Timber Framing Code

*Timber Framing Code.

INTRO TO ROOFING and PRELIMINARY CALCULATIONS

*Previously.We looked at the subject in general Discussed assessment criteriaSection 1. Scope & GeneralSection 2. Terminology & definitions.Section 7. Roof Framing

*FlowchartIt is recommended that design starts at the roof and works down to the foundation.Although the flowchart on page 17 tells us to- Determine wind classification.Consider the bracing and tie-down details.We will leave wind classification to the structures teachersConsider bracing details after roof and wall design.

*Revision Quiz1.AS 1684 specifies the requirements for building practices for what classes of building?

*2.List 5 limitations on building design using AS 1684

*4.Why is it necessary to determine the wind classification of a site prior to using AS 1684 to select section sizes of members?

*5. A site may be classified as N1, N2, N3, N4, C1, C2, C3 or C4. a.What do the letters N and C indicate?b.True or false : The higher the number the greater the wind risk

*6. What are racking forces and how are they resisted?

*7.How are overturning forces resisted?

*10. The amount of bearing of a member is.?

*11. What is stress grading and how is it achieved?

*Lets start.

Remember- throughout this module we will consider Coupled roofs With single row of underpurlin.Without ridge struts

*Roof (and ceiling) MembersCeiling joistsHanging beamsCounter beamsStrutting beamsCombined strutting/hanging beamsCombined counter/hanging beamsUnderpurlin

*Roof Members contdRaftersHipsRidges Valleys

*CalculationsIf you look at the supplement tables you see that you need to determineSpacing of membersSpans- single or continuousCeiling load widths CLWRoof area supported Roof load widths RLWRafter spanRafter overhang

*CalculationsSpacing of members such as ceiling joists are measured centre to centre or in to over"

*Member sizes Remember: The flow chart dictates that we first- Determine the wind classification Consider position and extent of wind bracing and tie downsLet us assume-That wind classification for all our exercises is N3There is sufficient room for bracing and tie-downs

*Preliminary calculationsSome calculations are required before we have sufficient data to use the span tables

*Preliminary calculationsYou MUST have a scientific calculator.You only need to be able to do very basic trigonometry.You must be able to use Pythagoras theorem.You need to be able to perform basic algebra

*Preliminary calculations

What do we mean by the term true length of rafter?We need to be able to calculate the true length of the rafter so that we can determine such things as- The span of the common rafterRLW Rafter overhangsAreas supported

*TrigonometryComes from the Greek words Trigon meaning triangle and metre meaning to measure.Trigonometry is based on right angled (900 ) triangles.It involves finding an unknown length or angle, given that we know a length or an angle or various combination of known and unknown data.We will also use the Pythagoras theorem

*Trig ratiosThe 3 basic trig ratios areSine(sin)Cosine(cos) Tangent(tan)

*TrigonometryThe ratios are related to parts of the right angled triangleThe Hypotenuse is always the longest side and is opposite the right angle.The other two sides are either the opposite or the adjacent depending on which angle is being considered.

*TrigonometrySin =opposite hypotenuseCos = adjacent hypotenuseTan = opposite adjacentORS= OHC= AHT= OA

*TrigonometrySome students remember this byforming the words-SOH CAH TOAOr by rememberingSome Old Hounds Cant Always Hide Their Old Age

*The Pythagoras theoremThe square on the hypotenuse equals the sum of the squares on the other two sides.Or A2 = b2 + c2

*Roofing calculations.If we know the roof pitch.And the run of the rafter.We will use the term RUN of rafter rather than half span.We can use trigonometry and Pythagoras to find the true length of the rafter And its overhang.

True length of the common Rafter*overhangunderpurlinOutside edge of top plateRafter lengthRun of rafterCentre of ridgeRise of roof

*NOTE!We are not calculating an ordering length.We require the length from ridge to birdsmouth.You may know this as the set out lengthWe need to find the Eaves overhang separately

*ProblemCalculate the true length of the rafterPitch is 270Run of rafter is 4000

*Example:Method 1. (using Tan)Trade students may be more comfortable with this methodFind the Rise per metre of C.RaftFind the True length per metre of C.RaftMultiply TLPM x Run= True length of rafter.

