savva
Well-known member
Anyone got any examples on how to do an overturning cal
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
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.
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.