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Tips & Tricks - Designing for Impact Loads (Round 2)

Energy absorption is the key to designing beams for impact strength. To increase energy absorption, choose sections which allow the maximum stress to be carried along the entire length of the beam, and maximise the volume of material under maximum stress.

variable_beam_164x30This was illustrated in the previous impact design tip - if you want to refresh your memory click here or visit our Tips & Tricks page.

Here are some rules of thumb for common scenarios:

  • In a simple tensile bar with a uniform cross section, increasing the length (L) will not alter the static stress yet it will decrease the stress due to impact.
  • In a simple horizontal beam, a decrease in length will decrease the static stress, but will increase the stress due to impact.
  • Place material so that the direction of hot rolling (for sheet or bar) is in line with the impact force. This will give higher impact strength that if impact occurs at right angles to the direction of rolling.
  • Increasing the section depth will increase the impact strength - however note the discussion below.

From the table below, the static strength is governed by I/c (where I = Second Moment of Area, and c = distance from the neutral axis to the outer fiber). Impact strength is governed by I/c2. For a rectangular beam, doubling the depth will double the impact load strength. However, the beam is well over-sized for static loads, heavier and more expensive, so keep this in mind when designing for impact.

Static and Impact Strength Comparison
Rectangular BeamDoubled Beam DepthFactor of Improvement
Bending FactorIbd3/12b(2d)3/128x
Static Load FactorI/cbd2/6b(2d)2/64x
Impact Load FactorI/c2bd/3b(2d)/32x
Sourced from Design of Weldments by Omer Blodgett


Other factors to consider are the material properties (ductility, notch toughness and yield strength) and supporting structure. Flexible supports are often used in earthquake design to absorb energy and reduce the loading on the structure.

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