Presentation on theme: "INTRO TO ROOFING and PRELIMINARY CALCULATIONS"— Presentation transcript:
1INTRO TO ROOFING and PRELIMINARY CALCULATIONS Timber Framing Code.INTRO TO ROOFING and PRELIMINARY CALCULATIONS
2Previously. We looked at the subject in general Discussed assessment criteriaSection 1. Scope & GeneralSection 2. Terminology & definitions.Section 7. Roof Framing
3FlowchartIt 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.
4Revision Quiz1. AS 1684 specifies the requirements for building practices for what classes of building?
52. List 5 limitations on building design using AS 1684
64. Why is it necessary to determine 4. Why is it necessary to determine the wind classification of a site prior to using AS 1684 to select section sizes of members?
75. 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
86. What are racking forces and how are they resisted?
15CalculationsIf you look at the supplement tables you see that you need to determineSpacing of membersSpans- single or continuousCeiling load widths CLWRoof area supportedRoof load widths RLWRafter spanRafter overhang
16CalculationsSpacing of members such as ceiling joists are measured centre to centre or “in to over"
17Member sizes Remember: The flow chart dictates that we first- Determine the wind classificationConsider 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
18Preliminary calculations Some calculations are required before we have sufficient data to use the span tables
19Preliminary calculations You 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
20Preliminary 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 rafterRLWRafter overhangsAreas supported
21TrigonometryComes 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
22Trig ratios The 3 basic trig ratios are Sine (sin) Cosine (cos) Tangent (tan)
23TrigonometryThe 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.
24Trigonometry Sin =opposite ÷ hypotenuse Cos = adjacent ÷ hypotenuse Tan = opposite ÷ adjacentORS= O÷HC= A÷HT= O÷A
25Trigonometry Some Old Hounds Can’t Always Hide Their Old Age Some students remember this byforming the words-SOH CAH TOAOr by rememberingSome Old HoundsCan’t Always HideTheir Old Age
26The Pythagoras theorem The square on the hypotenuse equals the sum of the squares on the other two sides.Or A2 = b2 + c2
27Roofing 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 rafterAnd it’s overhang.
28True length of the common Rafter Centre of ridgeOutside edge of top plateRafter lengthunderpurlinRise of roofoverhangRun of rafter
29NOTE! 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
30Problem Calculate the true length of the rafter Pitch is 270 Run of rafter is 4000
31Example: 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.
32Method 1. (using Tan) Rise per metre run = Tan 270 = 0.5095 = 0.510 T.L. per metre CR = √ R2 + 12= √= √ 1.260= 1.122mT.L.C.R. = T.L per metre X run= 1.122x 4.0= 4.489m
33Method 2. Using Cosine (Cos) Pitch is 270The run is 4.0mCos = adjacent ÷ hypotenuse= run ÷ rafter lengthRafter length = run ÷ cos 270= 4.0 ÷= 4.489m
34ExercisesCalculate the following rafter lengths. (choose either method)Pitch 370 , Run 3.750m4.696mPitch 230 , Run 4.670m5.073mPitch , Run 2.550m2.705m
35True 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 mm for brick and cavity = 600mm
37True 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
38True length of the common Rafter Centre of ridgeOutside edge of top plateRafter lengthunderpurlinRise of roofoverhangRun of rafter
39Student 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
40Span of the common rafter. The ‘Span of the rafter’ is the actual distance on the rake between points of support.
41Span of the common Rafter Centre of underpurlinspanOutside edge of top platespanoverhang
42Span of the common rafter. Span of rafter is the true length of the rafter divided by 2That is:- from our previous example= ÷ 2= m
43Student exercises 3.Calculate the span of the common rafter for the 3 roof pitches from previous exercise.
44Exercises (calculate span) Pitch 370 , Run 3.750m4.696m / 2 = 2.348mPitch 230 , Run 4.670m5.073m / 2 = 2.537mPitch , Run 2.550m2.705m / 2 = 1.353m
45Fan 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
46Fan 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
53Fan struts (use Tan) The formula is- ½ spread of the fan struts= Span of rafter x tan angle of pitchFor our previous example= x (tan 27 deg.)= 1.142m
54Fan 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.
55Supplement TablesOnce the preliminary calculations have been done we can start to use the span tables in the supplementBut which supplement????
56Using supplement tables Firstly 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)
57Class Exercise - N3 wind classification Using MGP 15 seasoned softwood Single storey buildingTile roofRoof load width 3.000mRafter spacing 600mmSelect a lintel size to span 2100m
60Class Exercise- Choose one of these 2/120x45 ? 2/140x35 170x35 ? Which one is the smallest cross section?But is this section commercially available?
61Class 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
62InterpolationSimply 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.
63Cross multiplication Before we start doing interpolation calcs. You need to be conversant with a mathematical procedure called cross multiplication.
64Cross multiplication In an “equation” such as A = C B D we can cross multiply so thatA x D = B x CSame as the ration A:B::C:D
65Class Exercise If RLW is now 3.300 Rafters still at 600 Required span 2.100We need to interpolate between two columns to find the most economical section size.
66Class ExerciseRun your fingers down the 3000m column and the 4500m column for 600 spacing.We can tell that 2/140x35 will probably span.