Steel Designers Manual - TheBestFriend.org

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Steel Designers' Manual - 6th Edition (2003)
858 Bearings and joints
bearings on piers 2 to 5. At each pier transversely lateral loads are taken by ‘C’
line bearings. Friction forces due to transverse expansion, etc. will tend to cancel
one another out.
(3) The arrangement in Fig. 28.9(c) is better for longitudinal effects than that in Fig.
28.9(b) as friction forces in this direction tend to cancel one another out. It has
the disadvantage that external loads are transmitted to an intermediate pier
rather than an abutment. The forces due to movement of the deck are minimized
in both horizontal directions. Occasionally, the line of fixed or guided
bearings with both horizontal and vertical capability such as at ‘C’ may be
replaced by two lines, one with bearings with vertical capability only, and one
with bearings with horizontal capability only.
28.2.4.3 Structures curved in plan
If the structure shown in Fig. 28.9(b) is curved in plan, then any expansion or contraction
movements longitudinally are accompanied by lateral movements also.This
effect can be controlled in two ways:
(1) set the bearing guides to permit radial expansion from a fixed point on the
structure,
(2) set the bearing guides tangential to the plan curvature, and so constrain the
structure to follow this line when it moves (see Fig. 28.10).
In radially-guided structures the accuracy of setting out and alignment becomes
more critical as the distance from the fixed point increases. In tangentially-guided
structures, the structure is constrained to move along a particular path, and the
horizontal forces developed in so doing must be taken into account in the design of
the structure and supports.
It should be noted that frequently bearings which are nominally ‘guided’ are
manufactured with a gap tolerance at the guides. The actual value of this tolerance
should be checked with the manufacturer of the particular bearing, but a value of
0.5 mm is typical.This tolerance can have a significant effect on the permissible accu-
fixed
(a) (b)
Fig. 28.10 Curved bridge deck: (a) radially-guided, (b) tangentially-guided