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Chapter 2.6 Trusses and Vierendeel Structures

Trusses are probably best known as long span, deep frameworks, which are used for bridging large voids in buildings or across wide bays and valleys. Other widely used applications are roof supports – as timber, iron or steel frameworks – from medieval times until today.

Vierendeel girders, named after the inventor – a Belgian engineer – date from the twentieth century. They utilise fixed connections. They are included with trusses since the two are most often substituted depending on circumstances. The chapter summarises the use of virtual cut sections for speedy resolution of forces; aspects of stiffness (by the use of beam analogy); trusses as deep beams; trusses as composite beam/pin jointed applications (king and queen post trusses); variations of roof trusses and vierendeel girders using approximate design methods.

Top and bottom chords balance the moment: referring to figure 2.6.9 it can be found, by resolving the diagonal cut strut into horizontal and vertical components, that the total force in the top chord (37.5) balances the force in the bottom chord.

Figure 2.6.15 Infolab Bridge

This footbridge comprises a top handrail which is the beam and tie rods placed within the balustrade. The top handrail (channel section) and bottom chord (angle section) is not particularly stiff to avoid visual intrusion.

The bridge has more trussness than beamness.

Approximation of End Strut Force (C) – ref. figure 2.6.6

RA = C.Cos30⁰….therefore

C= 25/0.866 = 28.8.

Calculation of chord force (CF) – ref figure 2.6.7

Calculate Moment () – assuming simply supported with UDL….

M= (10/10)x50²/8 = 312.5 Hence CF……

(d=depth of truss)……..

CF = M/d = 312.5/8.66 = 36.0

Figure 2.6.16 Traditional Kingpost Truss
Often the compression in the rafter will cause slipping of the joint. Also the central struts often shrink causing further bowing. A cautionary note for modelling existing trusses: Ensure the bottom chord is a continuous member from support to support without a central pin.

Demonstration Spreadsheets

The following figures are available to download in MS Excel format.