Roof Trusses Created By M.S.Martin 11/04, reviewed 11/05, reviewed 11/07.
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Transcript of Roof Trusses Created By M.S.Martin 11/04, reviewed 11/05, reviewed 11/07.
Roof Trusses
Created By M.S.Martin 11/04, reviewed 11/05, reviewed 11/07
Often used instead of pitched roofs
Not Always conventional shapes
What do you know about them ?
Types
Shapes / Limitations ? What supports them ? How are they joined ?
Definition / Design
A truss is a self contained frame, designed to transfer roof loads, typically to external walls
Most modern roofs can be adapted to a trussed system
Members are typically joined by gangnail plates
Types
Types continued
Types continued
Types – Standard truss
Types - other
There are many other types of trusses used in conventional roof shapes and we’ll look at them a little later
First we need to look at the components and how a standard truss works
Hand out – worksheet one
Identify the parts of a standard truss
Summarize in your own words, the truss components, use the terminology section of carp 15 text for reference
Components
Terminology – worksheet answers Bottom chord – the horizontal member forming
the base of the truss, which will also support the ceiling below
Top chord – the angled member at the top of the truss, in a standard truss this is where a rafter would be
Web – these are the internal members of the truss that help distribute the load to the external walls
Camber – An upward curvature built into the bottom chord to compensate for deflection
Terminology continued
Girder truss – A truss that supports other trusses or beams, differs in shape depending on where it is in the roof
Nail plates – the connectors made from a light gauge steel that join individual components of the truss
Gable end truss – the first truss at a gable end Raking truss – a gable end truss altered to suit
outriggers for a verge overhang
Terminology continued
Panel points – these are the connection points in a truss, (eg) where a web meets a top chord
Fish plate – an alternative bolted connection at a panel point
Load bearing walls – in a trussed roof these are still the support walls, but are typically the external walls
Truss panel points
Tension & Compression
Compression & Tension
When trying to understand how this works its easier to look at half the truss
Handout – Tension & Compression
Review handout to help understand how this process takes place
Parallel Chord trusses
Used where large raking or level spans are required to bridge intermediate support
They provide strength and versatility
Ground floor system
Alternative system
Roof Trusses
We mainly come across trusses in roof systems
Now we’ll have a look at the factors concerning them
Camber – P12
Trusses are built with a slight camber in the bottom chord
This is designed to provide maximum calculated deflection
Minimum clearance
Under no circumstances should trusses be supported along their span
With a trussed roof, internal walls are merely partitions / non-load bearing
Minimum clearance is 12mm
Support & Connections
The structure supporting the trusses must be level & square
External walls are load bearing with trussed roofs
Or intermediate panel points for cantilevered trusses
Either support directly over studs or change top plate
Support continued
Where not supported by studs
Use thicker top plate, or Double plate
Connections to trusses
Trusses require either factory connectors or nail or bolted connections on site, some of these are – Nail plates (gang nail)Triple grippesTruss saddlesMulti purpose anchorsVarious bolted bracketsJoist hanger bracketsWall brackets
Types of connections
Types of connections
Types of connections
Types of connections
Worksheet two
Handout Answer questions on handout to review
section on support and connections Use your text book to assist you in your
answers
Answers to worksheet two
Parallel chord trusses – used in ground floor, suspended floors and roofs
Camber – is there to provide maximum deflection when roof loads are placed upon them
Minimum clearance for internal non-load bearing walls – 12mm
Wall structure support – either directly over studs or change top plate to thicker or double one
Answers continued
Connection methods used in trussed roofing – nail plates, triple grips, truss saddles, multi-purpose anchors, bolted brackets, joist hanger brackets, wall brackets.
Answers continued
Changes for a cantilever
It changes here
Fixing to non load bearing walls
Lifting roof trusses
If lifting manually, you should always use support timbers to drag them up
If using a crane be careful to sling them correctly
The advantage of a crane, many trusses can be lifted at once
Lifting continued
Roof truss types
Its important to get familiar with the different types of trusses used in lieu conventional roof frames, some are –StandardTruncatedJack trussHip trussGirder trussAnd more
Some typical roofs- handout
Roofs continued
Breakup – P28 text
Standard roof truss
Other types
Location of trusses
Review text
Review text for other types of trusses.
Saddle trusses Girder trusses Etc. Review from P29
Bracing and Erection
Review text book for bracing requirements P24
Alternate roof styles P27 Cantilever trusses P32 Erection procedures P38
New alternative from Mitek
Alternative wall bracing
Mitek (gang nail) also make a timber wall bracing system they call Posi – brace
Suitable for most timber framed houses
Worksheet three
Answer questions to worksheet provided Review text for answers
Then complete handout on roof shape and select types of roof trusses to use on the conventional shaped roof shown.
Answers to worksheet three
Fixing to internal walls – by way of purpose made L-brackets, also, depending on layout of walls under, blocks may also have to be incorporated
Types – standard, truncated, jack truss, girder, hip truss, truncated girder
Support for saddle truss – timber ledger Typical bracing – speed bracing Picture for roof layout on next slide