dnv c202 : doubt in eqn(hoop)

[civil2013]

Hi,
Eqn 3.2.6 of dnv rp c202 states that the σho sd need not be taken if its positive (acting outwards of cylinder) while calculating the combined stress .. does this mean that the hoop pressure, if acting outward is not contributing to buckling ? please guide .screenshot attached frm code.
thanks,

 

[fegenbush]

The meaning here is that the failure mode of buckling does not apply when there is a net *internal* pressure acting on the cylinder. Buckling only applies when there is a net *external* pressure acting on the cylinder.

To illustrate the meaning, it is not possible for a flexible member such as a rod to buckle while it is in net tension. Buckling would only be a concern when you apply a compressive force.

 

[civil2013]

hi, thanks for the clarification, i do understand that buckling is not required for tensile loading on the cylinder.. but here, the loading with which i am concerned is internal/external pressure on the wall of cylinder.. how can we neglect the pressure on the walls while checking buckling - the eqn is such that only external pressure on the wall is considered active for buckling check.... will the internal pressure not help in initiating buckling ?? - correct me if my understanding is wrong

 

[dhengr]

Kr84:
From the combined stress standpoint the hoop stress caused by the positive internal pressure must be considered in arriving at the max. combined stresses. But, this same internal pressure hoop action tends to stabilize the cylinder from the buckling standpoint, it inhibits buckling rather than inducing buckling. A low internal pressure at the same time there is a high inward external radial loading would be a condition which would tend to induce shell buckling, as would a high internal vacuum. The hoop action and stresses are now in compression, and tend to induce shell buckling. Try this experiment: you can stand on a full/capped aluminum soda pop can, and it will not buckle or crush; but once you release the pressure in the can, this same exercise will allow the can to buckle or crush.

 

[civil2013]

@dhengr:
Thanks for the explanation, i was travelling and couldnt check the response... your example is convincing, but the case would be similar if i bury the soda can in sand and pump air into the can, the can wont buldge/buckle outwards as the sand outside the can is exerting pressure on the walls, but what happens if i dont have sand around and just pump in the air - it would buckle at some point - please correct me if i am wrong in counter stating.so whether the internal pressure or the ext pressure is relevant depends on the nature of loading/the boundary conditions. appreciate your response

 

[fegenbush]

In your scenario, the can would not buckle, it would rupture. Rupture is a tensile failure, buckling is a compressive failure.