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How to increase allowable bending moment ? Increase truss dimensions = most effective

How to increase allowable bending moment ? Increase truss dimensions = most effective Increase diameter main chords Increase wall thickness main chord. Be nding moment caused by Wind force. Loading force. Wind force. Wind loading on a canopy or projection screen.

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How to increase allowable bending moment ? Increase truss dimensions = most effective

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  1. How to increase allowable bending moment ? • Increase truss dimensions = most effective • Increase diameter main chords • Increase wall thickness main chord

  2. Bending moment caused by Wind force Loading force Wind force

  3. Wind loading on a canopy or projection screen Fh= 1,3 x wind Pressure * Wind surfaceFmembrane tension = Fh / 0,8

  4. Beaufort number Wind speed / wind pressure Land conditions km/h m/s kN/m^2 0 0 0-0.2 0 Calm. Smoke rises vertically. 1 1-6 0.3-1.5 0,001 Wind motion visible in smoke. 2 7-11 1.6-3.3 0,007 Wind felt on exposed skin. Leaves rustle. 3 12-19 3.4-5.4 0,02 Leaves and smaller twigs in constant motion. 4 20-29 5.5-7.9 0,04 Dust and loose paper raised. Small branches begin to move. 5 30-39 8.0-10.7 0,07 Smaller trees sway. 6 40-50 8.0-10.7 0,12 Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. 7 51-62 13.9-17.1 0,18 Whole trees in motion. Effort needed to walk against the wind. 8 63-75 17.2-20.7 0,27 Twigs broken from trees. Cars veer on road. 9 76-87 20.8-24.4 0,37 Light structure damage. 10 88-102 24.5-28.4 0,50 Trees uprooted. Considerable structural damage. 11 103-117 28.5-32.6 0,67 Widespread structural damage. 12 >117 >32.7 0,85 Massive and widespread damage to structures.

  5. Transversal force ( Also known as shear force )

  6. Example’s of situations when transversal forces will be determine the maximum loading: • A heavy load on a short span. • A heavy load near to the suspension point. • Truss constructions underneath a stage floor.

  7. How to increase allowable transversal force ? • Increase the diameter of the brace • Increase the wall thickness of the brace • Increase wall thickness of the main chord

  8. NORMAL FORCE

  9. Examples of situations when a normal force occurs: • Towers • Columns • Rafter trusses in a MPT or ST roof

  10. How to increase the normal force? • Increase tube diameter • Increase wall thickness of tubes

  11. TORSION FORCE

  12. Examples of situations when torsion will occur: • Equipment on a boom arm • Equipment which are all positioned on one tube of a truss

  13. How to increase the normal force? • Increase the amount of cross braces inside the truss. • Use a bigger truss.

  14. Deflection • The elastic movement of a structure under load • Deformation by self weight and pay load > the truss starts to bend/deflect

  15. How to minimise deflection ? • Increase truss dimensions = most effective • Use stiffer material e.g. steel • PRE-camber the truss

  16. Example of PRE-camber the truss

  17. When is deflection important ? • When rigging a fixed LED screen. • Personal feeling > a floor which deflects a lot “feels” unsafe

  18. Suspending load from a truss system. • Heavy loading should always be attached near or to or at the nodepoints • Small loadings up to 100 Kg can be attached anywhere on the truss.

  19. Possible mistakes of a Truss. • Parallel running braces • Miss nodding

  20. Miss Nodding

  21. Analising loading of truss structures. • The type of structure determines the allowable loading. • Loading tables are only valid for simply supported spans. • Other types of constructions or structures have to be calculated in a structural report.

  22. Simply supported span: • Truss on two hoists, • truss on two towers

  23. Continious span: • Truss on three or more hoists, • truss on three or more towers

  24. Span: • Free span is the total length between 2 suspension points Loading capacity: • The ability of the truss to withstand a load within its tolerances

  25. Reaction force on suspension points

  26. Situation 1:Spann of 20m loaded with 2000 Kg UDL ( including selfweight of the truss.) The spann is suspended on 2 chainhoist. Eache suspention point will be loaded by 1000 Kg, a 1 ton motor can be used.

  27. Situation 2:Spann of 20m loaded with 2000 Kg UDL ( including selfweight of the truss.)the spann is suspended on 3 chainhoist. The 2 outer suspension poins will be loaded with 380 Kg, and the middle suspention point will be loaded with 1240 Kg, the middle motor now needs to be a 2 ton chain hoist.

  28. Know your load: • Know the suspended loads from your Truss • Know the reaction forces on the suspension points • Make upfront a rigging plot of the loading setup

  29. Know your load: • Know the suspended loads from your Truss • Know the reaction forces on the suspension points • Make upfront a rigging plot of the loading setup

  30. Text underneath the loading table

  31. TRUSS STANDARDS Design standards • DIN 4113 German standard • VPLT SR 1.0 German “Code of practice” • BS 8118 British standard • BS 7906-1 / BS 7905-2 British standard for design, manufacture and use • Euro-code 9 European standard (not enforced yet)

  32. TRUSS STANDARDS Standards for temporary structures Design standards • DIN 4112 German standard • BS Temporary Demountable structures British “code of practice “ • EN 13814 European standard for temporary demountable structures

  33. TECHNICALTRAINING Questions?

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