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Presentation on theme: "1 Timber Framing Code. INTRO TO ROOFING and PRELIMINARY CALCULATIONS."— Presentation transcript:


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

3 3 Flowchart It is recommended that design starts at the roof and works down to the foundation. Although the flowchart on page 17 tells us to- 1. Determine wind classification. 2. Consider the bracing and tie-down details. We will leave wind classification to the structures teachers Consider bracing details after roof and wall design.

4 4 Revision Quiz 1. AS 1684 specifies the requirements for building practices for what classes of building?

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

6 6 4. Why is it necessary to determine the wind classification of a site prior to using AS 1684 to select section sizes of members?

7 7 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

8 8 6. What are racking forces and how are they resisted?

9 9 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?

12 12 Lets start. Remember- throughout this module we will consider Coupled roofs With single row of underpurlin. Without ridge struts

13 13 Roof (and ceiling) Members Ceiling joists Hanging beams Counter beams Strutting beams Combined strutting/hanging beams Combined counter/hanging beams Underpurlin

14 14 Roof Members contd Rafters Hips Ridges Valleys

15 15 Calculations If you look at the supplement tables you see that you need to determine Spacing of members Spans- single or continuous Ceiling load widths CLW Roof area supported Roof load widths RLW Rafter span Rafter overhang

16 16 Calculations Spacing of members such as ceiling joists are measured centre to centre or in to over"

17 17 Member sizes Remember: The flow chart dictates that we first- Determine the wind classification Consider position and extent of wind bracing and tie downs Let us assume- 1. That wind classification for all our exercises is N3 2. There is sufficient room for bracing and tie-downs

18 18 Preliminary calculations Some calculations are required before we have sufficient data to use the span tables

19 19 Preliminary 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

20 20 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 rafter RLW Rafter overhangs Areas supported

21 21 Trigonometry Comes from the Greek words Trigon meaning triangle and metre meaning to measure. Trigonometry is based on right angled (90 0 ) 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

22 22 Trig ratios The 3 basic trig ratios are Sine(sin) Cosine(cos) Tangent(tan)

23 23 Trigonometry The ratios are related to parts of the right angled triangle The Hypotenuse is always the longest side and is opposite the right angle. The other two sides are either theopposite or the adjacent depending on which angle is being considered.

24 24 Trigonometry Sin =opposite ÷ hypotenuse Cos = adjacent ÷ hypotenuse Tan = opposite ÷ adjacent OR S= O÷H C= A÷H T= O÷A

25 25 Trigonometry Some students remember this by forming the words- SOH CAH TOA Or by remembering S ome O ld H ounds C ant A lways H ide T heir O ld A ge

26 26 The Pythagoras theorem The square on the hypotenuse equals the sum of the squares on the other two sides. Or A 2 = b 2 + c 2

27 27 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.

28 True length of the common Rafter 28 overhang underpurlin Outside edge of top plate Rafter length Run of rafter Centre of ridge Rise of roof

29 29 NOTE! We are not calculating an ordering length. We require the length from ridge to birdsmouth. You may know this as the set out length We need to find the Eaves overhang separately

30 30 Problem Calculate the true length of the rafter Pitch is 27 0 Run of rafter is 4000

31 31 Example: Method 1. (using Tan) Trade students may be more comfortable with this method Find the Rise per metre of C.Raft Find the True length per metre of C.Raft Multiply TLPM x Run= True length of rafter.

32 32 Method 1. (using Tan) Rise per metre run = Tan 27 0 = = T.L. per metre CR= R = = = 1.122m T.L.C.R.= T.L per metre X run = 1.122x 4.0 = 4.489m

33 33 Method 2. Using Cosine (Cos) Pitch is 27 0 The run is 4.0m Cos 27 0 = adjacent ÷ hypotenuse = run ÷ rafter length Rafter length = run ÷ cos 27 0 = 4.0 ÷ = 4.489m

34 34 Exercises Calculate the following rafter lengths. (choose either method) Pitch 37 0, Run 3.750m 4.696m Pitch 23 0, Run 4.670m 5.073m Pitch , Run 2.550m 2.705m

35 35 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 mm for brick and cavity = 600mm

36 36

37 37 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

38 True length of the common Rafter 38 overhang underpurlin Outside edge of top plate Rafter length Run of rafter Centre of ridge Rise of roof

39 39 Student exercises 2. Calculate the true length of eaves overhang for each of the following (all brick veneer) Pitch 27 0, eaves width 450mm.673m Pitch 37 0, eaves width 500mm.814m Pitch 23 0, eaves width 480mm.684m

40 40 Span of the common rafter. The Span of the rafter is the actual distance on the rake between points of support.

41 Span of the common Rafter 41 overhang span Centre of underpurlin Outside edge of top plate

42 42 Span of the common rafter. Span of rafter is the true length of the rafter divided by 2 That is:- from our previous example = ÷ 2 = 2.245m

43 43 Student exercises 3. Calculate the span of the common rafter for the 3 roof pitches from previous exercise.

44 44 Exercises (calculate span) Pitch 37 0, Run 3.750m 4.696m / 2 = 2.348m Pitch 23 0, Run 4.670m 5.073m / 2 = 2.537m Pitch , Run 2.550m 2.705m / 2 = 1.353m

45 45 Fan struts We 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

46 46 Fan struts To estimate the spread of the fan strut we make 3 assumptions 1. The underpurlin is in the centre of the rafter length. 2. The plane of the fan strut is fixed perpendicular to the rafter. 3. The fans are at 45 0

47 47

48 48

49 49

50 50

51 51

52 52 Fan Struts

53 53 Fan struts (use Tan) The formula is- ½ spread of the fan struts= Span of rafter x tan angle of pitch For our previous example = x (tan 27 deg.) = 1.142m

54 54 Fan struts Therefore 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.

55 55 Supplement Tables Once the preliminary calculations have been done we can start to use the span tables in the supplement But which supplement????

56 56 Using supplement tables Firstly you must choose the correct supplement (see page 3 of the standard) Depends on wind classification, stress grade and species of timber Then choose the applicable table within the supplement (see list of tables page 3 of the supplement)

57 57 Class Exercise - N3 wind classification Using MGP 15 seasoned softwood Single storey building Tile roof Roof load width 3.000m Rafter spacing 600mm Select a lintel size to span 2100m

58 58 Class Exercise- Which supplement? 6 Which table? 18

59 59

60 60 Class Exercise- Choose one of these 2/120x45 ? 2/140x35 170x35 ? Which one is the smallest cross section? But is this section commercially available?

61 61 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

62 62 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.

63 63 Cross multiplication Before we start doing interpolation calcs. You need to be conversant with a mathematical procedure called cross multiplication.

64 64 Cross multiplication In an equation such as A= C B D we can cross multiply so that A x D = B x C Same as the ration A:B::C:D

65 65 Class Exercise If RLW is now Rafters still at 600 Required span We need to interpolate between two columns to find the most economical section size.

66 66 Class Exercise Run your fingers down the 3000m column and the 4500m column for 600 spacing. We can tell that 2/140x35 will probably span.

67 67

68 68 Class Exercise RLWRLW RLW ?2000

69 69 Class Exercise RLWRLW RLW

70 70 Class exercise Using Table 18 of supplement 6 Interpolate to find the maximum allowable span for a 290x45 lintel RLW 5300 Spacing of rafters 1200mm

71 71 Class Exercise RLWRLW RLW ?3100

72 72 Class Exercise RLWRLW RLW

73 73 Next week We will start work on selecting suitable roofing members from a given plan and specification

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