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?

#### 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?

• Posted by  Sudheer Kumar Raju
13 Jun, 2012

Question: 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?

Basically, there are mainly three reasons for this arrangement:
(i) The permanent structure is a statically indeterminate structure. During construction
by using span-by-span construction, if the first phase of construction consists of the
first span length L only, then the sagging moment in the mid span of the partially
completed bridge is larger than that of completed two-span permanent structure.
To avoid such occurrence, 0.25L of bridge segment is extended further from the
second pier which provides a counteracting moment, thereby reducing the mid-span
moment of the partially completed bridge.

(ii) The position of 1.25 L countering from the first pier is the approximate location of
point of contraflexure (assume that the two-span bridge is uniformly loaded) in
which the bridge moment is about zero in the event of future loaded bridge.
Therefore, the design of construction joint in this particular location has the least
adverse effect on the structural performance of the bridge.

(iii) In case of a prestressed bridge, prestressing work has to be carried out after the
construction of first segment of the bridge. If the prestressing work is conducted at
the first pier which is heavily reinforced with reinforcement, it is undesirable when
compared with the prestressing location at 1.25L from the first pier where there is
relatively more space to accommodate prestressing works.

Note: Span-by-span construction means that a bridge is constructed from one bridge span to another until its completion.