Adding holes to plates for added strength

[EspElement]

Hello guys, im still relitivly new in the engineering world and im just getting started with some of my classes however i got a few quick questions for ya guys

At the gym i see excerise equipment that has what appears to be 1/2" plate supporting the seats and they have a series of different sized holes in a line on it.. im not sure if it was for aesthetics or if it was also some added structure.. i was told before that some tubing is stronger then sq bar because of the added walls.. im not to sure about that statement because it appears when i do some standard beam calcs out of the mechinary's handbook the inertia of solid bars seem to be larger

i was just curious about this, i know how triangles strenthen just about anything you do, but circles are another very rigid structure too right? simular to what an arch would do?

 

[cvg]

reduces the weight and looks nice. doesn't add strength. May also allow the seat to be adjusted?

 

[EspElement]

nope, doesnt do anything for adusting... i suppose it takes some weight out... must be more for aesthetics like you said,.. so the tube theory that my boss tells me is not true either right? (he doesnt have an engineering degree, so i can see why he would say such things.. LOL)

 

[asixth]

Could be for aesthetics, if the gym equipment manufacturer is going for a nice modern design to make their equipment look flash (better sales).

It is not uncommon to see beams being modified as you have noticed. For example, castellated steel beams will often deepen the beam cross-section and increase the members stiffness (I value), while making a reduction of the shear capacity because the web has been modified. If the steel beam is governed by deflection checks and has plenty of capacity in shear than castellated beams will generally being a viable option (fabrication costs permitting).

Another example is precast prestressed concrete beams. These beams may be rectangular cross sections with polystyrene void formers cast in the center of the cross section. Because the units are prestressed they will have a higher shear capacity (compressive stresses) so a reduced shear area may be viable, but importantly they increase the I:A ratio, so the beam will deflect less under self weight.

 

[EspElement]

Hmm thats interesting.. so your staying in terms of sizes, they might us a 12" beam apposed to a 10" beam and take wieght out of it but still makes it just as stong as the original 10" beam but its self weight is lighter?

 

[asixth]

It may make it stronger for flexure (increased I value) but they also have some con's which may make them undesirable.

http://www.grunbauer.nl/eng/waarom.htm

 

[BAretired]

EspElement,

It is better to consider the question on the basis of what makes sense, rather than what you were told. The inertia of a solid bar is greater than that of a hollow tube with the same exterior dimensions. For the same yield strength, the solid bar is stronger. The strength of the tube is equivalent to the strength of the solid bar less the portion removed.

Perhaps what you were told is that the strength per unit weight is greater for the tube than for the solid bar. I would agree with that.

BA

 

[GregLocock]

If you look at aeroplanes which are designed by engineers (if not PEs) that care about weight, you'll often see spars with lightening holes in them. The reason is that for manufacturing reasons the thickness of the web cannot be too much less than the that of the flange, yet the main purpose of the web is to react shear forces, for which it is rather too thick.

So we drill big holes in the web. The remaining material is enough to react the shear forces.

However in the case of gym gear I suspect aesthetics has far more to do with it.



Cheers

Greg Locock

I rarely exceed 1.79 x 10^12 furlongs per fortnight

 

[EspElement]

I appreciate your guys posts.. You are right it never made since to me, but he always explained it that way.. He definitly was mistaken cause it wasnt a per weight strength he preached.. Your information i do agree with.. I do beam eqations alot, which involve the profiles inertia and section modulus to do calculations, how would i calculate for holes in various places along the Z axis per the unit length? (such as holes in the middle of the I beam's webbing) I use the machinary's handbook here at work.. I just recently bought some new books one of which is called "Engineering Formulas" which i hope to get a lot more information on this sort of thing in it.

 

[Bobber1]

Remember those old hot rods with the I-beam axles drilled full of holes? They told me it was to reduce wind resistance. Maybe they drill the members of the equipment to promote better air circulation in the room?

 

[EspElement]

Hmm, really? you would think that it cause more drag.. at least if you were blowing down the profile (z axis).. as they said im sure thats more for weight reduction on hot rods

 

[Ron]

Tubes are better than solids for torsion, not flexure...that's one reason drive shafts are tubular, not solid. As for the removal of material in your application, I suspect it is for aesthetics and no other good reason.

 

[msquared48]

The only time I can see to add holes to any structural member to increase the relative strength of the member (weight reduction not considered) would be to mitigate possible brittle fracture conditions at certain critical locations.

I could see, however, holes being added to alter the flexibility or stiffness for reasons of ductility.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[sandman21]

http://garagemonkey.net/assets/images/Fabrication/Dimple Dies -1.jpg

Dimple dies used a lot in auto applications, reduce weight and increase I.

 

[EspElement]

Sandman, when you say increase I.. ur meaning increase inertia?

intersting.. tubes are stronger in torque applications? why would that be? does it allow for more radial deflection before failing?

 

[apsix]

tubes are stronger in torque applications....if you are comparing weight, but not if you are comparing size.

 

[EspElement]

Oh, so again.. if the tube has the same external dimensions its not as strong as a shaft.. now that makes sense,. simular to what was stated above about sq tubing