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Structural
- Jan 8, 2018
- 197
Hi,
Does anyone have experience calculating the length of plastic hinges in steelwork? I have a cantilevered 168.3x8CHS bollard / post, approx. 500mm high (L = 500), which is acting a bumper / bollard to prevent a small forklift truck impacting existing pipework. The design philosophy is that if the total amount of energy absorbed by the bollard deflecting elastically + plastically is less than the impact energy of the forklift truck (at a specific velocity), then the bollard stops the truck (with some significant damage being observed).
I have calculated the length of the hinge as approx. 126mm, using similar triangles and the moment diagram. E.g. There is full plastic moment at the root of the cantilever, and at some distance 'up' the cantilever the moment is at the elastic limit. Using similar triangles, this would give the hinge length (e.g. distance from plastic moment to elastic moment) as: Lhinge = L - (Z / (S / L)) = 126mm, or about 1/4 the length of the cantilever post.
**Note Z and S above are the UK definitions - e.g. Z = elastic, S = plastic - yeah, we do it the other way around![[ponder]](/data/assets/smilies/ponder.gif "[ponder] [ponder]")
**
The hinge length is then multiplied by the maximum rupture strain for steel (0.15 in this case for S355 steel) to give an elongation of the member due to the hinge: E = Lhinge * 0.15 = 19mm.
This is then all used with standard deflection equations etc. to calculate the energy absorbed due to deflection.
I think all of the above is correct, but what I find striking is the length of the hinge - about 25% of the post length is fully plastic - enough that I see it as a possible 'red flag' and to ask for some other opinions? Does that hinge length sound reasonable?
**Note I still need to check the section class etc. to ensure it does not buckle before reaching the plastic moment, I've seen a note on this in AISC and need to follow up on it, just haven't done so yet**
Thanks.
Does anyone have experience calculating the length of plastic hinges in steelwork? I have a cantilevered 168.3x8CHS bollard / post, approx. 500mm high (L = 500), which is acting a bumper / bollard to prevent a small forklift truck impacting existing pipework. The design philosophy is that if the total amount of energy absorbed by the bollard deflecting elastically + plastically is less than the impact energy of the forklift truck (at a specific velocity), then the bollard stops the truck (with some significant damage being observed).
I have calculated the length of the hinge as approx. 126mm, using similar triangles and the moment diagram. E.g. There is full plastic moment at the root of the cantilever, and at some distance 'up' the cantilever the moment is at the elastic limit. Using similar triangles, this would give the hinge length (e.g. distance from plastic moment to elastic moment) as: Lhinge = L - (Z / (S / L)) = 126mm, or about 1/4 the length of the cantilever post.
**Note Z and S above are the UK definitions - e.g. Z = elastic, S = plastic - yeah, we do it the other way around
**The hinge length is then multiplied by the maximum rupture strain for steel (0.15 in this case for S355 steel) to give an elongation of the member due to the hinge: E = Lhinge * 0.15 = 19mm.
This is then all used with standard deflection equations etc. to calculate the energy absorbed due to deflection.
I think all of the above is correct, but what I find striking is the length of the hinge - about 25% of the post length is fully plastic - enough that I see it as a possible 'red flag' and to ask for some other opinions? Does that hinge length sound reasonable?
**Note I still need to check the section class etc. to ensure it does not buckle before reaching the plastic moment, I've seen a note on this in AISC and need to follow up on it, just haven't done so yet**
Thanks.
