1.0 Introduction
A roof, which is the 1 of the most indispensable parts of a edifice, is the covering on the topmost portion of the edifice that protects the edifice and its contents from the effects of conditions i.e. rainfall, heat, sunshine, cold and air current depending on the nature and intended design of the edifice ( Wiki n.d ; Foster and Greeno 2007 ) . The span of a roof is a major consideration amongst other factors including functional demands and considerations of velocity and economic system of hard-on. This can be classified in comparative footings as short ( up to 7.5m ) , medium ( 7.5 m – 25m ) and long-span ( over 25m ) harmonizing to ( Foster and Greeno 2007 ) . The focal point of this study will be on long-span roof constructions.
The thought of utilizing long-span roofing systems in constructions was likely developed based on a demand to fulfill aesthetical every bit good as functional demands of peculiar edifices such that a balance is reached. Buttressed by Indianetzone Construction ‘s ( n.d ) sentiment, a span is considered to be long-span when as a effect of its size ; proficient considerations are placed so high on the list of architectural precedences such that they significantly affect the aesthetic intervention of the edifice. Long-span edifices create unobstructed, column-free infinites greater than 30 meters ( 100 pess ) for a assortment of maps. These include activities where visibleness is of import for big audiences ( auditoriums and bowl ) , where flexibleness is of import ( exhibition halls and certain types of fabrication installation ) and topographic points where movable objects are housed ( Indianetzone Construction n.d ) .
‘Pushing the boundaries of long span constructions has ever been a field of involvement to the populace every bit good as to professional applied scientists. Of class lightweight and long-span are comparative footings and greatly influenced by the stuffs used and the engineering of the times. Westminster Hall was a immense effort of technology in the fourteenth century and in the nineteenth century ; St Pancras station roof was the largest span in the UK for many old ages. These spans seem really modest now with roofs crossing 200 or 300 m and Bridgess making several kilometres! ‘ ( Liddell 2007 ) .
An illustration of a fresh long-span roof designed by the designer Edward Durell Stone in the 1950s based on the steel overseas telegrams used in suspension Bridgess was the U.S. Pavilion at the 1958 Brussels World ‘s Fair ( Encyclop & A ; aelig ; Defense Intelligence Agency Britannica 2010 ) . Other more recent authoritative applications will be shown in Figures 1 – 3 below:
Figure 1: Reliant Stadium ( exposure by Russell Andorka ) , Source: www.orientalsheetpiling.com/Download/ReliantStadium-NASCC.pdf
.
Figure 2: Steel roof truss ( 114m span ) of the National Indoor Stadium, Bejing2008 ; Beginning: hypertext transfer protocol: //en.beijing2008.cn/cptvenues/venues/nis/headlines/n214204442.shtml
Figure 3: The Millennium Dome ( 1999 ) in London, exposure by Richard Rogers and Partners, Beginning: hypertext transfer protocol: //www.greatbuildings.com/buildings/Millennium_Dome.html
2.0 FUNCTIONAL REQUIREMENTS OF ROOFING SYSTEMS
It is known that a roof chiefly provides a covering over an enclosure, protecting it from the external environmental influence and action by air current, sunshine, snow, temperature, rainfall and other rough climatic effects. In order to adequately back up the actions of these natural perturbations imposed on it by the prevalent environmental conditions including the likely futuristic consequence of clime alteration, the roof has to be expeditiously designed to fulfill certain functional demands as outlined in the work by ( Foster and Greeno 2007 ; Harrison et Al. 2009 ) . These include the followers:
Strength and stableness, which is critical to the public presentation of the construction as a whole.
Weather opposition including bar and discharge of rain, snow and condensation.
Thermal opposition affecting modulating internal environments by solar heat loss balance, air temperatures, energy preservation and airing.
Fire opposition including fire safety steps and/or safeguards to maintain distribution of fire from beginning at a lower limit and proviso of equal lighting.
Sound insularity affecting keeping equal noise degrees.
2.1 Strength and stableness
The roof system maps to supply a great trade of structural rigidness and stiffness in edifices and other countries where they may be applied. A simple instance is the binding consequence the roof gives to simple edifices with short clear spans where the roof tends to keep the supporting walls together such that they do non rupture apart. The state of affairs is apparently different and more hard to manage when the country of infinite to be covered by the roof additions in dimensions. Harmonizing to ( Foster and Greeno 2007 ) , the chief factor impacting the choice of stuffs employed in the design of a peculiar roof system chosen from a broad scope of roof types is the span.
