Steel Buildings in Europe

Part 5: Detailed Design of Trusses 5 - 21 the internal connections rigid (diagonal and verticals fixed on their original end nodes). The comparison is summarized in Table 3.1, where it can be seen that the end moments are in the same range as the moments resulting from the self- weight of the diagonals. Table 3.1 Effect of rigid connection instead of pinned Horizontal web Vertical web End moment in a diagonal in tension (Double angles 120 x12) 1,03 1,17 End moment in a diagonal in compression (Double angles 150  15) 1,30 2,35 Moment resulting from the self-weight (for comparison) 1,36 1,36 Assumption of bi-hinged diagonals Acceptable Acceptable Note: the bending moments are given in kNm. 3.6 Effect of clearance of deflection When the connections between elements which make up a truss beam are bolted connections, with bolts in shear (category A in EN 1993-1-8 [2] ), the clearance introduced into these connections can have a significant effect on displacement of the nodes. In order to facilitate erection, the bolts are in fact inserted in holes which are larger than the bolts themselves. For standard bolt sizes, holes more than 2 mm bigger than the bolt are usually made (usually referred to as a 2mm clearance). In order for a connection with clearance to transmit to the node the load required by the attached member, the bolt must come into contact with one or other of the connected parts: this is called often referred to as ‘taking up slack’. For a connected tension member, this slack can be assimilated as an additional extension that is added to the elastic elongation of the member in tension. Likewise, for a connected compression member, the slack is assimilated as a reduction in length that is added to the elastic shortening of the compressed member. The total slack in the many different connections of a truss structure can lead to a significant increase in displacements, which can have various and more or less serious consequences. Amongst these, note:  In most of the cases, the visual effect is the worst consequence.  Increased deflection can lead to a reduction of free height under the bottom chord, which might prevent or upset the anticipated usage. For example, the additional deflection of a truss holding doors suspended in a gable of an aeroplane hangar could prevent the passage of the aeroplane.  Increase in the deflection can result in reduction in the slope of the supported roof and even, if the nominal slope were small, to a slope inversion; a risk of water accumulation is therefore associated with an inversion in pitch.  If the truss structure is not a statically determinate system, this may lead to unexpected internal forces.