Effective length of piers/columns and bearings

[Catjacob]

I found from some bridge design codes assume the effective length of a column to be 0.85 (say) x physical length if the bottom end is fixed against horiz movement and rotation and the top is restrained from any horiz movement but not the rotation (i.e. a pinned joint). However, I don’t understand how a bridge deck doesn’t experience any horiz movement or translation that both bearings are on the “same vertical line” at all times. I am just thinking whether a, say 30mm, thermal expansion and contraction of a concrete deck has to be considered as a “translation”??

 

[KnarfMan]

Bridges normally are constructed with expansion joints along the deck (i.e., strip seal, compression seal, tooth dam). The sizing of the joints is generally based on thermal movement.

In turn, the deck is supported on beams which sit on bearings (normally classified as fixed or expansion). Generally, fixed bearings prevent translation in any direction, and expansion bearings permit translation longitudinally, but are fixed against lateral translation. All bearings must be capable of permitting rotation in at least one direction.

However, these simple guidelines do not apply for every bridge. The most common (i.e., least expensive) bearings are steel-laminated neoprene (a.k.a elastomeric bearings). But I have also designed "metal" and "pot" bearings for bridges.

The use of all of these items are dependent on site-specific parameters such as movement-classification, end-rotation, skew, fixity, span, bridge-type, etc. From a design standpoint, the proper combination of joints & bearings is used to limit superstructure thermal forces acting on the substucture (i.e., abutments/piers).