# How to increase allowable bending moment ?

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

Bending moment caused by Wind force

Fh= 1,3 x wind Pressure * Wind surface Fmembrane tension = Fh / 0,8

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

Transversal force ( Also known as shear force )

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.

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

NORMAL FORCE

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

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

TORSION FORCE

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

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

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

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

Example of PRE-camber the truss

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

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.

Possible mistakes of a Truss.
Parallel running braces Miss nodding

Miss Nodding

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.

Simply supported span:
Truss on two hoists, truss on two towers

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

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

Reaction force on suspension points

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.

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.

Know the reaction forces on the suspension points Make upfront a rigging plot of the loading setup

Know the reaction forces on the suspension points Make upfront a rigging plot of the loading setup

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

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

TECHNICAL TRAINING Questions?