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null point for bridge deck


the null point is the point of zero movements. it is important to calculate the force in piers due to the change in bridge deck length. also, it is important to calculate the sliding movement for guided and free bearing. if the bridge deck is pinned at one pier location. then this the null point.
change in length for bridge deck due to temperature can be calculated using this equation


where 
ΔTG is the change in bridge deck length due to temparture change
L is the length of the bridge deck from fixed or null point to the point of calculation
α is the coefficient of thermal expansion for concrete
ΔT is the temperature change

This an example of the change in bridge deck due to temperature. creep,  shrinkage, elastic shortening due to prestressing and other factors can cause a change in deck length. this all will be calculated to estimate the change in deck length and the forces imposed due to these changes.






Figure 1


if the bridge fixed at more piers then the null point location should be determined by calculation. for the symmetrical bridge with two fixed piers. the null point will be at the midpoint between these two piers. if one pier is higher than another. the null point will be closer to the short pier.



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