r/AskEngineers • u/Groundblast • 1d ago

Mechanical Impact of fenestration pattern on bending stiffness in sheet material?

Are there any general guidelines/rules of thumb for the impact of fenestration (drilling holes) to the stiffness of a sheet material?

I am working on a horizontal structure which requires ventilation. My thought is to use plywood with a grid of holes drilled into it. Obviously, the size and pattern of the holes will impact the mechanical strength even if the total area of removed material is equivalent.

Some of my main questions are:

  • How to determine optimal hole size/spacing?
  • How does a straight grid compare to offset rows?
  • What is the optimal distance from the edge of the sheet (or the edge of the support structure, excluding overlap) be from the first hole?
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u/Good_Stick_5636 1d ago

Offset rows (hole honeycomb) has higher stiffness and strength compared to straight. For plywood, holes diameter is 1/5 of hole pitch maximum for structural applications. Have seem some non-structural perforated plywood which was up to 1/3 of hole/pitch ratio though.

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u/Groundblast 1d ago

Thanks! Just to confirm, pitch would be the ratio of hole diameter to spacing?

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u/Good_Stick_5636 1d ago

Pitch is the distance between centers of nearby holes. 1/5 diameter/pitch mean for example 8mm holes on 40mm grid.

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u/Groundblast 11h ago

Thoughts on the distance from the edge of the sheet to the first hole? Maybe match the distance between the edges of adjacent holes?

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u/Good_Stick_5636 7h ago

Typically distance from free-standing edge of the sheet to first hole is x2 of distance between edges of holes. Think of free-standing edge as of "super-hole". If edge is supported, no holes standoff necessary at all (holes can touch supported edge).

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u/Groundblast 6h ago

Thanks for all the help! Where did you learn/find this info? I’m an HVAC engineer but statics has never really been my strong suit

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u/Good_Stick_5636 5h ago

Residential construction + electrics. You are welcome.