Gable end trusses

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There have long been questions about whether a single gable end truss is suitable for fixing cladding to and, if so, what is the maximum height before a 2-ply gable end truss is required? For the answer, we look at the design of gable end trusses.

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Figure 1 Table 1A from MiTek’s Gable end bracing booklet.

A GABLE END TRUSS sitting directly over an external wall with no large openings is considered non-structural. It will generally have vertical webs only and no diagonals. This is similar in principle to gable end walls that are considered non-loadbearing in accordance with clause of NZS 3604:2011 Timber-framed buildings.

Need to be specific engineering design

As well as supporting external cladding, gable end trusses are subjected to face loads from wind and seismic actions. Most cladding manufacturers’ literature specifies that the timber framing support structure is to have a stiffness equivalent to, or more than, the provisions of NZS 3604:2011. There is no mention of timber size, length and grade.

Table 8.4 of NZS 3604:2011 does not provide for heights less than 2.4 m and framing sizes less than 90 × 35 mm. Anything smaller or shorter needs to be specific engineering design (SED). Hence, all gable end trusses are SED.

Truss design software

In MiTek’s Mi20/20 truss design software, gable end trusses are designed to AS/NZS 1170.0:2002 Structural design actions – Part 0: General principles, Table C1 Suggested serviceability limit state criteria.

The gable end truss webs are designed for face loads from wind with a serviceability deflection limit of h/150 (h is the length or height of web). The results are available in MiTek’s booklets Gable end bracing and Structural fixings on-site guide for Building Code compliance, which can be downloaded for free.

There are two tables in Gable end bracing – one for truss webs at 600 mm (see Figure 1) and the other for truss webs at 400 mm centres. Designs are carried out for 70 × 45 mm and 90 × 45 mm webs in SG6 and SG8 grade for all wind zones from low to extra high.

Using Table 1A for webs at 600 mm centres (see Figure 1):

  • The net pressure coefficient Cp,n is assumed to be 1.0 (Cp,e = 0.7; Cp,i = -0.3).
  • The first set of four columns gives the maximum height (h) of webs for 1-ply gable end trusses, which ranges from 1150 to 2200 mm.
  • The second set of four columns gives the maximum height (h) of webs for 2-ply gable end trusses, which ranges from 1600 to 2750 mm.
  • The third set of four columns gives the maximum height (h) of webs for wall framing and ranges from 1450 to 3450 mm. The ‘on flat’ description refers to truss fabrication terminology. The webs are designed to be on edge when resisting face loads from wind.
  • For brick veneer cladding, the recommendation is to use extra high wind zone and 2-ply gable end trusses or 90 × 45 wall frames (marked with *).
  • If any of these maximum height limits are exceeded, a wind beam or strongback will be required at mid-height. Depending on size and span, the strongback may need to be braced to ceiling runners at regular intervals. Four types of strongback options are shown in the Gable end bracing booklet.
Figure 1 Table 1A from MiTek’s Gable end bracing booklet.

BRANZ deflection limits incorporated

BRANZ Study Report SR168 (2007) Engineering basis of NZS 3604, Feb 2011 revision, section 6.2.2 uses a tighter serviceability deflection limit of h/180 with a maximum of 15 mm. This deflection criteria is mainly for wall studs with external cladding and internal plasterboard linings.

AS/NZS 1170.0:2002 has a stricter deflection limit of h/200 for plaster/gypsum walls under face loading. In comparison, most gable end trusses only support external cladding with no internal lining at all.

MiTek has adopted the deflection limits of SR168 (h/180 with a maximum of 15 mm) and is revising the maximum heights of gable end truss webs. The new tables and brochure will be released later in the year.

When should it be 2-ply?

1-ply gable end truss with 90 × 45 mm SG8 webs at 400 mm centres may be possible up to a height of 2.4 m for low wind zone. Over this height or in higher wind zones, a 2-ply gable end truss or wall frames will be required.

If cladding manufacturers have specific framing requirements (for example, 90 × 45 mm studs) that are greater than the above deflection criteria, their requirements will take precedence.

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Figure 1 Table 1A from MiTek’s Gable end bracing booklet.