*Method 1. (using Tan)Rise per metre run = Tan 270= 0.5095= 0.510T.L. per metre CR= R2 + 12= 0.5102 + 12= 1.260= 1.122mT.L.C.R.= T.L per metre X run= 1.122x 4.0= 4.489m

*Method 2. Using Cosine (Cos)Pitch is 270The run is 4.0m

Cos 270 = adjacent hypotenuse= run rafter lengthRafter length = run cos 270= 4.0 0.8910= 4.489m

*Exercises Calculate the following rafter lengths. (choose either method)Pitch 370 , Run 3.750m4.696mPitch 230 , Run 4.670m5.073mPitch 19.50 , Run 2.550m2.705m

*True length of eaves overhang.Firstly you must be aware of the difference between eave width and eaves overhang.For a brick veneer building with an eaves width of 450mm; the actual width to the timber frame is 450 + 150mm for brick and cavity = 600mm

*True length of eaves overhang.The true length of the eaves overhang is the measurement on the rake from the x y line to the back of the fascia along the top edge of the rafter.It can be calculated the same way you calculate you calculated the rafter length

True length of the common Rafter*overhangunderpurlinOutside edge of top plateRafter lengthRun of rafterCentre of ridgeRise of roof

*Student exercises 2.Calculate the true length of eaves overhang for each of the following (all brick veneer)Pitch 270, eaves width 450mm.673mPitch 370 , eaves width 500mm.814mPitch 230 , eaves width 480mm.684m

*Span of the common rafter.The Span of the rafter is the actual distance on the rake between points of support.

Span of the common Rafter*overhangspanspanCentre of underpurlinOutside edge of top plate

*Span of the common rafter.Span of rafter is the true length of the rafter divided by 2That is:- from our previous example= 4.489 2= 2.245m

*Student exercises 3.Calculate the span of the common rafter for the 3 roof pitches from previous exercise.

*Exercises (calculate span)Pitch 370 , Run 3.750m4.696m / 2 = 2.348mPitch 230 , Run 4.670m5.073m / 2 = 2.537mPitch 19.50 , Run 2.550m2.705m / 2 = 1.353m

*Fan strutsWe can make more economical use of underpurlin by using fan struts.The fan strut does not increase the allowable span of the underpurlin.It enables the points of support (walls or strutting beams) to be further apart

*Fan strutsTo estimate the spread of the fan strut we make 3 assumptionsThe underpurlin is in the centre of the rafter length.The plane of the fan strut is fixed perpendicular to the rafter.The fans are at 450

*Fan Struts

*Fan struts (use Tan)The formula is- spread of the fan struts= Span of rafter x tan angle of pitchFor our previous example = 2.245 x 0.510 (tan 27 deg.) = 1.142m

*Fan strutsTherefore distance apart of strutting points for a given u/purlin can be increased by 1.142m using a fan strut at one end of the underpurlin span.Distance apart of strutting points can be increased by 2.284m using a fan strut at both ends of the underpurlin span.

*Supplement TablesOnce the preliminary calculations have been done we can start to use the span tables in the supplementBut which supplement????

*Using supplement tablesFirstly you must choose the correct supplement (see page 3 of the standard)Depends on wind classification, stress grade and species of timberThen choose the applicable table within the supplement (see list of tables page 3 of the supplement)

*Class Exercise -N3 wind classification Using MGP 15 seasoned softwoodSingle storey buildingTile roof Roof load width 3.000mRafter spacing 600mmSelect a lintel size to span 2100m

*Class Exercise-Which supplement?6Which table?18

*Class Exercise-Choose one of these 2/120x45 ?2/140x35 170x35 ?Which one is the smallest cross section?But is this section commercially available?

*Class Exercise-Notice that in the last exercise the RLW, Rafter spacing and required span of lintel were all values included in the table.What if the RLW is 3300 or the rafters are spaced at 500mm or the lintel needs to span 2.250m?WE need to INTERPOLATE

*Interpolation Simply put, to interpolate is To estimate a value between known values.It is not possible to show every span or spacing related to member sizes.Convenient regular increments are used.Linear interpolation is allowed for calculation of intermediate values.

*Cross multiplicationBefore we start doing interpolation calcs.You need to be conversant with a mathematical procedure called cross multiplication.

*Cross multiplicationIn an equation such as A= CB Dwe can cross multiply so thatA x D = B x CSame as the ration A:B::C:D

*Class ExerciseIf RLW is now 3.300Rafters still at 600 Required span 2.100

We need to interpolate between two columns to find the most economical section size.

*Class ExerciseRun your fingers down the 3000m column and the 4500m column for 600 spacing.We can tell that 2/140x35 will probably span.

*Class ExerciseRLWRLW RLW300033004500

2300 ?2000


230022402000

*Class exerciseUsing Table 18 of supplement 6 Interpolate to find the maximum allowable span for a 290x45 lintelRLW 5300Spacing of rafters 1200mm


3400 ?3100


340032403100

*Next weekWe will start work on selecting suitable roofing members from a given plan and specification

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Timber Framing Code