Principles of modern edifice ( 1961 ) as cited in Harrison et Al. ( 2009 ) states that there are three basic structural systems that can be used over an gap: the concatenation, the arch and the beam, of which the concatenation is the best signifier for back uping tonss over long spans. Harmonizing to them, roofs can be made out of secondary systems derived by a careful mix of these three basic systems. ‘However, every roof needs to be sufficiently strong to transport the self-weight of the construction together with the intermittent loads – for illustration those due to environmental consequence ( e.g snow or air current ) or care – and it must make this without undue deformation or harm to the edifice, whether perceptible or unperceivable to its residents. ‘ ( Harrison et al. 2009 ) . These outlooks are codified in commissariats contained in assorted national edifice ordinances including the Building Regulations 2000 as cited in the work by ( Harrison et al. 2009 ) , which is specifically for application in England and Wales.
Table 3: Reported jobs with roof construction. Data from English House Condition Survey circa 1990 as cited in ( Harrison et al. 2007 ) .
Type of Problem
Proportion of homes ( approx. )
% of entire
All jobs
1:5
20.3
Saging
1:32
3.1
Hunching
1:166
0.6
Spreading
1:250
0.4
A casual expression at the history of roof public presentation in bing edifices ( Harrison et al. 2009 ) dating back to the 18th century, sing the consequence of lading reveals that prehistoric homes recorded a comparatively low public presentation with regard to the overall burden compared to more recent roof systems ( Table 1 ) . This is likely due to advancement in research and engineering in this country. Datas from a national house status study conducted in England as cited by ( Harrison et al. 2009 ) in Tables 2 and 3 severally shows inside informations of structural jobs recorded in homes more than a decennary before 2006 and within the twelvemonth 2006.
Table 1: Reported jobs with roof construction. Data from English House Condition Survey circa 1990 as cited in ( Harrison et al. 2007 ) .
Date of building
Proportion of homes ( approx. )
% of entire
Before 1850
1:9
11.0
1945-1964
1:70
1.4
Post 1965
1:35
2.8
All day of the months
1:15
6.6
Table 2: Reported jobs with roof construction. Data from English House Condition Survey circa 1990 as cited in ( Harrison et al. 2007 ) .
Type of Problem
Proportion of homes ( approx. )
% of entire
All jobs
1:15
6.6
Saging
1:90
1.1
Hunching
1:50
2.0
Spreading
Pre 1899
1918-1945
Post 1945
1:100
1:50
1:200
Negligible
1.0
2.0
0.5
All over the universe, applied scientists and builders are invariably faced with the challenge of set uping cost-efficient, adaptable solutions in the design of roof systems to back up the tonss that come on them. The purpose is to seek and happen the optimal, economically-feasible method of reassigning tonss on the roofs to the back uping super-structure beneath over spans of variable magnitudes ( Foster and Greeno 2007 ) . They further argue that, in order to do immense cost nest eggs in stuffs utilized in the design and building of the roof, a balance has to be reached such that there is an overall decrease in the entire dead burden to be carried by the roof, which will ensue in a state of affairs where light weight stuffs carry majorly imposed tonss over great spans. With the decrease in the entire burden to be carried by the roof, stuffs are saved and smaller, lighter subdivisions can be used to back up tonss over long spans. This nevertheless, will hold important deductions on the serviceableness demands of warp, which must be checked during design of the roof construction. As a corollary to this weight consequence, ( Foster and Greeno 2007 ) pointed out that one of the built-in structural troubles in the design of long-span roof constructions is cut downing the dead/live burden ratio, expressed as burden per square meter of country covered by the roof, to a safe degree thereby bettering the efficiency of maximal burden carried. Following their statement, addition in spans of roof systems by and large result in important addition in the dead weight of the roof which will take to a corresponding addition in the ratio and an overall lessening in the efficiency tonss carried by the construction. However, these jobs can be solved by maintaining two cardinal factors as discussed by ( Foster and Greeno 2007 ) in head when doing pick of stuffs to be employed in the design: the features of the stuff to be used including the strength, stiffness and weight and the signifier or form of the roof. They argued that if the strength is high, smaller volume of stuff is required to transport tonss ; besides if the stiffness is high the deepness of subdivision required will be little as the stuff will deform under little impact tonss ; eventually, a lightweight stuff will ensue in an overall decrease in the weight of the construction. These factors, if carefully considered in the choice of stuffs will assist to develop the most efficient burden transporting system where the dead/live burden ratio is reduced to a lower limit.
