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In the construction of a two-span bridge (span length = L) by using span-by-span construction, why is a length of about 1.25L bridge segment is constructed in the first phase of construction

The using of the span-by-span construction method is increased recently because it is reducing the time of construction, also it is an economic method compared to other methods used to erect bridge precast segments. the main reason for using 1.25L for the first bridge segments in the first phase of construction is:

  1. Using L only for the first segment for the bridge will produce a large sagging moment at the center of the first span compared to the sagging moment for the two spans of the bridge after the completion of construction. therefore using of 1.25L will reduce the sagging moment at the mid of span.
  2. assuming the bridge of two-spans is uniformly loaded, then the location of the construction joint using 1.25L from the first pier is at the contraflexure point, at this point the moment is zero, therefore,  the selection of this point to be the construction joint location will have the minimal effects on the bridge structure.
  3. in the event of the prestressed bridge. the prestressing will be carried out at L which located above the pier, in this location steel will be very congested and it will increase the difficulty of performing the prestressing work. The using of 1.25L will reduce these difficulties because at 1.25L the steel reinforcement is less congested and space will be available to perform prestressing works.


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