I am looking for a reference or a procedure to determine the allowable bending stress for laterally unbrased steel tubes and pipis sections. (particularly for Kl/r's up to 200).
I am in the US. I seem to remembered reading that the plactic moment will be reached before any buckling occurs in closed sections such as tubes and pipe, thus no addition reduction in the allowable bending stess would be required. Any thoughts?
You may find the specific procedure in LRFD appendix F where it is seen the unbraced length does not count. A circular pipe simply can't have what is called lateral buckling, lateral kink only develops as a consequence of local buckling; hence the determining factor is the local slenderness Diameter to thickness ratio that rules local buckling.
Whenever the Diameter to the thickness ratio is lesser than
2070/Fy
where Fy is to the number of ksi of yield strength of the steel, you can use the plastic capacity of the ring affected by the material strength reduction factor as the limit strength to compare with moments coming from factored forces of LRFD.
ASD for allowable stress design (service level design) is even simpler; if D/t is lesser than 3300/Fy, use 0.66·Fy as the Allowable strength to be compared with forces coming from the service level loadcases mandated in ASD.
