There seems to be some confusion around selecting lintels and deciding if uplift fixings are required using NZS 3604:2011 Timber-framed buildings Table 8.14, so grab your copy of NZS 3604 and work through the steps.
BEFORE USING NZS 3604:2011 Table 8.14, you will need to know the:
- wind zone
- loaded dimension on the lintel (see NZS 3604: 2011 section 1, Figures J, K and L)
- span of the lintel.
Generally, the loaded dimension of a lintel that is supporting a truss will be half the span of the truss.
NZS 3604:2011 does not cover girder truss loads, which will be greater than an ordinary truss. If a girder truss lands on a lintel, the lintel will require specific engineering design.
Using NZS 3604:2011 Figures 8.7 to 8.11, identify the lintel location in the building and then select the appropriate lintel from the table above the chosen figure. The lintel tables relate to all wind zones.
Only lintels that directly support trusses or roofs must be secured against uplift (clause 220.127.116.11), and not all of these require uplift fixings (see below).
Worked example 1
This example is a 2-storey construction but with the window opening in the upper level, so NZS 3604:2011 Figure 8.7 and Table 8.9 are chosen (see Figure 1). Only lintels from Figure 8.7 and Table 8.9 require uplift fixings because they are the only ones that relate to lintels that support a roof or truss.
Let’s assume we have a window 2.5 m wide supporting a loaded dimension of 6 m and a light roof in a medium wind zone. Use these numbers in Table 8.9. Note there is no option for a 2.5 m span so choose the next larger span on that line, that is, 2.7.
Now read up the table to obtain the lintel size required, in this case, 240 × 90 mm.
Are uplift fixings needed?
Now we have the lintel size, but does it require uplift fixings? To work this out, go to NZS 3604:2011 Table 8.14 and enter the same information – light roof, medium wind zone, loaded dimension of 6 m (see Figure 2).
In the relevant row, under the third column ‘Uplift fixings not required, use fixings from table 8.19’ is NA (not applicable). This means that this column is not applicable, it does not mean uplift fixings are not required.
We move to the fourth column ‘Uplift fixings required, see 18.104.22.168 for fixings’. This indicates that the maximum span of a lintel in a medium wind zone supporting a loaded dimension of 6 m is 3.7 m. Our lintel is within that span (2.5 m) so it is within the scope of NZS 3604 and does require uplift fixings.
Clause 22.214.171.124 requires the lintel to be secured against uplift by being fixed at each end to the trimming stud, which in turn is attached to the floor framing as shown in Figure 8.12 or an alternative fixing of 7.5 kN capacity in tension along the line of the trimming stud.
An example that doesn’t require uplift fixings in Table 8.14 would be: light roof, medium wind zone, loaded dimension 3 m and lintel span 1.8 m.
Using this information in Table 8.14 shows fixings from NZS 3604:2011 Table 8.19 should be used. This has the standard attachment requirements of trimming studs to lintels, studs to bottom plates and bottom plates to floor. In this case, no additional strapping is required.
NZS 3604:2011 is available for purchase from www.standards.co.nz.
Articles are correct at the time of publication but may have since become outdated.