ROOF SYSTEMS
Manage rainfall and provide an aesthetic element to structure. Classified into flat which has pitch (1-3 degrees) need to have angle to allow water to drain Pitched and sloping roofs timber or steel rafters beams and purlins high slope 30 – 45 degrees, medium 15-30, low < 15. Concrete slabs used in flat roofs for strength and fire rating, have membrane for waterproofing. Structural steel frames: flat have combination of primary and secondary or beams and purlins for lighter sheet roofing. Sloping have beams and purlins with lighter sheet metal roofing. Portal frames rigid frames (columns and beam) with purlins and girts with sheet metal. Trussed frames very common, shape determined by materials and function. Space frames: 3D trusses various cross section types welded bolted or threaded together forming matrix like structures. Gable roofs: vertical triangular section at one or both ends. Hip roof: vertical triangular section of wall at one or both ends, folds around a corner. Constructed of rafters and ceiling joists.
Materials
Ferrous metals – iron. Non ferrous metals – all others. Alloys – combination of two or more metals. (Ferrous alloy – iron). Hard, low fragility, high ductility, generally impermeable, high density, good conductors, strong, recyclability, cost effective. Most prone corrosion magnesium, zinc, alimnium, structural steels, cast iron, lead, tin, copper brass bronze, nickel, titanium, stainless steels less prone to corrode. Galvanized – zinc coating (protection). Corrosion and oxidation from water.
Ferrous Metals
Iron – magnetic, corrodes easily, good compressive strength. Wrought iron used for years (1000BC) Cast iron used 19th century, melted and cooled. Rarely used now heavy and brittle. Steel (iron and carbon) very strong, good conductor, forms many shapes. Long lasting. Structural steel – framing (purlins, stud frames, columns, beams). Hot rolled steel – elements shaped while metal hot, primary structures, joints welded or bolted. Cold formed steel – folded from sheets previously produced and cooled, secondary structures, joints bolted or screwed. Reinforcing bars – used with concrete to form reinforced concrete. Steel sheeting – cladding and roofing, needs galvanisation. Stainless steel alloys – high cost, used for finish quality or in harsh conditions.
Non ferrous metals
Aluminium – lighter metal, non magnetic and non sparking, easily formed, expensive, used window frames door handles, self protective oxide, other treatments also applied (powder coating, anodisation). Copper – roofing material, natural wreathing causes copper to go green, used electric cabling. Zinc – cladding material, galvanising other metals, brittle, reasonable conductor. Lead – less used can be toxic. Tin – rare, was used for lining of lead pipes and protective covering iron plates and small pipes. Titanium – expensive, strong light weight alloys cladding material, corrosion resistance, high strength to weight ratio. Bronze – bearing, clips electrical connectors and springs, tough, resistant to corrosion. Brass - locks, gears, screws, frictions type uses.
SPANNING SPACES
Architecture about enclosing space. Main problem is spanning space: Stone: beam or slab (brittle – breaks) or corbel to overlap till meets in middle. Arches created with bricks – hard to support while building (timber framework) Major interior space created through parallel columns supporting roofs
Plates and grids
Plates: rigid, planar structures that disperse loads in multidirectional pattern eg: concrete slab. Grid: lines of support for structural system. Critical points are columns and load bearing walls. Plates and walls interact as plates will sit upon the grid and transfer loads through columns in the grid.
Truss
Structural frame using stable and strong triangular shape with linear members under compression, tension or no force. Top and bottom chords with web members connecting. Composed of steel and timber mainly, sometimes concrete. Steel: members bolted or welded, can have concrete reinforcement, lateral bracing. Trusses used in roofing to span long distances.