Assessment task 3: Timber design- Design of a bolted truss connection

An exposed roof truss is to be fabricated from kiln-dried Blackbutt for a restaurant in Bathurst, NSW. A sketch of the truss geometry is shown in

Figure below. The truss is one of a number of trusses supporting 20 m² of roof that will be fabricated from stained seasoned hardwood and

connected with galvanised bolts. The architect is keen to express the structural form.

An analysis of connection load cases indicates that the critical loads for the strength limit state will be the construction imposed load combination:

Nominal permanent load G = 9 KN axial compression in the top chord and

Nominal imposed load Q = 9 KN (with a duration of load of 5 days) axial compression in the top chord

Using 140 x 45 mm timber for the top chord and two 140 x 35 mm pieces for the bottom chord, design a bolted heel connection using M12 bolts.

(The heel connection is at the junction of the top chord and the bottom chord (see Figure (a) below) and is supported by the wall top plate.) Inclined member,

top chord

Heel joint

Horizontal member -

bottom chord - 2 x 140 x 35

(a) Roof truss

Inclined member,

top chord

Bolt force

30

Design load in top

chord

Horizontal member,

bottom chord

(c) Forces on connection

Inclined member,

top chord, 140 x 45

Horizontal member,

bottom chord - 2 x 140 x 35

(b) Detail of connection