Another of import action apart from effects of weight which is critical in the design of roof constructions is wind consequence. Gales, highly strong air currents, pose inauspicious effects on edifices particularly roofs in the UK ( Harrison et al. 2009 ) . Records by them show that since the aftermath of the early 90 ‘s up boulder clay now, about 1.1million houses have affected adversely by gales. This resulted in pronounced alterations in the codifications of pattern to give a more robust codification – Bachelor of science 6399 Part 2 as cited in ( Harrison et al. 2009 ) for air current burden computations on roof, which takes into consideration assorted constructing parametric quantities necessary for a good design unlike the old publications. The application of the codification in the design of roof guarantee that certain factors like speed of air current, tallness of edifice land degree, vicinity of the edifice, height, blast, wind way and seasonal factors ( Foster and Greeno 2007 ; Harrison et Al. 2009 ) . There is some grounds ( Foster and Greeno 2007 ) that wind force per unit area and suction has a harmful consequence on roofs supported by edifices particularly on the windward terminal where its consequence is greatly felt. As such, for lightweight roofs peculiarly 1s with distinguishable overhangs, the upheaval is highly unwanted and should be designed with careful consideration given to the articulations and connexions to the ties, walls and columns as the instance may be to forestall the roof from being thrown off ( Foster and Greeno 2007 ) .
2.2 Weather opposition
As may be given in the commissariats of the Building Regulations ( 2000 ) papers H3 for England and Wales as cited in Harrison et Al. ( 2009 ) , a roof should be adequately designed to execute such that there is zero-tolerance on ooze of rainfall, snow and/or any signifier of wet into edifices. In order to accomplish this, Harrison et Al. ( 2009 ) suggests that drainage systems ( troughs ) with equal drain capacities be installed in line with the commissariats of the edifice ordinances above by sing factors such as: the rainfall strengths ( litres/sec/m2 ) , the orientation of the roof and the effectual drained surface country. Furthermore, they stressed that the orientation of the troughs should be such that it slopes to the closest drain mercantile establishment to forestall inordinate burden of the construction in the event of an overspill. They recommend that in instances where overspills are expected, equal commissariats should be made for the design of the drain in conformity with the public presentation demands as stated in BS EN12056-3 and design counsel including proving, care and commissioning in BS 8490 both cited in ( Harrison et al. 2009 ) .
2.3 Thermal opposition
Thermal opposition of a roof, which could besides be expressed as thermic insularity is a cardinal consideration made in the design of roof so as to strike a perfect balance between bar of heat loss and remotion of inordinate unwanted heat from homes when necessary. Thermal public presentation of any roof is an of import demand for the design of roof against thermic effects ( Harrison et al. 2009 ) . These demands as encapsulated in the new Approved Document ( AD ) L as cited in ( Harrison et al. 2009 ) are to be adopted in a more flexible manner in a command to conserving energy, advancing more energy-efficient edifices and roofs every bit good as making C emanation marks as stipulated in the relevant criterions. This, as stipulated by ( Harrison et al. 2009 ) can be maintained by installing of roof visible radiations and roof Windowss. For the instance of solar radiation on roofs ( Harrison et al. 2009 ) has suggested that the roof stuffs should be 1s with brooding surfaces such that in periods of summer where the strength of the Sun radiation on the Earth is greater attendant upon the consequence of planetary heating, there is an overall decrease in heat soaking up transmitted to the interior parts of the edifice.
2.4 Fire opposition
The major safety demand for roofs is to make an optimal public presentation that fire onslaught will non instantly convey down the roof and will non impact all other parts as in a Domino consequence ( Harrison et al. 2009 ) . The demand for covering with roof fires as cited by ( Harrison et al. 2009 ) is covered by trial methods in BS 476-3. This trial process determines the fire public presentation in roofs by effects of incursion and spread of fire which is denoted by two letters. In order to forestall fire, ( Harrison et al. 2009 ) have stipulated speedy counsel for fire protection including pit barriers, fume sensors, sprinklers and smoke extraction systems, which help to keep an acceptable degree of fire safety.
