Study On The Effect Of Opening On The Basement-roof To The Fixed End Of High-rise Building

There are many rules when the basement roof used as the fixed-end of the high-rise buildings in current specifications. For example, the smallest stiffness ratio of basement and upper layer is limited, and structural measures should be taken to ensure the roof is rigid. Avoiding setting the big hole in the basement roof is mentioned in several locations of specifications. But there is no quantitative regulation for the size of the hole, what’s more, hole’s position and arrangement form of the hole is lack of specific rules.Thanks to its using function, high-rise buildings sometimes open hole in the basement roof inevitably, and the hole sometimes can be very large. The opening on the roof undoubtedly will cause a certain degree of weakened on stiffness of the slab, decrease the lateral stiffness of the structure, thus affect the basement roof build-in effect of upper structure.Using the method of finite element analysis, this paper discusses questions above by establishing two groups of the frameworks, shear-frame structures and shear wall structures with two layers of basement, one group of which are models with basement roof fixed, and the another one with basement roof non-fixed. The dynamic performance, displacement, internal force characteristic of the two groups of models are compared and analyzed to draw a conclusion. This paper include mainly:(1) The effect of the hole sizes to the fixed end. In this part, holes are set in the center of the roof, and the sizes are varied via changing its width and length proportionately.(2) The effect of arrangement of the holes to the fixed end. Using a symmetric way, a total of11different forms of open hole is designed.(3) The effect of position of the holes to the fixed end. In this part, holes are set in different positions. Through the research, the conclusion can be drawn as the following:(1) Compared with build-in model, when the rate of hole’s area and whole floor is less than36%, the dynamic characteristics, internal force and displacement characteristic of non-fixed models changes smoothly, and the error rate remains stable, and the value is small, the basement roof can be used as the fixed end of upper structure; and when the rate is more than36%, the characteristics change violently, and error rate increases rapidly, at this time basement roof can’t be used as fixed end.(2) When two holes are set on the edge of the roof symmetrically or diagonally, the basic period and the maximum top displacement of non-fixed models are larger, and when compared with fixed models the error rate is higher, what’s more, the local stress concentration around the hole or the edge of the roof is more serious. But the change caused by the holes inside roof is smallest. When big hole will be set inside basement roof unavoidably, in order to ensure the safety of fixed end, opening is advised to be arranged symmetrically in the middle of the roof and holes set on the edge of the roof symmetrically or diagonally should be avoided as far as possible.(3) In the case of smaller reasonable opening rate, different opening location would have an effect only on the margins between non-fixed models and fixed models in basic period and maximum top displacement. But local stress concentration around the opening will be caused obviously when the hole arranged up along one edge of the roof. Appropriate advise is drawn as that the hole arranges far away from edge for the safety of the fixed end.