, approximate designs for their roof systems, reference can be made to BS5950-6 (1995) as cited in (Corus 2010).5.2 Loading limitsDesigns will be done ordinarily based on the flexural strength at ultimate limit states and deflection will be checked to ensure that it is satisfactory at serviceability states by applying the appropriate serviceability loads such that the roof system performs satisfactorily and fulfils its intended target without collapse during its entire design action (Corus 2010)5.3 Serviceability and deflection limits(Corus 2010) advices that significant distortions or deflections in the structure is dead undesirable and must be checked at design stage in order to prevent complications such asPoor drainage systems and ponding in specific locationsDamage to sealants at overlap sections of the roof systemExcessive strains at regions of overlaps or other interconnected parts such as interior coveringsGeneral external deformations or distortion in the regular shape or profile of the roof systems.Corus (2010) has specified, according to the code BS 5950 Part6 (1995), the permissible values of deflection for satisfying the serviceabi lity limits as shown in the Table 4 below. A limiting value of L/200 is however recommended for use where L is the span which is a function of the span of the structure as will be obtained from the load/span tables used by the respective manufacturer of the particular roof system employed in construction.5.4 Ultimate limit statesAt ultimate limit states, the critical load or the worst load case is used to determine the design value of load at failure where the material yield or the structure collapses. Corus (2010) has specified two likely modes of failure tensile fracture and compressive buckling, concluding that the probability of the former occurring is close to zero while the latter is prevalent in web-strengthened flanges subjected to high compressive stress levels leading to buckling at yield. This must be taken into account when carrying out design calculations.For shear, Corus (2010) documented that shear failure is improbable for small sections of long span members but coul d be present in deeper sections especially when used over short spans. This can be controlled by use of web stiffeners.5.5 ceiling load calculations5.5.1 Concentrated imposed loadThough relevant parcel packages are now available for calculation of these loads, Corus (2010) has specified quick guidance for conniving loads from human activities in line with provisions of BS 6399-3 as cited in (Corus 2010)Roof with adit (for maintenance purposes only) greater of 0.9kN or effective snow loadRoof load for all purpose access greater of 1.8kN or the effective snow load.5.5.2 Dead loadLoad due to the self weight of the entire roof system which acts downwards like a gravity load.5.5.3 Uniform imposed loadThis relates to snow loading which is extremely difficult to calculate due to the variance of meteorological data. Corus (2010) suggests that extra concern should be given to estimation of this load especially for application at altitudes greater than 500m. As cited in (Corus 2010), B S 6399-3 (1988) is the recommended code for calculating uniform imposed loading on roof systems.5.5.4 Wind loadWind force has two momentous effects the positive lateral imposed wind pressure acting on the walls and the negative vertical suction pressure acting majorly on the roof (Foster and Greeno 2007). Roof system as such must be designed against these effects. BS 6399-2(1997 or 2002 latest version) as cited in (Corus 2010) is the recommended code for calculating these loads.5.6 Design loadsCorus (2010) has summarized a quick reference in Table 5 for determining design loads to be applied to buildings by confirming the relevant load case and calculating the design load using the worst loading situationLoading combination/situationLoad caseWind load (imposed or suction)Snow load (uniformly distributed or redistributed)Uniformly distributed load (kN/m2)Concentrated load (kN)Roof with accessDetermined from BS 6399 Part 2Determined from BS 6399 Part31.51.8Roof without accessDetermine d from BS 6399 Part 2Determined from BS 6399 Part30.60.9WallsDetermined from BS 6399 Part 2