2.5 Sound insularity
Unwanted sound, which could be termed as noise can be unwanted to inhabitants particularly when it emanates from an external beginning. Sound degree which is described on a logarithmic graduated table in dBs ( dubnium ) vary in volume, frequence and clip ( Harrison et al. 2009 ) . They opined that noise could originate from assorted conditions generated beginnings like rain, snow, Sun, air current or hail. However, they pointed out that these effects can be controlled by using some general noise decrease rules like surfacing the bottom of the roof with a thicker bed of a weaker stuff, muffling and debut of PTFE washers between articulations.
3.0 DESIGN CONSIDERATIONS/GUIDE ROOF ONSTRUCTION/ERECTION
( Griffis 2004 ) highlights some of the factors which should be taken into history in the design and building of long-span roofs. He every bit outlined schemes, cognition of which in add-on to a reasonably good apprehension of the structural behavior of long span constructions and careful execution, will cut down the incidence of prostration of long span constructions every bit good as extinguish some of the attendant jobs of hard-on of long span constructions. These schemes are presented below:
Major undertaking forces and their functions and duties should be identified at the start of the undertaking in order to find the correct concatenation of bid and describing hierarchy – This will guarantee that proper undertaking direction processs are applied to forestall clash amongst parties concerned, extinguish budget overproductions and guarantee that undertaking bringing timelines are met.
It is advisable to affect the fabricator/erector squad at the start of the undertaking – This will non merely be good to the undertaking cost and clip agendas but besides enable the squad adequately familiarise themselves with certain building demands, specifications and inside informations which have been prepared in line with the codifications of pattern at design phase. These include, but are non limited to understanding on the class of steel, connexion type, bolt size and class, welding processs and procedures, hard-on sequence and method, pigment type and building divergence allowances.
Huge overall cost nest eggs can be made on the construction from stuffs used in the building e.g steel by using high strength steel of the best quality such that light weight stuffs are used.
Adequate environmental surveies should be conducted and consequences of these should be employed in the appraisal of the air current and snow burden on the construction. Accuracy of burden appraisal has a long-run economy consequence in cost of the construction.
Whether utilizing strengthened concrete or strictly steel work, prances and truss chord of the roof construction should be framed in order to bring forth light weight constructions.
It is ne’er advisable to utilize motion articulations in roof construction because of the built-in troubles it brings along.
Allowance should ever be made in the initial design of the roof system to take into cognizance extra dead tonss which may originate from replacing of roof facing and other stuffs in the hereafter.
Careful idea should be given to factors such as material shrinking, support colonies and temperature consequence including hard-on processes when doing initial designs and building planning processs.
So long as the architectural form and line of vision of the roof construction is non impaired, much attending should non be paid to warps and camber effects of long span roofs.
Careful intervention should be given to diaphragm emphasiss, pick of stop brace of structural members and diaphragm fond regard, which are of import for defying sidelong effects of air current and seismal tonss by making a determination on the system to utilize based on considerations of economic system and hazard.
Bolted field connexions on shop-welded/built steel members are ever the best and should be employed in the building of long span roof systems. This is good pattern which can cut down holds and downtime in building taking to timely completion of undertaking.
In every bit much as the interior decorator needs to get down pass oning with the storyteller early plenty to integrate store patterns to back up design computations, he should ne’er let the storyteller to take on his primary duty of planing the roof system. This may ensue in struggles on site.
For simpleness of design/details and turning away of confusion on site, steel subdivisions should be selected such that one size fits all! This will cut down overall cost of stuffs and facilitate fiction.
Where possible a elaborate documented hard-on method should be outlined to guarantee lucidity to all parties concerned and uniformity of installing process.
The structural applied scientist should bear in head that any construction designed should be analyzed and that built should be designed. Besides he must guarantee that careful supervising of the hard-on procedure on site is carried out decently to corroborate that consequences of the design are reflected on site.
