Abstract: Scheduling delay is common in construction projects, and effective time claim is the guarantee of the interests of all parties and project benefits when the project duration is delayed. Therefore, it is of great significance to make time claim properly. However, the methods for time claim at present have certain shortcomings and limitations although they have different strengths respectively as well. The key problem is that the principle of interaction among delayed activities and the effect on total duration have not been clarified when several activities are delayed. As a result, time claim is often accompanied by disputes and difficult to deal with. This paper is to propose a new method for time claim analysis based on float which reveals the laws of network change in scheduling delay.
Firstly, the "combination effect", which refers to the phenomenon that the actual delay of total duration is not always equal to the sum of the values that each delay postpones total duration alone, is proposed in this paper. Then,the principle of interaction among delayed activities is studied based on float characteristics in CPM. And the laws of combination effect in multi-activities delay are revealed. The proportion of combination effect for each delayed activity is determined according to the influencing factors of combination effect, and the responsibility of each delayed activity in time claim is further derived. Therefore, a time claim method based on float is proposed.
Finally, a case project is used to demonstrate its capabilities and the advantages of the proposed method are analyzed. Results show that this method can make up for the shortcomings of current common methods. As for the analysis content, the float ownership is more reasonable, and the claim result is fairer for the proposed method. As for the analysis tool, the method can be programmed, thus avoiding the multiple updates of the network.
The contribution of this paper is twofold.First, the combination effect is revealed,and the influence relationship among delayed activities and how this interaction affects the total duration in CPM are figured out. Second, the time claim method proposed in this paper can provide project managers with a powerful tool which can make time claim fairer.

Key words: floats, construction delay, time claim, responsibility sharing