True pin connections-how to maintain? 2

[IJR]

Everytime I try to use a true pin in my connection design, my advisors(usually a professor from a university) would warn me that a true frictionless pin is difficult to implement in that after a few days rust will get in the way and the "pin" action will get lost. I assume they are right but since they never told me "never use pins" I also assume there is a way to minimize this rust

What do you pals use? bushings? etc.

Thanks in advance

regards
IJR

 

[flamby]

IJR,

I agree that true pin is not possible to achieve in practice. Though one could think of several ways of minimising the friction in tedious and costly ways, I do not stand in favour of such efforts because I have experienced that wherever you give pins, you are designing a truss. If there is some friction, that is not going to give great moments, but going to relieve some axial stress which is of the critical order in trusses. I have seen many structural joints designed as pinned but in practice there are a couple of pins and the release is left to the deformation of the connected members instead.

 

[Ron]

IJR...True pinned connections are maintenance problems. If a true pinned connection were to be necessary for the viability of the structure, maintenance instruction would have to be included and you would need to use a bushing or similar to achieve the freedom to rotate.

In practicality, a connection acts like a pinned connection when there is enough flexibility to only resist rotation under large deformations. For example, a flat baseplate on a column acts as a pinned connection only when the plate is thin enough to be flexible under lateral column loads. This flexible thickness might be less than the thickness required for other design characteristics and is usually not considered... we make the baseplate thick enough to resist the moment imposed by its bending. When we do so in our design, we force the connection to assume some moment resistance capability, like it or not, but then it is not truly a pinned connection.

The error in these assumptions is not necessarily great. We design as a pinned connection and force the moments to occur (theoretically) in other parts of the member, thus we might actually be overdesigning the member, assuming that no rotation is attenuated at the connection. Similarly, we often use couples to represent some moment action so that we can assess the bolt tension or weld stresses in a connection, so, in effect, we almost always analyze some moment resistance into a connection, whether we truly model it as such in our programs or not.

Sorry for the long-winded response.

Ron

 

[IJR]

Thanks Ron, thanks flame

My problem is not in design assumptions, but is on when you elect to use a physical pin and the consequent stickiness that will arrive later.

If you find time please go on elaborate on the detailing side of this problem.

regards
IJR

 

[flamby]

IJR,
One another aspect of connections... How to details true "fixed" connections? This is equally difficult as is to provide a true pin. If a beam is connected to a column in fixed contunuity, the column flanges should also be fixed, so that no deformation of flanges takes place. If the column flanges, to which the beam is bolted/welded, is not properly stiffened, the joint will still behave more to the pinned side than to the fixed side inspite of being welded or bolted by n number of bolts by way of local deformation taking place at flages as described by Ron.

As to your worries, if the supports of the pinned-member are really un-yielding, and you fear residual moments, the computer programs allow you to simulate this condition as well. You model such connection as "fixed" and partially release the moments (say by 50% or 80%). This partial release of moments is widely practiced and I think you should consider this to match the modelling and the actual structure.

Regards,

flame

 

[Ron]

IJR...if you are in fact installing a true pin connection, you will need to provide for maintenance in your detailing. This would include a bushing that could be re-saturated with oil periodically, or a machined sleeve with a "grease fitting" (Alemite fitting) installed into a threaded hole in the side of the sleeve. This would allow someone with a portable grease gun to inject bearing grease into the sleeve periodically to keep the pin moveable.

This type of connection would typically provide full "X" and "Y" restraint, but the "Z" restraint (assuming Z is perpendicular to the plane of the truss or member)might be less than full, depending upon the type of pin and its restraining device (clevis, cotter pin, spring clip, etc.). The connection would allow full rotation in the X-Y plane and no rotation in the Z-Y or Z-X planes.