A roof, which is the one of the most essential parts of a building, is the covering on the uppermost part of the building that protects the building and its contents from the effects of weather i.e. rainfall, heat, sunlight, cold and wind depending on the nature and intended design of the building (Wiki n.d; Foster and Greeno 2007). The span of a roof is a major consideration amongst other factors including functional requirements and considerations of speed and economy of erection. This can be classified in relative terms as short (up to 7.5m), medium (7.5 m - 25m) and long-span (over 25m) according to (Foster and Greeno 2007). The focus of this report will be on long-span roof structures.
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The idea of utilizing long-span roofing systems in structures was probably developed based on a need to satisfy aesthetical as well as functional requirements of particular buildings such that a balance is reached. Buttressed by Indianetzone Construction's (n.d) opinion, a span is considered to be long-span when as a consequence of its size; technical considerations are placed so high on the list of architectural priorities such 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 include activities where visibility is important for large audiences (auditoriums and stadia), where flexibility is important (exhibition halls and certain types of manufacturing facility) and places where movable objects are housed (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 professional engineers. Of course lightweight and long-span are relative terms and greatly influenced by the materials used and the technology of the times. Westminster Hall was a huge feat of engineering in the 14th century and in the 19th century; St Pancras station roof was the largest span in the UK for many years. These spans seem very modest now with roofs spanning 200 or 300 m and bridges reaching several kilometers!'(Liddell 2007).
An example of a novel long-span roof designed by the architect Edward Durell Stone in the 1950s based on the steel cables used in suspension bridges was the U.S. Pavilion at the 1958 Brussels World's Fair (Encyclopædia Britannica 2010).
2.0 FUNCTIONAL REQUIREMENTS OF ROOFING SYSTEMS
It is known that a roof primarily provides a covering over an enclosure, protecting it from the external environmental influence and action by wind, sunlight, snow, temperature, rainfall and other harsh climatic effects. In order to adequately support the actions of these natural disturbances imposed on it by the prevailing environmental conditions including the likely futuristic effect of climate change, the roof has to be efficiently designed to satisfy certain functional requirements as outlined in the work by (Foster and Greeno 2007; Harrison et al. 2009). These include the following:
- Strength and stability, which is vital to the performance of the structure as a whole.
- Weather resistance including prevention and discharge of rain, snow and condensation.
- Thermal resistance involving regulating internal environments by solar heat loss balance, air temperatures, energy conservation and ventilation.
- Fire resistance including fire safety measures and/or precautions to keep distribution of fire from source at a minimum and provision of adequate lighting.
- Sound insulation involving maintaining adequate noise levels.
2.1 Strength and stability
The roof system functions to provide a great deal of structural rigidity and stiffness in buildings and other areas where they may be applied. A simple case is the tying effect the roof gives to simple buildings with short clear spans where the roof tends to hold the load-bearing walls together such that they do not tear apart. The situation is seemingly different and more difficult to handle when the area of space to be covered by the roof increases in dimensions. According to (Foster and Greeno 2007), the main factor affecting the selection of materials employed in the design of a particular roof system 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 systems that can be used over an opening: the chain, the arch and the beam, of which the chain is the best form for supporting loads over long spans. According 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 carry the self-weight of the structure together with the intermittent loads - for example those due to environmental effect (e.g snow or wind) or maintenance - and it must do this without undue distortion or damage to the building, whether perceptible or imperceptible to its occupants.' (Harrison et al. 2009). These expectations are codified in provisions contained in various national building regulations including the Building Regulations 2000 as cited in the work by (Harrison et al. 2009), which is specifically for application in England and Wales.
A cursory look at the history of roof performance in existing buildings (Harrison et al. 2009) dating back to the eighteenth century, considering the effect of loading reveals that prehistoric dwellings recorded a relatively low performance with respect to the overall loading compared to more recent roof systems (Table 1). This is probably due to advancement in research and technology in this area. Data from a national house condition survey conducted in England as cited by (Harrison et al. 2009) in Tables 2 and 3 respectively 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 cost-effective, adaptable solutions in the design of roof systems to support the loads that come on them. The aim is to seek and find the optimum, economically-feasible method of transferring loads 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 construction of the roof, a balance has to be reached such that there is an overall reduction in the total dead load to be carried by the roof, which will result in a situation where light weight materials carry majorly imposed loads over great spans. With the reduction in the total load to be carried by the roof, materials are saved and smaller, lighter sections can be used to support loads over long spans. This however, will have significant implications on the serviceability requirements of deflection, 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 inherent structural difficulties in the design of long-span roof structures is reducing the dead/live load ratio, expressed as load per square metre of area covered by the roof, to a safe level thereby improving the efficiency of maximum load carried. Following their argument, increase in spans of roof systems generally result 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 solved by keeping two key factors as discussed by (Foster and Greeno 2007) in mind when making choice 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, smaller 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 impact 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 reduced to a minimum.
Another important action apart from effects of weight which is critical in the design of roof structures is wind effect. Gales, extremely strong winds, pose adverse effects on buildings especially roofs in the UK (Harrison et al. 2009). Records by them show that since the wake of the early 90's up till now, about 1.1million houses have affected adversely by gales. This resulted in marked modifications in the codes 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 direction and seasonal factors (Foster and Greeno 2007; Harrison et al. 2009). There is some evidence (Foster and Greeno 2007) that wind pressure and suction 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 designed 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 resistance
As 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 zero-tolerance on seepage of rainfall, snow and/or any form of moisture into buildings. In order to achieve this, Harrison et al. (2009) suggests that drainage 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 orientation of the roof and the effective drained surface area. Furthermore, they stressed that the orientation of the gutters should be such that it slopes to the closest drain outlet to prevent excessive 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 guidance including testing, maintenance and commissioning in BS 8490 both cited in (Harrison et al. 2009).
