An analysis of structure design of storey - increase of certain building on natural foundation

Transcription

1 An analysis of structure design of storey - increase of certain building on natural foundation H. Xuanjiangl, C. Haosheng2 & Zh,Shudong2 lnanjing UniversiQ of Technology, China 2Jiangsu Foundation Engineering General Company, China Abstract This paper introduces an example of engineering construction work; the reforming of a 3-storey office building constructed on a natural foundation more than forty years ago into a 7-storey residence to meet the needs of social development. The structural system with an outer covered frame storey-increase was chosen, as it allowed the area to be enlarged. According to the principle of biomimetics, the plant root has a better stability, so an artificial excavated-bellied pile foundation was made on natural foundation. At the same time, the design was carried out according to the method of deformation control. Its actual value of settlement is found to be consistent with the estimated one, thus showing that the design theory is consistent with engineering practice, 1 Introduction This L-type office building was built in 1954 and was used as a guesthouse in the 1970s. The main body is a 3-storey brick masonry building having a reinforcement concrete strip foundation and partial basement. In order to solve the housing problem for the younger staff and workers, the building was reformed into a 7-storey residence. However, the building had been out of repair for years and there were leaks in the basement and bathrooms, which could not be used any more, In addition, there was a 6-storey building in close vicinity. Therefore gradual retreating step-up had to be made, so as to let light enter this neighboring building. It was such a complex job that even water pipes, electric wiring had to be renewed.

2 286 Design and Nature 2 Site condition The site is flat, while the ground distribution consists of 4 layers as follows: ls layer: fill soil being 2. O 2.7m thick, yellowish brown, loose and damp, containing a small amount of broken bricks. 2nd layer: mainly silt clay, but varies rapidly, as it subdivides into 4 layers, namely, 2-1 layer O 2.6m thick, grayish brown, being of plastic state, loading capacity fk=202kpa; 2-2 layer, O 2. 1 m thick, grayish brown to yellowish brown, of soft plastic state, loading capacity fk=llokpa;2-3 layer O 6.Om thick, grayish brown to yellowish brown, being of plastic to hard plastic state, loading capacity fk=274kpa; 2-4 layer being very thick, not drilled through. This layer contains gravel, being of hard plastic state. 3rdlayer: silt clay layer and it can be subdivided into 2 layers, namely, 3-1 layer, O 2. 1 m thick, brownish yellow, being of plastic state; 3-2 layer O 9.5 m thick, being of hard plastic state. 4ti layer: residual soil, not drilled through, being of hard plastic state. As seen above, this layer of soil is very uneven, There is generally layer lacking and there is even cross-stratification, the underground water is about m below the ground level and belongs to phreatic water type. So it has to fulfill the requirements of uneven settlement of seriously controlled housing. 3 The rationale of the structure design According to the above housing situation and site condition, the engineering structure design had following rationale: (1) It is necessary to renovate the seriously damaged brick masonry structure of the old 3-storey building. (2) It is necessary to consider the whole structural system of storeyincrease regarding its antiseismic reinforcement condition in order to meet the requirements of protection against the earthquakes that frequently occur along the coast of the Pacific Ocean. (3) It is necessary to make the structure design of the outer frame for storey-increase so as to meet the requirements of the distance of the building position and its gradual retreating step-up layers ensuring its stability and durability. 4 The design project of the structure 4.1 Design principle (1) To combine the new renovated brick-masonry storey-increase structure with the old one forming a new structural system to protect against earthquakes and other natural disasters.

