Summary - lecture 7 - Roof framing PDF

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Summary

Summary of roof framing...

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BLDG1211 – Lecture 7

ROOF FRAMING PITCHED ROOFS - Traditional roofing that is erected on site - Rely on rafters to provide main skeleton of the frame - Transfers loads from roofs into internal/external walls Rafters - Provide the shape of the roof and support roof coverings - Size determined by roof span, stress grade and loading requirements Birdsmouth cut - A birdsmouth cut is placed at the rafter/top plate intersection to assist in the secure connection of the rafter on the top plate (the triangular cut is placed a maximum depth of 1/3 the depth of the rafter perpendicularly) Ridge board - Member placed between the top ends of rafters to provide a fixing point for them (joins them together) - Also a member against which bracing is fixed - A brace should extend from the ridge to a wall top plate or ceiling joint fixed securely to a wall Collar ties - Act as secondary (to ceiling joists) restraint against the spreading forces of the rafters - Placed within the lower 2/3rds of the rafter length (toward ridge board) to provide resistance to spreading action of the rafters - Placement in upper 1/3rd would see the rafters bend along their own length - Fixed to every second rafter with 10mm bolts Underpurlin - Breaks rafter lengths into two spans Underpurlin strut (roof strut) - Transfers the roof load from the underpurlin to a load bearing support beneath (bearing either a load bearing wall or strutting beam) - Struts can be plumb (vertical) or perpendicular (approx. 90O to the rafter) - They must be housed over the upper side of the underpurlin to prevent collapse in the event of nails rusting - Can be configured as fan struts where they are inclined in two planes.

Strutting beam

Collar ties are the second row of a roof frame; the ceiling joists are the props/front row

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Used where no load bearing walls exist beneath roof struts Not placed in contact with ceiling joists (25mm gap) as the load they carry would bend the joists Spans over large openings to carry the roof load to supporting load bearing walls or columns at each end Ends of beam supported on engineered timber or steel posts

Counter beam * diagram in exam * - Breaks span of hanging beam - Runs in same plane as hanging beam (runs through it) - Used to support hanging or strutting beams at mid span - Runs perpendicular to the hanging or strutting beam and reduces their span thereby allowing economy of size - Supports top level of housing over large spanning areas (open spaced lounge rooms etc.) - Can be made of steel - Requires studs underneath to support loads they carry, or steel posts that are bolted to the slab with stud running either side to it Cable ties/trusses - Used to replace underpurlin struts - Steel cables fixed to the rafter at the top plate and ridge points and stressed to push against the underpurlin or rafter in a similar fashion to the forces found in a bow and arrow if the arrow was jammed between the bow and the bowstring - Sags in conventionally framed roofs can usually be easily rectified with the use of these cables - Adjustable steel bar length pushes the rafter in an upward direction straightening out any direction Eave and verge framing - Four basic types of eave configurations:

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1. Boxed eave Consists of soffit bearers and soffit hangers Soffit bearers may extend from fascia board (board connected to the end of rafters supporting gutters) across to wall studs. Alternative is to have the bearer extend from the fascia board across to the outside of the brickwork and instead have it supported by a soffit hanger 2. Raked eave lined on the underside of the rafter Involves nailing soffit lining (4.5mm fibre cement sheet) to the underside of the rafters. Eave is running the same direction as rafter 3. Raked eave lined on the top of the rafter Lining material placed on top of rafters Requires blocking between rafters to stop birds, possums etc. from entering roof void

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Eave is running the same direction as rafter 4. Flush eave

TRUSSED ROOFS - Alternative to roof frames - Prefabricated frames delivered to the site ready for installation, makes construction faster - Engineered to specific design for each project - Consume large areas of useable roof space - Difficult to install over large spans (due to their fixed height and configuration, e.g. on home units etc.) - Various designs and configurations for different purposes (e.g. to carry extra loads at ceiling level, carrying extra loads at the roof covering level such as solar water heaters etc.) - Can be easily distinguished from pitched roofs from: the large number of nail plate connectors (hold pieces of timber together), very little space for movement within frame and the presence of internal web members (often in a “W” configuration) Members of roof trusses - Top chord – replaces rafters - Bottom chord – replaces ceiling joists (placed 10mm from internal walls so incase roof settles it will have room) - Webs – members in between chords that replace underpurlins and struts - Truss girder – supports other trusses/supports large loads Truss design - Trusses are designed to transfer the entire load they support to the external walls. This allows the internal walls to be non load bearing and removed safely, saves on cost and allows freedom of planning - Each piece of timber in a truss is designed to be axially loaded (stretching or squashing it along its axis) instead of bending like a beam - If timber members span a large distance unsupported, they will sag and bend. Bottom chords will bend first as they are longer than top chords. To prevent this, a web is placed mid way through the bottom chord connecting to the apex of the truss (junction between the two top chords). This web is in tension as it is pulling down on the apex joint. Top chords will similarly bend as they widen. To stop this, more webs are installed from the base of the vertical web connected to the apex to the middle of the top chord span. These webs are in compression like a roof strut - As the span of the truss widens, the bottom chord exceeds its bending limits and starts to sag between the points of support, this then leads to additional webs replacing the vertical web in the centre. Advantages of trussed roofs

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Fabricated off site whilst slab floor or floor framing is being constructed – reduces construction time Engineered design Cost effective Simple and quick to erect

Disadvantages - Cannot be altered on site without affecting engineering certification - Difficult to use and handle on very large roof spans without the use of a crane – due to the height of the truss - Cannot be used effectively on complicated roof shapes – less effective than conventional roof framing Internal walls - Set 10mm below external walls to ensure they do not carry any loads - Top plates of internal walls are attached to trusses via slip brackets External walls - Trusses are fixed to external wall top plates using Trip L Grips, a special three way steel connection plate that provides fixing into three surfaces - Trip L Grips are nailed to the side and top of the top plate and into the side of the truss - Minimum of 10 nails required to secure the Trip L Grip Truss bracing - Steel straps used to stabilise racking forces - Works in similar fashion to steel wall braces...