MadSci Network: Engineering

Re: Why is a hollow steel member harder to bend than a solid one?

Date: Wed Oct 18 18:36:56 2000
Posted By: Bob Gibilisco, Staff, Process Engineering, Retired
Area of science: Engineering
ID: 971372370.Eg


Your question is a good one and it very much misunderstood. Oddly enough, I 
cannot locate any information to explain the bending mechanics specific to 
hollow and solid bodies. So, we need to apply the rules of mechanics to 
understand the mechanical forces involved.

To directly answer your question, we need to visualize a 2" round solid 
steel bar and a 2" O.D. steel tube both clamped to a support on one end. For 
this example, both members are 4 long and we will use a 1/32" wall for the 
2" tube. Now suspend a 1 lb. weight on the unsupported ends of each member. 
Chances are neither member will bend much at all. 
Lets start to increase the weights in 10 lb. increments. As the weight 
increases the solid bar will start to bend before the tube because the top 
surface of the tube is in tension and the bottom surface is in compression, 
thus opposing stresses. The stresses are the same for the solid bar however 
the stresses are spread out over much more material so the solid bar will 
deflect more than the tube at the lower weight range. Increase the weight to 
500 lb. and the solid bar will bend more but guess what! I would not want to 
be standing beneath the tube under this amount of weight. As the weight 
increases, the tube will start to bend but since there is less material, it 
will reach its ultimate yield strength, progress into the region of plastic 
deformation and finally catastrophically fail much sooner than the solid 
 In final analysis, the resistance to bending prior to failure depends upon 
the mass of the member and the weight applied.
Please note the example above is just that, a paper example. The dimensions 
and weights are theoretical and must be proven. However the concept is 
correct. If it were not, we would be out there drilling a lot of holes 
through a lot of construction members.
I hope this gives you better insight on the workings of solid and hollow 
bodies. If you decide to conduct an experiment, I would appreciate you 
letting me know the results. 

Thank You. 

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