4.0 PROBLEMS WITH LONG SPAN ERECTION/CONSTRUCTION.
The design of long span constructions for hard-on with constructability in head frequently poses challenges on the interior decorators which are related to both technological and aesthetical facets ( Kawaguchi 1991 ) . Some of the cardinal inquiries a interior decorator should happen replies to in order to get the better of these challenges as outlined by Ruby ( 2007 ) are:
What is the lading flight for the structural system to be developed?
How can the productive usage of the structural members in footings of span, size, measure of store pieces and constructability be optimized?
How can the brace system determined from a structural position be expeditiously incorporated into the initial architectural layout?
How can shop fiction be expeditiously utilized to cut down draw cost, if it will be shipped and non field-built?
What will be most effectual building flow order?
At what strategic locations would ephemeral braces be placed while building and hard-on is still in advancement?
How will the determined building flow order be applied to minimise the usage of impermanent props for truss during hard-on?
All these inquiries, carefully evaluated will steer the interior decorator in fixing functional designs which can easy be integrated in the building and hard-on procedure to accomplish the best consequences at decreased overall costs with prompt undertaking bringing.
A expression at the typical jobs associated with long span roof building will be presented below utilizing a instance survey of a big individual floor constructing with long span roof as presented by Khup ( 2009 ) .
4.1 Description of the full construction
This instance survey illustrates the building of a big single-storey, long-span industrial edifice with external dimensions 200m ten 60m. The 10.8m high roof which is sustained by rc beams and columns is a 59m span construction with 29 single steel constituents at 10.8m maximal tallness.
Chief members were dual angle steel subdivisions connected back to endorse.
4.2 Erection of the truss
The truss as shown in Figure 4 below was erected by raising truss units, 3 at a clip, to the needed tallness get downing from the Centre of the edifice and efficaciously back uping next truss units against each other while supplying impermanent shore towers for props at the bottom chords of the truss assembly.
Figure 4: Erection of the first 3 trusses
4.3 Analysis of the failure
Shortly after the first two trusses were erected, they failed and all came down Figure 5 shows the inside informations. The immediate cause of the ruinous prostration of the slender truss was the remotion of the impermanent shore towers shortly after installing of the truss in place.
Some of the distant causes include:
get downing installing at the Centre of the edifice instead than at the house gable terminal wall,
skip of a figure of tie beams and purlins near to the shoring towers in order to make allowance for the great lift,
non-utilization of impermanent diagonal braces to supply sufficient sidelong support and torsional rigidness sing the slender nature of the truss,
no continuity in the web angle cleats at the knee-joint support due to obstructor from the holding-down bolts at that point which made the support behave as a pin-joint,
Figure 5: Collapsed trusses
bizarre burden and non-uniform distribution of emphasiss and forces at the articulations due to the irregular order of building,
angle cleats which connects the purlins to the truss every bit good as all cardinal truss members were non provided as a uninterrupted strip along the its length to keep the dual angles in place and
skip of a diagonal prance which made the truss collapse/fail in flection.
4.3 Lessons learned
Khup ( 2009 ) has drawn out larning points for farther action which could be noted for rectification and application in future occupations. These are:
The consequence of overall dimensions and subdivision belongingss of the truss must be considered when covering with trusses to avoid issues linked with tortuosity and sidelong
Adequate site monitoring and effectual supervising should be the ultimate duty of the applied scientist as has been highlighted as one of the design considerations given earlier in this study by ( Griffis 2004 ) to guarantee hard-on is done to plan specification.
Members with slender signifiers e.g. purlins with angle subdivisions should be decently battened along its full length to supply sufficient stiffness and braced for sidelong stableness.
Impermanent props, if used for hard-on of the truss should be supported on comparatively stiff members like concrete nucleuss within the edifice frame.
All shoring towers should be designed against inadvertent sidelong or gravitation tonss that may happen during hard-on of the truss.
Detailss of connexions at articulations should be clearly provided such that there are no bizarre minutes originating from induced forces as consequence of misunderstanding of inside informations by the storytellers.
5.0 DESIGN GUIDANCE FOR LONG-SPAN ROOF SYSTEMS
5.1 Structural design regulations
For the design of roof systems, The Corus ( 2010 ) has recommended BS 5950-6 ( 1995 ) for full design regulations and trial processs used by assorted makers of roof systems, the footing on which the respective load/span tabular arraies are generated. The design regulations for metal roof cladding systems have non yet been included in the Eurocode 3 published earlier in the twelvemonth, April, 2010. As a usher for helping applied scientists and practicians particularly in the UK to do speedy, approximative designs for their roof systems, mention can be made to BS5950-6 ( 1995 ) as cited in ( Corus 2010 ) .
