Slip joint

A slip joint is a mechanical construction allowing extension and compression in a linear structure.

Slip joints in large structures are used to allow independent motion of large components while enabling them to be joined in some way. For example, if two tall buildings are to be joined with a pedestrian skyway at some high level, there are two options in structural engineering. If the buildings are identical in mass and elasticity they will tend to respond similarly to ground motion induced by earthquakes. In this case it may be appropriately to construct a rigid connection between the buildings, although this may require additional supporting members within the structures. On the other hand, a lower cost connection may be made by using a lightweight structure that is not coupled rigidly but instead which is allowed to slide or "float" relative to one or both structures. This is especially suitable where the two structures may respond differently to ground motion. The structure will not be completely free to move but rather may use elastic materials to locate it near the center of its range of motion and viscous shock absorbers to absorb energy and to restrict the speed of relative motion. When a sliding connection is used it is extremely important that there be sufficient range of motion without failure to accommodate the maximum credible relative motion of the structures. Additional "fail safe" flexible connections may be added to ensure that the structure does not fall, although it may be damaged to a point of unusability or irreparability.

Slip joints are sometimes found in tubular structures such as piping, but are generally avoided for this application due to requirements for sealing against leakage, instead using either a large loop or a semi-rigid bellows. An exception is the use of slip joints in underwater transport tubes that are buried or tunneled through mud in seismically active regions. One such slip joint is used at the San Francisco end of the trans-bay BART tube. In an unfortunate engineering underestimation the joint was not made sufficiently long to survive the maximum credible motion of the components, and so could fail catastrophically in a large earthquake. The correction of this oversight will require an extensive (and expensive) seismic retrofit. Owing to the related cost overruns on replacement of the eastern span of the San Francisco-Oakland Bay Bridge, combined with the state of California's fiscal crisis, Governor Arnold Schwarzenegger has proposed taking money from the BART retrofit to construct the eastern replacement span of the Bay Bridge, rather than either propose raising tolls or the contribution of state funding. State legislators from the Bay Area are likely to be able to block this proposal, which would essentially rob one public safety project to pay for another.