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Explore different examples of long span roof construction, including simple span roofs, lattice trusses, and cellular beams. Learn about the factors to consider in the design and selection of a suitable roof structure. Discover the advantages and disadvantages of various roof coverings and recommend an appropriate solution for your project.
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Categories of buildings • Large span, low rise buildings • Multi-storey structures Can you think of examples of buildings with long span roof construction?
Simple Span Roof support When a beam spans between two simple supports and carries a uniformly distributed load (such as a roof covering) it tends to bend in the centre
Can a simple span roof support be used for distances in excess of 15m? The beams themselves become more costly due to the amount of material in them. The supports (columns) require to be larger since the self weight of the beams increases as they get deeper. In essence, we want to increase a beam’s resistance to bending whilst minimising the self weight of the structural member and maximising its efficiency both economically and structurally.
Lattice Beams, Trusses and Girders • Large spans in excess of 25m can be formed using these systems. • These are effectively deep beams with only a lightweight central section that serves to connect the top structural members that are carrying the compressive forces, with the bottom chords which are carrying the tension forces. • However, designers must always give consideration to how components are delivered to site and how they are handled once there. Trusses having spans greater than 15m may be difficult to transport to site and therefore may have to be transported in sections rather than as complete assemblies. Connections on site may be bolted or welded. • This means that large distances can be spanned economically with structural members which are very much lighter a solid beam of the same overall depth.
Different ways of roof framing It is necessary to design a roof structure for a square building. Figure 1a and b shows two different ways of roof framing. The roof system shown in Figure 1a is a complex roof composed of planar latticed trusses. Each truss resists the load acting on it independently and transfers the load to the columns on each end. To ensure the integrity of the roof system, usually purlins and bracings are used between trusses. In Figure 1b latticed trusses are laid orthogonally to form a system of space latticed grids that will resist the roof load through its integrated action as a whole and transfer the loads to the columns along the perimeters. Since the loads can be taken by the members in three dimensions, the corresponding forces in space latticed grids are usually less than that in planar trusses and hence the depth can be decreased in a space frame.
Proprietary sections are manufactured in standardised section sizes, however, these may not be appropriate either in terms of achievable span, or in terms of architectural style. Therefore, many truss sections are bespoke, or built up specifically for specific building structures.
Cellular and Castellated Beams An incidental advantage of these beams is that they permit the straightforward integration of services ducts through the webs of beams. This means that the floor to ceiling height can be minimised. Castellated beams and cellular beams can be efficiently and economically used to provide long span roof structures (spans in excess of 25m). You should note that the use of both castellated and cellular beams is not limited to roof systems but they can be used in frames too.
The use of columns to increase the effective clear span of main roof beams Umbel or Tree Columns (Sources- StructureAE & Syska.com) This can be done by enlarging the area or support positions at the column head.
Purlins or secondary Beams in long span roofs Load transfer paths are as follows: roof covering purlins or secondary beams roof beams (truss, lattice girder etc.) columns foundations.
4-RoofCovering 1. 2. 3. 4. 5. 6. Appropriateroofcoveringscanincludesheetcladding,tilesandflatroofs, greenroofs Discussionofadvantagesanddisadvantages,keyfeaturesandcomponents ofeachsystem Understandthefunctionsofroofcoverings Compareandcontrastallthedifferentformsofroofcoverings Recommendaroofcoveringsolutionappropriatetotheprojectscenario Sketchdifferentformsofroofcoveringshowingacrosssectionofhowthe roofingsolutionwouldlookandwork 21
What are the factors that must be taken into account in the design and selection of a suitable roof structure? • Function of building • Span of the roof • Height of the space • Aesthetic and design requirements • Economic considerations • Construction considerations. • The environment
Roofing Manyfunctionalrequirementsandfactorswill affectthechoiceofroofcovering: Weatherresistance Riskofcondensationandentrappedmoisture Durability FireResistance Thermalinsulation Resistancetomaintenancetraffic Strengthofsubstructure Degreeofexposure Therelationshipofcostsanddurability Surfacefinish Colourandsolarreflectivity Specialconsiderationse.g.differenttypesof traffic
‘Warm’ & ‘Cold’ Roofs A 'cold' roof construction is one where the insulation is laid between the ceiling joists (ie over a normal flat ceiling). This then means that everything above the insulation, such as the rafters, is colder than the living space. Warm, moisture-laden air can permeate up through the ceiling. When it reaches the cold roof space (ie the loft) the change in temperature can mean that condensation will form on timbers, underlay etc. Roof space ventilation is used to remove this damp air and thus prevent condensation forming. There are different interpretations of what a 'warm' roof is. However, the generally accepted design of a 'warm' roof is one where the insulation, in the form of rigid sarking, is laid on top of the rafters. Therefore, everything below the insulation is as warm as the living rooms in the house. This form of construction has been adopted from flat roofing, where a structural deck is covered by a sealed, vapour-proof membrane. Rigid insulation is laid over this, the gaps between it are carefully sealed and then a waterproof roof covering is laid on top.
What are the benefits of green roofs? • Reduce heating and cooling • Filter pollutants and CO2 out of the air • Grow fruits, vegetables, and flowers • Reduce run-off storm water • Insulate for sound and heat • Filter pollutants and heavy metals out of rainwater • Increase wildlife habitat
When considering the installation of a green roof, what design factors should be considered? • Additional dead weight on the existing or proposed structural system; • Insulation and waterproofing system; • Root barrier (typically polythene or PVC); • Water-retaining layer to reduce quantity of watering required by irrigation or sprinkling; • Water drainage layer (typically gravel, lightweight aggregate or profiled plastic sheeting); • Filter sheet to prevent soil and organic material blocking the drainage system (typically polyester or polypropylene); • Vegetation layer of soil upgraded with organic and mineral additives; • Planting — the anticipated planting dictates the depth of the soil substrate.
