Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 23 The deflection of a truss due to the slack can be evaluated by considering the effect of a unit load applied at mid span, using the Bertrand Fontviolant equation. -0,5 0,66 -0,68 0,66 -0,68 0,71 -0,75 0,17 -0,75 0,72 -0,68 0,66 -0,68 0,66 -0,5 2,85 Figure 3.8 Worked example – Axial forces ( N 1,i ) under unit load The deflection is given by:     i b i i i i i ES v N F l 1 1, Where : N 1, i is the axial force produced in the member i by a unit force applied at the point where the deflection is required i l is the length of member i i S is the section area of the member i b is the number of members with bolted connection(s). i i i ES F l is the variation in length of member i due to the slack recovery = ±4 mm according to whether the chord is in compression or tension. Then: v = 4 × (2,31 + 2,85 + 0,5 + 0,66 + 0,68 + 0,66 + 0,68 + 0,71 + 0,75 +… + 0,17 + 0,75 + 0,72 + 0,68 + 0,66 + 0,68 + 0,66 + 0,5) v = 58,4 mm This is a significant additional deflection, compared with the deflection due to the ULS combination (127 mm). 3.7 Modification of a truss for the passage of equipment It frequently occurs that it is necessary to modify the form of a truss in order to allow equipment to pass (a large section duct for example). Several solutions can be provided (Figure 3.9):  Either to increase the passage area available by an eccentricity in the connection of one of the chords (case 1)  Or “break” the straight form of a diagonal, by triangulating the breakage point (case 2).