Steel Buildings in Europe

Part 2: Concept Design 2 - 39 4.4 Preliminary design At the preliminary design stage, the following process is recommended: 1. Determine the loading on the truss. See Section 1.4.1. At the preliminary design stage it is sufficient to convert all loads, including self weight, to point loads applied at the nodes and assume that the entire truss is pin-jointed. This assumption is also generally adequate for final design. As an alternative, the roof loads may be applied at the purlin positions and the chords assumed to be continuous over pinned internal members, but the precision is rarely justified. 2. Determine a truss depth and layout of internal members. A typical span : depth ratio is approximately 20 for both W- and N-trusses. Internal members are most efficient between 40° and 50°. 3. Determine the forces in the chords and internal members, assuming the truss is pin-jointed throughout. This can be done using software, or by simple manual methods of resolving forces at joints or by taking moments about a pin, as shown in Figure 4.5. d d d p p p V V V V L L L L x x Resolving forces at joints V L A B C D C 1 p Taking moments around node D determines the force CB Figure 4.5 Calculation of forces in a pin-jointed truss A very simple approach is to calculate the maximum bending moment in the truss assuming that it behaves as a beam, and divide this moment by the distance between chords to determine the axial force in the chord. 4. Select the compression chord member. The buckling resistance is based on the length between node points for in-plane buckling. The out-of-plane