2.3 Thermal resistance
Thermal 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 strike 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 installation 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 intensity of the sun radiation on the earth is greater consequent upon the effect of global warming, there is an overall reduction in heat absorption transmitted to the interior parts of the building.
2.4 Fire resistance
The major safety requirement for roofs is to reach an optimum performance that fire attack will not immediately bring down 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 cited by (Harrison et al. 2009) is covered by test methods in BS 476-3. This test procedure determines the fire performance in roofs by effects of penetration and spread 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, smoke detectors, sprinklers and smoke extraction systems, which help to maintain an acceptable level of fire safety.
2.5 Sound insulation
Unwanted 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, 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 underside of the roof with a thicker layer of a weaker material, damping and introduction of PTFE washers between joints.
3.0 DESIGN CONSIDERATIONS/GUIDE 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 behaviour of long span structures and careful implementation, will reduce the incidence of collapse of long span structures as well as eliminate some of the concomitant problems of erection of long span structures. These strategies are presented below:
Major project personnel and their roles and responsibilities should be identified at the start of the project in order to determine the correct chain of command and reporting hierarchy - This will ensure that proper project management procedures are applied to prevent friction amongst parties concerned, eliminate budget overruns and ensure that project delivery timelines are met.
It is advisable to involve the fabricator/erector team 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 prepared in line with the codes of practice at design stage. These include, but are not limited to agreement on the grade of steel, connection type, bolt 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 steel by employing high strength steel of the best quality such that light weight materials are used.
Adequate environmental studies should be conducted and results of these should be employed in the estimation of the wind and snow 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 truss chord of the roof structure should be framed 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.
Allowance 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 cladding and other materials in the future.
Careful 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 paid to deflections and camber effects of long span roofs.
Careful treatment should be given to diaphragm 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 leading to timely completion of project.
In as much as the designer 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 responsibility of designing the roof system. This may result in conflicts on site.
For simplicity 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 materials 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) are:
- What is the loading trajectory for the structural system to be developed?
- How can the productive use of the structural members in terms of span, size, quantity of shop pieces and constructability be optimized?
- How can the bracing system determined from a structural perspective be efficiently incorporated 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 flow order?
- At what strategic locations would ephemeral bracings be placed while construction and erection is still in progress?
- How will the determined construction flow order be applied to minimize the use of temporary props 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 reduced 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 single storey building with long span roof as presented by Khup (2009).
4.1 Description of the entire structure
This 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 angle steel sections connected back to back.
4.2 Erection of the truss
The 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 Analysis of the failure
Shortly 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 slender truss was the removal of the temporary shoring towers soon after installation of the truss in position.
Some of the remote causes include:
- commencing 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 length to hold the double angles in position and
- omission of a diagonal strut which made the truss collapse/fail in flexure.
4.3 Lessons learned
Khup (2009) has drawn out learning points for further action which could be noted for correction and application in future jobs. These are:
- The effect of overall dimensions and section properties of the truss must be considered when dealing with trusses to avoid issues linked with torsion and lateral
- Adequate site monitoring and effective supervision should be the ultimate responsibility of the engineer as has been highlighted as one of the design considerations given earlier 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 designed 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 SYSTEMS
5.1 Structural design rules
For 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 published 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 limits
Designs will be done normally 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 purpose without collapse during its entire design life (Corus 2010)
5.3 Serviceability and deflection limits
(Corus 2010) advices that significant distortions or deflections in the structure is absolutely undesirable and must be checked at design stage in order to prevent complications such as:
- Poor drainage systems and ponding in specific locations
- Damage to sealants at overlap sections of the roof system
- Excessive strains at regions of overlaps or other interconnected parts such as interior coverings
- General 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 serviceability 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 states
At 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 could be present in deeper sections especially when used over short spans. This can be controlled by use of web stiffeners.
5.5 Roof load calculations
5.5.1 Concentrated imposed load
Though relevant software packages are now available for calculation of these loads, Corus (2010) has specified quick guidance for calculating loads from human activities in line with provisions of BS 6399-3 as cited in (Corus 2010):
Roof with access (for maintenance purposes only) -- greater of 0.9kN or effective snow load
Roof load for all purpose access --- greater of 1.8kN or the effective snow load.
5.5.2 Dead load
Load due to the self weight of the entire roof system which acts downwards like a gravity load.
5.5.3 Uniform imposed load
This relates to snow loading which is extremely difficult to calculate due to the variability 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), BS 6399-3 (1988) is the recommended code for calculating uniform imposed loading on roof systems.
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5.5.4 Wind load
Wind 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 loads
Corus (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 situation:
- Wind load (imposed or suction)
- Snow load (uniformly distributed or redistributed)
- Uniformly distributed load (kN/m2)
- Concentrated load (kN)
Roof with access
Determined from BS 6399 Part 2
Determined from BS 6399 Part3
Roof without access
Determined from BS 6399 Part 2
Determined from BS 6399 Part3
Determined from BS 6399 Part 2
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