over turning calc (1 Viewer)

Hi Savva,

The basics behind overturning moments are relatively simple, it is basically the horizontal load at a lift (for example from wind) x the distance to the pivot point (usually the ground). For example, say 2kN of lateral load acts at a node position at 2.000m from ground, this gives an overturning moment of 2 x 2 x 1.5FOS = 6kNm. Noting, the 1.5FOS is included in almost all OTM calcs.

The resistance moment is then taken from the OTM. This is calculated by taking the self weight of the scaffold alone at each leg (no super) and multiplying the dead load by the distance at which the self weight is resisting overturning. For example, say you have 2kN of self weight on the inside leg which acts at 1.200m (a standard scaffold width) then the resistance moment would = 2.4kNm.

To work out the kentledge you would take the (OTM - RESM) / Lever Arm. The lever arm is usually the distance from c/c of the uprights outside to outside or c/c of the kentledge (can vary depending on the circumstances and how the system is designed). Let's say c/c of the uprights. This would give a required kentledge value of (6 - 2.4) / 1.2 = 3kN.

This would mean a 2 lift high scaffold x 1.2m wide would need 3kN on the inside standard and 3kN on the soutside. This is a very crude example but hopefully broadly illustrates the method.

R
 
would this basic cal work for overtunring moments ?-
1/2 the smallest base dimension x the self weight and then divided by the height ?

would this tell you what the tipping point of the tower would be and freestanding scaffol?
 
Overturning moment is;

(Force in kN x height in m) divide by two

gives overturning in KNm then times by 1.25 or 1.5 for a factor of safety.

Working out the righting moment is

Self weight inside standard in kN x distance from pivot point in m

added together with the similar for each standard.

usually done for four standards and coming nowhere near the overturning moment.

Which is a difficult one to explain to clients when you say you need a tonne on the inside and a tonne on the outside to keep the structure up.
 
Hi Savva,

The basics behind overturning moments are relatively simple, it is basically the horizontal load at a lift (for example from wind) x the distance to the pivot point (usually the ground). For example, say 2kN of lateral load acts at a node position at 2.000m from ground, this gives an overturning moment of 2 x 2 x 1.5FOS = 6kNm. Noting, the 1.5FOS is included in almost all OTM calcs.

The resistance moment is then taken from the OTM. This is calculated by taking the self weight of the scaffold alone at each leg (no super) and multiplying the dead load by the distance at which the self weight is resisting overturning. For example, say you have 2kN of self weight on the inside leg which acts at 1.200m (a standard scaffold width) then the resistance moment would = 2.4kNm.

To work out the kentledge you would take the (OTM - RESM) / Lever Arm. The lever arm is usually the distance from c/c of the uprights outside to outside or c/c of the kentledge (can vary depending on the circumstances and how the system is designed). Let's say c/c of the uprights. This would give a required kentledge value of (6 - 2.4) / 1.2 = 3kN.

This would mean a 2 lift high scaffold x 1.2m wide would need 3kN on the inside standard and 3kN on the soutside. This is a very crude example but hopefully broadly illustrates the method.

R


I Knew that ;)
 
Overturning moment is;

(Force in kN x height in m) divide by two

In addition to AshReactive suggestion above that the force x height (lever arm) is divided by 2, it should be noted that this is assuming that the applied load is a UDL, which it normally would be for wind loads. However there may be instances where the applied force, which could cause overturning, is not a UDL and a such needs to be reviewed slightly differently.

Generally when determining the overturning moment (Mot) the structure should be considered to be a cantilever beam where Mot is equal to the maximum moment in a cantilever beam,

i.e Mot= Force (kN) x lever arm (m).

In addition a minimum factor of safety of 1.5 should also be applied.

Below is a link to a very useful document which has standard formulae for determining max moment & max shear for various beam configurations / load types. As mentioned above, for overturning a cantilever should be considered (Fig 12, 13 & 14 in the document).

http://www.awc.org/pdf/DA6-BeamFormulas.pdf
 
But they're wooden beams.

The formulae are correct regardless of the material, whether the beams are timber, steel, concrete, aluminium, platinum.
These formulae are also published in the Steel designers manual (6th Edition pages 1077-1101)

The formula used to determine the Max Moment in a simply supported beam
M= force (kN) x Lever arm (m). Nowhere in this equation does it ask what material is being used.

Only once you have worked out the moments, shear, deflection etc. do you then consider the material properties, (i.e. for steel; grade of steel, yield stress, section shape / size) to determine the beam size required.
 
Only once you have worked out the moments, shear, deflection etc. do you then consider the material properties, (i.e. for steel; grade of steel, yield stress, section shape / size) to determine the beam size required.

Just in case this gets quoted, it is worth pointing out that you can't work out deflection without first knowing both the type and grade of material and the section size of the material. In some cases (like scaffold beams) you also need to know how the section is built up so that you can include something known as shear stiffness in the deflection calculation.
 
Just in case this gets quoted, it is worth pointing out that you can't work out deflection without first knowing both the type and grade of material and the section size of the material. In some cases (like scaffold beams) you also need to know how the section is built up so that you can include something known as shear stiffness in the deflection calculation.

TG6 is correct (thanks TG6); deflection is assessed after a member & material has been selected and not prior;

Sorry people for that basic error.:embarrest:
 
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