In addition to the requirements for the beam to safely carry the intended design loads, there are other factors that have to be considered including assessing the likely deflection of the beam under load. If beams deflect excessively, then this can cause visual distress to the users of the crane and can lead to failiure.
Beam design is carried out according to principles set out in Codes of Practice. Typically, the maximum deflection is limited to the beam’s span length divided by 250. Hence, a 5m span beam can deflect as much as 20mm without adverse effect. Thus, in many situations it is necessary to calculate, using numerical methods, the actual beam deflection under the anticipated design load and compare this figure with the allowable value to see if the chosen beam section is adequate.
So using the ratio method and this principle the allowable deflection in the beam would be:
5:0.02=2.3:x
5x=0.02*2.3
X=(0.02*2.3)/5
X=9.2*10^-3m
Earlier calculations indicated that the beam was going to deflect only about 4.2*10^-3 which shows that the beam will withstand the required deflection and the material used is appropriate.
For something like a crane, beam deflection in itself is not going to be an issue - we are not talking about mm precise lifting. Stress will be the thing which kills the crane normally by simple bending or buckling. The use of deflection is however a useful field indicator of the stress experienced by a beam - it is easy to approximately measure deflection, difficult to measure stress. Designing a product from the outset should always entail stress calcs.
ReplyDelete