Abstract
Statistically,equipment pipelines installed under beams will occupy onesixth to one-fifth of storey height of a building,reducing clear height of the building storey.If equipment pipelines cross beams through the openings in beam webs,they will not occupy the space under the beams.As a result,the storey height and the total height of the building will be reduced significantly,which will bring about great economic benefit.
A commercial and office building on the Middle Huaihai Road in Shanghai is designed as a reinforced concrete frame structure,with 3 floors below ground,6 floors above ground,and a total height of 24 meters.The typical storey height is 3.65 meters.The typical beam height is 650 mm.And the typical beam span is about 9 meters.In order to satisfy the quality of Grade A,all equipment pipelines of the building are required to be installed through the openings in beam webs.Therefore,in each storey of the structure,more than 50% of the frame beams are designed to open a rectangular hole of 220 mm H×400 mm W at both ends.The cross section of RC beam is weakened due to the openings,which may reduce the ultimate bearing capacity of the beam,causing the change of the failure mode of RC beam from ductile bending failure to brittle diagonal tension failure on hole side or shear failure of chord.Due to the stress concentration on the corners,rectangular holes may severely crack in advance,affecting the normal use of the beam and causing psychological anxiety to the user.The flexural stiffness of the beam with openings is reduced,resulting excessive deformation of the beam and the whole structure.
Based on test results of 61 simply supported RC beams with web openings,Cai J.et al.[1]summarized four typical kinds of failure modes.By regression analysis,the calculation method of shear bearing capacity of RC beams with web openings was proposed.Liu R.et al.[2]conducted six simply supported RC beams with web openings and four restrained ones.Openings in beam webs were divided into two dif ferent kinds,one subjected to bending and the other subjected to shearing,according to the position of openings on beams.Based on experimental results,calculation methods of shear bearing capacity and bending bearing capacity RC beams were proposed.Zhang S.et al.[3]studied the stress distribution characteristics of the circular,elliptical and rectangular openings of beams by photo-elastic experiment method.Huang T.et al.[4]analyzed effects of beam stiffness,opening height,opening width and opening eccentricity on deformation of beams by 42 RC beams'experiment.The relationship between crack width in the compressive corner of rectangular openings and the opening height was analyzed in the same study.Wu Y.et al.[5]conducted a pseudo-dynamic experiment to study the seismic performance of two single-storey RC frame structures with different sizes of beam opening.
Calculation methods of ultimate bearing capacity and deformation of RC beams proposed in above research were only applied to beams with small opening,whose height less than one third of beam height.The typical height of beam opening of the commercial and office building in this paper is 220 mm,higher than one third of the typical beam height,650 mm.Considering the large number of beams with two big openings,the long typical span of beams,and the unfavorable location of openings at beam's shearing segments,it is significant to prove the mechanical performance of the typical beams with big openings,and the performance of the overall RC frame structure.
Two full-scale RC beams with openings(Figure 1 and Figure 2)and one without opening(Figure 3)are designed according the original RC frame structure.Loading experiments of the beams are conducted to study the mechanical performance of crack resistance,flexural stiffness,failure mode,ultimate bearing capacity and so on(Figure 4).The finite element numerical models for the beams are established and verified by the experimental results.Using the proved finite element model to analyze the bending stiffness of other beams with openings different from experimental beams.Considering the stiffness degradation of beams,the overall structure performance is analyzed,and compared with the analysis result without considering the stiffness degradation of beams due to big openings.
Figure 1 Experimental beam 1
Figure 2 Experimental beam 2
Figure 3 Un-opened experimental beam
Figure 4 The mechanical test of full scale member