![]() This exceeds the rafter span proposed without using the multiplier so is suitable. This rafter is a single span, so the 10% increase in span over two supports cannot be used.Ĭhoosing 2.5 m span from the 600 mm spacing column:Ģ.5 × 1.1 multiplier = 2.75 m does not give the required span of 3.000 m. This example is in a high wind zone, so the ‘High and Very High’ multiplier of 1.1 can be used.īack to top Step 3 – Continuous rafter 10% increase? The most likely choice here would be for 600 mm spacing or less, to accommodate ceiling battens and sheet linings (see Figure 4).īack to top Step 2 – Rafter span multipliers This is a skillion roof, so consider roof cladding, how it will be fixed, whether it is to purlins, battens or sarking, and the proposed ceiling lining requirements. This is a heavy roof, so the 1200 mm column on the right side of the table is excluded – it only applies to light roofs. We’re changing the parameters in this second example to:Ĭhoose rafter spacing from Table 10.1 – the options are 480, 600 or 900 mm. (Provided by Standards New Zealand under licence 001094.) Example 2 Read off the fixing type in the same row, in this case Type E (2/90 × 3.15 mm skew nails + 2 wire dogs) or an alternative fixing of 4.7 kN.įigure 2 Example 1 using NZS 3604:2011 Table 10.1. This indicates a 140 × 45 mm rafter size. Read the rafter size in the left column of Table 10.1 for the 2.2 m span chosen. This is more than the span in Figure 1, so a 2.2 m rafter span is suitable. Look at the values in the span column for 1200 mm rafter spacing.Īpply the additional 10% permitted in Note 1:Ģ.86 + 10% (0.286) = 3.146. ![]() ![]() This rafter extends over two spans, so the 10% increase can also be used to select the most economic rafter.īack to top Step 4 – Work out rafter span Table 10.1 Note 1 also allows rafter spans to be increased by 10% where rafters are continuous over two or more spans, and are not birdsmouth seated. This example is a light roof in a low wind zone, so the Low and Medium multiplier of 1.3 can be used.īack to top Step 3 – Continuous rafter 10% increase Table 10.1 has span length multipliers for ‘Low and Medium’ and ‘High and Very High’ wind zones. Is 1200 mm rafter spacing appropriate?īack to top Step 2 – Rafter span multiplier ![]() When choosing the rafter spacing, also consider the proposed roof type and the support it requires – battens, purlins or sarking. The rafter span is 3.000 m, and because this is a light roof, the far right column of NZS 3604:2011 Table 10.1 Rafters for all wind zones (see Figure 2) can be used for rafter spacings at 1200 mm (see Note 4 in Table 10.1). rafter span 3.000 m continuous over two spans.In the first example, the building’s parameters are: single span or continuous over two spans (without birdsmouth over mid support).rafter span, taken along the pitch line of the rafter (see Figure 1).(I sleep well that way.TO USE TABLE 10.1 for a building, you need to know: You never know when the client will call and want to add an air conditioner on the roof, or they’re going to invite you to their 50th anniversary out on their deck. I like a very safe size rather than figuring everything too tight. Ultimate stress design figures exactly when the beam will fail and then uses an allowable safety factor.sometimes 1.5, or 1.8%. Working stress leaves a BIG safety factor because they know a home owner will be using it. That’s why carpenters like to hand pick beams, (they know they have an extra safety factor so the beams won’t twist, Bend, etc.)īeams are calculated using the “working stress design” or “ultimate stress design”. Grade of beam can be upgraded if more of the beam (edges of beam) can be observed during grading. Wood is somewhat elastic, so it depends on how many allowable factors you care to investigate to determine its maximum span.Įven the same grade and species can have its “allowable stresses” increased based on “duration of load.” That is to say that if you put a heavy short duration load on a beam it will act the same as a smaller long term load. There’s lots of factors that determine the size of wood beams.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |