Hope there isn't a thread on this topic—I haven't managed to find one through searching...
I have an idea for a construction where I want/need to reduce the weight of a beam-like device. (I understand it sounds mysterious, but right now I want to say no more than necessary.) It's a home project in wood and I want to liken it to a wooden beam where I want to reduce the weight by using a hole saw to reduce the amount of material in the beam, without significantly affecting the load-bearing capacity.
Is it possible and is there any formula regarding hole size and "placement" that can be used generally?
Big thanks for all input!
//Ronnie
I have an idea for a construction where I want/need to reduce the weight of a beam-like device. (I understand it sounds mysterious, but right now I want to say no more than necessary.) It's a home project in wood and I want to liken it to a wooden beam where I want to reduce the weight by using a hole saw to reduce the amount of material in the beam, without significantly affecting the load-bearing capacity.
Is it possible and is there any formula regarding hole size and "placement" that can be used generally?
Big thanks for all input!
//Ronnie
Tool enthusiast
· Stockholm
· 1 399 posts
Kerto-balk is otherwise a good alternative when it comes to weight vs strength.
Hehe, sounds like a "perpetual motion machine question" 
the short answer is of course that you affect the load-bearing capacity with a hole. Often, a hole impacts the stress field by a factor of 2-3, meaning the stresses around the hole become 3 times as high if you have a large homogeneous surface. This applies to steel. For wood, it is likely worse because you cut through a lot of fibers when making the hole. If you can make a sketch with the assumed loads and support points for the beam, we might be able to figure something out. But with the scant information you've given us, it becomes difficult.
Use a steel beam...
the short answer is of course that you affect the load-bearing capacity with a hole. Often, a hole impacts the stress field by a factor of 2-3, meaning the stresses around the hole become 3 times as high if you have a large homogeneous surface. This applies to steel. For wood, it is likely worse because you cut through a lot of fibers when making the hole. If you can make a sketch with the assumed loads and support points for the beam, we might be able to figure something out. But with the scant information you've given us, it becomes difficult. Use a steel beam...
If Kerto doesn't fit, there are more options.
HB beam is a real classic from the times when everything could be built with simple means. Essentially an I-beam made of wood. In principle, it should be possible to build a modern HB beam with a web of plywood.
There are now also different types of lightweight beams with a web of masonite and flanges of solid wood. They likely provide the most bending stiffness in relation to weight.
Then there are glued truss beams that are normally used as girder backs for arch forms. They are not approved for permanent constructions, but if you need a portable, extremely lightweight construction, it could be an option.
There are calculation models for the strength of wooden beams with holes in the middle. The big problem is that the holes reduce the beam's shear strength. If we imagine a simply supported beam with somewhat evenly distributed load, the greatest bending stress is in the middle, and the greatest shear force is near the ends. It is thus only along part of the length where it is suitable to make holes. When a beam fails in shear, it starts to function like a leaf spring pack or a bundle of boards and thus has poor load-bearing capacity.
HB beam is a real classic from the times when everything could be built with simple means. Essentially an I-beam made of wood. In principle, it should be possible to build a modern HB beam with a web of plywood.
There are now also different types of lightweight beams with a web of masonite and flanges of solid wood. They likely provide the most bending stiffness in relation to weight.
Then there are glued truss beams that are normally used as girder backs for arch forms. They are not approved for permanent constructions, but if you need a portable, extremely lightweight construction, it could be an option.
There are calculation models for the strength of wooden beams with holes in the middle. The big problem is that the holes reduce the beam's shear strength. If we imagine a simply supported beam with somewhat evenly distributed load, the greatest bending stress is in the middle, and the greatest shear force is near the ends. It is thus only along part of the length where it is suitable to make holes. When a beam fails in shear, it starts to function like a leaf spring pack or a bundle of boards and thus has poor load-bearing capacity.
I think it might be possible to solve with a truss of some kind. However, it will require great precision if space is limited.
Thank you so much for the input!
I realize it might be more problematic than I thought.
It's not about a beam/girder supported at each end. For that reason, Kertobalk, I-beams, and the like are out.
We've all been to the dentist where there's a big lamp on an articulated boom/arm/whatever suspended from the ceiling. Welding fume extractors are a similar construction. Festool’s contraption for holding up the hose is a primitive analogy.
I want to create something similar in wood and keep the weight down. When I'm done, I'll post the result on Byggahus.
The question is, can I reduce the material without losing a lot of load-bearing capacity? Simply put — how much. Can I drill out a plank that stands on edge without it losing its primary load-bearing capacity? Is there a formula to use?
Thanks for the response for now!
//Ronnie
I realize it might be more problematic than I thought.
It's not about a beam/girder supported at each end. For that reason, Kertobalk, I-beams, and the like are out.
We've all been to the dentist where there's a big lamp on an articulated boom/arm/whatever suspended from the ceiling. Welding fume extractors are a similar construction. Festool’s contraption for holding up the hose is a primitive analogy.
I want to create something similar in wood and keep the weight down. When I'm done, I'll post the result on Byggahus.
The question is, can I reduce the material without losing a lot of load-bearing capacity? Simply put — how much. Can I drill out a plank that stands on edge without it losing its primary load-bearing capacity? Is there a formula to use?
Thanks for the response for now!
//Ronnie
Tool enthusiast
· Stockholm
· 1 399 posts
If you want the construction to be in wood, you can build it in, for example, björkplyfa, a boxed construction that you can then hole perforate.
Yes, that's what I'm thinking. I'm considering a sandwich construction with ply/eps/ply/eps/ply. A simple prototype indicates a torsional rigidity that is quite good. Now I want to reduce the weight and wonder how much plywood I can remove by using a hole saw.
Is there a rule of thumb/formula?
//Ronnie
Is there a rule of thumb/formula?
//Ronnie
Completely unscientifically, I would suggest elongated holes, like these. According to my gut feeling. https://www.google.se/search?q=vatt...UIBygB&biw=1280&bih=800#imgrc=e3StSKpA35Gi7M:
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