Expansion joints


Expansion joints must accommodate movements produced by concrete shrinkage and creep, post-tensioning shortening, thermal variations, dead and live loads, wind and seismic loads, and structure settlements. Concrete shrinkage, post-tensioning shortening, and thermal variations are generally taken into account explicitly in design calculations. Because of uncertainties in predicting, and the increased costs associated with accommodating large displacements, seismic movements are usually not explicitly included in calculations.

Expansion joints should be designed to accommodate all shrinkage occurring after their installation. For unrestrained concrete, ultimate shrinkage strain after installation,

?, may be estimated as 0.0002. More-detailed estimations can be used which include the effect of ambient relative humidity and volume-to-surface ratios. Shrinkage shortening of the bridge deck,

? shrink, in mm,is calculated as

?shrink = (?) · (µ) · (Ltrib) · (1000 mm/m)

where
Ltrib  =  tributary length of structure subject to shrinkage; m
? = ultimate  shrinkage  strain  after  expansion  joint  installation;  estimated  as  0.0002  in  lieu  of more-re?ned calculations
µ =  factor accounting for restraining effect imposed by structural elements installed before slab is cast
= 0.0 for steel girders, 0.5 for precast pre-stressed concrete girders, 0.8 for concrete box girders and T-beams, 1.0 for ?at slabs Thermal displacements are calculated using the maximum and minimum anticipated bridge deck temperatures. These extreme values are functions of the geographic location of the structure and the bridge type. Thermal movement, in mm, is calculated as

?temp = (?) · (Ltrib) · (?T) · (1000 mm/m) (25.2)

where
? = coef?cient of thermal expansion; 0.000011 m/m/°C for concrete and 0.000012 m/m/°C for steel
Ltrib = tributary length of structure subject to thermal variation; m
?T = temperature variation; °C

Any  other  predictable  movements  following  expansion  joint  installation,  such  as  concrete  post-tensioning shortening and creep, should also be included in the design calculations.


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