Bee KANGSAOVANG

Analysis on the Limit state of Composite Girder bridge to Positive Bending 
 
Takeshi MIYASHITA

In the revision of the Specifications for Highway Bridges (November 2017), the limit state design method was shifted from the allowable stress design method to the limit state design method based on the partial factor design method, and new limit states were defined. The limit states are defined to classify the conditions of bridges for verifying the load-bearing capacity of bridges and are defined in terms of the combination of situations and the degree of feasibility of each combination. Then, as a standard for load-bearing capacity of bridges, the bridge condition is classified appropriately, the limit state of the classification is defined, the bridge condition is evaluated under the design condition, and the reliability that this condition does not exceed the limit state is measured. Here, each limit state is defined as follows:
 Limit state 1: The limit state in which the load-bearing capacity of a member is secured.
 Limit state 2: The limit state in which the load-bearing capacity as a member is reduced but still within the assumed capacity.
 Limit state 3: The limit beyond which the load-bearing capacity as a member is completely lost.
The H29 Guidelines allow the limit state of a bridge to be represented by the limit state of each member, but the limit state of each member and the method of verification are not clear.
Therefore, this research is conducted to analyze the experimental data of composite girders subjected to positive bending, based on the data of static bending tests of composite girders conducted in the past, and to extract the events of each member. Then, candidates for limit states 1 to 3 are considered and trial calculations are made for each limit state. The purpose of this research is to examine the limit states and verification methods.
As limit states and verification methods, the following limit states are assumed: limit state 1, where the lower flange reaches yield strain; limit state 2, where the lower flange and part of the girders yield and the strain on the upper surface of the slab reaches 2000μ; and limit state 3, where the total plastic moment is reached.
As a result, in the present research, the calculated values are almost the same as the experimental values, and the errors between the calculated and experimental values are small.
In conclusion, the calculated results are close to the experimental results for all the limit states, and the proposed method is considered to be a candidate for one of the limit states.