3 287 (2) To ensure its stability and durability so the building can be continuously used for more than 50 years. (3) To fully satisfy the functions of the new storey-increase building. 4.2 The design of the storey-increase structural system To meet the requirements of the storey-increase, it was necessary to enlarge the span of the original building; so an outer enlarged frame was chosen. The outer frame structure can stand as firm as the skeleton of an animal shell thus ensuring that the renovated interior building structure can withstand the action of outside load given by wind, earthquakes and other such forces, The root foundation of the outer frame column is designed according to the principle of biomimetics, for the plant root has a better stability. Therefore the foundation is made to be an excavated bellied pile with an enlarged diameter, deeply penetrating a certain depth of soil layer to ensure the stability of the main body. In order to lessen the number of storeys and to reduce its own weight, each frame unit adopts the truss type strut bracing, Then, the space fills with wall material to form a wall separating the rooms of the building. Along each truss unit, 6 connection beams are used to form a frame structure system. According to its position and the requirements of the enlarged construction area, the outer frame column is designed as a rectangular type, T-type, or L-type cross-section (its position is shown in figure 1). The outer frame structure after having made storeyincreases is a complete set of multi-storey structure building, such as 4-storey, 5- storey, 6-storey, 7-storey, The original 3-storey building having an area of 2445m2 is enlarged to an area of 45652m, being enlarged by times the original one, and it fulfills all the requirements of the housing function as a new building. 4.3 Foundation engineering design There are 3 kinds of excavated piles designed with enlarged root, namely, JC, JD and JF, and their diameters respectively, while the enlarged diameters respectively. Entering the 2-4 layer or 4 layer bearing stratum supporting course at the anchoring soil depth, will control the horizontal displacement and vertical settlement of the outer columns. 4.4 The construction measure of the storey-increase structure Because the outer frame structure is designed with a variable rigidity system of unequal height, the connecting structure of various parts of the structure system are strengthened in this step, especially the settlement seam set at the connecting place between the rear part of the outer frame column and the original old building board, After the load of the outer frame structure is fully added and a certain amount of settlement is produced, the last filled strap at the settlement seam is then connected as a whole, to effectively control the absolute settlement and the differential settlement of the foundation.

4 288 Design and Nature 5 Observation of the settlement and the analysis 5.1 Arrangement of the observation points At the original building wall body and storey-increase column wall body respectively 16 settlement observation points were placed, so as to monitor, compare and analysis, (the observation points are shown in fig. 1), 5.2 Observation results and analysis Observing when the beam plate of the second layer is cast in place, until 4-7 storeys of the storey-increase structure and the whitewash is done. The result of the observation shows no settlement change i.e. no settlement at the S1 Slfj settlement observation points of the original building wall body, only a little settlement of about 2 3rnrn at the settlement observation points S 1 S lb of the storey-increase outer frame column, and only at point S 10 is the settlement is 4rnm. The average settlement is 2,6mrn and the maximum differential settlement is 2mrn. This shows that the design for the structure system of the storey-increase is reasonable, The foundation deformation monitor has provided the construction of building storey-increase with a reliable guarantee, 6 Conclusions (1) By storey-increase and reforming, the low buildings built in the 1950s can be effectively reformed to get antiseismic protection and reinforcement and can be made much bigger. Social and economic benefit can be obtained, and it is an effective way of solving the housing problem by developing space resources. (2) It is worthwhile considering and summarizing making reasonable use of the principle of biomimetics, to successfully carry out the design of the outer frame type storey-increase structural system and to strengthen the corresponding measure of structure connecting construction as an effective way of ensuring the stability and durability of the storey-increase and reform the new main body structure, (3) Designing the storey-increase structure of unequal height and reform engineering on the uneven foundation by strengthening foundation deformation monitoring is a reliable way of ensuring safety constructions, It also provides the reasonable arrangement of construction order and controlling loading with a reliable basis. References [1] Tang, N. & Xianjiang, H., The application of underpinning technology on foundation engineering of storey-increase and reform building, Nanjang University Publishing House, Nanjing, 1992.

5 Design and Nature 289 w& tti$-!q ml-- YHl- 1 +! R@ +--k#-t@ ~:-w Figure 1: The arrangement diagram for settlement observation points if storey - increase and reform engineering. 1. S1 Slb are the settlement observation points arranged on the original building wall body of the reform engineering, 2, S1 Slfj are the settlement observation points arranged on the storey-increase structure outer frame column and wall body.