Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 24 Case 1 Case 2 Figure 3.9 Passage of a duct – Local modification of the truss In case 1, the secondary moments which result from the introduction of an eccentricity increase in relation to the size of the eccentricity. If there is a choice, it is always preferable to introduce an eccentricity in the least stressed chords. In case 2, care must be taken with several phenomena:  The axial force can increase significantly in certain chords situated in the immediate proximity of the modified panel (as a result of modification to the position of the members).  “Secondary” moments appear as a result of the lack of stiffness in a broken diagonal compared with a straight diagonal, even if the breakage point is triangulated.  The breakage point must of course be triangulated in the plane of the truss; it must also be restrained out-of-plane (where three members meet) if the broken diagonal is in compression. These two phenomena (case 1 and case 2) are illustrated using the worked example. 3.7.1 Introduction of an eccentricity axis in a diagonal (case 1) The truss panel through which the passage of equipment is required is the second panel from the support on the right. Figure 3.10 shows a part of the truss, with the eccentricity of a diagonal. 300 mm Figure 3.10 Passage of a duct – Eccentricity of a diagonal Changes in axial forces in the modified area are represented on the Figure 3.11.