5.2 Loading bounds
Designs will be done usually based on the flexural strength at ultimate bound provinces and warp will be checked to guarantee that it is satisfactory at serviceableness provinces by using the appropriate serviceableness tonss such that the roof system performs satisfactorily and carry through its intended intent without prostration during its full design life ( Corus 2010 )
5.3 Serviceability and warp bounds
( Corus 2010 ) advices that important deformations or warps in the construction is perfectly unwanted and must be checked at design phase in order to forestall complications such as:
Poor drainage systems and ponding in specific locations
Damage to sealers at overlap subdivisions of the roof system
Excessive strains at parts of convergences or other interrelated parts such as interior coverings
General external distortions or deformation in the regular form or profile of the roof systems.
Corus ( 2010 ) has specified, harmonizing to the codification BS 5950 Part6 ( 1995 ) , the allowable values of warp for fulfilling the serviceableness limits as shown in the Table 4 below. A restricting value of L/200 is nevertheless recommended for usage where L is the span which is a map of the span of the construction as will be obtained from the load/span tabular arraies used by the several maker of the peculiar roof system employed in building.
Load status
Permissible warp as a multiple of span
Roof facing
Wall facing
Dead
L/500
–
Dead and imposed
L/200
–
Dead and air current ( excepting roof visible radiations )
L/90
L/120
Table 4: Deflection bounds as stated by BS5950-6 as cited in ( Corus 2010 ) .
5.4 Ultimate bound provinces
At ultimate bound provinces, the critical burden or the worst burden instance is used to find the design value of burden at failure where the material output or the construction collapses. Corus ( 2010 ) has specified two likely manners of failure: tensile break and compressive buckling, reasoning that the chance of the former occurring is close to zero while the latter is prevailing in web-strengthened rims subjected to high compressive emphasis degrees taking to clasping at output. This must be taken into history when transporting out design computations.
For shear, Corus ( 2010 ) documented that shear failure is unlikely for little subdivisions of long span members but could be present in deeper subdivisions particularly when used over short spans. This can be controlled by usage of web stiffeners.
5.5 Roof burden computations
5.5.1 Concentrated imposed burden
Though relevant package bundles are now available for computation of these tonss, Corus ( 2010 ) has specified speedy counsel for ciphering tonss from human activities in line with commissariats of BS 6399-3 as cited in ( Corus 2010 ) :
Roof with entree ( for care intents merely ) — greater of 0.9kN or effectual snow burden
Roof burden for all purpose entree — – greater of 1.8kN or the effectual snow burden.
5.5.2 Dead burden
Load due to the ego weight of the full roof system which acts downwards like a gravitation burden.
5.5.3 Uniform imposed burden
This relates to snow lading which is highly hard to cipher due to the variableness of meteoric informations. Corus ( 2010 ) suggests that excess concern should be given to appraisal of this burden particularly for application at heights greater than 500m. As cited in ( Corus 2010 ) , BS 6399-3 ( 1988 ) is the recommended codification for ciphering uniform imposed lading on roof systems.
5.5.4 Wind burden
Wind force has two momentous effects: the positive lateral imposed wind force per unit area moving on the walls and the negative perpendicular suction force per unit area moving 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 codification for ciphering these tonss.
5.6 Design tonss
Corus ( 2010 ) has summarized a speedy mention in Table 5 for finding design tonss to be applied to edifices by corroborating the relevant burden instance and ciphering the design burden utilizing the worst loading state of affairs:
Loading combination/situation
Load instance
Wind burden ( imposed or suction )
Snow burden ( uniformly distributed or redistributed )
Uniformly distributed burden ( kN/m2 )
Concentrated burden ( kN )
Roof with entree
Determined from BS 6399 Part 2
Determined from BS 6399 Part3
1.5
1.8
Roof without entree
Determined from BS 6399 Part 2
Determined from BS 6399 Part3
0.6
0.9
Walls
Determined from BS 6399 Part 2
